Inhibitory interactions in the human vision system revealed in pattern-evoked potentials.

1. Visual evoked potentials (v.e.p.s) were recorded from human adults to investigate orientation-specific neural interactions. The stimuli were the sum of two gratings, sinusoidally modulated in space and time at different frequencies. Recordings were made for one grating (test) alone, and with another superimposed grating (mask), oriented parallel or orthogonal to the test. The amplitude and phase of the v.e.p.s at twice the test modulation frequency (second harmonic) was measured as a function of test contrast to produce contrast-response curves. 2. Orthogonal masks attenuated considerably the amplitude of v.e.p.s. The attenuation at any given contrast was approximately proportional, or multiplicative, lowering the slope of the contrast-response curve, without affecting significantly the extrapolated threshold. Parallel masks also attenuated v.e.p. amplitudes but in a different way, leaving the slope of the contrast-response curves unchanged, while elevating threshold. 3. The attenuation by orthogonal masks occurred over a wide range of test spatial frequencies, from 0.8 to 8 cycles/deg. For any given test spatial frequency, the most effective masks were those of spatial frequency similar to or lower than the test. Masks of spatial frequency 1.5 octaves higher than the test did not attenuate v.e.p. amplitudes. 4. The mask temporal frequency for maximal attenuation of v.e.p. amplitude was around 12 Hz, with stationary masks having little effect. 5. Under most conditions, the phase of the second harmonic of the v.e.p., increased with increasing contrast (phase advance). Superimposition of a parallel mask abolished phase advance, while orthogonal masks increased it. 6. Comparisons with single cortical unit and evoked potential recordings in cats suggest that the attenuation by orthogonal masks reflects intracortical inhibitory interactions between cell populations of different orientation preference.