Chapter 22. Neurologic Disorders Caused by Lesions in Specific Parts of the Cerebrum

The long-standing controversy about cerebral functions, whether they are diffusely represented in the cerebrum with all parts roughly equivalent, or localized to certain lobes or regions, has been resolved to the satisfaction of most neurologists. Clinicians have demonstrated beyond doubt that particular functions are assignable to certain cortical regions. For example, the pre- and postrolandic zones control motor and sensory activities, respectively, the striate occipital zones control visual perception, the superior temporal gyri are auditory, and so on. Beyond these broad correlations, however, there is a notable lack of precision in the cortical localization of most of the behavioral and mental operations described in Chaps. 20 and 21. In particular, of the higher order functions, such as attention, vigilance, apperception, and analytic and synthetic thinking, none has a precise and predictable anatomy; or, more accurately, the neural systems on which they depend are widely distributed among several regions.

One may inquire into what precisely is meant by cerebral localization. Does it refer to the physiologic function of a circumscribed group of neurons in the cerebral cortex, indicated clinically by a loss of that function when the neurons in question are destroyed? This is the way in which neurologists have assigned functions to particular areas of the cerebral cortex. However, from what we know of the rich connectivity of all parts of the specialized cortical centers, one must assume that this is only partly the case. Most who ponder this subject believe that the organization of cerebral function is based on discrete networks of closely interconnected afferent and efferent neurons in several regions of the brain. These ensembles must be linked by both regional and more widespread systems of fibers. This is especially apparent in the discussion of the anatomy of complex cognitive properties such as intelligence, as described in Chap. 21. Thus, many basic functions are anchored in one cortical region and a lesion there causes loss of a particular ability. But it is apparent from physiologic studies such as functional imaging and electromagnetic stimulation that widely distributed networks are engaged, which nonetheless encompasses the region that can be ablated and eliminate the function in question.

These aspects of cerebral localization—brought out so clearly in the writings of Wernicke, Déjerine, and Liepmann, have been elaborated by Luria (1966 and 1969) and the Russian school of physiologists and psychologists and extended by Geschwind (1965). In keeping with the model of interconnected networks, they viewed function not as the direct property of a particular, highly specialized region of the cerebrum but as the product of complex, diffusely distributed activity by which sensory stimuli are analyzed and integrated at various levels of the nervous system and then united, through a system of temporarily acquired connections, into a working mosaic adapted to accomplish a particular task. To some extent, this model has been corroborated by functional imaging studies, which show increased metabolic activity in several cortical regions during almost every form of human behavior, including willed motor acts, language tasks, and those coinciding with perceptive and apperceptive sensory experiences. Within such a functional system, the initial and final points (the task and the effect) remain unchanged, but the intermediate links (the means of performance of a given task) may be modified within wide limits and will never be exactly the same on two consecutive occasions. Thus, when a certain act is called for by a spoken command, the dominant temporal lobe must receive the message and transmit it to the premotor areas. Or it may be initiated by the intention of the individual, in which case the first measurable cerebral activity (a "readiness potential") occurs anterior to the premotor cortex. The motor cortex is also always under the dynamic control of the proprioceptive, visual, and vestibular systems. Thus, a lesion that affects any one of several elements in the act may cause loss of a skilled ability, either the motor centers themselves or their connections with the other elements.

Another theoretical scheme of cerebral function identifies cortices of similar overall structure and divides the cerebral mantle into three longitudinally oriented zones, the triune brain articulated by Paul Maclean. A central vegetative neuronal system (allocortex and hypothalamus) provides the mechanisms for all internal functions, the milieu intérieur of Bernard and Cannon. An outer zone, comprising the sensorimotor and association cortices and their projections, provides the mechanisms for perceiving the external world and interacting with it, and a region between them (limbic–paralimbic cortices) that provides the bridges that permit the adaptation of the organism's needs to the external environment. This ostensibly metaphoric concept of central nervous system function, first proposed by Broca, was elaborated by Yakovlev and has been adopted more recently by Benson and by Mesulam (1998). Such a model retains to a large degree the cytoarchitectural similarities among areas that serve similar functions (i.e., the scheme of Brodmann discussed further on) and also respects the sequence of brain maturation (myelination) of connecting pathways proposed by Flechsig (see Fig. 28-3). In this way, localization may be viewed as the product of genetic patterns of structure, which mature during development, and their synaptic formations, which permit the development of complex circuits during lifelong learning and experience.

It is worthwhile to point out that these broadened concepts of cerebral function, which apply to all mental activities, contradicts both the historical notion that there is a functional equivalence of all cerebral regions and also the more recently developed one that assumes strict localization of any given activity.

From these remarks, it follows that subdivision of the cerebrum into frontal, temporal, parietal, and occipital lobes is somewhat of an abstraction in terms of landmarks and cerebral function. Some of these delineations were made long before our first glimmer of knowledge about the function of the cerebrum. Even when neurohistologists began parceling the neocortex, they found that their areas did not fall neatly within zones bounded by sulci and fissures. Therefore, when the terms frontal, parietal, temporal, and occipital are used, it is largely to provide the clinician with familiar and manageable anatomic landmarks for localization (Fig. 22-1).

The current method of study of cortical activity is by functional imaging techniques (positron emission tomography [PET] and functional magnetic resonance imaging [fMRI]). Invariably, an ensemble of areas, a "network" of the variety described earlier, is activated to perform even seemingly simple tasks such as recalling a name, visualizing or identifying an object, or carrying out a commanded task. The fact that multiple areas of the cortex are entrained may seem at odds with the classic view of lesional neurology, but as already stated, the discrepancy is one of epistemology in that normal function does not equate with abnormal function as exposed by a focal lesion. A lesion in the cerebrum merely exposes the site at which damage results in the greatest loss of that particular function but does not reveal the much wider area that is essential for the full normal operation of that function. Imaging studies similarly demonstrate that certain regions of the cortex are necessary to fully conduct particular behaviors, but they are not sufficient for their enactment.

Pertinent to this subject are a number of morphologic and physiologic observations. Along strictly histologic lines, Brodmann distinguished 47 different areas of cerebral cortex (Fig. 22-2), and von Economo identified more than twice that number. Although this parceling was severely criticized by Bailey and von Bonin (and the data upon which Brodmann based his system were never published), it is still used by physiologists and clinicians, who find that the Brodmann areas do indeed approximate certain functional zones of the cerebral cortex (Fig. 22-3). Also, the cortex has been shown to differ in its various parts by virtue of connections with other areas of the cortex and with the thalamic nuclei and other lower centers. Hence, one must regard the cortex as a heterogeneous array of many anatomic systems, each with highly organized intercortical and diencephalic connections.

The sheer size of the cortex is remarkable. Unfolded, it has a surface extent of about 4,000 cm2, about the size of a full sheet of newsprint (right and left pages). Contained in the cortex are many billions of neurons (estimated at 10 to 30 billion) and five times this number of supporting glial cells. The intercellular synaptic connections number in the trillions. Because nerve cells look alike and presumably function alike, the remarkable diversity in human intelligence, store of knowledge, and behavior must depend on the potential for almost infinite variations in neuronal interconnectivity.

Most of the human cerebral cortex is phylogenetically recent, hence the term neocortex. It was also referred to as isocortex by Vogt because of its uniform embryogenesis and morphology. These latter features distinguish the neocortex from the older and less uniform allocortex ("other cortex"), which comprises mainly the hippocampus and olfactory cortex. Concerning the detailed histology of the neocortex, six layers (laminae) can be distinguished—from the pial surface to the underlying white matter they are as follows: the molecular (or plexiform), external granular, external pyramidal, internal granular, ganglionic (or internal pyramidal), and multiform (or fusiform) layers (illustrated in Fig. 22-4). Two cell types—relatively large pyramidal cells and smaller, more numerous rounded (granular) cells—predominate in the neocortex, and variations in its lamination are largely determined by variations in the size and density of these neuronal types.

Many variations in lamination have been described by cortical mapmakers, but two main types of neocortex are recognized: (1) the homotypical cortex, in which the six-layered arrangement is readily discerned, and (2) the heterotypical cortex, in which the layers are less distinct. The association cortex—the large areas (75 percent of the surface) that are not obviously committed to primary motor or sensory functions—is generally of this latter type. Homotypical areas are characterized by either granular or agranular nerve cells. The precentral cortex (Brodmann areas 4 and 6, mainly motor region) is dominated by pyramidal rather than granular cells, especially in layer V (hence the term agranular). Agranular cortex is distinguished by a high density of large pyramidal neurons. In contrast, the primary sensory cortices, postcentral gyrus (areas 3, 1, 2), banks of the calcarine sulcus (area 17), and the transverse gyri of Heschl (areas 41 and 42), where layers II and IV are strongly developed for the receipt of afferent impulses, has been termed granular cortex because of the marked predominance of granular cells a preponderance of which are small neurons (Fig. 22-5).

Beyond these morphologic distinctions, the intrinsic organization of the neocortex follows a pattern elucidated by Lorente de Nó. He described vertical chains of neurons arranged in cylindrical modules or columns, each containing 100 to 300 neurons and heavily interconnected up and down between cortical layers and to a lesser extent, horizontally. Figures 22-4 and 22-5 illustrate the fundamental vertical (columnar) organization of these neuronal systems. Afferent fibers activated by various sensory stimuli terminate mainly in layers II and IV. Their impulses are then transmitted by internuncial neurons (interneurons) to adjacent superficial and deep layers and then to appropriate efferent neurons in layer V. Neurons of lamina III (association efferents) send axons to other parts of the association cortex in the same and opposite hemisphere. Neurons of layer V (projection efferents) send axons to subcortical structures and the spinal cord. Neurons of layer VI project mainly to the thalamus. In the macaque brain, each pyramidal neuron in layer V has about 60,000 synapses, and one afferent axon may synapse with dendrites of as many as 5,000 neurons; these figures convey some idea of the wealth and complexity of cortical connections. These columnar ensembles of neurons, on both the sensory and motor sides, function as the elementary working units of the cortex.

Whereas certain regions of the cerebrum are committed to special perceptual, motor, sensory, mnemonic, and linguistic activities, the underlying intricacy of the anatomy and psychophysical mechanisms in each region are just beginning to be envisioned. The lateral geniculate-occipital organization in relation to vision and recognition of form, stemming from the work of Hubel and Wiesel, may be taken as an example. In area 17, the polar region of the occipital lobe, there are discrete, highly specialized groups of neurons, each of which is activated in a small area of lamina 4 by spots of light or lines and transmitted via particular cells in the lateral geniculate bodies; other groups of adjacent cortical neurons are essential for the perception of color. Lying between the main unimodal receptive areas for vision, audition, and somesthetic perception are zones of integration called heteromodal cortices. Here neurons respond to more than one sensory modality or neurons responsive to one sense are interspersed with neurons responsive to another.

The integration of cortical with subcortical structures is reflected in volitional or commanded movements. A simple movement of the hand, for example, requires activation of the premotor cortex (also called accessory motor cortex), which projects to the striatum and cerebellum and back to the motor cortex via a complex thalamic circuitry before the direct and indirect corticospinal pathways can activate certain combinations of spinal motor neurons, as described in Chaps. 3 and 4.

Interregional connections of the cerebrum are required for all natural sensorimotor functions; moreover, as indicated above, their destruction disinhibits or "releases" other areas. Denny-Brown referred to the latter as cortical tropisms. Thus, destruction of the premotor areas, leaving the precentral and parietal lobes intact, results in release of sensorimotor automatisms such as groping, grasping, and sucking. Parietal lesions result in complex avoidance movements to contactual stimuli. Temporal lesions lead to a visually activated reaction to every observed object and its oral exploration, and limbic sexual mechanisms are rendered hyperactive.

Another group of disorders known as disconnection syndromes depend not merely on involvement of certain cortical regions but more specifically on the interruption of inter- and intrahemispheric fiber tracts. Extensive white matter lesions may virtually isolate certain cortical zones and result in a functional state that is the equivalent of destruction of the overlying cortical region. Some of these disconnections are indicated schematically in Fig. 22-6; the usually involved fiber systems include the corpus callosum, anterior commissure, uncinate temporofrontal fasciculus, occipito- and temporoparietal tracts. An example is the isolation of the perisylvian language areas from the rest of the cortex, as occurs with anoxic–ischemic infarction of border zones between major cerebral arteries (see "Disconnection Syndromes" further on).

Anatomic and Physiologic Considerations

The frontal lobes lie anterior to the central or rolandic sulcus and superior to the sylvian fissure (see Fig. 22-1). They are larger in humans (30 percent of the cerebrum) than in any other primate (9 percent in the macaque). Several systems of neurons are located here, and they subserve different functions. Brodmann areas 4, 6, 8, and 44 relate specifically to motor activities. The primary motor cortex, i.e., area 4, is directly connected with somatosensory neurons of the anterior part of the postcentral gyrus as well as with other parietal areas, thalamic and red nuclei, and the reticular formation of the brainstem. The supplementary motor cortex, a portion of area 6, shares most of these connections. As pointed out in earlier chapters, all motor activity requires sensory guidance, and this comes from the somesthetic, visual, and auditory cortices and from the cerebellum via the ventral tier of thalamic nuclei.

Area 8 is concerned with turning the eyes and head contralaterally. Area 44 of the dominant hemisphere (Broca area) and the contiguous part of area 4 are "centers" of motor speech and related functions of the lips, tongue, larynx, and pharynx. Left-sided lesions cause a distinctive articulatory and language syndrome, and bilateral lesions in these areas cause paralysis of articulation, phonation, and deglutition. The medial-orbital gyri and anterior parts of the cingulate and insular gyri, which are the frontal components of the limbic system, take part in the control of respiration, blood pressure, peristalsis, and other autonomic functions. The most anterior parts of the frontal lobes (areas 9 to 12 and 45 to 47), sometimes referred to as the prefrontal areas, are particularly well developed in human beings but have imprecisely determined functions. They are not, strictly speaking, parts of the motor cortex in the sense that electrical stimulation evokes no direct movement (the prefrontal cortex is said to be inexcitable). Yet these areas are involved in the initiation of planned action and executive control of all mental operations, including emotional expression.

The frontal agranular cortex (areas 4 and 6) and more specifically, pyramidal cells of layer V of the pre- and postcentral convolutions provide most of the cerebral efferent motor system that forms the pyramidal, or corticospinal, tract (see Figs. 3-2 and 3-3). Another massive projection from these regions is the frontopontocerebellar tract. In addition, there are several parallel fiber systems that pass from frontal cortex to the caudate and putamen, subthalamic and red nuclei, brainstem reticular formation, substantia nigra, and inferior olive, as well as to the ventrolateral, mediodorsal, and dorsolateral nuclei of the thalamus. Areas 8 and 6 are connected with the ocular and other brainstem motor nuclei and with identical areas of the other cerebral hemisphere through the corpus callosum. A tract, the fronto-occipital fasciculus, connects the frontal with the occipital lobe and the uncinate bundle connects the orbital part of the frontal lobe with the temporal lobe.

