The influence of grating contrast on the human cortical potential visually evoked by motion.

A cortical potential was evoked by the onset of horizontal drift of a previously stationary vertical square-wave grating (motion-on VEP). The influence of contrast on the principal VEP waves N2 and P2 (peak latencies 180-220 ms and 250-350 ms, respectively) was investigated in five subjects for velocities between 0.3 and 4.1 deg/s, a spatial frequency of 2.6 c/deg and a mean luminance of 17 cd/m2. For low contrasts an ascending linear relation could be found between wave amplitude and logarithm of contrast, and a descending one between wave peak latency and log contrast. The waves remained constant for moderate and high contrasts. The lowest contrast value at which wave constancy occurred (saturation contrast) was determined by a method of least squares. All the data summarized yielded a saturation contrast of 0.058 for N2 and 0.084 for P2. The saturation contrast of the pattern VEP already known from the literature is essentially greater under comparable experimental conditions.