Bifurcation of subcortical afferents to visual areas 17, 18, and 19 in the cat cortex

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Abstract
The pattern of axon bifurcation in the thalamic and claustral afferents to visual areas 17, 18 and 19 in the adult cat neocortex was examined by injecting 2 fluorescent retrograde tracers, fast blue and diamidino yellow, in retinotopically corresponding regions of 2 of these 3 cortical areas. The pattern of single- and double-labeled cells was examined in subcortical structures and the presence of double-labeled cells was interpreted as indicating that these neurons seed bifurcating axons to the 2 injected areas. The size of the cortical region surrounding the injection site where each fluorescent dye is taken up was studied by making side-by-side injections of the 2 tracers in area 17 and examining the size and the separation of the 2 groups of labeled cells in the lateral geniculate nucleus (LGN). Evidently, the uptake region is < 1 mm and is included in the region of dense coloring surrounding the track of the injection needle. Injections were made in cortical regions which were in retinotopic correspondence as determined by electrophysiological recording. The double-labeled neurons were always found in the zone of overlap of the 2 populations of colored cells and no double-labeled neurons were found when there was no overlap between these populations. Apparently, the bifurcating axons send branches to strictly retinotopically corresponding regions in the 2 cortical areas. After injections in areas 18 and 19, numerous double-labeled cells were observed in laminae C of the LGN, in the medial interlaminar nucleus (MIN), the posterior nucleus (PN) and the lateral part of the lateral posterior nucleus (LP), in the retinorecipient zone of the pulvinar (RRZ-Pul), the intralaminar nuclei (ILN) and the claustrum. The proportions of double-labeled cells with respect to the total number of labeled neurons were computed in the region of overlap of the 2 populations of labeled cells. These percentages ranged between 5 and 20% and were highest in the C laminae of the LGN, the intralaminar nuclei, and the claustrum. After injection of areas 17 and 18, similar proportions of double-labeled cells were observed in the same structures, as well as in the A laminae of the LGN. The intralaminar nuclei and the claustrum tended to have slightly higher (20-30%) proportions of double-labeled cells. When the nonadjacent areas 17 and 19 were injected, doubled-labeled neurons were also observed in all these structures, except the A laminae of the LGN. They were generally less frequent than in the case of injections in adjacent visual areas, with the exception of the ILN, which returned similar proportions. The 3 cortical areas studied receive a substantial amount of shared subcortical input terminating in strictly retinotopically corresponding regions. Apparently, the functional specificity of these areas does not stem from each receiving its afferent input from a particular subset of neurons in each subcortical nucleus. It is argued that the functional specificity of different cortical areas must largely be elaborated by mechanisms dependent on intracortical connectivity.

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