Visual post-stimulus excitability changes in freely moving rats indicated by amplitude and peak time of evoked potentials.

Visual post-stimulus excitability changes (visual recovery cycles = VRC) were investigated in 25 freely moving hooded rats by means of double-flash stimuli with varying intervals in 10 ranks from 25 to 400 ms. The amplitudes and peak times of visually evoked potentials (VEP)-components were evaluated from averaged double-flash responses of the visual cortex under conditions of stationary spontaneous behaviour during the averaging time. The amplitudes of component N 31 showed the slowest increase: still below 60% after 400 ms flash interval during relaxed wakefulness, but approximately 85% during exploratory behaviour. P 45 was equalized after 150 ms and component N 49 never decreased but increased. All components of the test VEP were influenced by oscillating excitability changes in the course of the photically evoked afterdischarges triggered by the first flash. The peaks of all test VEP components were delayed by about 15-25 ms with a maximum at the 50 ms intervals and an exponential course of normalization with the prolongation of flash intervals to 300-400 ms. The results demonstrate that rather complicated neuronal interactions take place in the post-stimulus period, which is expressed by the dynamic changes of different test VEP components.