In the macaque monkey the parvocellular and magnocellular layers of the lateral geniculate nucleus (LGN) relay qualitatively different types of information. These two channels of visual information have been extensively studied and are known to have separate projections to striate cortex, and to remain largely segregated within that area. Although many lines of evidence indicate that these channels retain their distinctions in later stages of cortical processing, as yet there is no conclusive evidence regarding their fate in extrastriate cortex. Given the longterm objective of understanding information processing in primate visual cortex, it is important to know if these channels contribute directly to the pronounced physiological differences seen among extrastriate visual areas. Experiments will be carried out to determine the relative contributions of the parvocellular and magnocellular channels and degree of mixing in two extrastriate visual areas in the macaque: V4 and MT. These areas have been selected because they are the best candidates for receiving differential inputs from the parvocellular and magnocellular channels, they are relatively well characterized in terms of other properties, they are at similar stages in cortical processing, and they are readily accessible. Several approaches will be employed to study these areas: 1) measurement of cortical responses following reversible or irreversible inactivation of selected layers of the LGN, 2) determination of the subcortical conduction speed of the axons which indirectly feed the two areas, and 3) comparing the contrast sensitivity and visual response latency and transience of neurons in the two areas. A further objective is to examine V2, which projects to both V4 and MT, in order to examine the role it plays in determining the degree of segregation of these channels in extrastriate cortex. In addition to answering a fundamental question about information processing primate visual cortex, these experiments could provide a link between channels which exist in the early stages of the visual system and differences in physiological properties which are seen among the various areas in cortex. Such a link could provide valuable information for psychological studies of the contributions of various cortical areas to specific visual capabilities.
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