Direct and indirect visual inputs to superficial layers of cat superior colliculus: a current source-density analysis of electrically evoked potentials

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Abstract
The spatiotemporal pattern of visual inputs to the stratum griseum superficiale (SGS) and stratum opticum (SO) of the cat superior colliculus (SC) was determined by an analysis of the current sinks occurring during postsynaptic activity following stimulation of each optic nerve (ON) and the optic chiasm (OX). Electrolytic lesions were used to determine the locations of the 5 major current sinks. Direct SC afferents from the contralateral ON induced 3 current sinks whose maxima were located in the upper part of the SGS, in the middle part of the SGS and in the lower part of the SGS and upper part of the SO. These 3 sinks were generated by 3 afferent fiber groups conducting in the optic nerve with modal and maximum velocities, respectively, of 4 and 5 m/s (slow W-group), 7 and 10 m/s (fast W-group) and 32 and 43 m/s (Y-group). Indirect SC inputs from the contralateral ON via the ipsilateral visual cortex were identified by comparing the pattern of current sinks generated by OX stimulation before and after cortical ablation. The most prominent and fastest indirect sink (Y-group) was found in the lower half of the SGS and uppermost part of the SO. Low-amplitude, long-latency indirect current sinks were also found in the upper and lower thirds of the SGS. The SGS is divisible into 3 physiologic regions according to the spatiotemporal pattern of excitatory synaptic activity generated by the afferent inputs. There is a spatiotemporal matching of the direct collicular afferents from the contralateral retina and the indirect retinal afferents relaying through the ipsilateral visual cortex.