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Evoked cortical potentials were used to test neural models of binocular interactions in humans. The critical feature that distinguished between basic models was whether the two monocular signals combined linearly or nonlinearly. Stimuli were presented separately and simultaneously to two eyes with the relative temporal phase parametrically varied. Fourier analysis was applied to the recorded electroencephalogram to measure the frequency response of the visual cortex to this stimulation. Comparisons of the models' predictions with the experimental results indicate that the greatest contribution to the cortical response originates in a neural pathway that contains nonlinear binocular combination, and a smaller contribution originates in a pathway with linear binocular combination. Current models, based on psychophysical studies of humans and on single-cell neurophysiological investigations of cats, are inadequate to explain these results. A model of binocular interaction is proposed that does account for the present findings and is also supported by neurophysiological studies on monkeys. This model, which contains both nonlinear and linear binocular interactions, has profound implications for perceptual processing.

Electrical potentials evoked in the human brain by visual stimulation can easily be recorded by using electrodes attached to the scalp. It is difficult, however, to relate these visual evoked potentials (VEPs) to specific neural processes: scalp electrodes, far removed from the brain, sum potentials from large areas of cortex. We improved identification and localization of lateral interactions by differentially modulating small neighboring parts of a "windmill-dartboard" stimulus pattern-a central disc surrounded by three 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 second harmonic of the frequency of modulation--as expected. Temporal contrast reversal of the segments in the central disc and second annulus, with contrast of segments held constant in the first and third annuli, unexpectedly amplifies the VEP at the fundamental frequency of modulation and attenuates it at the second harmonic. Slight spatial separation of static and dynamic zones reduces both the amplification of the fundamental and the attenuation of the second harmonic. Thus, both phenomena appear to result from strong lateral interactions over relatively short distances. Nevertheless, different neural mechanisms must be involved; fundamental and second-harmonic components of the VEP are different functions of spatial separation and relative contrast of the segments in contiguous static and dynamic zones.

Visual evoked potentials (VEPs), elicited by modulation of luminance of homogeneous fields of light, were recorded from the scalp and from the surface of the visual cortex of cats before and after topical application of bicuculline to the cortex. The application of this drug drastically altered the VEP: the amplitude of a normally small negative component was increased greatly, and a normally prominent late positive component was diminished. Bicuculline is known to block the action of gamma-aminobutyric acid, which is thought to be the primary inhibitory neurotransmitter in the visual cortex. We suggest, therefore, that the affected negative wave reflects an excitatory process in the visual cortex and that the affected late positive wave reflects an intracortical inhibitory process.