Visual evoked potentials: evidence for lateral interactions.

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Abstract
Electrical potentials evoked in the human brain by visual stimulation easily be recorded by using electrodes attached to the scalp. It is difficult to relate these visual evoked potentials (VEP) to specific neural processes: scalp electrodes, far removed from the brain, sum potentials from large areas of cortex. Identification and localization of lateral interactions was improved by differentially modulating small neighboring parts of a windmill-dartboard stimulus pattern.sbd.a central disc surrounded by 3 contiguous annuli, all radially divided into light and dark segments. With temporal contrast reversal of all segments in the pattern, the major component of the VEP is at the 2nd harmonic of the frequency of modulation, as expected. Temporal contrast reversal of the segments in the central disc and 2nd annulus, with contrast of segments held constant in the 1st and 3rd annuli, unexpectedly amplifies the VEP at the fundamental frequency of modulation and attenuates it at the 2nd harmonic. Slight spatial separation of static and dynamic zones reduces the amplification of the fundamental and the attenuation of the 2nd harmonic. Both phenomena appear to result from strong lateral interactions over relatively short distances. Different neural mechanisms must be involved; fundamental and 2nd-harmonic components of the VEP are different functions of spatial separation and relative contrast of the segments in contiguous static and dynamic zones.