Short-term changes in the response characteristics of the human visual evoked potential

The present study examined how the response characteristics of the visual evoked potential (VEP) varied during the course of trials using a sinusoidal grating stimulus that reversed contrast in a square-wave manner. To accomplish this, amplitude and phase values were derived in short segments during the course of continuous stimulation for three subjects. When stimulus spatial frequencies of 0.77 or 1.55 c/deg were used, VEP amplitude remained at a stable value throughout the trial. At 3.1 c/deg, 6-12 sec were required for VEP amplitude to increase to a stable value, which was on average 204% greater than the value noted during the first few seconds of the trial. At 6.2 and 12.4 c/deg, VEP amplitude changes were more complex, first increasing and then decreasing substantially, to levels that were on average 63.8% and 38% of the peak reached earlier in the trial. In all cases, VEP phase decreased during the trial. The magnitude of this decrease ranged up to 50 deg, corresponding to an approx. 10.5 msec delay for the 6.65 Hz stimulation rate used. Prior exposure to an adapting grating diminished the changes in VEP amplitude and advanced the phase changes. Therefore, these changes appear to represent a form of contrast adaptation that is restricted to responses to high spatial frequencies. In addition, the present results provide evidence against a fundamental assumption of signal averaging--that an invariant stimulus will evoke an invariant response.