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HARVARD NEUROBIO 204 - Segregation of Form, Color, Movement, and Depth

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Segregation of Form, Color, Movement, and Depth: Anatomy, Physiology, and Perception Anatomical and physiological observations in monkeys indicate that the primate visual system consists of several separate and independent subdivisions that analyze differ- ent aspects of the same retinal image: cells in cortical visual areas 1 and 2 and higher visual areas are segregated into three interdigitating subdivisions that differ in their selectivity for color, stereopsis, movement, and orienta- tion. The pathways selective for form and color seem to be derived mainly from the parvocellular geniculate subdivi- sions, the depth- and movement-selective components from the magnocellular. At lower levels, in the retina and in the geniculate, cells in these two subdivisions differ in their color selectivity, contrast sensitivity, temporal prop- erties, and spatial resolution. These major differences in the properties of cells at lower levels in each of the subdivisions led to the prediction that different visual functions, such as color, depth, movement, and form perception, should exhibit corresponding differences. Human perceptual experiments are remarkably consistent with these predictions. Moreover, perceptual experiments can be designed to ask which subdivisions of the system are responsible for particular visual abilities, such as figurdground discrimination or perception of depth from perspective or relative movement-functions that might be difficult to deduce from single-cell response properties. P EOPLE WITH NORMAL COLOR VISION WILL PROBABLY FIND the left illustration in Fig. 1 less clear and three-dimensional than the one on the right. But it springs forth if you look at it through a blue filter, such as a piece of colored glass or cellophane. In the left version the gray and yellow are equally bright, or luminant, for the average person, whereas the right version has luminance-contrast information. The ability to infer distance and three-dimensional shape from a two-dimensional image is an exam- ple of a visual fiunction that can use luminance but not color differences. Depth from perspective and color perception are thus aspects of vision that seem to be handled by entirely separate channels in our nervous system. Even though intuition suggests that our vision can plausibly be subdivided into several components--color, depth, movement, form, and texture perception-our perception of any scene usually seems well unified. Despite this apparent wholeness, studies of anatomy, physiology, and human perception are converging toward the conclusion that our visual system is subdivided into several -- - The authors are members of the faculty, Depamnent of Neurobiology, Harvard Medical School, Boston, MA 02115. separate parts whose functions are quite distinct. In this article we summarize some of these anatomical, physiological, and human- perceptual observations. Physiological and Anatomical Studies Occasionally people with strokes suffer surprisingly specific visual losses-for example, loss of color discrimination without impair- ment of form perception, loss of motion perception without loss of color or form perception, or loss of face recognition without loss of the ability to recognize most other categories of objects or loss of color or depth perception (1). Such selectivity seems to indicate that the visual pathway is functionally subdivided at a fairly gross level. Anatomical and physiological studies in monkeys also support this idea of functional divergence within the visual pathway. They reveal major anatomical subdivisions at the earliest peripheral stages in the visual system as well as segregation of function at the highest known cortical stages, but until recently there was little information about corresponding subdivisions in the intermediate levels, the first and second cortical visual areas. Subdivisions at early stages in the miualpathway. It has been known for a century that the nerve fibers leaving the eyes diverge to provide input both to the lateral geniculate bodies and to the superior colliculi. The colliculus seems to be relatively more important in lower mammals than it is in primates, in which its main role is probably orientation toward targets of interest; here we will be Fig. 1. The same image at equiluminance (left) and non-equiluminance (right). Depth from perspective, spatial organization, and figurelground segregation are diminished in the equiluminant version. To convince yourself that the left version does indeed contain the same information as the other, look at it through a piece of blue cellophane or glass. These two colors may not be close enough to your eqduminance point to be effective. Changing the light source may help.Fig. 2. The primate lateral genicu- late body. This six-layered structure is the first stage in the visual system after the retina, and it consists of two distinct subdivisions, the ventral two magnocellular layers and the dorsal four pan~ocellular layers. The two eyes project to different layers in the interdigitating fashion shown: c indicates layers that are innervated by the contralateral eye; i indicates layers with input from the ipsilateral eye. Fig. 3. Receptive fields for (left) typical color-oppo- nent parvocellular genicu- late neuron, excited over a small region by red light and inhibited over a larger region by green light and (right) typical broadband magnocellular neuron, excited by all wavelengths in the center and inhibited by all wavelengths in its surround. concerned exclusively with the geniculo-cortical part of the visual system, which seems to be directly concerned with visual perception (2)-what we think of as seeing. The primate lateral geniculate body is a six-layered structure, with two obviously different subdivisions: the four dorsal, small-cell (parvocellular) layers and the two ventral, large-cell (magnocellular) layers; these two subdivisions differ both anatomically and physio- logically. In 1920 Minkowski (3) discovered that each eye projects to three of the six layers in the peculiar alternating fashion shown in Fig. 2: each half-retina is mapped three times onto one geniculate body, twice to the parvocellular layers and once to the magnocellu- lar, and all six topographic maps of the visual


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