Motion VEPs with simultaneous measurement of perceived velocity

The dependency of the N200 amplitude of the motion-onset VEP evoked by a pa rafoveal grating of variable speed (0.25 - 13.5 deg/s, corresponding to 0.5 - 27 Hz) and constant contrast (4%) was studied. Additional measurements w ere made with parafoveally presented gratings of constant speed (2 deg/s, c orresponding to 4 Hz) and a variable contrast (0.5 64%) before and after ad aptation to a stationary or drifting grating. In this latter experiment, si multaneous psychophysical measurements were made of the perceived speed. Th e amplitude of the N200 wave increased with increasing stimulus speed withi n the slow speed range up to 1.5 deg/s (corresponding to 3 Hz). Adaptation to a stationary grating had no significant effect on the relationship betwe en the N200 amplitude and stimulus contrast. Contrary to this, adaptation t o a slowly drifting grating (1 deg/s, corresponding to 2 Hz) or to a rapidl y drifting grating (4 deg/s, corresponding to 8 Hz) reduced the N200 amplit ude significantly. Adaptation to a stationary grating slightly reduced the perceived speed of subsequently viewed gratings. Adaptation to a slowly dri fting grating increased the perceived speed of the subsequently viewed grat ings, whereas adaptation to a rapidly drifting grating decreased the percei ved speed. The findings can be best explained by a two-channel model of spe ed perception. While the motion VEP reflects the sum of both channel activi ties, the psychophysical measures point to the antagonistic encoding of low and high velocities.