The granular frontal cortex has a rich system of connections both with lower levels of the brain (medial and ventral nuclei and pulvinar of the thalamus) and with virtually all other parts of the cerebral cortex, including its limbic and paralimbic parts. As to its limbic connections, the frontal lobe is unique among cerebrocortical areas in that electrical stimulation of the orbitofrontal cortex and cingulate gyrus has manifest effects on respiratory, circulatory, and other vegetative functions. These parts of the frontal cortex also receive major afferent projections from other parts of the limbic system, presumably to mediate the emotional responses to sensory experiences; they, in turn, project to other parts of the limbic and paralimbic cortices (hippocampus, parahippocampus, anterior pole of the temporal lobe), amygdala, and midbrain reticular formation. Chapter 25 describes these frontal–limbic connections in greater detail.

Most of the popular notions relating to the function of the frontal lobes are oversimplified. In the frontal lobe, are presumed to reside the mechanisms that govern personality, character, motivation, and our unique capacities for abstract thinking, introspection, and planning. These qualities and traits do not lend themselves to easy definition and study and certainly not to discrete localization. Most are too subtle to isolate or even to measure accurately. Except for the more posterior frontal mechanisms subserving motility, motor speech, and certain behaviors relating to impulse (conation), neurologists recognize that the other features of frontal lobe disease are more abstruse.

Blood is supplied to the medial parts of the frontal lobes by the anterior cerebral artery and to the convexity and deep regions, by the superior (rolandic) division of the middle cerebral artery. The underlying deep white matter is supplied by a series of small penetrating arteries, called lenticulostriate vessels that originate directly from the initial portion (stem) of the middle cerebral artery, as detailed in Chap. 34.

Clinical Effects of Frontal Lobe Lesions

For descriptive purposes, the clinical effects of frontal lobe lesions can be grouped under the following categories: (1) motor abnormalities related to the prerolandic motor cortex; (2) speech and language disorders related to the dominant frontal lobe, which are described in the next chapter; (3) incontinence of bladder and bowel; (4) impairment of capacity for goal-directed sustained mental activity, and the ability to shift from one line of thought or action to another, i.e., aspects of attention manifest as impersistence and perseveration; (5) akinesia and lack of initiative and spontaneity (apathy and abulia); (6) changes in personality, particularly in mood and self-control (disinhibition of behavior); and (7) an abnormality of gait that has proved difficult to characterize (see also Chap. 7 on disorders of gait).

With regard to behavior and the frontal lobe, the anterior half of the brain is in a general sense committed to the planning, initiation, monitoring, and execution of all cerebral activity. This was aptly summarized by Luria (1966 and 1973) as "goal-directed behavior." Of necessity in such a scheme, there must also be inhibitory mechanisms that control or modulate behavior. Thus, aside from the overt abnormalities of motor, speech, and voluntary movement, lesions of the frontal lobes give rise to a loss of drive, impairment of consecutive planning, an inability to maintain serial relationships of events, and to shift easily from one mental activity to another. These are combined with sucking, grasping, and groping reflexes and other obligate behaviors. In the emotional sphere, frontal lobe lesions may cause anhedonia (lack of pleasure), apathy, loss of self-control, disinhibited social behavior, and euphoria, as described further on.

Motor Abnormalities

Voluntary movement involves the motor cortex in its entirety or at least large parts of it, and of the various effects of frontal lobe lesions, most is known about the motor abnormalities. Electrical stimulation of the motor cortex elicits contraction of corresponding muscle groups on the opposite side of the body; focal seizure activity has a similar effect. Stimulation of Brodmann area 4 produces movement of discrete muscle groups or, if sufficiently refined, of individual muscles. Repertoires of larger coordinated movements are evoked by stimulation of area 6, the premotor and supplementary motor cortices.

Lesions in the posterior part of the frontal lobe cause spastic paralysis of the contralateral face, arm, and leg. Motor impulses from the frontal lobe are conducted by the direct corticospinal tract and by tracts that descend from the motor, premotor, supplementary motor, and anterior parietal cortex to the spinal cord, either directly or via the red and reticular nuclei in the brainstem. Lesions of the more anterior and medial parts of the motor cortex result in less paralysis and more spasticity, as well as a release of sucking, groping, and grasping reflexes, the actual mechanisms for which probably reside in the parietal lobe and which, as conceptualized by Denny-Brown, are tropisms or automatisms that are normally inhibited by the frontal cortex. When the lesions of the motor parts of the frontal lobe are bilateral, there is a tetraparesis in which the weakness is not only more severe but also more extensive than in unilateral lesions, affecting both spinal and cranial muscles (pseudobulbar palsy).

Ablation of the right or left supplementary motor areas (the parts of area 6 that lie on the medial surfaces of the cerebral hemispheres) was found by Laplane and colleagues (1977b) to cause mutism, contralateral motor neglect, and impairment of bibrachial coordination. On the basis of blood flow studies, Roland and colleagues and Fuster suggest that an important function of the supplementary motor area is the ordering of motor tasks or the recall of memorized motor sequences, further evidence of the executive functions of the frontal lobes. Some insight into organization in supplementary motor cortex is given by seizures originating there; they give rise to curious postures such as a fencing position or flailing of the opposite arm.

Temporary paralysis of contralateral eye turning and sometimes of head turning follows a destructive lesion in area 8, on the dorsolateral aspect (convexity) of the cerebral cortex, often referred to as the frontal eye field (see Fig. 22-1). Seizure activity in this area causes a tonic deviation of the head and eyes to the opposite side.

Destruction of the Broca convolution (areas 44 and 45) and the adjacent insular and motor cortex of the dominant hemisphere result in a reduction or loss of motor speech, and of agraphia, and apraxia of the face, lips, and tongue, as described in Chap. 23.

The gait condition described by Bruns that is caused by a frontal lobe lesion was designated by him as an ataxia of gait; he made no reference to an ataxia of limb movements. This disorder is often also referred to now as an apraxia of gait, inappropriately in our opinion, because the term apraxia is best used to describe an inability to carry out a commanded or learned motor task, not an ingrained one (see Chap. 3). What is meant by these terms in application to gait has never been clearly specified, but broadly speaking, they signify a loss of the ability to use the lower limbs in the act of walking that cannot be explained by weakness, loss of sensation, or ataxia. The patient may retain these fundamental motor and sensory functions when examined in bed and can even make motions that simulate walking while seated or reclining. As detailed in Chap. 7, the resultant pattern is a slowed, slightly imbalanced, and short-stepped gait with the torso and legs not properly in phase when placed in motion, to which may be added the feature of "magnetic" gait, where one or both feet appear to be stuck to the ground as the body moves forward. Probably the basal ganglia and their connections to the frontal lobes are involved in these cases. The steps are shortened to a shuffle and balance is precarious; with further deterioration, the patient can no longer walk or even stand. Cerebral paraplegia in flexion is the most advanced stage; the affected individual lies curled up in bed, unable even to turn over (see Chap. 7 for further discussion).

Damage to the cortices anterior to areas 6 and 8, i.e., areas 9, 10, 45, and 46, the prefrontal cortex, and also the anterior cingulate gyri, has less easily defined effects on motor behavior. The prefrontal cortex is heteromodal and has strong reciprocal connections with the visual, auditory, and somatosensory cortices. Of these, the visuomotor relationships are the most powerful. These frontal areas as well as the supplementary motor areas are involved in the planning and initiation of sequences of movement, as indicated in Chap. 4. In the monkey, for example, when a visual signal evokes movement, some of the prefrontal neurons become active immediately preceding the motor response; other prefrontal neurons are activated if the response is to be delayed. With prefrontal lesions on one side or the other, a series of motor abnormalities occur, for example, slight grasping and groping responses, a tendency to imitate the examiner's gestures and to compulsively manipulate objects that are in front of the patient (imitation and utilization behavior described by Lhermitte [1983]), reduced and delayed motor and mental activity (abulia), motor perseveration or impersistence (with left and right hemispheric lesions, respectively), and paratonic rigidity on passive manipulation of the limbs (oppositional resistance, or gegenhalten).

Incontinence is another manifestation of frontal lobe disease. Right- or left-sided lesions involving the posterior part of the superior frontal gyrus, the anterior cingulate gyrus, and the intervening white matter result in a loss of control of micturition and defecation (Andrew and Nathan). There is no warning of fullness of the bladder or of the imminence of urination or bowel evacuation, and the patient is surprised at suddenly being wet or soiled. Less-complete forms of the syndrome are associated with frequency and urgency of urination during waking hours. The patient is embarrassed unless an element of indifference is added when the more anterior (nonmotor) parts of the frontal lobes are the sites of disease.

In the spheres of speech and language, a number of abnormalities other than Broca's aphasia appear in conjunction with disease of the frontal lobes: laconic speech, lack of spontaneity of speech, telegrammatic speech (agrammatism), loss of fluency, perseveration of speech, a tendency to whisper instead of speaking aloud, and dysarthria. These are more prominent with left-sided lesions and are fully described in Chap. 23.

Cognitive and Intellectual Changes

In general, when one speaks of cognitive and behavioral aspects of frontal lobe function, reference is made to the more anterior (prefrontal) parts rather than the motor and linguistic parts. These most recently developed parts of the human brain, called the "organ of civilization" by Halstead and repeated by Luria, have the most elusive functions.

The effects of lesions of the frontal lobes were nicely divulged by Harlow's famous case of Phineas Gage, published in 1868; it has been the subject of numerous monographs ever since. His patient was a capable foreman of a railroad gang who became irreverent, dissipated, irresponsible, and vacillating (he also confabulated freely) following an injury in which an explosion drove a large iron-tamping bar into his frontal lobes. In Harlow's words, "he was no longer Gage." Another similarly dramatic example was Dandy's patient (the subject of a monograph by Brickner), who underwent a bilateral frontal lobotomy during the removal of a meningioma. Feuchtwanger, in a clinical study of 200 cases of frontal lobe injury, was impressed most with the lack of initiative, changes in mood (euphoria), and inattentiveness, without intellectual and memory deficits. Rylander, in a classic monograph, described similar changes in patients with unilateral and bilateral frontal lobectomies (see later). Kleist (1934), under the heading of alogia, stressed the importance of loss of capacity for abstract thought, as shown in tests of analogies, proverbs, definitions, etc. In chimpanzees, Jacobsen observed that the removal of the premotor parts of the frontal lobes led to social indifference, tameness, placidity, forgetfulness, and difficulty in problem solving, findings that led Egas Moniz, in 1936, to perform prefrontal lobotomies on psychotic patients (see Damasio). This operation and its successor, prefrontal leukotomy (undercutting of the prefrontal white matter) reached their height of popularity in the 1940s and (tragically) provided the opportunity to study the effects of a wide range of frontal lobe lesions in a large number of patients.

The findings in patients who underwent frontal leukotomy have been the subject of endless controversy. Some workers claimed that there were few or no discernible effects of the operation, even with bilateral lesions. Others insisted that if the proper tests were used, a series of predictable and diagnostic changes in cognition and behavior could be demonstrated. The arguments pro and con and the inadequacies of many of the studies, both in methods of testing and in anatomic verification of the lesions (the extent and location of the lesions varied considerably, and this influenced the clinical effects), have been well summarized by Walsh. Admittedly, in patients who underwent bilateral frontal lobotomy, there was little if any impairment of memory function or of cognitive function as measured by intelligence tests, and certainly no loss of alertness and orientation. And some patients who had been disabled by schizophrenia, anxious depression, obsessive–compulsive neurosis, or a chronic pain syndrome did improve with respect to their psychiatric and pain symptoms. However, many were left with changes in personality, much to the distress of their families. They were indifferent to the feelings of others; gave no thought to the effects of their conduct; were tactless, distractible, and socially inept; and were given to euphoria and emotional outbursts. El-Hai has written a fascinating historical account of the procedure in the United States and a portrait of its main proponent at the time, Dr. Walter Freeman. Although no longer undertaken, the procedure must be viewed in the context of the understanding of, and limited options for, psychiatric disease at that time.

Luria (1973) had another interesting conception of the role of the frontal lobes in intellectual activity. He postulated that problem solving of whatever type (perceptual, constructive, arithmetical, psycholinguistic, or logical, definable also as goal-related behavior) proceeds in four steps: (1) the specification of a problem (in other words, a goal is perceived and the conditions associated with it are set); (2) formulation of a plan of action or strategy, requiring that certain linguistic activities be initiated in orderly sequence; (3) execution, including implementation and control of the plan; and (4) checking or comparing the results against the original plan to see if it was adequate.

Obviously, such complex psychologic activity must implicate many parts of the cerebrum and will suffer to some extent from a lesion in any of the parts that contribute to the functional system. Luria found that when the frontal lobes are injured, there was not only a general psychomotor slowing and easy distractibility but also an erroneous analysis of the above-listed conditions of the problem. "The plan of action that is selected quickly loses its regulating influence on behavior as a whole and is replaced by a perseveration of one particular link of the motor act or by the influence of some connection established during the patient's past experience." Furthermore, there was a failure to distinguish the essential sequences in the analysis and to compare the final solution with the original conception of the problem. Plausible as this scheme appears, like Goldstein's "loss of the abstract attitude" (the patient thinks concretely, i.e., he reacts directly to the stimulus situation), such psychophysiologic analyses of the mental processes are highly theoretical, and the factors to which they refer are not easily measured.

Finally, a lesion that includes the frontal eye field may engender a type of reduced attention to the contralateral visual environment. This probably is the result of a defect in visually guided attention and it is seen only irregularly in clinical practice. The degree of neglect seen with a nondominant parietal lobe lesion is not observed with frontal lobe lesions, and it is difficult to differentiate the frontal defect from the simple impediment of being unable to direct gaze in one direction.

In modern parlance, the frontal lobe, particularly its prefrontal components, is said to exert an executive function, referring here to the overall control and sequencing of other cognitive functions. This allows for a type of self-monitoring that guides the selection of strategies to solve problems, the inhibition of incorrect responses, the ability to deal with change in focus and novelty in tasks, and probably to be able to generalize from experience. Indeed, all ability to adapt to changes in circumstance and to learn from experience requires this executive function. Unlike some of the psychic properties mentioned above, these are subject to measurement by testing and they are observable during the clinical examination as deterioration in problem solving, by repetitiousness and stereotypes, and by ineptitude in managing simple social situations. Probably, the trouble all individuals experience in maintaining a stream of thought when interrupted, a type of loss of attention, tests this function.

Other Alterations of Behavior and Personality

A lack of initiative and spontaneity is the most common effect of frontal lobe disease, and it is much easier to observe than to quantitate. With relatively mild forms of this disorder, patients exhibit an idleness of thought, speech, and action, and they lapse into this state without complaint. They are tolerant of most conditions in which they are placed, although they may act unreasonably for brief periods if irritated, seemingly unable to think through the consequences of their actions. They let members of the family answer questions and "do the talking," interjecting a remark only rarely and unpredictably. Questions directed to such patients may evoke only brief, unqualified answers. Once started on a task, they may persist in it ("stimulus bound"); i.e., they tend to perseverate. Fuster, in his studies of the prefrontal cortex, emphasizes the failure over time to maintain events in serial order (impairment of temporal grading) and to integrate new events and information with previously learned data. Placidity is a notable feature of the behavior. Worry, anxiety, self-concern, hypochondriasis, complaints of chronic pain, and depression are all reduced by frontal lobe disease, as they were to some extent by frontal lobotomy.

