Antidromic activity in visual radiation during evoked cortical responses.

1. Antidromic stimulation of the visual radiation of cats has been used to investigate the possibility that some of the activity in principal geniculate cells following an optic tract stimulus is antidromic. Single spikes were selected from two classes of poststimulus activity to condition the antidromic test spike--the undoubted orthodromic postsynaptic spike and the later spikes, occurring up to 100 ms after the optic tract stimulus. 2. In 15 of 39 cells the minimum antidromic activation times and the minimum spike-spike intervals were found to be shorter and latencies for antidromic stimulation were longer when the conditioning spikes belonged the class of late poststimulus activity. The differences are in accord with the assumption that some of the conditioning spikes were antidromic. 3. Test spikes were frequently found to have long and variable latency when the conditioning spike occurred more than 45 ms after the optic tract stimulus. Possible reasons are briefly discussed. 4. It is suggested that antidromic activity may occur in conditions of the cortex that are more physiological than those associated with a penicillin-induced seizure focus. Some possible mechanisms and functional significance are briefly discussed.