Branching and laminar origin of projections between visual cortical areas in the cat

Abstract

The laminar distribution and branching pattern of corticocortical neurons were studied in areas 17, 18, 19, 20, 21, and the lateral suprasylvian areas of the adult cat neocortex. This was done by examining the laminar position of single‐labelled neurons and the proportions of double‐labelled cells in these areas after paired injections of the fluorescent retrograde labels fast blue and diamidino yellow in areas 17, 18, and 19 of the ipsilateral hemisphere. After injections in areas 18 and 19, the labelled neurons in area 17 were mostly confined to the supragranular layers, with a small proportion of labelled cells in lamina 5 and upper lamina 6. Double‐labelled neurons were rare and were found in the region of overlap between the two populations of labelled cells. They were mostly found in the upper laminae but a few were observed in laminae 5 and 6. The cells projecting to either area were often grouped in patches which were seen to overlap or interdigitate depending on the region examined. As a population, the neurons projecting to area 18 occupied a deeper position in laminae 2 and 3 than those projecting to area 19. Labelled cells in area 18 after injections in areas 17 and 19 were mostly found in the upper laminae with a few double‐labelled cells which were restricted to the region of overlap between the two populations of labelled cells. The pattern of labelling in area 19 after injections in areas 17 and 18 was different from the one seen in areas 17 and 18. Neurons were almost equally distributed between the supra‐ and infragranular layers and there was a substantial proportion of double‐labelled neurons (10%) which tended to belong mostly to lamina 5 and upper lamina 6. In area PMLS, the laminar position of corticocortical cells was somewhat similar to the one observed in area 19, in that a substantial number of labelled neurons were found in the deep laminae, especially after injections in 17 or 18. After injections in area 19, labelled cells were mostly found in the upper layers. Double‐labelled cells were numerous (20%) when the injections were placed in areas 17 and 18 but quite rare in the other cases (17–19 and 18–19). Most of the double‐labelled neurons were found in the deep layers. After injections in areas 17, 18, and 19, labelled cells were found in area 20, thus demonstrating a hitherto unknown projection from area 20 to areas 17 and 18. Labelled cells in area 20 were almost exclusively confined to the infragranular layers. Double‐labelled neurons were numerous.(1–20%) when areas 17–18 and 18–19 were injected but quite rare when areas 17 and 19 received the injections. The results are discussed in terms of different levels of processing of information in the visual cortex. It is argued that areas 17 and 18 occupy a similar level and that areas 19 and PMLS are at higher levels while area 20 occupies a still higher level in the course of sensory information processing. Corticocortical connections from low levels tend to arise from supragranular layers whereas the feedback from higher to lower levels is mostly done via infragranular laminae. It is proposed that, contrary to the subcortical afferents which branch profusely to innervate all three visual areas (Bullier et al., '84), the corticocortical projections are generally poorly bifurcated except in the case of feedback to areas both occupying a similar level in the information processing.