The effect of contrast and spatial frequency on the visual evoked potential of the hooded rat.

Transient and steady-state visual evoked potentials, obtained with counterphase modulated grating patterns, were recorded through chronically implanted electrodes in lightly anesthetized and unanesthetized hooded rats. As a function of spatial frequency, the latency of the major positive wave (P1) of the transient VEP was minimal with a 0.1 cycles per degree (cpd) grating as stimulus. The amplitude of the P1-N2 component remained constant from 0.04 to 0.20 cpd and then decreased with higher spatial frequencies. The function relating amplitude of steady-state VEP to spatial frequency at high contrast depended on temporal frequency: It had a band-pass form at 5.0 Hz and a low-pass form at 8.4 Hz. As a function of contrast, the latency of the P1 wave linearly decreased and its amplitude increased with increments of contrast up to 55%. The contrast function of the amplitude of the second harmonic of the steady-state VEP (8.4 Hz) showed two linear segments: one for low contrasts with a shallow slope and one for high contrasts with a steep slope, suggesting two distinct contrast mechanisms.