Responses of cortical neurones to stimulation of the visual afferent radiations

Extracellular spikes of visual cortical neurones in unanesthetized cats were recorded. The latency after electric stimuli of the optic radiation to the onset of firing of cells with concentric fields, simple receptive fields and complex receptive fields was measured. Simple receptive field neurones had response latencies averaging 0.93 msec longer than neurones with concentric fields. The majority of the latter represented responses of geniculate axones terminating in the visual cortex and a few were action potentials of first-order cortical cells. Cells with complex receptive fields had longer latencies than simple field neurones and often showed prolonged cessation of firing following electrical stimulation. Many neurones showed a period of firing arrest after radiation shock. The duration and variability of this period also varied according to receptive field type. The data were consistent with the hypothesis that neurones with simple receptive fields receive the initial contact of incoming geniculocalcarine afferent fibres. Complex receptive field neurones appear to derive their input from other cortical cells rather than from the direct geniculocortical connections.