Visual evoked potentials for red-green gratings reversing at different temporal frequencies: Asymmetries with respect to isoluminance

Human visual evoked potentials (VEPs) were recorded for abrupt reversals of 2 cycles/deg (c/deg) square-wave gratings combining high red-green contrast with different levels of luminance contrast. Response characteristics-2nd harmonic amplitudes and peak latencies as a function of luminance contrast-were compared for four different reversal rates ranging from 6.25 Hz to 12.5 Hz. At every reversal frequency, the VEP amplitude and latency plots were nonsymmetrical with respect to isoluminance. The amplitude dropped to a minimum within a region of rapid phase change, always at a red-green luminance contrast for which the green color had the higher luminance, at about 40% or 50% Michelson luminance contrast. The rapid phase shift around this contrast suggested a sudden change in the relative impact of VEP generators with different latencies, possibly dominated by parvocellular or magnocellular input. The most prominent VEP waveform through most of the luminance contrast range, P110, is interpreted in terms of a parvo-mediated response that is attenuated with increasing reversal frequency. Contrast-dependent changes in the P110 amplitude appear to be responsible for the VEP asymmetries reported here.