Extensive and bilateral frontal lobe disease is accompanied by a quantitative reduction in all psychomotor activity. The number of movements, spoken words, and thoughts per unit of time diminish. Milder degrees of this state, associated with only a delay in responses, are called abulia as described earlier. The most severe degrees take the form of akinetic mutism wherein a nonparalyzed patient, alert and capable of movement and speech, lies or sits motionless and silent for days or weeks on end. It has been attributed to bilateral lesions in the ventromedial frontal regions or frontal-diencephalic connections (but focal lesions in the upper midbrain do the same). Laplane found that the lack of motivation of the patient with bifrontal lesions and bipallidal lesions to be the same, although one would expect the latter to manifest more as a bradykinesia than as a bradyphrenia (slowness of thinking).

The opposite state, in a sense, is a behavioral disinhibition that in extreme form becomes a hyperactivity syndrome, or "organic drivenness," described by von Economo in children who had survived an attack of encephalitis lethargica. Disinhibition occurs largely with dorsolateral frontal lesions. In our patients, this syndrome has been produced most often by combined frontal and temporal lobe lesions, usually traumatic but also encephalitic, although exact clinicopathologic correlations could not be made. Such patients may also exhibit brief but intense involvement with some meaningless activity, such as sorting papers in an attic or hoarding objects or food. Possibly, compulsive behavior is related in some manner to this state and more particularly to lesions damaging the caudate-frontal connections. Combativeness and extreme insomnia or an otherwise disrupted sleep cycle are often part of the syndrome.

Pathological collecting behavior (hoarding) may be related to this type of drivenness and has been attributed to medial frontal lobe damage, including the cingulate gyri, by Anderson and colleagues based on a series of 13 patients. These patients, otherwise displaying mental clarity and despite negative personal and social consequences, collect massive amounts of useless items such as newspapers, junk mail, catalogs, food, clothing, and appliances, often encompassing several categories.

In addition to the disorders of initiative and spontaneity, frontal lobe lesions result in a number of other changes in personality and behavior. These, too, are easier to observe in the patient's natural environment than to measure by psychologic tests. It has been difficult to find a term for all these personality changes. Some patients, particularly those with inferofrontal lesions, feel compelled to make silly jokes that are inappropriate to the situation, witzelsucht or moria; they are socially uninhibited and lack awareness of their behavior. The patient is no longer the sensitive, compassionate, effective human being that he once was, having lost his usual ways of reacting with affection and consideration to family and friends. In more advanced instances, there is an almost complete disregard for social conventions and an interest only in immediate personal gratification. The patient at the same time seems to lose an appreciation of the motivations and thought processes of other sapient persons ("theory of mind"); this results in the inability to incorporate these factors into his responses. These changes, observed characteristically in lobotomized patients, came to be recognized as too great a price to pay for the loss of anxiety, pain, depression, and "tortured self-concern," hence the procedure became obsolete.

In general, the greatest cognitive-intellectual deficits relate to lesions in the dorsolateral parts of the prefrontal lobes and that the greatest personality, mood, and behavioral changes stem from lesions of the medial-orbital parts, although the two disorders often merge with one another. Benson (and Kleist and others before him) related the syndrome of apathy and lack of initiative to lesions in the dorsolateral frontal cortex, and a facetious, unguarded, and socially inappropriate state (see below) to orbital and medial frontal lesions. This distinction has held up only broadly in our experience. Some studies of penetrating brain injuries have reported an inconsistent but interesting relationship between left dorsal frontal lesions and anger with hostility, and right side orbitofrontal lesions, with anxiety and depression. Again, in clinical work, few lesions have this degree of localizability, making conclusions about emotional states somewhat uncertain.

Although the frontal lobes are the subject of a vast literature and endless speculation (see reviews of Stuss and Benson and of Damasio), a unified concept of their function has not emerged, probably because they are so large and include several heterogeneous systems. There is no doubt that the mind is greatly altered by disease of the prefrontal parts of the frontal lobes, but often it is difficult to say exactly how it is changed. Perhaps at present it is best to regard the frontal lobes as the part of the brain that quickly and effectively orients and drives the individual, with all the percepts and concepts formed from past life experiences, toward action that is projected into the future.

Psychologic Tests of Frontal Lobe Function

These are of particular value in establishing the presence of frontal lobe disease and are generally constructed to detect the ability to persist in a task and the opposite, to switch mental focus on demand. They include the Wisconsin card-sorting test, the Stroop color-naming test, sequencing of pictures, "trail making" (a two-part test in which the patient draws lines, first connecting randomly arrayed numbers on a paper in order and then connecting numbers and letters that correspond in order), the verbal equivalent of trail making, and the "go-no-go" test, both of which are used regularly in the mental status examination (see below), and the three-step hand posture test of Luria. The alphabet-number verbal trailmaking test requires the patient to give each letter of the alphabet followed by the corresponding number (A-1, B-2, C-3, etc.). In the Luria test and its variants, the patient is, for example, asked to imitate, then reproduce, a sequence of three hand gestures, typically making a closed fist, holding the open hand on its side, and then opening an outstretched palm. Patients with frontal lesions on either or both sides have difficulty performing the test in correct sequence, often perseverating, balking, or making unwanted gestures. Luria suggested testing this with the sequence of arm thrusting forward, clenching the fist, and forming a ring with the first two fingers—derivatives of this test are now used. He also pointed out (Luria, 1969) that the natural kinetic "melody," or smoothness of transition from one hand position to the next is disrupted and there is a tendency to perseverate. This has been termed "kinetic limb apraxia" by some behavioral neurologists.

It should be kept in mind that similar impairments of performance may occur with all manner of confusional and inattentive states so that no conclusion can be made if the patient is less than fully attentive. More complex mental acts that may be easily tested and betray frontal lobe disease but are less specific, in that they are also disordered by lesions in other brain regions, include serial subtraction ("working memory"), interpretation of proverbs, tests of rapid motor response, and others.

Effects of frontal lobe disease may be summarized as follows:

1. Effects of unilateral frontal disease, either left or right

   1. Contralateral spastic hemiplegia

   2. Contralateral gaze paresis

   3. Apathy and loss of initiative or its opposite, slight elevation of mood, increased talkativeness, tendency to joke inappropriately (witzelsucht), lack of tact, difficulty in adaptation

   4. If entirely prefrontal, no hemiplegia; but grasp and suck reflexes or instinctive grasping may be released

   5. Anosmia with involvement of orbital parts

2. Effects of right frontal disease

   1. Left hemiplegia

   2. Changes as in I.B, C, and D

3. Effects of left frontal disease

   1. Right hemiplegia

   2. Broca's aphasia with agraphia, with or without apraxia of the lips and tongue (see Chap. 23)

   3. Sympathetic apraxia of left hand (see "Apraxia" in Chap. 3)

   4. Changes as in I.B, C, and D

4. Effects of bifrontal disease

   1. Bilateral hemiparesis

   2. Spastic bulbar (pseudobulbar) palsy

   3. If prefrontal, abulia or akinetic mutism, lack of ability to sustain attention and solve complex problems, rigidity of thinking, bland affect, social ineptitude, behavioral disinhibition, inability to anticipate, labile mood, and varying combinations of grasping, sucking, obligate imitative movements, utilization behavior

   4. Decomposition of gait and sphincter incontinence

Anatomic and Physiologic Considerations

The sylvian fissure separates the superior surface of each temporal lobe from the frontal lobe and anterior parts of the parietal lobe. There is no natural anatomic boundary between the temporal lobe and the occipital or the parietal lobe but the angular gyrus serves as a landmark for the latter. Figure 22-1 indicates the boundaries of the temporal lobes. The inferior branch of the middle cerebral artery supplies blood to the convexity of the temporal lobe, and the temporal branch of the posterior cerebral artery supplies the medial and inferior aspects, including the hippocampus.

The temporal lobe includes the superior, middle, and inferior temporal, lateral occipitotemporal, fusiform, lingual, parahippocampal, and hippocampal convolutions and the transverse gyri of Heschl. The last of these constitutes the primary auditory receptive area and is located within the sylvian fissure. It has a tonotopic arrangement: fibers carrying high tones terminate in the medial portion of the gyrus and those carrying low tones, in the lateral and more rostral portions (Merzenich and Brugge). The planum temporale (area 22), an integral part of the auditory cortex, lies immediately posterior to the Heschl convolutions, on the superior surface of the temporal lobe. The left planum is larger in right-handed individuals. There are rich reciprocal connections between the medial geniculate bodies and the Heschl gyri. These gyri project to the unimodal association cortex of the superior temporal gyrus, which, in turn, projects to the paralimbic and limbic regions of the temporal lobe and to temporal and frontal heteromodal association cortices and the inferior parietal lobe. There is also a system of fibers that project back to the medial geniculate body and to lower auditory centers. The cortical receptive zone for labyrinthine impulses is less well demarcated than the one for hearing but is probably situated on the inferior bank of the sylvian fissure, just posterior to the auditory area. Least well delimited is the role of the medial parts of the temporal lobe in olfaction and gustatory perception, although seizure foci in the region of the uncus (uncinate seizure) often excite hallucinations of these senses.

The middle and inferior temporal gyri (areas 21 and 37) receive a massive contingent of fibers from the striate cortex (area 17) and the parastriate visual association areas (areas 18 and 19). These temporal visual areas make abundant connections with the medial limbic, rhinencephalic (olfactory), orbitofrontal, parietal, and occipital cortices, allowing for an intimate interconnection between the cortices subserving vision and hearing.

The superior part of the dominant temporal lobe is concerned with the acoustic or receptive aspects of language, as discussed in Chap. 23, which is devoted to this subject. The middle and inferior convolutions are sites of visual discriminations; they receive fiber systems from the striate and parastriate visual cortices and, in turn, project to the contralateral visual association cortex, the prefrontal heteromodal cortex, the superior temporal cortex, and the limbic and paralimbic cortex. Presumably, these systems subserve such functions as spatial orientation, estimation of depth and distance, stereoscopic vision, and hue perception. Similarly, the unimodal auditory cortex is closely connected with a series of auditory association areas in the superior temporal convolution, and the latter are connected with prefrontal and temporoparietal heteromodal areas and the limbic areas (see Mesulam, 1998). Most of these auditory connections have been worked out in the macaque but the limited number of well-studied lesions in patients suggests that they are also involved in complex verbal and nonverbal auditory discriminations in humans.

The most important functions of the hippocampus and other structures of the hippocampal formation (dentate gyrus, subiculum, entorhinal cortex, and parahippocampal gyrus) are learning and memory, already discussed in Chap. 21. There is an abundance of connections between the medial temporal lobe and the entire limbic system. For this reason, MacLean referred to these parts as the "visceral brain," and Williams, as the "emotional brain." Also included in this anatomic concept are the hippocampus, the amygdaloid nuclei, the fornices and limbic portions of the inferior and medial frontal regions, the cingulate cortices, and the septal and associated subcortical nuclei referred to as the limbic system (see Chap. 25).

Most of the temporal lobe cortex, including Heschl gyri, has nearly equally developed pyramidal and granular layers. In this respect, it resembles more the granular cortex of the frontal and prefrontal regions and inferior parts of the parietal lobes. Unlike the six-layered neocortex, the hippocampus and dentate gyrus are typical of the phylogenetically older three-layered allocortex.

A massive fiber system projects from the striate and parastriate zones of the occipital lobes to the inferior and medial parts of the temporal lobes. The temporal lobes are connected to one another through the anterior commissure and middle part of the corpus callosum; the inferior or uncinate fasciculus connects the anterior temporal and orbital frontal regions. The arcuate fasciculus connects the posterosuperior temporal lobe to the motor cortex and Broca area.

Physiologically, the temporal lobe is an integrator of "sensations, emotions, and behavior" in so far as it relates the organism's sensory experiences to emotional meaning thought it's proximity to the limbic system. Similar integrative mechanisms are operative in the parietal lobe, but only in the temporal lobe are they brought into close relationship to one's instinctive and emotional life. Self-awareness also requires a coherent and sequential stream of thought. Where the inner "stream of thought" (William James' term for constant thinking) is perceived is still an open question. Given the requirement that it be close to other integrated sensory experiences and that it incorporate the temporal lobe functions of both language and memory, a locus in the temporal lobes seems likely. Some hint of the role of the temporal lobe in our personal and emotional lives was suggested by Hughlings Jackson in the nineteenth century, derived from his insightful analysis of the psychic states accompanying temporal lobe seizures. Later, the observations of Penfield and his collaborators on the effects of stimulating the temporal lobes in the conscious patient undergoing surgical correction of epilepsy revealed something of its complex functions. The seminal writings on this subject include Williams' chapter on temporal lobe syndromes in the Handbook of Clinical Neurology and the monographs by Penfield and Rasmussen (The Cerebral Cortex of Man) and by Alajouanine and colleagues (Les Grandes Activités du Lobe Temporale).

Clinical Effects of Temporal Lobe Lesions

The symptoms that arise as a consequence of disease of the temporal lobes may be divided into disorders of: (1) special senses (visual, auditory, olfactory, and gustatory), (2), language, (3) memory and time perception, (4) emotion, and behavior. Of central importance also are the roles of the superior part of the dominant (usually left) temporal lobe in language and handedness. Several of these functions and their derangements are of such scope and importance that they are accorded separate chapters. Language is discussed in Chap. 23, memory in Chap. 21, and the neurology of emotion and behavior in Chap. 25; these subjects are omitted from further discussion here.

Visual Disorders

In Chap. 13 (on vision), it was pointed out that lesions of the white matter of the central and posterior parts of the temporal lobe characteristically involve the lower arching fibers of the geniculocalcarine pathway (Meyer loop). This results in an upper homonymous quadrantanopia, usually not perfectly congruent. However, there is considerable variability in the arrangement of visual fibers as they pass around the temporal horn of the lateral ventricle, accounting for the smallness of the field defect in some patients after temporal lobectomy or stroke and extension into the inferior field in others. Quadrantanopia from a dominant (left-sided) lesion is often combined with aphasia.

Bilateral lesions of the temporal lobes render a monkey psychically blind. It can see and pick up objects but does not recognize them until they are explored orally. Natural emotional reactions such as fear are lost. This syndrome, named for Klüver and Bucy, has been identified only in partial form in humans. Using special tests, lesser degrees of visual imperception were uncovered in patients by Milner (1971) and by McFie and colleagues. This syndrome is further discussed in Chap. 25.

Visual hallucinations of complex form, including ones of the patients himself (autoscopy), appear during temporal lobe seizures. Penfield was able to induce what he called "interpretive illusions" (altered impressions of the present) and to reactivate past experiences completely and vividly in association with their original emotions. Temporal lobe abnormalities may also distort visual perception; seen objects may appear too large (macropsia) or small (micropsia), too close or far away, or unreal. Some visual hallucinations have an auditory component: an imaginary figure may speak and move and, at the same time, arouse intense emotion in the patient. The entire experience may seem unnatural and unreal to the patient.

