The effect of grating contrast on the human visually evoked potential

Visually evoked potentials (VEPs) were recorded from the human occiput at the appearance of sinusoidal gratings. Grating spatial frequency was 4 c/deg, at which both the early and late negative waves, N1 and N2 were present in VEP. Increasing grating contrast from a near-threshold value, 0.006, to the value of 0.3, the maximum in the linear range of the device, resulted in a peak-latency shortening and an amplitude increase of both N1 and N2. While the latencies changed in parallel, the amplitudes did not. The N1 amplitude increased almost linearly with log contrast in the whole range tested. The amplitude of N2 was large at the lowest contrast level, increased by a factor of about 2 and saturated at the contrast of 0.05. The results suggest a greater contrast sensitivity of the generator of N2 than of the generator of N1 and support the assumption that these generators belong to structures with different properties and functions.