Visual evoked potentials during spontaneously occurring spike-wave discharges in rats.

Flash evoked visual potentials (VEPs) were recorded in freely moving WAG/Rij rats. These rats show spontaneously occurring spike-wave discharges in the EEG, interpreted as absence-like seizures. VEPs recorded during the presence of spike-wave discharges were compared with those obtained during normal states of vigilance as quiet wakefulness, slow-wave sleep and REM sleep. Almost similar VEPs were recorded during wakefulness and REM sleep, whereas during slow-wave sleep the second positive peak (P2) was considerably larger. In comparison with normal sleep-wake states, VEPs during spike-wave discharges showed unique changes, such as a decrease in the N1 amplitude, an increase of the P4 amplitude and an enhanced afterdischarge. Other characteristics were similar to those seen during slow-wave sleep, such as an increase of the P2 amplitude and a diminished P2-N3-P3 complex. These findings indicate sensory alterations during a spike-wave discharge. As expressed in the decrease of N1, afferent information cannot enter the thalamus during the rhythmic oscillatory mode. Such alterations may underlie the lowered responsiveness to external stimuli during spike-wave activity.