Effect of seizures in visual cortex on response to geniculate stimulation

¹ Division of Neurology, State University of New York College of Medicine, Brooklyn, New York

Electrical seizures elicited in the visual cortex of unanesthetized cats by direct cortical stimulation have been found to be associated with two phases of depression of the primary response evoked by single stimuli to the visual radiation fibers at their origin in the lateral geniculate body. Depression in the first phase, accompanying what has been called the after-discharge proper, was relatively small and probably in large measure the result of tetanization of the visual radiation fiber terminals. It is suggested, therefore, that the primary response and the after-discharge utilize different portions of the cortical circuitry. Depression in the later phase coincided with the appearance of grouped bursts of irregular spikes and slow waves, was of severe degree, and lasted several minutes. Depression of the radiation spike was prominent during this phase if the seizure was long. This phase has been identified with the spreading depression of previous investigators and was shown to depend, in part at least, upon the seizure itself and not simply upon the direct effects of the tetanic stimulating current.