Visual cortical inputs to deep layers of cat's superior colliculus

In the superior colliculi of cats anesthetized with ketamine, 84% of identified output cells of the deep layers could be driven by shocks to the contralateral optic disk, optic chiasm, or ipsilateral optic tract; 75% of these deep-layer cells had response latencies reflecting a polysynaptic influence of retinal Y-cells. Following large, acute lesions of the ipsilateral occipital cortex (including visual areas 17, 18, 19, and the posteromedial lateral suprasylvian area (PMLS), only 18% of deep-layer output cells were driven by electrical stimulation of the optic pathway and only 4% exhibited an indirect Y-cell influence. Thus, one or more of these visual areas may be important for the relay of retinal information, and particularly of Y-cell information, to the deep layers of the superior colliculus. This hypothesis is supported by the observation that intracortical stimulation in areas 17, 18, 19, and PMLS activated many cells of the ipsilateral, deep tectal layers at latencies consistent with those exhibited by the indirect Y-cell pathway. The distributions of activation latencies were similar to those observed in the superficial layers, raising the possibility that at least some of the cortical influence on the deep layers may be mediated by direct connections. Cells of the deep layers were more likely to be excited by a cortical stimulus that activated cells immediately above them in the superficial layers than by a stimulus that did not. This indicates that the functional connections between visual cortex and the deep collicular layers exhibit a topographic orderliness similar to that previously described for corticotectal projections to the superficial layers. These results provide further evidence that the visual cortex exerts a significant influence on cells of the deep collicular strata and that the pathways involved are capable of mediating the indirect, retinal Y-cell input to these neurons.