MOTION-ONSET VISUAL-EVOKED POTENTIALS AS A FUNCTION OF RETINAL ECCENTRICITY IN MAN

Visual-evoked potentials were elicited by the motion-onset of a black- and-white square-wave grating of 2.4 cycles/deg that drifted from righ t to left at a velocity of 3 deg/s. The center of the 2 x 2 deg stimul us field was binocularly viewed either foveally or at eccentricities o f 6, 12, or 20 deg in the lower visual field along the vertical meridi an. Peak-to-peak amplitudes PI-N2 and N2-P2 were found to decrease non -linearly as a function of eccentricity. The VEP-amplitudes were stand ardized by setting each foveal value to 100%, and a relative measure w as derived for peripheral values given by the ratio of the peripheral to the foveal values. The decrease of the relative VEP-values with ecc entricity was significantly smaller than that of the relative cortical magnification factor of striate cortex in man, whereas it agreed fair ly well with that of the relative point-image size of the area MT in M acaque monkey. In this respect, the motion-onset VEP is distinct from the pattern-reversal VEP, the amplitude of which decreases much more r apidly with retinal eccentricity; hence, it may involve different gene rating structures of the brain.