Steady-state visual evoked responses in anesthetized monkeys.

Steady-state stimuli are repeated continuously at a frequency that evokes averaged EEG waveforms with components at a frequency which is the same or some multiple of the stimulus rate. A variety of stimulus parameters are readily manipulated and controlled. We wished to assess the suitability of this approach for animal experimentation, and chose to evaluate steady-state visual evoked potential (VEP) responses to counterphased checkerboard stimuli in anesthetized adult male Rhesus monkeys. Large VEPs were reliably produced over the range of 3-6 Hz reversal rate; at 10 Hz, the VEP spectral power was clearly diminished in all monkeys. At each given reversal rate, monkeys differed substantially in variability and amount (up to 3-fold) of spectral power. These large inter-subject differences paralleled what we have seen in similar studies of unanesthetized humans. All monkeys developed large VEPs at the midline, but the topographic distribution of maximum VEP was not uniform; in 3 monkeys the power was much greater over one hemisphere than the other. Fatigue or habituation was not evident; in each monkey, there were usually no significant differences in VEP power over the 3 consecutive 30 sec segments of a given stimulation. We conclude that the anesthetized monkey can be a suitable model for study of steady-state phenomena, as long as the proper stimulus parameters are employed and as long as the level of variance is taken into account.