Visual and auditory evoked responses during penicillin induced generalized spikewave activity in cats

In 10 healthy adult cats chronically implanted with parasagittal electrocorticographic electrodes applied to the dural surface, curarization was performed and baseline recordings of the visual evoked response (VER) and the auditory evoked response (AER) were made. Following the procedures of Prince and Farrell (1969), the animals were then given intramuscular doses of 300,000 to 500,000 U of penicillin G per kg with the subsequent development of diffuse, bilaterally symmetrical, photosensitive spike/wave discharges in the EEG from 1 to 1.5 hr later and concomitant facial myoclonus, arrest of movement, and 'absence like' staring in noncurarized animals. The VER and AER were monitored at 15 min intervals for several hours during which time the VER consistently decreased in amplitude up to the time at which the first spike/wave bursts could be elicited by photic stimulation, approximately 1 hr after injection, after which time all early components (0 to 200 msec) of the VER were progressively increased from 150 to 300% until spontaneous spike/wave bursts were consistently recorded (2 to 2.5 hr.) Coincident with this change, a marked increase in late components (200 to 500 msec) was also observed. Both the early diminution and later augmentation of the VER were equally observable and of the same degree in both visual and nonvisual cortex. Changes in the AER were also recorded with the development of this model, and were similar to those of the VER, but of a lesser degree. These data and the similarity of this model to human petit mal epilepsy argue against increased inhibitory impulses from subcortical structures, specifically midbrain tegmentum, to the visual system during the ictal discharge being responsible for the subjective loss of visual information during petit mal absence, although it is intriguing that a gross diminution of the VER was observed before any electrographic manifestations of the model became evident.