SPATIAL-FREQUENCY SPECIFIC CONTRAST GAIN AND FLICKER MASKING OF HUMANTRANSIENT VEP

We studied the effects of grating contrast and luminance-flicker maski ng on the early waves of human visually evoked potentials (VEPs) recor ded at the onset-offset of sinusoidal gratings of varying spatial freq uencies (SFs). At high SFs, the response waveform was simple and VEP w as dominated by a negative wave (N110). At low SFs, several positive-n egative deflections were recorded, the earliest dominating wave being positive (P90). The amplitude of P90 was saturated at a contrast of ab out 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 a t SFs higher than 2-4 c/deg. At medium and high SFs, VEPs were obtaine d at higher contrast levels. No saturation (max contrast tested 0.5) a nd no flicker masking of N110 were observed. These results suggest tha t the early VEP components recorded at low and high SFs are related to different types of neuronal activity. Correlation between VEP propert ies and properties of magnocellular and parvocellular pathways is cons idered with an emphasis on recent morphological data about the human r etina.