After studying this chapter, the student should be able to:
Diagram the function and central projections of each major cell class.
Describe the functions of retinal recipient brain areas.
Describe the neural processing in V1 and higher cortical areas.
THE OUTPUT OF THE RETINA: WHAT THE EYE TELLS THE BRAIN
The common metaphor that the eye is like a camera works reasonably well for some of the optical components of the eye such as the lens and pupil but fails dramatically in describing the output of the retina. A camera is a recording device whose purpose is to register (and, in the video case, transmit) a true rendition of the light distribution focused on the film or detector. Virtually all cameras have a homogeneously dense pixel array, whereas the photoreceptor density in the retina varies markedly from center to periphery. More importantly, a camera records or transmits the light intensity at each pixel, but in the retina, most ganglion cells receive inputs from many photoreceptors, and many ganglion cells extract a highly processed signal from these photoreceptor inputs, such as motion occurring in a particular direction or the presence of an edge.
A Review of Ganglion Cell Classes
The mammalian retina, including that of primates such as humans, contains at least 20 to 25 classes of ganglion cells whose axons terminate in at least 15 different retinal recipient zones. The responses of these different ganglion cells mediate many different types of visual functions by virtue of their receptive field properties (what causes them to respond) and central projections (where in the brain the ganglion cell axons terminate). Many retinal ganglion cells terminate in multiple retinal recipient zones and thus participate in multiple visual processing pathways.
Functions of Visual Output
There are several visually mediated functions of which we are conscious, and many of which we are not conscious. Conscious visual functions include perception, identification, and visually controlled movement guidance. Unconscious visually mediated processes include the control of eye position and pupil size and circadian rhythms. These processes are mediated by different ganglion cell classes in different visual pathways.
Ganglion Cell Response Selectivity
Different ganglion cell classes differ considerably in their response selectivity, which is related to the function of the visual circuit to which they project. At one extreme are the midget or parvocellular ganglion cells of the fovea whose input arises primarily from a single bipolar cell, which in turn is driven mostly by a single red or green cone. The intracellular voltage response of these midget ganglion cells closely resembles that of the single bipolar cell driving them. In the case of off-center ganglion cells, the hyperpolarizing bipolar cell response itself resembles that of the cone to which it connects. Because the spike output of the midget ganglion cell follows its ...