Human visual evoked potentials (VEPs) were elicited by alternately increasing and decreasing contrast by 0.1 Michelson units in the presence of an in-phase grating mask with static contrast between 0 and 0.6. Gratings were achromatic or isoluminant (R/G). Coarse achromatic gratings elicit VEPs of equal amplitude irrespective of the static contrast, like some transient-type magno-units in macaque V1. Conversely, much finer achromatic gratings elicit human VEPs, which attenuate at high static contrast reflecting nonspecific activation of sustained mechanisms. VEPs to red/green contrast increment were negative and attenuate at high-static contrast consistent with chromatic pattern adaptation of sustained parvocellular mechanisms. R/G contrast decrement elicits positive waves that are more resilient to the effects of adaptation. (C) 2000 John Wiley & Sons: Inc.