The idea that the primate visual system contains parallel streams of processing that arise in the retina remain relatively independent throughout visual cortex has recently commanded considerable attention, and has led to far-reaching speculation about its functional consequences, Parallel organization is obvious at all levels of the geniculocortical system, including the sub-cortical pathway leading to cortex, the early stages of cortex, and the highest levels of cortical processing. However, the mapping of parallel components in one level onto those at the next has not been established conclusively. It is not known whether components in each stage map directly onto those at the next level, creating truly parallel pathways, or if instead a gradual intermixing eliminates any exclusive relationship between components in the different stages. While many lines of evidence suggest that the geniculocortical system in fact does contains largely independent pathways. there is also clear evidence of intermixing. The proposed experiments will continue on-going work that is aimed at evaluating the degree of parallel organization in monkey visual cortex. Conclusive answers cannot be provided either by anatomical studies or by those that examine similarities in response properties at different levels. Instead, the proposed experiments will record from cortical cells while reversibly inactivating individual magnocellular or parvocellular layers in the lateral geniculate nucleus (LGN). This is a method that the investigators have applied successfully to demonstrate that neurons in the middle temporal visual area (MT) depend primarily on the magnocellular layers on the LGN for their excitatory drive. The proposed experiments will address several specific questions about segregation of magnocellular (M channel) and parvocellular (P channel) contributions to visual cortex. 1) Does the degree of P and M channel segregation seen at the level of MT persist throughout visual cortex, or does it gradually erode in later stages? 2) Given that the P and M channels have been shown to mix to some extent, what are the special properties conferred by mixing? 3) How are the (two) P and M channels distributed across the three pathways that have been identified in the early stages of visual cortex using cytochrome oxidase stain?
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