PARVOCELLULAR AND MAGNOCELLULAR CONTRIBUTIONS TO VISUAL-EVOKED POTENTIALS IN HUMANS - STIMULATION WITH CHROMATIC AND ACHROMATIC GRATINGS AND APPARENT MOTION

Psychophysical evidence suggests that two major parallel pathways, the parvocellular (P) and the magnocellular (M) pathways, exist in humans . We herein report that responses specific to the P and M systems can be recorded in human visual evoked potentials (VEPs) by using the appr opriate stimuli. The onset of isoluminant chromatic (red-green) and hi gh contrast achromatic sinusoidal gratings were used far stimulating t he P-system. A chromatic stimulation evoked a characteristic negative wave (N1) with peak latencies around 120 msec. The amplitude showed an inverse U-shaped function as a function of spatial frequency with a p eak at 2 c/deg. In contrast, VEPs to achromatic (black-white) gratings showed different spatial frequency characteristics with a peak at 5.3 c/deg. By varying the luminous intensity ratio between the red and gr een gratings, N1 was found to reach a maximum during isoluminant stimu lation. An apparent motion display was used for stimulating the M-syst em. The speed of alternation (i.e., the interstimulus interval (ISI)) was varied to record both the transient and steady-state VEPs. Transie nt VEPs showed triphasic waves with the major positive peak (PI) at ar ound 120 ms. Steady-state VEPs were quasi-sinusoidal waveforms, depend ing on the ISI, and were quite stable across all subjects. There was a also high correlation between the motion threshold and the VEP amplit ude. The above observations indicate that characteristic potentials ma y distinguish between these two parallel visual systems in humans. Thu s, the combined use of isoluminant color and high contrast achromatic gratings and an apparent motion display is considered to be useful for evaluating both systems electrophysiologically.