Spatial-frequency specific contrast gain and flicker masking of human transient VEP.

We studied the effects of grating contrast and luminance-flicker masking on the early waves of human visually evoked potentials (VEPs) recorded at the onset-offset of sinusoidal gratings of varying spatial frequencies (SFs). At high SFs, the response waveform was simple and VEP was dominated by a negative wave (N110). At low SFs, several positive-negative deflections were recorded, the earliest dominating wave being positive (P90). The amplitude of P90 was saturated at a contrast of about 0.1 and it was attenuated by flicker masking. Masking involved to a lesser extent the waves following P90. It was weaker at the flicker frequency of 5 Hz than at 10 and 20 Hz. No flicker masking was found at SFs higher than 2-4 c/deg. At medium and high SFs, VEPs were obtained at higher contrast levels. No saturation (max contrast tested 0.5) and no flicker masking of N110 were observed. These results suggest that the early VEP components recorded at low and high SFs are related to different types of neuronal activity. Correlation between VEP properties and properties of magnocellular and parvocellular pathways is considered with an emphasis on recent morphological data about the human retina.