Electrophysiological evidence for interhemispheric transmission of visual information in man

1. Electrophysiological evidence is presented of interactions between two stimuli (sinusoidal gratings of equal spatial frequency but different contrast, phase-reversed sinusoidally at different temporal frequencies) located on opposite side of, and within a few degrees from, the vertical meridian. 2. These interactions are revealed by a depression of the cortical visual evoked potential (VEP) evoked by the grating of lower contrast in the presence of the grating of higher contrast. They are similar to, albeit weaker than, those obtained with superimposed asynchronously modulated gratings. 3. The VEP reduction occurs also if the stimuli are presented dichoptically. 4. It does not occur if the gratings are located one above the other either on the same or on opposite sides of the vertical meridian. 5. The strength of the VEP reduction depends on the relative contrast of the two gratings and vanishes for spatial frequencies beyond 4 cycles/deg and temporal frequencies of the high-contrast grating beyond 10 Hz. 6. The results are in agreement with data on visual callosal connections in animals and confirm previous psychophysical findings (Berardi & Fiorentini, 1987) indicating the particular properties of the interhemispheric cross-talk between symmetric regions of the visual field astride the vertical meridian in man.