Cortical Deafness

Bilateral lesions of the transverse gyri of Heschl, while rare, are known to cause a central deafness. Henschen, in his extensive review of 1,337 cases of aphasia that had been reported up to 1922, found 9 in which these parts were destroyed by restricted vascular lesions, with resulting deafness. There are now many more cases of this type in the medical literature; lesions in other parts of the temporal lobes have no effect on hearing. These observations are the basis for the localization of the primary auditory receptive area in the cortex of the transverse gyri (chiefly the first) on the posterosuperior surface of the temporal lobe, deep within the sylvian fissure (areas 41 and 42). Subcortical lesions, which interrupt the fibers from both medial geniculate bodies to the transverse gyri, as in the two cases described by Tanaka and colleagues, have the same effect. With left-sided superotemporal lesions, there is usually an aphasia because of the proximity of the transverse gyri to the superotemporal association cortex. Hécaen has remarked that "cortically deaf" persons may seem to be unaware of their deafness, a state similar to that of blind persons who act as though they could see (the latter, called Anton syndrome is described further on).

For a long time, unilateral lesions of Heschl gyri were believed to have no effect on hearing; it has been found, however, that subtle deficits can be detected with careful testing. If very brief auditory stimuli are delivered, the threshold of sensation is elevated in the ear opposite the lesion. Also, while unilateral lesions do not diminish the perception of pure tones or clearly spoken words, the ear contralateral to a temporal lesion is less efficient if the conditions of hearing are rendered more difficult (binaural testing). For example, if words are slightly distorted (electronically filtered to alter consonants), they are heard less well in the ear contralateral to the lesion. In addition, the patient has more difficulty in equalizing the volume of sounds that are presented to both ears and in perceiving rapidly spoken numbers or different words presented to the two ears (dichotic listening). Few of these changes are evident by clinical examination.

Auditory Agnosia

Lesions of the secondary (unimodal association) zones of the auditory cortex—area 22 and part of area 21—have no effect on the perception of sounds and pure tones. However, the appreciation of complex combinations of sounds is severely impaired. This impairment, or auditory agnosia, takes several forms: inability to recognize sounds, different musical notes (amusia), or words and presumably each has a slightly different anatomic basis.

In agnosia for sounds, auditory sensations cannot be distinguished from one another. Such varied sounds as the tinkling of a bell, the rustling of paper, running water, and a siren all sound alike. The condition is usually associated with word deafness ("Pure Word Deafness" in Chap. 23 and below) or with amusia. Hécaen observed an agnosia for sounds alone in only two cases; one patient could identify only half of 26 familiar sounds, and the other could recognize no sound other than the ticking of a watch. Yet in both patients, the audiogram was normal, and neither had trouble understanding spoken words. In both, the lesion involved the right temporal lobe and the corpus callosum was intact.

Amusia proves to be more complicated, for the appreciation of music has several aspects: the recognition of a familiar melody and the ability to name it (musicality itself); the perception of pitch, timbre, and rhythm; and the ability to produce, read, and write music. There are many reports of musicians who became word-deaf with lesions of the dominant temporal lobe but retained their recognition of music and their skill in producing it. Others became agnosic for music but not for words, and still others were agnosic for both words and music. According to Segarra and Quadfasel, impaired recognition of music results from lesions in the middle temporal gyrus and not from lesions at the pole of the temporal lobe, as had been postulated by Henschen. Many other studies implicate the superior temporal gyrus in these deficits. A loss of the ability to perceive and produce rhythm may or may not be associated. In any case, the temporal lobe opposite that responsible for language (i.e., the right) is implicated in almost all cases.

That the appreciation of music is impaired by lesions of the nondominant temporal lobe finds support in Milner's studies of patients who had undergone temporal lobectomy. She found a lowering of the patient's appreciation of the duration of notes, timbre, intensity of sounds, and memory of melodies following right temporal lobectomy; these abilities were preserved in patients with left temporal lobectomies, regardless of whether Heschl gyri were included. Shankweiler had made similar observations, but in addition found that patients had difficulty in denominating a note or naming a melody following left temporal lobectomy.

More recent observations permit somewhat different interpretations. Tramo and Bharucha examined the mechanisms mediating the recognition and discrimination of timbre (the distinctive tonal quality produced by a particular musical instrument) in patients whose right and left hemispheres had been separated by callosotomy. They found that timbre could be recognized by each hemisphere, somewhat better by the left than by the right. Also, it was observed that lesions of the right auditory cortex impaired the recognition of melody (the temporal sequence of pitches) and of harmony (the sounding of simultaneous pitches). However, if words were added to the melody, then either a left- or right-sided lesion impaired its recognition (Samson and Zatorre). From functional imaging studies, it appears that the left inferior frontal region is activated by tasks that involve the identification of familiar music (Platel et al), as if this were a semantic test, but passively listening to melodies activates the right superior temporal and occipital regions (Zatorre et al).

By way of summary, Stewart and colleagues systematically reviewed the subject and were able to separate disorders of musical listening into the following categories: appreciation of pitch (including interval, pattern, and tonal structure), timbre, temporal structure, emotional content, and memory for music. The authors present clinical cases, mostly strokes that illustrate each defect. Taken together, these data suggest that the nondominant hemisphere is important for the recognition of harmony and melody (in the absence of words), but that the naming of musical scores and all the semantic (writing and reading) aspects of music require the integrity of the dominant temporal and probably the frontal lobes as well.

Word Deafness (Auditory Verbal Agnosia)

In essence, word deafness is a failure of the left temporal lobe function in decoding the acoustic signals of speech and converting them into understandable words. This is the essential element of Wernicke's aphasia and is discussed in Chap. 23. However, word deafness can occur by itself, without other features of Wernicke's aphasia. Other aspects of language such as reading, are not affected. The syndrome is sometimes seen as patients are improving from Wernicke's aphasia. Also, as mentioned earlier, verbal agnosia may be combined with agnosia for sounds and music, or the two may occur separately.

Auditory Illusions and Hallucinations (See Also Chap. 15)

(See also Chap. 15)

Temporal lobe lesions that leave hearing intact may cause a hearing disorder in which sounds are perceived as being louder or less loud than normal. Sounds or words may seem strange or disagreeable, or they may seem to be repeated, a kind of sensory perseveration. If auditory hallucinations are also present, they may undergo similar alterations. Such paracusias may last indefinitely and, by changing timbre or tonality, alter musical appreciation as well.

With lesions of the temporal lobes, these may be elementary (murmurs, blowing, sound of running water or motors, whistles, clangs, sirens) or complex (musical themes, choruses, voices). Usually sounds and musical themes are heard more clearly than voices. Patients may recognize hallucinations for what they are, or they may be convinced that the voices are real and respond to them with intense emotion. Hearing may fade before or during the hallucination.

In temporal lobe epilepsy, the auditory hallucinations are known to occur alone or in combination with visual or gustatory hallucinations, visual distortions, dizziness, and aphasia. There may be hallucinations based on remembered experiences (experiential hallucinations, in the terminology of Penfield and Rasmussen).

The anatomy of lesions underlying auditory illusions and hallucinations, formerly the province of study by ablative lesions, is currently being studied using functional imaging techniques. In some instances, these sensory phenomena have been combined with auditory verbal (or nonverbal) agnosia; the superior and posterior parts of the dominant or both temporal lobes were then involved. Clinicoanatomic correlation is difficult in cases associated with tumors that distort the brain without completely destroying it and that also cause edema of the surrounding tissue. Moreover, it is often uncertain whether symptoms have been produced by destruction of tissue or by excitation, i.e., by way of seizure discharges. Elementary hallucinations have been reported with lesions of either temporal lobe, whereas the more complex auditory hallucinations and particularly polymodal ones (visual plus auditory) occur more often with left-sided lesions. It should also be noted that complex but unformed auditory hallucinations (e.g., the sound of an orchestra tuning up), as well as entire strains of music and singing, also occur, inexplicably, with lesions that appear to be restricted to the pons (pontine auditory hallucinosis, as noted in Chap. 15).

It is tempting to relate complex auditory hallucinations to disorders in the auditory association areas surrounding the Heschl gyri, but the available data do not clearly justify such an assumption. In schizophrenic patients, the areas activated during a period of active auditory hallucinosis include not only Heschl gyri but also the hippocampus and other widely distributed structures mainly in the dominant hemisphere (see Chap. 53).

Vestibular Disturbances

In the superior and posterior part of the temporal lobe (posterior to the primary auditory cortex), there is an area that responds to vestibular stimulation by establishing one's sense of verticality in relation to the environment. If this area is destroyed on one side, the only clinical effect may be a transient illusion that the environment is tipped on its side or is upside down; more often, there is only subtle change in eye movements on optokinetic stimulation. Epileptic activation of this area induces vertigo or a sense of disequilibrium. As pointed out in Chap. 15, pure vertiginous epilepsy does occur but is a rarity, and if vertigo precedes a seizure, it is usually momentary and quickly submerged in other components of the seizure.

Autoscopy and Out‐of-Body Experiences

Recently, there has been interest in the cortical vestibular area and states of autoscopy (seeing one's self from an external perspective) and the associated but not identical "out-of-body experience" that has been reported by patients who have near-death episodes. Stimulation of this cortical area for the treatment of intractable tinnitus has elicited autoscopy (DeRidder et al) and seizures originating in the same or adjacent areas have produced out of body sensations. These observations suggest that one's mental perspective of corporeal place may be mediated by the cortex at the temporal-parietal junction. This is not surprising as the representation of extrapersonal space is found in the parietal lobes as described further on (see Blanke et al).

Disturbances of Time Perception

In a temporal lobe seizure originating on either side, time may seem to stand still or to pass with great speed. On recovery from such a seizure, the patient, having lost all sense of time, may repeatedly look at the clock. Assal and Bindschaedler have reported an extraordinary abnormality of time sense in which the patient invariably placed the day and date 3 days ahead of the actual ones. There had been aphasia from a left hemispheral stroke years before, but the impairment of time sense occurred only after a left temporal stroke that also produced cortical deafness.

Certainly, the most common disruptions of the sense of time occur as part of confusional states of any type. The usual tendency is for the patient to report the current date as an earlier one, much less often as a later one. Characteristically, in this situation, the responses vary from one examination to the next. The patient with a Korsakoff amnesic state is unable to place events in their proper time relationships, presumably because of failure of retentive memory, a function assignable to the medial temporal lobes.

Disturbances of Smell and Taste

(See also Chap. 12)

The central anatomy and physiology of these two senses in humans have been elusive. Brodal concluded that the hippocampus was not involved; however, seizure foci in the medial part of the temporal lobe (in the region of the uncus) often evoke olfactory hallucinations. This type of "uncinate fit," as originally pointed out by Jackson and Stewart, is often accompanied by a dreamy state, or, in the words of Penfield, an "intellectual aura." The central areas identified physiologically with olfaction are the posterior orbitofrontal, subcallosal, anterior temporal, and insular cortices, i.e., the areas that mediate numerous visceral functions.

In comparison, hallucinations of taste are less common. Stimulation of the posterior insular area elicited a sensation of taste along with disturbances of alimentary function (Penfield and Faulk). There are cases in which a lesion in the medial temporal lobe caused both gustatory and olfactory hallucinations. Sometimes the patient cannot decide whether he experienced an abnormal odor, taste, or both. The anatomy and physiology of smell and taste are discussed further in Chap. 12. Alterations or loss of taste and smell with temporal lobe lesions has not been adequately studied, and these do not appear to be common in clinical practice.

Other (Nonauditory) Temporal Lobe Syndromes

There is a large inferolateral expanse of temporal lobe that has only vaguely assignable integrative functions. With lesions in these parts of the dominant temporal lobe, a defect in the retrieval of words (amnesic dysnomia) has been frequently observed. Stimulation of the posterior parts of the first and second temporal convolutions of fully conscious epileptic patients can arouse complex memories and visual and auditory images, some with strong emotional content (Penfield and Roberts).

The loss of certain visual integrative abilities, particularly face recognition (prosopagnosia), is usually assigned to lesions of the inferior occipital lobes, as discussed further on, but the area implicated borders on the adjacent inferior temporal lobe as well.

Careful psychologic studies disclose a difference between the effects of dominant and nondominant partial (anterior) temporal lobectomy (Milner, 1971). With the former, there is dysnomia and impairment in the learning of material presented through the auditory sense; with the latter, there is impairment in the learning of visually presented material. In addition, about 20 percent of patients who have undergone temporal lobectomy, left or right, show a syndrome similar to that which results from lesions of the prefrontal regions. Perhaps more significant is the observation that the remainder of the cases show little or no defect in personality or behavior.

Disorders of Memory, Emotion, and Behavior

Finally, attention must be drawn to the central role of the temporal lobe, notably its hippocampal and limbic parts, in memory and learning and in the emotional life of the individual. As indicated earlier, these functions and their derangements have been accorded separate chapters. Memory is discussed in Chap. 21 and the neurology of emotion and behavior in Chap. 25.

To summarize, human temporal lobe syndromes include the following:

1. Effects of unilateral disease of the dominant temporal lobe

   1. Homonymous contralateral upper quadrantanopia

   2. Wernicke's aphasia (word deafness; auditory verbal agnosia)

   3. Dysnomia or amnesic aphasia

   4. Amusia (some types)

   5. Visual agnosia

   6. Occasionally, amnesic (Korsakoff) syndrome

2. Effects of unilateral disease of the nondominant temporal lobe

   1. Homonymous upper quadrantanopia

   2. Inability to judge spatial relationships in some cases

   3. Impairment in tests of visually presented nonverbal material

   4. Agnosia for sounds and some qualities of music

3. Effects of disease of either temporal lobe

   1. Auditory, visual, olfactory, and gustatory hallucinations

   2. "Dreamy" states with seizure (focal temporal lobe seizure)

   3. Emotional and behavioral changes

   4. Delirium-confusional states (usually nondominant)

   5. Disturbances of time perception

4. Effects of bilateral disease

   1. Korsakoff amnesic defect (hippocampal formations)

   2. Apathy and placidity

   3. Klüver-Bucy syndrome: compulsion to attend to all visual stimuli, hyperorality, hypersexuality, blunted emotional reactivity; the full syndrome is rarely seen in humans

Anatomic and Physiologic Considerations

This part of the cerebrum, lying behind the central sulcus and above the sylvian fissure, is the least well demarcated (see Fig. 22-1). Its posterior boundary, where it merges with the occipital lobe, is obscure, as is part of the inferior-posterior boundary, where it merges with the temporal lobe. On its medial side, the parietooccipital sulcus marks the posterior border, which is completed by extending the line of the sulcus downward to the preoccipital notch on the inferior border of the hemisphere. Within the parietal lobe, there are two important sulci: the postcentral sulcus, which forms the posterior boundary of the somesthetic cortex, and the interparietal sulcus, which runs anteroposteriorly from the middle of the posterior central sulcus and separates the mass of the parietal lobe into superior and inferior lobules (see Fig. 22-1). The inferior parietal lobule is composed of the supramarginal gyrus (Brodmann area 40) and the angular gyrus (area 39). The superior parietal lobule is that remaining part of the lobe that is bounded below by interparietal sulcus, anteriorly by the postcentral sulcus, and extends onto the medial surface of the brain in Brodmann areas 5 and 7 (Fig. 22-2). The architecture of the postcentral convolution is typical of all primary receptive areas (homotypical granular cortex). The rest of the parietal lobe resembles the association cortex, both unimodal and heteromodal, of the frontal and temporal lobes.

