Memory allows us to store, retain, and retrieve information. These three processes influence and are modified by the type of information that is to be remembered, the duration of time over which it must be retained, and the way in which the brain will use the information in the future. The neural circuits underlying these processes are dynamic, reflecting the flexibility of memory itself. To delineate the neural circuitry underlying it, it is helpful to break down memory into simpler components. This categorization, however, need not lead to the assumption that memory is not a unitary phenomenon.
In an effort to explain why focal brain damage affects some aspects of memory but not others, a fundamental distinction has been made between declarative memory, which refers to the conscious memory for facts and events, and nondeclarative memory, which refers to memory for skills, habits, or other manifestations of learning that can be expressed without awareness of what was learned (Fig. e9-1). Patients with bilateral medial temporal lobe (MTL) damage show declarative memory deficits in the face of intact nondeclarative memory. For example, such a patient may learn to play the piano over time without remembering a single practice session or even recognizing the teacher who patiently works with him everyday.
Fractionation of long-term memory. (Adapted from LR Squire, SM Zola: Proc Natl Acad Sci U S A, 24: 13515, 1996.)
Within the declarative memory system, episodic and semantic memory can be distinguished. Episodic memory allows the recollection of unique personal experiences. With episodic memory, the person reexperiences the sights, sounds, smells, and other details of a specific event. Many episodic memories are kept for minutes and hours but soon discarded. Others remain for the course of a lifetime. This temporal difference in storage probably reflects different physiologic processes at work (see below). Semantic memory, in contrast, refers to knowledge about the world; generic information that is acquired across many different contexts and accessed without accompanying details of the time when the words or facts were remembered. One's vocabulary and knowledge of the associations between verbal concepts make up the bulk of semantic memory. This fractionation of declarative memory is supported by evidence that episodic and semantic memory have distinctive anatomic substrates.
In the MTL, the hippocampal formation receives processed sensory information from association areas in the frontal, parietal, and occipital lobes via the parahippocampal cortex. Given these multiple cortical neuroanatomic connections, the hippocampus is well placed to create associations between simultaneously occurring stimuli in our sensory world. Key structures involved with episodic memory include the hippocampus, entorhinal cortex, mammillary bodies, and thalamus. Alterations of episodic memory can be devastating. Overly intense or painful episodic memories can result in posttraumatic stress disorder, a devastating illness in which patients repeatedly reexperience unpleasant ...