The effect of standing contrast and pattern adaptation on the human visually evoked potential.

Human visually evoked potentials (VEPs) were recorded with sinusoidal gratings of low (0.5 c/deg), medium (4 c/deg) and high (16 c/deg) spatial frequency (SF). Simultaneous recording from Oz-A1, O2-A1 and Oz-O2 allowed the separation of the early and late VEP waves for most subjects. The effect of standing contrast and pattern adaptation on these waves was studied. At low SF both early and late waves were not affected by standing contrast or adaptation with motionless grating, while adaptation with a drifting grating reduced them. At medium and high SFs both principal negative waves. N1 and N2, were reduced by standing contrast and pattern adaptation. The amplitude of N2 saturated at low contrast level (0.1) and was reduced after adaptation regardless of the SF of test and adapting stimuli, while the amplitude of N1 did not saturate up to the contrast level of 0.3 and exhibited SF-selective adaptation. The adapted onset-VEP was similar to the offset-VEP. The data suggest that: the early wave is generated by multiple narrowly tuned SF-selective structures that are medially positioned while the generators of the late wave are not SF-selective and occupy a wider area centered laterally; the effects of standing contrast and pattern adaptation on VEPs, as well as the relationship of onset and offset VEPs, reflect the time-course of neural activity evoked by long-lasting stimuli.