The superior and inferior parietal lobules and adjacent parts of the temporal and occipital lobes are relatively much larger in humans than in any of the other primates and are relatively slow in attaining their fully functional state (beyond age 7 years). This area of heteromodal cortex has large fiber connections with the frontal, occipital, and temporal lobes of the same hemisphere and, through the middle part of the corpus callosum, with corresponding parts of the opposite hemisphere.

The postcentral gyrus, or primary somatosensory cortex, receives most of its afferent projections from the ventroposterior thalamic nucleus, which is the terminus of the ascending somatosensory pathways. The contralateral half of the body is represented somatotopically in this gyrus on the posterior bank of the rolandic sulcus. It has been shown in the macaque that spindle afferents project to area 3a, cutaneous afferents to areas 3b and 1, and joint afferents to area 2 (Kaas). Stimulation of the postcentral gyrus elicits a numb, tingling sensation and sense of movement. Penfield (1941) remarked that rarely are these tactile illusions accompanied by pain, warmth, or cold. Stimulation of the motor cortex may produce similar sensations, as do discharging seizure foci from these regions. The primary sensory cortex projects to the superior parietal lobule (area 5), which is the somatosensory association cortex. Some parts of areas 1, 3, and 5 (except the hand and foot representations) probably connect, via the corpus callosum, with the opposite somatosensory cortex. There is some uncertainty as to whether area 7 (which lies posterior to area 5) is unimodal somatosensory or heteromodal visual and somatosensory; certainly, it receives a large contingent of fibers from the occipital lobe.

In humans, electrical stimulation of the cortex of the superior and inferior parietal lobules evokes no specific motor or sensory effects. Overlapping here, however, are the integrative zones for vision, hearing, and somatic sensation, the supramodal integration of which is essential to our awareness of space and person and certain aspects of language and calculation (apperception), as described below.

The parietal lobe is supplied by the middle cerebral artery, the inferior and superior divisions supplying the inferior and superior lobules, respectively, although the demarcation between the areas of supply of these two divisions is quite variable.

Despite Critchley's pessimistic prediction that establishing a formula of normal parietal function would prove to be a "vain and meaningless pursuit," our concepts of the activities of this part of the brain have assumed some degree of order, in part from his own work. There is little reason to doubt that the anterior parietal cortex contains the mechanisms for tactile percepts. Discriminative tactile functions, listed below, are organized in the more posterior, secondary sensory areas. But the greater part of the parietal lobe functions as a center for integrating somatosensory with visual and auditory information in order to construct an awareness of one's own body (body schema) and its relation to extrapersonal space. Connections with the frontal and occipital lobes provide the necessary proprioceptive and visual information for movement of the body and manipulation of objects and for certain constructional activities (constructional apraxia). Impairment of these functions implicates the parietal lobes, more clearly the nondominant one (on the right).

The conceptual patterns on which complex voluntary motor acts are executed also depend on the integrity of the parietal lobes, particularly the dominant one. Defects in this region give rise to ideomotor apraxia, as discussed in Chap. 3 and further on. The understanding of spoken and written words is partly a function of the supramarginal and angular gyri of the dominant parietal lobe as elaborated in Chap. 23. The recognition and utilization of numbers, arithmetic principles, and calculation, which have important spatial attributes, are other functions integrated principally through these structures.

Clinical Effects of Parietal Lobe Lesions

Within the brain, perhaps no other territory surpasses the parietal lobes in the rich variety of clinical phenomena exposed under conditions of disease. Our current understanding of the effects of parietal lobe disease contrasts sharply with that of the late nineteenth century, when these lobes, in the textbooks of Oppenheim and Gowers, were considered to be "silent areas." However, some of the clinical manifestations of parietal lobe disease may be subtle, requiring special techniques for their elicitation.

Close to the core of the complex behavioral features that arise from lesions of the parietal lobes is the problem of agnosia. Allusion has already been made to agnosia in the discussion of lesions of the temporal lobes that affect language, and similar findings occur with lesions of the occipital lobe as discussed further on. In those contexts, agnosia refers to a loss of recognition of an entity that cannot be attributed to a defect in the primary sensory modality. The term agnosia extends to a loss of more complex integrated functions and mental symbolism as described below, a number of intriguing deficits arise. These syndromes expose properties of the parietal lobe that have implications regarding a map of the body schema and of external topographic space, of the ability to calculate, to differentiate left from right, to write words, and other problems discussed below. The fact that apraxia, an inability to carry out a commanded task despite the retention of motor and sensory function, may also arise from parietal lobe damage, and the relationship of the apraxias to language and to agnosias, exposes some of the most complicated issues in behavioral neurology. Some of the theoretical aspects of agnosia, particularly those related to the disturbances of visual processing, are discussed later in the chapter.

Cortical Sensory Syndromes

The effects of a parietal lobe lesion on somatic sensation were first described by Verger and then more completely by Déjerine, in his monograph L'agnosie corticale, and by Head and Holmes. The latter, in their important paper of 1911, noted the close interrelationships between the thalamus and the sensory cortex. Although difficult to study, it is apparent that a large lesion of the primary sensory cortex, or beneath it, results in a circumscribed loss or reduction in sensation on the opposite side of the body. When primary sensory perception is altered, analysis of more complex and integrative sensory function is rendered less accurate.

However, as pointed out in the discussion of the organization of the sensory systems in Chap. 9, the parietal postcentral cortical defect is essentially one of sensory discrimination, i.e., impairment of the ability to integrate and localize stimuli that is reflected by an inability to distinguish objects by their size, shape, weight, and texture (astereognosis); to recognize figures written on the skin (agraphesthesia); to distinguish between single and double contacts (impairment of two-point discrimination); and to detect the direction of movement of a tactile stimulus. This type of sensory defect is sometimes referred to as "cortical," although it can be produced just as well by lesions of the subcortical connections. Clinicoanatomic studies indicate that parietocortical lesions that spare the postcentral gyrus produce only transient somatosensory changes or none at all (Corkin et al; Carmon and Benton). In other words, the primary perception of pain, touch, pressure, vibratory stimuli, and thermal stimuli is relatively intact in lesions of the parietal cortex that does not involve the postcentral gyrus.

The question of bilateral sensory deficits as a result of lesions in only one postcentral convolution was raised by the studies of Semmes and of Corkin and their associates. In tests of pressure sensitivity, two-point discrimination, point localization, position sense, and tactile object recognition, they found bilateral disturbances in nearly half of their patients with unilateral lesions, but the deficits were always more severe contralaterally and mainly in the hand and therefore the ipsilateral effect is rarely evident in clinical work. These disturbances of discriminative sensation and the subject of tactile agnosia are discussed more fully in Chap. 9.

Déjerine and Mouzon described the sensory syndrome in which touch, pressure, pain, thermal, vibratory, and position sense are lost on one side of the body or in a limb. This syndrome, typically the result of a thalamic lesion and not of a parietal one, may nonetheless occur with large, acute lesions (infarcts, hemorrhages) in the central and subcortical white matter of the parietal lobe. In this case, the symptoms partially recede in time, leaving more subtle defects in sensory discrimination. Smaller lesions, particularly ones that result from a glancing blow to the skull or a small infarct or hemorrhage, may cause a defect in cutaneous–kinesthetic perception in a discrete part of a limb, e.g., the ulnar or radial half of the hand and forearm; these cerebral lesions may mimic a peripheral nerve or root lesion (Dodge and Meirowsky).

A pseudothalamic pain syndrome on the side deprived of sensation by a parietal lesion has been described (Biemond). In a series of 12 such patients described by Michel and colleagues, burning or constrictive pain, identical to the thalamic pain syndrome (described in Chap. 9), resulted from vascular lesions restricted to the cortex. The discomfort involved the entire half of the body or matched the region of cortical hypesthesia; in a few cases, the symptoms were paroxysmal.

Head and Holmes drew attention to a number of interesting points about patients with parietal sensory defects: the easy fatigability of their sensory perceptions; the inconsistency of responses to painful and tactile stimuli; the difficulty in distinguishing more than one contact at a time; the disregard of stimuli on the affected side when the healthy side is stimulated simultaneously (tactile inattention or extinction); the tendency of superficial pain sensations to outlast the stimulus and to be hyperpathic; and the occurrence of hallucinations of touch. Of these, the testing of sensory extinction by the presentation of two tactile stimuli simultaneously on both sides of the body has become a component of the routine neurologic examination for parietal lesions. In modern parlance, these are "cortical sensory" defects of extinction of double simultaneous stimulation—astereognosis and agraphesthesia.

With anterior parietal lobe lesions, there is sometimes an associated mild hemiparesis, as this portion of the parietal lobe contributes a considerable number of fibers to the corticospinal tract. Occasionally there is such a large degree of inability or disinclination to use the limb that it simulates a hemiplegia. More often, there is only a poverty of movement or a weak effort of the opposite side. The affected limbs, if involved with this apparent weakness, tend to remain hypotonic and the musculature may undergo slight atrophy of a degree possibly not explained entirely by inactivity alone. In some cases, as noted below, there is clumsiness in reaching for and grasping an object under visual guidance (optic ataxia), and exceptionally, at some phase in recovery from the hemisensory deficit, there is incoordination of movement and intention tremor of the contralateral arm and leg that closely simulates a cerebellar deficit (pseudocerebellar syndrome). While relatively rare, this type of ataxia is authenticated by our own case observations.

In instances of cortical sensory disturbance, the outstretched hand may display small random "searching" movements of the fingers that simulate playing a piano (pseudoathetosis); these are exaggerated when the eyes are closed. Fixed dystonic postures and asterixis have also been described after parietal lesions with sensory loss, but these are most often the result of thalamic damage.

Agnosia

A conceptual inability to recognize objects, persons, or sensory stimuli in the absence of a primary deficit in the sensory modality is termed agnosia, derived from the Greek for lack of knowledge. It was included as a form of loss of insight as part of the confusional state in Chap. 20.

The idea that visual and tactile sensory information is synthesized into a body schema or image (perception of one's body and the relations of bodily parts to one another) was first formulated by Pick and elaborated by Brain. Long before their time, however, it was suggested that such information was the basis of our emerging awareness of ourselves, and philosophers had assumed that this comes about by the constant interplay between inherent percepts of ourselves and of the surrounding world.

The formation of the body schema is considered to be based on the constant influx and storage of sensations from our bodies as we move about; hence, motor activity is important in its development. A sense of extrapersonal space is central to this activity, and this also depends upon visual and labyrinthine stimulation. The mechanisms of these perceptions are best appreciated by studying their derangements in the course of neurologic disease of the parietal lobes.

Denny-Brown and Banker introduced the idea that the basic disturbance in all these defects is an inability to integrate a series of "spatial impressions"—tactile, kinesthetic, visual, vestibular, or auditory—a defect they referred to as amorphosynthesis. Examples of the loss of concept in their schema include finger agnosia, right-left confusion, acalculia, and all the apperceptive losses that attend damage of integrative sensory areas of the brain. The theoretical problem presented by agnosia is taken up in a later section.

Anosognosia and Hemispatial Neglect (Anton–Babinski Syndrome)

The observation that a patient with a dense hemiplegia, usually of the left side, may be indifferent to a paralysis, or is entirely unaware of it, was first made by Anton; later, Babinski named this disorder anosognosia. It expresses itself in several ways. For example, a lack of concern regarding paralysis was called anosodiaphoria by Babinski, an interesting term that is now little used. The term "denial" was introduced by Freud to explain the problem but is laden with psychic and psychoanalytical meaning and is less precise than "neglect."

With regard to parietal lobe disease, the term "anosognosia," using "anos," disease, is used to describe a group of disorders in which there is an unawareness of a deficit. While used most frequently to describe a lack of recognition, neglect, or indifference to a left sided paralysis or even to ownership of the limb, the term anosognosia is appropriate to denote the inability to perceive a number of deficits based on cerebral disease including blindness, hemianopia, deafness, and memory loss. Anosognosia is usually associated with a number of additional abnormalities. Often there is a blunted emotionality. The patient is inattentive and apathetic, and shows varying degrees of general confusion. There may be an indifference to performance failure, a feeling that something is missing, visual and tactile illusions when sensing the paralyzed part, hallucinations of movement, and allochiria (one-sided stimuli are felt on the other side).

The patient may act as if nothing were the matter. If asked to raise the paralyzed arm, he may raise the intact one or do nothing at all. If asked whether the paralyzed arm has been moved, the patient may say "yes." If the fact that the arm has not been moved is pointed out, the patient may admit that the arm is slightly weak. If told it is paralyzed, the patient may deny that this is so or offer an excuse: "My shoulder hurts." If asked why the paralysis went unnoticed, the response may be, "I'm not a doctor." Some patients report that they feel as though their left side had disappeared, and when shown the paralyzed arm, they deny it is theirs and assert that it belongs to someone else or even take hold of it and fling it aside. The mildest form of anosognosia is reflected by an imperfect and reduced appreciation of the degree of weakness. On the other extreme of the conceptual negation of paralysis are instances of self-mutilation of the paralyzed limb (apotemnophilia). It should be pointed out that the loss of body schema and the lack of appreciation of a left hemiplegia are separable, some patients displaying only one feature.

The lesion responsible for the various forms of one-sided anosognosia lies in the cortex and white matter of the superior parietal lobule. Rarely, a deep lesion of the ventrolateral thalamus and the juxtaposed white matter of the parietal lobe will produce a similar contralateral neglect. Unilateral asomatognosia is many times more frequent with right (nondominant) parietal lesions as with left-sided ones (seven times more often according to Hécaen). The apparent infrequency of right-sided agnosic symptoms with left parietal lesions is attributable in part, but not entirely, to their obscuration by an associated aphasia.

Another common group of parietal symptoms consists of neglect of one side of the body in dressing and grooming, recognition only on the intact side of bilaterally and simultaneously presented stimuli (sensory extinction) as mentioned above, deviation of head and eyes to the side of the lesion (transient), and torsion of the body in the same direction. The patient may fail to shave one side of the face, apply lipstick, or comb the hair only on one side.

Unilateral spatial neglect is brought out by having the patient bisect a line, draw a daisy or a clock, or name all the objects in the room. Homonymous hemianopia and varying degrees of hemiparesis may or may not be present and interfere with the interpretation of the lack of application on the left side of the drawing.

Clinical observations indicate that patients with right parietal lesions show variable but lesser elements of ipsilateral neglect in addition to the striking degree of contralateral neglect, suggesting that, in respect to spatial attention, the right parietal lobe is truly dominant (Weintraub and Mesulam). Damage of the superior parietal lobule, in addition to producing agnosias and apraxias, may interfere with voluntary movement of the opposite limbs, particularly the arm, as pointed out by Holmes. In reaching for a visually presented target in the contralateral visual field, and to a lesser extent in the ipsilateral field, the movement is misdirected and dysmetric (the distance to the target is misjudged).

Another subtle aspect of parietal lobe physiology revealed by human disease is the loss of exploratory and orienting behavior with the contralateral arm and even a tendency to avoid tactile stimuli. Mori and Yamadori call this rejection behavior. Denny-Brown and Chambers attributed the released grasping and exploring that follow frontal lobe lesions to a disinhibition of inherent parietal lobe automatisms but there is no way of confirming this. It is of interest that demented patients with prominent grasp reflexes tend not to grasp parts of their own bodies, but if there has been an additional parietal lesion, there is "self-grasping" of the forearm opposite the lesion (Ropper).

