A double-labeling investigation of the afferent connectivity to cortical areas V1 and V2 of the macaque monkey.

The afferent connectivity of areas V1 and V2 was investigated using the fluorescent dyes fast blue and diamidino yellow. Simultaneous injection of each dye in retinotopically corresponding regions of these areas gave rise to two afferent populations of labeled neurons in subcortical and cortical structures which project to both areas. These two populations showed a variable degree of overlap in their spatial distribution. Neurons labeled by both dyes (double-labeled neurons) which, therefore, project to both areas, were found in substantial numbers in these overlap zones. When the injections were made in non-retinotopically corresponding regions in the two areas, both populations of labeled cells overlapped extensively in the cortex but not in subcortical structures, suggesting that the laws governing the topography of these two types of connections are different. In the cortex, the labeled neurons extended from the fundus of the lunate sulcus to the fundus of the superior temporal sulcus. A few labeled neurons were also found in the inferior temporal cortex and the parahippocampal gyrus. In all cortical regions, corticocortical neurons projecting to V1 and V2 were found in both supra- and infragranular layers, although double-labeled neurons were more numerous in infragranular layers. With increasing distance from V1 there was an increase in the proportion of neurons labeled in infragranular layers. The comparative strength of input to V1 and V2 was computed and was found to be higher to V2 in all cortical regions except the superior temporal sulcus which projected equally heavily to both areas. The superior temporal sulcus also stood out in that of all cortical regions it contained the highest proportion of double-labeled neurons. Single- and double-labeled neurons were found in a number of subcortical structures including the lateral geniculate nucleus, the inferior and lateral pulvinar, the intralaminar nuclei, the nucleus basalis of Meynert, and the amygdala. The pattern of labeling in the lateral pulvinar was in agreement with the suggestion that this structure has a complex topographical organization containing at least a dual representation of the visual field (Bender, D. B. (1981) J. Neurophysiol. 46: 672-693). In the pulvinar complex, densities of labeled neurons permitted evaluation of the strength of input to V1 and V2, the latter being the strongest. These results demonstrate that areas V1 and V2 share a vast amount of common input from the same cortical and subcortical structures and that a number of neurons project to both areas via branching axons.