SPATIAL RECIPROCITY OF CONNECTIONS BETWEEN AREA-17 AND AREA-18 IN THECAT

We examined whether the interconnections between areas 17 and 18 are s patially reciprocal, i.e., whether a column of cells in area 17 receiv es from the same region of area 18 as it sends projections to, and vic e versa. We addressed this question by making side by side injections of retrograde fluorescent tracers in area 18, calculating the converge nce and divergence of the connections from area 17 to 18. We compared these values with previously reported values of divergence and converg ence of the projections from area 18 to area 17. The results demonstra te that there is a good match between the convergence and divergence o f the area 17 to area 18 connection and, respectively, the divergence and convergence of the reverse connection. We confirmed directly the s patial reciprocity by injecting simultaneously in area 17 a retrograde and an anterograde tracer and by analyzing quantitatively the density of anterograde and retrograde labeling across the surface of area 18. There was an excellent match between the density maps of retrogradely labeled cells and anterogradely labeled axon terminals in area 18. Co nnections between areas 17 and 18 therefore exhibit large degrees of c onvergence and divergence and are spatially reciprocal. Thus, a given column of cells within one of these two areas is reciprocally intercon nected with a large region of the opposite area. Such an organization may provide the basis for synchronization of firing of neurons across these two areas, as revealed by cross-correlation studies.