Cat lateral suprasylvian cortex: Y-cell inputs and corticotectal projection

Abstract

Retinal Y-cells activate most cells in the deep layers of the cat's superior colliculus via an indirect pathway involving the occipital cortex. The lateral suprasylvian area seems to be an important source of visual input to the deep collicular strata but it is unclear whether Y-cell influences reach this extrastriate area and, hence, whether this area participates in the indirect Y-cell pathway. In this study, retinal influences on the posteromedial lateral suprasylvian area (PMLS) were studied in anesthetized cats. Responses to electrical stimulation of the optic disk (OD) and optic chiasm (OX) were recorded in single units in PMLS and in neurons of the dorsal lateral geniculate nucleus (LGNd) that were antidromically driven from PMLS. Virtually all PMLS cells (99%; 99/100) exhibited small differences (less than or equal to 0.8 ms) between OD- and OX-activation latency, indicating that they were driven by a pathway originating in rapidly conducting Y-cell axons. A small number of PMLS cells (17%; 20/118) had very short activation latencies (less than or equal to 3.2 ms from OX), comparable to those of cells in areas 17 and 18 receiving monosynaptic inputs from geniculate Y-cells. Further, LGNd cells with latency behaviors typical of Y-cells could be antidromically driven from PMLS, confirming that geniculate Y-cells project directly to PMLS. Most PMLS cells (83%; 98/118), though exhibiting small OD-OX latency differences, had absolute latencies too long to be attributed to direct inputs from geniculate Y-cells (3.3-8.5 ms from OX). Thus Y-cells in the LGNd influence most PMLS cells by way of a multisynaptic pathway. PMLS cells antidromically activated from the superior colliculus were driven only by this multisynaptic Y-cell input. Total conduction time from the retina through PMLS to the colliculus corresponds closely to the latency of the indirect Y-cell activation observed in the deep collicular layers. These results support the view that the lateral suprasylvian cortex constitutes an important source of visual input to the cat's deep collicular layers and, more generally, that the extrastriate visual cortex may figure prominently in the cortical control of gaze.