An electrophysiological study of the superior colliculus and visual cortex.

In cats anesthetized with Nembutal, simultaneous recording of evoked potentials from superior colliculus and visual cortex to single light flashes gave mean onset latencies of 36.5 msec in the former and 27 msec in the latter. Onset latency to electric stimulation of optic nerve was 8 msec in colliculus and over 1 msec in cortex. Since similar latency differences were obtained from colliculus and cortex to both retinal (photic) and postretinal (electric) stimulation, latency differences were attributed to slower conduction in the retinocollicular pathway. The thresholds were higher in superior colliculus than visual cortex to light flashes of short duration or low intensity, or electric pulses applied to the optic nerves. Simultaneously recorded collicular and cortical evoked potentials differed from each other in shape, duration, and several other respects. With microelectrodes, three functionally differentiated layers of the superior colliculus were distinguished: a dorsal layer in which evoked potentials and single units responding to optic stimuli were obtained; an intermediate layer in which evoked potentials of unchanged amplitude, but no driven units, were recorded; and a ventral layer in which evoked potentials were greatly reduced, and where only “spontaneously” discharging units were found. The optically driven units were classified as those firing with short latency, with long latency, to light-on only, to light-on and light-off, and those inhibited by photic or optic nerve stimuli.