PERIODIC EXTRINSIC CONNECTIONS IN VISUAL CORTEX

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Extrastriate visual cortex is made up of multiple areas differing in their topography, anatomical connectivity, and neuronal response properties. Combined behavioral-physiological experiments, in conjunction with other lines of research, suggest that these differences are functionally significant. In particular, they are thought to be associated with selective processing of different types of visual information ("streams"), and to represent different stages in sensory-perceptual transformations. The characterization of physiological transformations in extrastriate cortex is still at an early stage. Certain general tendencies have been observed in contrast with primary visual cortex; for example, a decrease in specificity for retinal position and stimulus parameters, along with an increased complexity in response properties. Still not clear, however, is whether particular processes might be uniquely associated with a given area, or whether different areas may share common computational strategies. One approach to elucidating the functional architecture of extrastriate areas is to investigate the "wiring" of an area, as has systematically been done in area V1, at the level of single neurons and axons. The present project has taken this approach, and begun by labeling small populations f efferent axons from area V1 with PHA-L. Among observations to date, we report 1) that cortical axons are not stereotyped. Even within a single projection focus in area V2, individual axons vary in the number and, to some extent, the size of their terminal clusters; 2) although terminal arbors vary in size in area V2, the majority are smaller (about 200um) than the dimensions of CO stripes; and 3) although not absolute, there is surprising size constancy of terminal arbors in both V2 and MT (i.e., about 200um), but total arborizations may be more divergent in MT than in V2. In the next grant period, we will 1) define the terminal loci of PHA-L labeled extrinsic axons, from area V1 to V2, and from area V2 to V1, in relation to modular structure (demonstrated by clusters of retrogradely labeled neurons, by CO histochemistry, or, in V1, by GABA-immuno- reactivity); 2) investigate PHA-L labeled axons from area V2 to certain extrinsic cortical targets, as well as within V2; 3) characterize ultrastructurally cortical axons efferent to area MT and to different compartments in V2, in terms of physiologically significant features such as axon diameter, and size and density of terminal specializations; and 4) begin to investigate axon-target cell interactions by an ultrastructural survey of elements postsynaptic to V1 axons in layers 3 and 4 of V2. In addition to possible functional relevance, it is hoped that this research will also provide a baseline for future studies on ontogenetic, pathological, or activity-related modifications in axonal arborization and ultrastructure, and provide data for theoretical modeling of cortical function.