Conventional treatments for hemispatial neglect use prismatic glasses and training in visual exploration of the left side. Another approach demonstrates improvement by the application of vibratory stimulation to the right side of the neck, as reported by Karnath and colleagues, or of the ipsilateral labyrinth by caloric or electrical means (a similar treatment has been successful in some cases of dystonic torticollis, see Chap. 6). Based on the work of Ramachandran and colleagues, mirrors have been used to assist recovery of the side with agnosia. With a mirror in the right parasagittal plane, the patient observes the mirror image of their neglected hand and space and is induced to use that side more naturally. The larger problem is that these patients may not respond to rehabilitation if they lack an innate body schema.

Ideomotor and Ideational Apraxia

(See also Chap. 3)

As discussed extensively in Chap. 3, patients with parietal lesions of the dominant hemisphere who exhibit no defects in motor or sensory function, lose the ability to perform learned motor skills on command or by imitation. They can no longer use common implements and tools, either in relation to their bodies (e.g., brushing teeth, combing hair) or in relation to objects in the environment (e.g., a doorknob or hammer). The patient holds the implement awkwardly or seems at a loss to begin the act. It is as though the patient had forgotten the sequences of learned movements. The effects are bilateral. When defects of apraxia are intertwined with agnosic defects, the term apractognosia seems appropriate. A special type of visuospatial disorder, separable from neglect but also associated with lesions of the nondominant parietal lobe, is reflected in the patient's inability to reproduce geometric figures (constructional apraxia). A number of tests have been designed to elicit these disturbances, such as indicating the time by placement of the hands on a clock, drawing a map, copying a complex figure, reproducing stick-pattern constructions and block designs, making three-dimensional constructions, and constructing puzzles.

From the above descriptions, it is evident that the left and right parietal lobes function differently. The most obvious difference, of course, is that language and arithmetical functions are centered in the left hemisphere. It is hardly surprising, therefore, that verbally mediated spatial and praxic functions are more affected with left-sided than with right-sided lesions. This is ostensibly because language function, sited in the left hemisphere, is central to all cognitive functions. Hence cross-modal matching tasks (auditory–visual, visual–auditory, visual–tactile, tactile–visual, auditory–tactile, etc.) are most clearly impaired with lesions of the dominant hemisphere. Such patients can read and understand spoken words but cannot grasp the meaning of a sentence if it contains elements of relationship (e.g., "the mother's daughter" versus "the daughter's mother," "the father's brother's son," "Jane's complexion is lighter than Marjorie's but darker than her sister's"). There are similar difficulties with calculation. The recognition and naming of parts of the body and the distinction of right from left and up from down are learned, verbally mediated spatial concepts that are disturbed by lesions in the dominant parietal lobe.

Gerstmann Syndrome

This syndrome, caused by a left (dominant) inferior parietal lesion, provides the most striking example of what might be viewed as a bilaterally manifest agnosia (the previously mentioned asomatognosia of Denny-Brown and Banker). The characteristic tetrad of features is (i) inability to designate or name the different fingers of the two hands (finger agnosia), (ii) confusion of the right and left sides of the body, (iii) inability to calculate (acalculia), and (iv) inability to write (dysgraphia). One or more of these manifestations may be associated with word blindness (alexia) and homonymous hemianopia or a lower quadrantanopia. The lesion is in the left inferior parietal lobule (below the interparietal sulcus), particularly involving the angular gyrus or subjacent white matter of the left hemisphere.

There has been a dispute as to whether the four main elements of the Gerstmann syndrome have a common basis or only an association. Benton states that they occur together in a parietal lesion no more often than do constructional apraxia, alexia, and loss of visual memory and that every combination of these symptoms and those of the Gerstmann syndrome occurs with equal frequency in parietal lobe disease. Others, including the authors, tend to disagree and have the experience that right–left confusion, digital agnosia, agraphia, and acalculia have special significance, possibly being linked through a unitary defect in spatial orientation of fingers, body sides, and numbers. The relationship between the finger agnosia and the inability to enumerate is especially intriguing and relates to other arithmetic difficulties, discussed below. Attempts to clarify a common or fundamental source for all the elements of the Gerstmann syndrome by functional imaging have been difficult to comprehend. In healthy subjects, Rusconi and colleagues were unable to find a shared cortical substrate that could give rise to the features of the Gerstmann syndrome.

Dyscalculia has attracted little critical attention, perhaps because it occurs most often as a by-product of aphasia and an inability of the patient to appreciate numerical language. Primary dyscalculia is usually associated with the other elements of the Gerstmann syndrome. Computational difficulty may also be part of the more complex visuospatial abnormality of the nondominant parietal lobe; there is then difficulty in the placing of numbers in specific spatial relationships while calculating. In such cases, there is no difficulty in reading or writing the numbers or in describing the rules governing the calculation, but the computation cannot be accomplished correctly with pencil and paper. Hécaen has made a distinction between this type of anarithmetia and dyscalculia. In the latter, the process of calculation alone has been disturbed; in the former, there is an inability to manipulate numbers and to appreciate their ordinal relationships. Recognition and reproduction of numbers are intact in both. An analysis of how computation goes awry in each individual case is therefore required.

Visual Disorders with Parietal Lesions

A lesion deep to the inferior part of the parietal lobe, at its junction with the temporal lobe, involves the geniculocalcarine radiations and results in an incongruous homonymous hemianopia or an inferior quadrantanopia on the opposite side; but just as often, in practice, the defect is complete or almost complete and congruous. If the lesion is small and predominantly cortical, optokinetic nystagmus is usually retained; with deep lesions, it is abolished, with the target moving ipsilaterally (see Chap. 14).

Visual neglect is a typical feature of posterior parietal lesions on either side, more prominent with right-sided lesions. The problem that often arises is of distinguishing visual hemineglect (particularly of the left side) from a hemianopia. In its more severe forms the neglect is evident from casual observation of the patient's behavior or in drawings made by the patient that omit features on the left side; but here a more pervasive syndrome of hemispatial neglect, discussed earlier, may underlie the visual behavior. Occasionally, severe left-sided visual neglect results from a lesion in the right angular gyrus (see Mort et al). Visual neglect can also occur after focal lesions in the posterior medial temporal lobe (supplied by a branch of the posterior cerebral artery, in contrast to the middle cerebral artery supply of the angular gyrus of the inferior parietal lobule).

With posterior parietal lesions, as noted by Holmes and Horrax, there are deficits in localization of visual stimuli, inability to compare the sizes of objects, failure to avoid objects when walking, inability to count objects, disturbances in smooth-pursuit eye movements, and loss of stereoscopic vision. Cogan observed that the eyes may deviate away from the lesion upon forced lid closure, a "spasticity of conjugate gaze."

A common disorder of motor behavior of the eyelids is seen in many patients with large acute lesions of the right parietal lobe. Its mildest form is a disinclination to open the lids when the patient is spoken to. This gives the erroneous impression that the patient is drowsy or stuporous, but it will be found that a quick reply is given to whispered questions. In more severe cases, the lids are held shut and opening them is strongly resisted, to the point of making an examination of the pupils and fundi impossible.

Visual Disorientation and Disorders of Extrapersonal Space (Topographic Localization)

Spatial orientation depends on the integration of visual, tactile, and kinesthetic perceptions, but there are instances in which the defect in visual perception predominates. Patients with this disorder are unable to orient themselves in an abstract spatial setting (topographagnosia). Such patients cannot draw the floor plan of their house, a map of their town, or of the United States and cannot describe a familiar route, as from home to work, for example, or find their way in familiar surroundings. In brief, such patients have lost topographic memory. This disorder is almost invariably caused by lesions in the white matter deep to the inferior and superior parietal lobules and it is separable from anosognosia as summarized by Levine and colleagues.

A clever mental experiment posed to patients by Bisiach and Luzzatti has suggested that the loss of attention to one side of the environment extends to, or perhaps is derived from, the mental representation of space. Their patient with a right parietal lesion was asked to describe from memory the buildings lining the Piazza del Duomo, first as if seen from one corner of the piazza and then from the opposite corner. In each instance, the description omitted the left side of the piazza from the observer's perspective.

An important and not infrequent disorder of visual agnosia, a disorder of visually directed reaching with the hand, difficulty directing gaze, and simultanagnosia, is given the name Balint syndrome. It is, strictly speaking, a bilateral disorder of the parietal lobes but we discuss it below for convenience in order to append it to the clinically similar entity of cortical blindness.

Auditory Neglect

This defect in appreciation of the left side of the environment is less apparent than is visual neglect, but it is no less striking when it occurs. Many patients with acute right parietal lesions are initially unresponsive to voices or noises on the left side, but the syndrome is rarely persistent. Special tests demonstrate a displacement of the direction of the perceived origin of sounds toward the right. This defect is separable from visual agnosia (see De Renzi et al); curiously, it may be worsened by the introduction of visual cues. Subtle differences between the allocation of spatial attention to sound (auditory neglect) and a distortion in its localization may be found in different cases, but the main lesion usually lies in the right superior lobule.

In summary, the effects of disease of the parietal lobes are as follows:

1. Effects of unilateral disease of the parietal lobe, right or left

   1. Corticosensory syndrome and sensory extinction (or total hemianesthesia with large acute lesions of white matter)

   2. Mild hemiparesis or poverty of movement (variable), poverty of movement, hemiataxia (seen only occasionally)

   3. Homonymous hemianopia or inferior quadrantanopia (incongruent or congruent) or visual inattention

   4. Abolition of optokinetic nystagmus with target moving toward side of the lesion

   5. Neglect of the opposite side of external space (more prominent with lesions of the right parietal lobe; see later)

2. Effects of unilateral disease of the dominant (left) parietal lobe (in right-handed and most left-handed patients); additional phenomena include

   1. Disorders of language (especially alexia)

   2. Gerstmann syndrome (dysgraphia, dyscalculia, finger agnosia, right–left confusion)

   3. Tactile agnosia (bimanual astereognosis)

   4. Bilateral ideomotor and ideational apraxia (see Chap. 3)

3. Effects of unilateral disease of the nondominant (right) parietal lobe

   1. Visuospatial disorders

   2. Topographic memory loss

   3. Anosognosia, dressing, and constructional apraxias (these disorders may occur with lesions of either hemisphere but are observed more frequently and are of greater severity with lesions of the nondominant one)

   4. Confusion

   5. Tendency to keep the eyes closed, resist lid opening, and blepharospasm

4. Effects of bilateral disease of the parietal lobes

   1. Balint syndrome: visual-spatial imperception (simultagnosia), optic apraxia (difficulty directing gaze), and optic ataxia (difficulty reaching for objects)

With all these parietal syndromes, if the disease is sufficiently extensive, there may be a reduction in the capacity to think clearly as well as inattentiveness and slightly impaired memory.

It does seem reasonably certain that, in addition to the perception of somatosensory impulses that arrive in the postcentral gyrus, the parietal lobe participates in the integration of all sensory data, especially those that provide an awareness of one's body as well as a percept of one's surroundings and of the relation of one's body to extrapersonal space and of objects in the environment to each other. In this respect, the parietal lobe may be regarded as a special high-order sensory organ, the locus of transmodal intersensory, integration, particularly tactile and visual ones, which are the basis of our concepts of spatial relations. In this way, parietal lesions cause disorders of specific types of self-consciousness or self-awareness that are tied to sensory modalities. This is distinctly different from the distortions of perception caused by lesions of the temporal lobes.

Authoritative references on parietal function include Critchley's monograph on the parietal lobes and the chapter by Botez and Olivier in the Handbook of Clinical Neurology.

Anatomic and Physiologic Considerations

The occipital lobes are the termini of the geniculocalcarine pathways and are essential for visual perception and recognition. This part of the brain has a large medial surface and smaller lateral and inferior surfaces (see Fig. 22-1). The parietooccipital fissure creates a noticeable medial boundary with the parietal lobe, but laterally the occipital lobe merges with the parietal and temporal lobes. The large calcarine fissure courses in an anteroposterior direction from the pole of the occipital lobe to the splenium of the corpus callosum; area 17, the primary visual receptive cortex, lies on its banks (see Figs. 22-1 and 22-2). Area 17 is a typical homotypical cortex but is unique in that its fourth receptive layer is divided into two granular cell laminae by a greatly thickened band of myelinated fibers, the external band of Baillarger. This stripe, also called the line or band of Gennari, is grossly visible and has given this area its name, striate cortex. The largest part of area 17 is the terminus of the retinal macular fibers that arrive via the lateral geniculate (see Fig. 13-2). The parastriate cortex (areas 18 and 19) lacks the line of Gennari and resembles the granular unimodal association cortex of the rest of similar areas in the cerebrum. Area 17 contains cells that are activated by the homolateral geniculocalcarine pathway (corresponding, of course, exclusively to the contralateral visual field); these cells are interconnected and project also to cells in areas 18 and 19. The latter are connected with one another and with the angular gyri, lateral and medial temporal gyri, frontal motor areas, limbic and paralimbic areas, and corresponding areas of the opposite hemisphere through the posterior third (splenium) of the corpus callosum.

The occipital lobes are supplied almost exclusively by the posterior cerebral arteries and their branches, either directly in most individuals or through an embryologically persistent branch of the internal carotid arteries ("fetal" posterior cerebral artery). A small area of the occipital pole receives blood supply from the inferior division of the middle cerebral artery. This assumes importance in the clinical finding of "macular sparing," discussed in Chap. 13.

The connections among these several areas in the occipital lobe are complex, and the notion that area 17 is activated by the lateral geniculate neurons and that this activity is then transferred and elaborated in areas 18 and 19 is surely not complete. Actually, 4 or 5 occipital receptive fields are activated by lateral geniculate neurons, and fibers from area 17 project to approximately 20 other visual areas, of which only 5 are well identified. These extrastriate visual areas lie in the lingula and posterior regions of the occipital lobes. As Hubel and Wiesel have shown, the response patterns of neurons in both occipital lobes to edges and moving visual stimuli, to on-and-off effects of light, and to colors reflects this complexity. Hence form, location, color, and movement each have separate localizable hierarchical arrangements of neurons in series. The monographs of Polyak and of Miller contain detailed information about the anatomy and physiology of this part of the brain.

Beyond the effects on vision of lesions in the occipital lobes, monkeys with bilateral lesions in the temporal visual zones lose the ability to identify objects; with posterior parietal lesions, there is loss of ability to locate objects.

