Monocular and binocular visual evoked potentials were studied as a function of modulation depth (contrast) of a counterphase sinusoidal grating stimulus. A range of spatial and temporal frequencies of stimulation were used. The contrast functions showed many different forms, and were in some cases nonmonotonic. The binocular response usually had a steeper slope than the monocular response. Extrapolating the slope to the zero intercept gave electrophysiological thresholds which did not correspond with psychophysical threshold at any spatial or temporal frequency used. The binocular response was reduced to the monocular level when the orientation difference between the two eyes exceeded about 20 degrees. The data show that the pattern evoked potential is highly specific to many stimulus variables.