Postsynaptic potentials in the cat's visual cortex following electrical stimulation of afferent pathways

SummaryPostsynaptic potentials (PSP) of neurones in the visual cortex after electrical stimulation of optic tract and radiation fibres were analysed. Special attention is drawn to the disappearance of excitatory reactions, if cells are partially depolarized to a membrane potential of 30–40 mV, as it is the case in many visual cortex cells after impalement. This might lead to misinterpretation of the records obtained. In those cells in which excitatory responses were still visible, stimulation of the optic tract lead to sub- and suprathreshold EPSP's at an average latency of 2.8±0.3 msec. This mean latency could be divided into two sub-groups (2.4 and 3.8 msec). After radiation stimulation the latency for excitation was 1.2±0.3 msec; antidromic responses due to stimulation of corticofugal fibres were sometimes observed. The excitatory responses were followed by a gradable IPSP which sometimes was the first recordable event after a stimulus. Because of the above mentioned limitations, the number of cells with true primary inhibition could not be evaluated. Latencies of IPSP's after optic tract and radiation stimulation were, in the average 1 msec longer than those of EPSP's and both latency groups were clearly separated. After optic tract stimulation two different groups of IPSP-latencies could be distinguished (3.4 and 4.6 msec), which again were 1 msec longer than the two groups of EPSP-latencies. The findings suggest a recurrent collateral inhibitory feedback mechanism within the visual cortex. No indication for special inhibitory interneurones or for direct inhibitory geniculocortical fibres was found. The distinction of two latency groups after optic tract stimulation can be explained by two fibre groups with different conduction velocity in the optic radiation. Interaction of PSP's induced by light and electrically was demonstrated.