PERIODIC EXTRINSIC CONNECTIONS IN VISUAL CORTEX

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The laminar and tangential organization of extrinsic corticocortical connection from primary visual cortex (area l7 or Vl) is known, but only as revealed by bulk injections of 3H-amino acids or HRP. These orthograde transport methods are relatively crude and cannot easily distinguish between axon preterminals and terminals, or otherwise resolve axon morphology. Thus, we do not currently know whether, as in the case of geniculostriate terminations, there are several morphologically distinct classes of striate efferent axons, perhaps arborizing in distinct sublaminae. The proposed experiment will analyse the arborization of striate efferent axons in prestriate areas V2 and MT, by injecting kidney bean lectin (PHA) in area Vl of the macaque monkey. Knowing the precise morphology of these axonal arborizationsis a prerequisite to better understanding how visual information is dispersed from Vl, and is further essential for defining the structural basis of modular organization in prestriate cortex. A periodic organization has been described for several anatomical connections in V2 and MT; but, partly because the periodic spacing varies widely among the differnt systems, it is not presently clear whether there is a basic anatomical-functional unit in prestriate cortex comparable to the hypercolumn in V1. We will specifically examine whether striate axon arborizations exhibit recurrent preterminal widths or form tangential patches of regular periodicity. These parameters might point to an underlying cortical substructure, perhaps shared by other anatomical systems. Detailed comparisons will be made of how striate axons arborize in V2 and MT in order to ascertain whether these fibers have a sterotyped anatomy, or whether, alternately, their anatomy relects the particular funtional prpoperties of a given area. Additional experimental goals include: 1) an investigation, using PHA and florescent beads, of reciprocal connections between striate and prestriate area; 2) an EM-PHA study of neuronal targets postsynaptic to striate termnations in layer 4 of area V2 in the macaque; and 3) a comparative anatomical investigation in the ferrent, using PHA injections, of striate effernet arborizations in V2 of this species. In addition to relevance for issues of visual cortical organization, this research will also provide a baseline for future work on developmental stages of cortical connections. Also, a more detailed animal model of cortical organiztion will be necessary for interpreting human pathological conditions, including mental retardation.