Clinical Effects of Occipital Lobe Lesions

Visual Field Defects

The most familiar clinical abnormality resulting from a lesion of one occipital lobe, a contralateral homonymous hemianopia, has already been discussed in Chap. 13. Extensive destruction abolishes all vision in the corresponding opposite half of each visual field. With a neoplastic lesion that eventually involves the entire striate region, the field defect may extend from the periphery toward the center, and loss of color vision (hemiachromatopsia) often precedes loss of black and white. Destruction of only part of the striate cortex on one side yields characteristic field defects that accurately indicate the loci of the lesion. A lesion confined to the pole of the occipital lobe results in a central hemianopic defect that splits the macula and leaves the peripheral fields intact. This observation indicates that half of each macula is unilaterally represented and that the maculae may be involved (split) in hemianopia. Bilateral lesions of the occipital poles, as in embolism of the posterior cerebral arteries, result in bilateral hemianopias and cortical blindness as detailed below. Unilateral quadrant defects and altitudinal field defects due to striate lesions indicate that the cortex on one side, above or below the calcarine fissure, is damaged. The cortex below the fissure is the terminus of fibers from the lower half of the retina; the resulting field defect is in the upper quadrant, and vice versa. Most bilateral altitudinal defects, either superior or inferior, are traceable to incomplete bilateral occipital lesions (cortex or terminal parts of geniculocalcarine pathways). Head and Holmes described several such delimited cases caused by gunshot wounds; embolic infarction is now the common cause.

As indicated in Chap. 13, the homonymous hemianopia that results from ablation of one occipital lobe is not absolute. In monkeys, visuospatial orientation and the capacity to reach for moving objects in the defective field are preserved (Denny-Brown and Chambers). In humans also, flashing light and moving objects can sometimes be seen in the blind field even without the patient's full awareness. Weiskrantz and colleagues have referred to these preserved functions as blindisms or blindsight. It is useful as a practical matter to note that the optokinetic responses are usually spared in hemianopic deficits of occipital origin.

Many of the complex behavioral defects involving visual function are caused by lesions at the junctions of the occipital and parietal or temporal lobes. They are discussed here with the occipital lobe syndromes for convenience but should be considered as transcending the largely arbitrary boundaries of these three lobes of the brain.

Cortical Blindness

With bilateral lesions of the occipital lobes (destruction of area 17 of both hemispheres), there is a loss of sight that can be conceptualized as bilateral hemianopia. The degree of blindness may be equivalent to that which follows severing of the optic nerves. The pupillary light reflexes are preserved because they depend upon visual fibers that terminate in the midbrain, but reflex closure of the eyelids to threat or bright light may, or may not, be preserved (see Fig. 14-9). No changes are detectable in the retinas. The eyes are still able to move through a full range and, if there is macular sparing as there usually is with vascular lesions, optokinetic nystagmus can be elicited. Visual imagination and visual imagery in dreams are preserved. With rare exceptions, no cortical potentials can be evoked in the occipital lobes by light flashes or pattern changes (visual evoked response), and the alpha rhythm is lost in the electroencephalogram (EEG; see Chap. 2).

Less-complete bilateral lesions leave the patient with varying degrees of visual perception. There may also be visual hallucinations of either elementary or complex types. The mode of recovery from cortical blindness has been studied carefully by Gloning and colleagues, who describe a regular progression from cortical blindness through visual agnosia and partially impaired perceptual function to recovery. Even with recovery, the patient may complain of visual fatigue (asthenopia) and difficulties in fixation and fusion.

The usual cause of cortical blindness is occlusion of the posterior cerebral arteries (most often embolic) or the equivalent, occlusion of the distal basilar artery. The above-mentioned macular sparing may leave the patient with an island of barely serviceable central vision. The infarct may also involve the mediotemporal regions or thalami, which share the posterior cerebral artery supply, with a resulting Korsakoff amnesic defect and a variety of other neurologic deficits referable to the high midbrain and diencephalon (drowsiness, akinetic mutism as described in Chap. 17).

Visual Anosognosia (Anton Syndrome)

The main characteristic of this disorder is the denial of blindness by a patient who obviously cannot see. These patients act as though they could see, and in attempting to walk, collide with objects, even to the point of injury. They may offer excuses for the difficulties—"I lost my glasses," "The light is dim"—or may only evince indifference to loss of sight. The lesions in cases of negation of blindness extend beyond the striate cortex to involve the visual association areas.

Rarely, the opposite condition arises: a patient is able to see small objects but claims to be blind. This individual walks about avoiding obstacles, picks up crumbs or pills from the table, and catches a small ball thrown from a distance. This simulates the condition of hysterical blindness (see further on).

Visual Illusions (Metamorphopsias)

These may present as distortions of form, size, movement, or color. In a group of 83 patients with visual perceptual abnormalities, Hécaen found that 71 fell under one of four headings: deformation of the image, change in size, illusion of movement, or a combination of all three. Illusions of these types have been reported with lesions confined to the occipital lobes but are more frequently caused by shared occipitoparietal or occipitotemporal lesions; consequently, they are also considered in earlier sections of this chapter as well as in Chaps. 13 and 16. The right hemisphere appears to be involved more often than the left. Illusions of movement occur more frequently with posterior temporal lesions or seizures, polyopia (one object appearing as two or more objects) more frequently with occipital lesions (it also occurs in hysteria), and palinopsia (perseveration of visual images, as in the frames of a celluloid film) with both posterior parietal and occipital lesions. Visual field defects are present in many of the cases. In all these conditions, the anatomic correlates are imprecise.

It is likely that an element of cortical vestibular disorder underlies the metamorphosis of parietooccipital lesions. The vestibular and proprioceptive systems are represented in the parietal lobes of each side and the lesions there are probably responsible for misperceptions of movement and spatial relations. The illusion of tilting of the environment or upside-down vision is known to occur with parietooccipital lesions, but occurs more often with abnormalities of the vestibular system.

Visual Hallucinations

These phenomena may be elementary or complex, and both types have sensory as well as cognitive aspects. Elementary (or unformed) hallucinations include flashes of light, colors, luminous points, stars, multiple lights (like candles), and geometric forms (circles, squares, and hexagons). They may be stationary or moving (zigzag, oscillations, vibrations, or pulsations). They are much the same as the effects that Penfield and Erickson obtained by stimulating the calcarine cortex in a conscious patient. Complex (formed) hallucinations include objects, persons, or animals and infrequently, more complete scenes that are indicative of lesions in the visual association areas or their connections with the temporal lobes. They may be of natural size, Lilliputian, or grossly enlarged. With hemianopia, they appear in the defective field or move from the intact field toward the hemianopic one. The patient may realize that the hallucinations are false experiences or may be convinced of their reality. Because the patient's response is usually in accord with the nature of the hallucination, he may react with fear to a threatening vision or casually if its content is benign.

The clinical setting for the occurrence of visual hallucinations varies. The simplest black-and-white moving scintillations are part of migraine. Others, some colored, occur as a seizure aura (see Chap. 16). Often, they are associated with a homonymous hemianopia, as already indicated. Frequently, they are part of a confusional state or delirium (see Chap. 20). Similar phenomena may occur as part of hypnagogic hallucinations in the narcolepsy–cataplexy syndrome. In the "peduncular hallucinosis" of Lhermitte (1932), the hallucinations are purely visual, appear natural in form and color, sometimes in pastels, move about as in an animated cartoon, and are considered by the patient to be unreal, abnormal phenomena (preserved insight). Ischemia in the territories of the posterior cerebral arteries is the usual cause. Lhermitte used the term peduncle to represent the midbrain as the source of the hallucinations was ischemia in the high mesencephalon, creating images that may be akin to those experienced in dreaming. The hallucinations as mentioned are purely visual; if hallucinations are polymodal, the lesion is always in the occipitotemporal parts of the cerebrum.

A special syndrome of ophthalmopathic hallucinations occurs in persons with reduced vision, as discussed in Chap. 13. A similar phenomenon in elderly patients with partially impaired vision has been called the Charles Bonnet syndrome, following his description of visual hallucinations in a "sane" person. The topic of senile hallucinosis has been reviewed by Gold and Rabin, and 60 such patients with Bonnet syndrome were reported in detail by Teunisse and colleagues. The latter authors found that 11 percent of older persons with reduced vision experienced these phenomena at one time or another. Further comments are to be found in Chap. 13.

It is usually the case that the lesions responsible for visual hallucinations are situated in the occipital lobe or posterior part of the temporal lobe and that elementary hallucinations have their origin in the occipital cortex, and complex ones in the temporal cortex. However, the opposite may pertain; in some cases, formed hallucinations are related to lesions of the occipital lobe and unformed ones to lesions of the temporal lobe, according to Weinberger and Grant. Also, as emphasized by these authors, lesions that give rise to visual hallucinations, simple or elaborate, need not be confined to central nervous system structures but may be caused by lesions at every level of the neurooptic apparatus (retina, optic nerve, chiasm, etc.).

The Visual Agnosias (See Also Lesions of the Parietal Lobe and Temporal Lobe)

Several syndromes involving visual dysfunction are due to lesions that span the occipital lobe and either the adjacent temporal or parietal lobes. They have been divided conceptually and anatomically into a dorsal and a ventral stream of information processing, the former running from the occipital to the parietal lobe and the latter from the occipital to the temporal lobe. Those of the temporal lobe include visual object agnosia, prosopagnosia, alexia, and color agnosia. In this way, the ventral stream may be considered to represent the "what" of visual processing to identify objects. The parietal-occipital, or dorsal stream syndromes are visual simultanagnosia, Balint syndrome and the earlier mentioned topographagnosia, that reflect disorders of "where" in visual behavior.

Visual Object Agnosia

This rare condition, first described by Lissauer in 1890, consists of a failure to name and indicate the use of a seen object by spoken or written word or by gesture. The patient cannot even determine the generic class of the object presented. Visual acuity is intact, the mind is clear, and the patient is not aphasic—conditions requisite for the diagnosis of agnosia. If the object is palpated, it is recognized at once, and it can also be identified by smell or sound if it has an odor or makes a noise. Moving the object or placing it in its customary surroundings facilitates recognition. In most reported instances of object agnosia, the patient retains normal visual acuity but cannot identify, match, or name objects presented in any part of the visual fields; if misnamed, the object is used in a fashion that reflects the incorrect perception. Amazingly, one encounters patients who have lost the capacity to recognize only one class of objects, e.g., animals or vegetables—suggesting that, in the human, stored information is grouped and classified in a way that is necessary for visual perception (we are unable to corroborate this from our own material). Lissauer conceived of visual object recognition as consisting of two distinct processes, the construction of a perceptual representation from vision (perception) and the mapping of this perceptual representation onto stored percepts or engrams of the object's functions and associations (apperception), and he proposed that impairment of either of these processes could give rise to a defect in visual object recognition.

As indicated in Chap. 13, visual object agnosia is usually associated with visual verbal agnosia (alexia) and homonymous hemianopia. Prosopagnosia (the inability to identify faces; see further on) is also present in most cases. The underlying lesions are usually bilateral, although McCarthy and Warrington have related a case with a restricted lesion of the left occipitotemporal region (by MRI). Two of our patients with visual object agnosia had an incomplete amnesic syndrome from a left-sided inferior occipital and mediotemporal infarction, reflecting a proximal occlusion of the posterior cerebral artery.

Prosopagnosia

This term (from the Greek prosopon, "face," and gnosis, "knowledge") was introduced by Bodamer for a type of visual defect in which the patient cannot identify a familiar face by looking at either the person or a picture, even though he knows that a face is a face and can point out its features. Such patients also cannot learn to recognize new faces. They may also be unable to interpret the meaning of facial expressions or to judge the ages or distinguish the genders of faces. In identifying persons, the patient depends on other data, such as the presence and type of glasses or moustache, the type of gait, or sound of the voice. Similarly, species of animals and birds and specific models or types of cars cannot be distinguished from one another, but the patient can still recognize an animal, bird, or car as such. Other agnosias may be present in such cases (color agnosia, simultanagnosia) and there may be topographic disorientation, disturbances of body schema, and constructional or dressing apraxia. Visual field defects are nearly always present. Some neurologists have interpreted this condition as a simultanagnosia involving facial features. Another view is that the face, though satisfactorily perceived, cannot be matched to a memory store of faces. Levine has found a deficit in perception, characterized by insufficient feature analysis of all visual stimuli.

The small number of cases that have been studied anatomically and by CT scanning and MRI indicate that prosopagnosia is most often associated with bilateral lesions of the ventromedial occipitotemporal regions (Damasio et al) including the inferior occipital or midfusiform gyri, but there are exceptions that are attributable to unilateral damage, almost always on the right side. The notion that there is a "face area" in the fusiform gyrus is expressed uncritically in the literature and seems to be an oversimplification.

A variant of this disorder is characterized by specific difficulty with facial matching or discrimination from partial cues, such as portions of the face or a profile. The distinction between this deficit and the usual type of prosopagnosia rests on the use of tests that do not require memory of a specific face. This difficulty with facial matching and discrimination is more likely to be seen with lesions of the right than of the left posterior hemisphere.

Closely allied and often associated with prosopagnosia is a subtle syndrome of loss of environmental familiarity, in which the patient is unable to recognize familiar places. The patient may be able to describe a familiar environment from memory and locate it on a map, but he experiences no sense of familiarity and gets lost when faced with the actual landscape. In essence, this is an environmental agnosia. This syndrome is associated with right-sided, medial temporooccipital lesions, although in some patients, as in those with prosopagnosia, the lesions are bilateral (Landis et al).

Environmental agnosia can be distinguished from the visual disorientation and disorder of spatial (topographic) localization discussed earlier. Patients with the latter disorder are unable to orient themselves in an abstract spatial setting (topographagnosia, or loss of topographic memory). They cannot draw the floor plan of their house or a map of their town or the United States and cannot describe a familiar route, as from their home to their place of work, or find their way in familiar surroundings.

Visual Agnosia for Words (Alexia Without Agraphia)

See Chap. 23 and further on in this chapter in the discussion of alexia without agraphia, under the "Disconnection Syndromes."

Color Agnosia

Here one must distinguish several different aspects of identification of colors, such as the correct perception of color (the loss of which is called color blindness) or the naming of a color. The common form of retinal color blindness is congenital and is readily tested by the use of Ishihara plates. Acquired color blindness caused by a cerebral lesion, with retention of form vision, is referred to as central achromatopsia. Here the disturbance is one of hue discrimination; the patient cannot sort a series of colored wools according to hue (Holmgren test) and may complain that colors have lost their brightness or that everything looks gray. Achromatopsia is frequently associated with visual field defects and with prosopagnosia. Most often, the field defects are bilateral and tend to affect the upper quadrants. However, full-field achromatopsia may exist with retention of visual acuity and form vision. There may also be a hemi- or quadrant-achromatopsia without other abnormalities, although special testing is required to reveal this defect. These features, together with the usually associated prosopagnosia, point to involvement of the inferomedial, occipital, and temporal lobe(s) and the lower part of the striate cortex or optic radiation (Damasio et al, 1974a). The existence of a central achromatopsia is not surprising in view of the animal studies of Hubel, which identified sets of cells in areas 17 and 18 that are activated only by color stimuli.

A second group of patients with color agnosia have no difficulty with color perception (i.e., they can match seen colors), but they cannot reliably name them or point out colors in response to their names. They have a color anomia, of which there are at least two varieties. One is typically associated with pure word blindness, i.e., alexia without agraphia, and is best explained by a disconnection of the primary visual areas from the language areas (see further on). In the second variety, the patient fails not only in tasks that require the matching of a seen color with its spoken name but also in purely verbal tasks pertaining to color naming, such as naming the colors of common objects (e.g., grass, banana). This latter disorder is probably best regarded as a form of anomic aphasia, in which the aphasia is more or less restricted to the naming of colors (Meadows, 1974b). According to Damasio and associates, the lesion has involved the medial part of the left hemisphere at the junction of the occipital and temporal lobes, just below the splenium of the corpus callosum. All their patients also had a right homonymous hemianopia as a result of destruction of the left lateral geniculate body, optic radiation, or calcarine cortex.

