Neural processing of orientation differences between the eyes' images

The aim of this study was to explore the neural mechanisms underlying visual processing of brief stimuli that were either the same in the two eyes or differed in orientation between the two eyes. To examine the neural mechanisms, I measured event-related potentials (ERPs) to 200-ms sine-wave gratings differing in orientation between the eyes from 0° to 90°. The gratings were either both of high contrast or both of low contrast. They elicited typical ERPs at occipital electrodes, with a first major component (P100) 100 ms after stimulus onset and a second major component (N170) 170 ms after stimulus onset. Global electrical field strength and focal amplitudes of both components were affected by grating contrast: High-contrast gratings elicited larger amplitudes than low-contrast gratings, confirming that neural responses depend on stimulus salience. P100 amplitude followed a U-shaped function: It was larger when the orientations were the same in the two eyes (yielding binocular fusion), intermediate when the orientations were maximally different between the eyes (leading to binocular rivalry), and smallest for in-between orientation differences. N170 amplitude followed a linear function: It was smallest when the orientations were the same and increased with orientation difference between the eyes. These results suggest that the P100 reflects processes in which the binocular input are offset against each other, and that the N170 reflects binocular rivalry. I argue that the N170 shows the effects of reciprocal inhibition and adaptation—both critical factors in theories of binocular rivalry.