Coincidence of patchy inputs from the lateral geniculate complex and area 17 to the cat's Clare-Bishop area

Abstract

Pathways from a variety of structures to the largest of the cat's suprasylvian visual areas, the Clare-Bishop area, were found to patchy. These inputs arose from the lateral geniculate complex, from area 18, from area 19, and, as noted by Montero (Brain Behav. Evol. 18:194-218, '81), from area 17. The Clare-Bishop area was previously delineated on the basis of its uniform pattern of connections with cortex and thalamus (Sherk: J. Comp. Neurol. 247:1-31, '86) and found to incorporate pieces of several retinotopically defined areas (Tusa, Palmer, Rosenquist: Cortical Sensory Organization. Vol 2. Multiple Visual Areas. Clifton, NJ: Humana Press, pp. 1-31, '81). However, since individual patches did not correspond to particular retinotopically defined areas, other explanations of afferent patchiness were sought. An obvious question is whether the patches originating from different sources are systematically related to each other. Two hypotheses were considered. First, different inputs--for example, from the lateral geniculate nucleus (LGN) and from area 17--might terminate in intermingled but mutually exclusive zones in the Clare-Bishop area. Second, multiple patches of input might reflect duplicate representations of the corresponding visual field segment in the Clare-Bishop area. Both hypotheses were tested by injecting the lateral geniculate complex and either area 17 or area 19 with different anterograde tracers. In each case the two injections involved regions of the visual field that coincided to some degree, ranging from near-total overlap to almost complete exclusion. The first hypothesis predicted that the different labels in the Clare-Bishop area would never be found to overlap, while the second hypothesis predicted that when injections were closely matched retinotopically, there would be extensive overlap between patches. The results supported the second hypothesis: the better the retinotopic match between injections, the greater the overlap found between labeled geniculate and cortical input in the Clare-Bishop area. However, the multiplicity of patches seen in some experiments, and the close spacing between some patches, suggested that an additional, nonretinotopic mechanism also contributes to patchiness in the projections to the Clare-Bishop area.