Visual Simultanagnosia

This describes an inability to grasp the sense of the multiple components of a total visual scene despite retained ability to identify individual details. Wolpert pointed out that there was an inability to read all but the shortest words, spelled out letter by letter, and a failure to perceive simultaneously all the elements of a scene and to properly interpret the scene, which Wolpert called simultanagnosia. A cognitive defect of synthesis of the visual impressions was thought to be the basis of this condition. Some patients with this disorder have a right homonymous hemianopia; in others, the visual fields are full but there is one-sided extinction when tested with double simultaneous stimulation. This is an integral part of the Balint syndrome described below.

Through tachistoscopic testing, Kinsbourne and Warrington (1963) found that reducing the time of stimulus exposure permits single objects to be perceived, but not two objects. Rizzo and Robin proposed that the primary defect is in sustained attention to incoming visuospatial information. There is consistent localization. Nielsen has described it with a lesion of the inferolateral part of the dominant occipital lobe (area 18). In a patient who presented with an isolated "spelling dyslexia" and simultanagnosia, Kinsbourne and Warrington (1962) found the lesion to be localized within the inferior part of the left occipital lobe. In other instances, the lesions have been bilateral in the superior parts of the occipital association cortices.

Balint Syndrome

(See also Chap. 13.) In this not uncommon syndrome, the appreciation of a coherent and detailed visual world is disrupted and the patient perceives only disconnected individual parts of the scene, as in the visual simultanagnosia described earlier. While it is due to lesions that span the occipital and parietal lobes, it is presented here for ease of exposition. Balint, a Hungarian neurologist, was the first to recognize this constellation. The defect is noted when the patient describes a complex scene in a disjointed way, single objects being pointed out, others missed entirely, the relationships and context of parts of the picture remaining unappreciated. The entire syndrome consists of (1) a disorder of visual attention mainly to the periphery of the visual field, in which the totality of a scene is not perceived despite preservation of vision for individual elements (visual simultanagnosia as discussed earlier); (2) difficulty in grasping or touching an object under visual guidance, as though hand and eye were not coordinated (called by Balint optic ataxia); and (3) an inability to project gaze voluntarily into the peripheral field and to scan it despite the fact that eye movements are full (termed psychic paralysis of fixation of gaze by Balint, incorrectly called optic apraxia).

An essential feature of the Balint syndrome appears to be a failure to properly direct oculomotor function in the exploration of space. This psychic paralysis of gaze is apparent when the patient is unable to turn his eyes to fixate an object in the right or left visual field or to consistently follow a moving object. The pattern in which the patient scans a picture is haphazard and fails to encompass on entire areas. Normal individuals accomplish visual scanning in a fairly uniform manner beginning paracentrally and moving clockwise, then to the corners. Thus, the mechanism of simultanagnosia may be in part the result of this abnormality of eye movements as pointed out by Tyler.

Optic ataxia is detected when the patient reaches for an object, either spontaneously or in response to verbal command. To reach the object, the patient engages in a tactile search with the palm and fingers, presumably using somatosensory cues to compensate for a lack of visual information. The disorder may involve one or both hands and give the erroneous impression that the patient is blind. In contrast, movements that do not require visual guidance, such as those directed to the body or movements of the body itself, are performed naturally. The presence of visual inattention is tested by asking the patient to carry out tasks such as looking at a series of objects or connecting a series of dots by lines; often only one of a series of objects can be found, even though the visual fields seem to be full.

In almost all reported cases of the Balint syndrome, the lesions have been bilateral, mainly in the vascular border zones (areas 19 and 7) of the parietooccipital regions, although instances of optic ataxia alone have been described within a single visual field contralateral to a right or left parietooccipital lesion, and visual simultanagnosia, as noted earlier, has had variable localization. The neuropsychologic aspects of the syndrome and several interesting historical notes, including the attribution of original reporting to Inouye, can be found in the review by Rizzo and Vecera.

The effects of disease of the occipital lobes may be summarized as follows:

1. Effects of unilateral disease, either right or left

   1. Contralateral (congruent) homonymous hemianopia, which may be central (splitting the macula) or peripheral; also homonymous hemiachromatopsia

   2. Elementary (unformed) hallucinations—usually because of irritative lesions

2. Effects of left occipital disease

   1. Right homonymous hemianopia

   2. If deep white matter and splenium of corpus callosum is involved, alexia without agraphia

   3. Visual object agnosia

3. Effects of right occipital disease

   1. Left homonymous hemianopia

   2. With more extensive lesions, visual illusions (metamorphopsias) and hallucinations (more frequent with right-sided than left-sided lesions)

   3. Loss of topographic memory and visual orientation

4. Bilateral occipital disease

   1. Cortical blindness bilateral hemianopias,

   2. Anton syndrome (visual anosognosia, denial of cortical blindness)

   3. Loss of perception of color (achromatopsia)

   4. Prosopagnosia (impaired face recognition, bilateral temporooccipital including fusiform gyrus)

   5. Balint syndrome (bilateral dorsal parietooccipital) (see description in text)

A line of disagreement, as old as neurology itself, pertains to the relationship between the two cerebral hemispheres. Fechner, in 1860, speculated that since the two hemispheres, joined by the corpus callosum, were virtual mirror images of one another and functioned in totality in conscious life, separating them would result in two minds. William McDougall rejected this idea and is said to have offered to have his own brain divided by Charles Sherrington should he have an incurable disease. He died of cancer, but the callosotomy was considered unnecessary, for already there were indications from the work of Sperry and colleagues that when separated, the two hemispheres had different functions.

The practice of surgical sectioning of the corpus callosum for the control of epilepsy greatly stimulated interest in the special functions of the right cerebral hemisphere when isolated from the left. It is in the sphere of visuospatial perception that right hemispheral dominance is most convincing. Lesions of the right posterior cerebral region result in an inability to utilize information about spatial relationships in making perceptual judgments and in responding to objects in a spatial framework. This is manifest in constructing figures (constructional apraxia), in the spatial orientation of the patient in relation to the environment (topographic agnosia), in identifying faces (prosopagnosia), and in relating a scattering of visual stimuli to one another (simultanagnosia). Also, there are claims that the right hemisphere is more important than the left in visual imagery, attention, emotion (both in feeling and in the perception of emotion in others), and manual drawing (but not writing); in respect to these functions, however, the evidence is less firm. The idea that attention is a function of the right hemisphere derives from the neglect of left visual space and of somatic sensation in the anosognosic syndrome and also from the apathy that characterizes such patients. Certainly, the popular notion of the right hemisphere as "emotional" in contrast to the left one as "logical" has no basis in fact and represents a gross oversimplification of brain function and localization.

Similar issues arise, of course, in relation to handedness and language dominance in the left hemisphere as discussed in the following chapter. Here we comment only on how intriguing it is that praxis and linguistic skill are aligned on the same side of the brain, suggesting that an essential property of the dominant hemisphere is its ability to comprehend and manipulate symbolic representations of all types. At the same time, the colocalization of gnosis and visuospatial ability in the nondominant hemisphere has salience in that the two are so often interdependent in normal functioning.

Following the insightful clinical observations and anatomic studies of Wernicke, Déjerine, and Liepmann, the concept of disconnection of parts of one or both cerebral hemispheres as a cause of neurologic difficulty was introduced to neurologic thinking. In recent years, these ideas were resurrected and modernized by Geschwind (1965) and greatly extended by Sperry and by Gazzaniga. Geschwind called attention to several clinical syndromes resulting from interruption of the connections between the two cerebral hemispheres in the corpus callosum or between different parts of one hemisphere. Some of these are illustrated in Fig. 22-6.

When the entire corpus callosum is destroyed by tumor or surgical section, the language and perception areas of the left hemisphere are isolated from the right hemisphere. Patients with such lesions, if blindfolded, are unable to match an object held in one hand with that in the other. Objects placed in the right hand are named correctly, but not those in the left. Furthermore, if rapid presentation is used to avoid bilateral visual scanning, such patients cannot match an object seen in the right half of the visual field with one in the left half. They are also alexic in the left visual field, because the verbal symbols that are seen there and are projected to regions of the right hemisphere have no access to the language areas of the left hemisphere. If given a verbal command, such patients will execute it correctly with the right hand but not with the left; if asked to write from dictation with the left hand, they will produce only an illegible scrawl. Many remarkable conclusions regarding the nature of behavior and the special roles of each cerebral hemisphere have been drawn from clever observations of patients with callosal section. Extensive discussion of these neuropsychologic abnormalities cannot be undertaken here; suffice it to say that these are not features seen in patients with the usual neurologic diseases, but they are nonetheless of interest to neurologists and are discussed in the writings of Gazzaniga.

In most lesions confined to the posterior portion of the corpus callosum (splenium), only the visual part of the disconnection syndrome occurs. Cases of occlusion of the left posterior cerebral artery provide the best examples. Because infarction of the left occipital lobe causes a right homonymous hemianopia, all visual information needed for activating the speech areas of the left hemisphere must thereafter come from the right occipital lobe. The patient with a lesion of the splenium of the corpus callosum or the adjacent white matter cannot read or name colors because the visual information cannot reach the left language areas. There is, however, no difficulty in copying words; presumably, the visual information for activating the left motor area crosses the corpus callosum more anteriorly. Spontaneous writing and writing to dictation are also intact because the language areas, including the angular gyrus, Wernicke and Broca areas, and the left motor cortex, are intact and interconnected, but after a delay, the patient is unable to read what he has previously written (unless it was memorized). This is the syndrome of alexia without agraphia mentioned earlier.

Surprisingly, a lesion that is limited to the anterior third of the corpus callosum (or a surgical section of this part, as in patients with intractable epilepsy) does not result in an apraxia of the left hand. A section of the entire corpus callosum does result in such an apraxia, i.e., a failure of only the left hand to obey spoken commands, the right one performing normally, indicating that the fiber systems that connect the left to the right motor areas cross in the corpus callosum posterior to the genu (but anterior to the splenium). Object naming and matching of colors without naming them are also done without error. However, when blinded, the patient cannot name a finger touched on the left hand or use it to touch a designated part of the body.

Of interest to the authors is the fact that one sometimes encounters patients with a lesion in all or some part of the corpus callosum without being able to demonstrate any aspect of the aforementioned disconnection syndromes. Notable is the observation that in some patients with a congenital agenesis of the corpus callosum (a developmental abnormality), none of the interhemispheral disconnection syndromes can be found. One must suppose that in such patients, information is transferred by another route—perhaps the anterior or posterior commissure—or that dual dominance for language and praxis was established during early development. (See a review of this subject by Lassonde and Jeeves.)

In addition to alexia without agraphia, the following intrahemispheral disconnections have received the most attention. They are mentioned here only briefly and are considered in more detail in the following chapter.

1. Conduction (also called "central") aphasia. The patient has severely impaired repetition, but fluent and paraphasic speech and writing and relatively intact comprehension of spoken and written language. The Wernicke area in the temporal lobe is putatively separated from the Broca area, presumably by a lesion in the arcuate fasciculus or external capsule or subcortical white matter. However, most often the lesion is in the supramarginal gyrus, as discussed in Chap. 23.

2. Sympathetic apraxia in Broca's aphasia. By destroying the origin of the fibers that connect the left and right motor association cortices, a lesion in the more anterior parts of the corpus callosum or the subcortical white matter underlying Broca area and contiguous frontal cortex causes an apraxia of commanded movements of the left hand (see Chap. 3 and earlier discussion).

3. Pure word deafness. Although the patient is able to hear and identify nonverbal sounds, there is loss of ability to discriminate speech sounds, i.e., to comprehend spoken language. The patient's speech may be paraphasic, presumably because of the inability to monitor his own speech. This defect has been attributed to a subcortical lesion of the left temporal lobe, spanning the Wernicke area and interrupting also those auditory fibers that cross in the corpus callosum from the opposite side. Thus, there is a failure to activate the left auditory language area (Wernicke area). Bilateral lesions of the auditory cortex have the same effect (see Chap. 23).

4. Furthermore, all of the syndromes that span the occipital and either parietal or temporal lobes are, in effect intrahemispheral disconnections in the stream of visual information as discussed earlier.

In the study of focal cerebral disease, there are two complementary approaches: the clinical-neurologic and the neuropsychologic. The first consists of the observation and recording of qualitative changes in behavior and performance and the identification of syndromes from which one may deduce the locus and nature of certain diseases. The second consists of recording a patient's performance on a variety of psychologic tests that have been standardized in a large population of age-matched normal individuals. These tests provide data that can be graded and treated statistically. An example is the deterioration index, deduced from the difference in performance on subtest items of the Wechsler Adult Intelligence Scale that hold up well in cerebral diseases (vocabulary, information, picture completion, and object assembly) and those that undergo impairment (digit span, similarities, digit symbol, and block design). A criticism of this index and others is the implicit assumption that cerebrocortical activity is a unitary function. However, it cannot be denied that certain psychometric scales reveal disease in certain parts of the cerebrum more than in others. These tests allow comparison of the patient's deficits from one point in the course of an illness to another. Walsh has listed the ones that he finds most valuable. In addition to the Wechsler Adult Intelligence Scale, Wechsler Memory Scale, and an aphasia screening test, he recommends the following for quantifying particular psychologic abilities and skills:

1. Frontal lobe disorders

   1. Milan Sorting Test, Halstead Category Test, and Wisconsin Card-Sorting Test as tests of ability to abstract and shift paradigms

   2. The Porteus Maze Test, Reitan Trail-Making Test, and the recognition of figures in the Figure of Rey as tests of planning, regulating, and checking programs of action

   3. Benton's Verbal Fluency Test for estimating verbal skill and verbal regulation of behavior

2. Temporal lobe disorders

   1. Figure of Rey, Benton Visual Retention Test, Illinois Nonverbal Sequential Memory Test, Recurring Nonsense Figures of Kimura, and Facial Recognition Test as modality-specific memory tests

   2. Milner's Maze Learning Task and Lhermitte-Signoret amnesic syndrome tests for general retentive memory

   3. Seashore Rhythm Test, Speech-Sound Perception Test from the Halstead-Reitan battery, Environmental Sounds Test, and Austin Meaningless Sounds Test as measures of auditory perception

3. Parietal lobe disorders

   1. Figure of Rey, Wechsler Block Design and Object Assembly, Benton Figure Copying Test, Halstead-Reitan Tactual Performance Test, and Fairfield Block Substitution Test as tests of constructional praxis

   2. Several mathematical and logicogrammatical tests as tests of spatial synthesis

   3. Crossmodal association tests as tests of suprasensory integration

   4. Benson-Barton Stick Test, Cattell's Pool Reflection Test, and Money's Road Map Test, as tests of spatial perception and memory

4. Occipital lobe disorders

   1. Color naming, color form association, and visual memory, as tests of visual perception; recognition of faces of prominent people, map drawing

It is the authors' opinion that the data obtained from the above tests should be used to supplement clinical observations. Taken alone, they cannot be depended upon for the localization of cerebral lesions.