Visual evoked potentials during the development of a spiking cobalt focus in rat neocortex.

Averaged visual evoked potentials recorded from three electrode placements in visual cortex in response to single and paired light flashes were acquired from rats with epileptogenic cobalt implants or with stainless-steel control implants. Evoked potentials acquired after computer-detected epileptic spikes were also recorded from rats in the cobalt group. Epileptic spiking began between the 6th and 9th postoperative days and flashes then began to evoke epileptic spikes. Excitability cycles were assessed. On the day showing the maximum rate of spiking, excitability at the site of the cobalt was markedly reduced following flash-triggered or spontaneous epileptic spikes in almost all cases. By contrast, the electrode placement in contralateral homotopic cortex showed nearly normal excitability despite the presence of large amplitude epileptiform spikes and sharp waves projected from the site of the cobalt. The results are discussed with reference to other experimental findings.