Under conditions in which the visual system cannot reconcile dissimilar images from the two eyes, perception typically alternates between the two half-images - a process known as binocular rivalry. We report a real-time, steady-state VEP method that is a sensitive detector of the continuous alternations in perceptual dominance across the eyes. This method works by labeling each half-image with a slightly different temporal frequency so that the record generated by each can be recovered from the EEG by spectrum analysis. In this way, one can track the 'waxing' and 'waning' of the VEP amplitudes for each eye simultaneously during spontaneous rivalry, permitting an analysis of the relative physiological dominance of each eye in real-time. Such alternations were clearly observed in the VEP amplitudes generated by each half-image during rivalry (the amplitudes for the two eyes correlated negatively). In contrast, VEP amplitudes for the two eyes varied either synchronously or randomly when the half-images were allowed to fuse. The instances of physiological dominance of each eye as evidenced by the VEP correlated well with the subjects' report of perceptual dominance. This purely electrophysiological method appears to be suitable for measuring rivalry in non-verbal human or animal subjects, as it does not require active participation from them.
Brown, R. J., & Norcia, A. M. (1997). A method for investigating binocular rivalry in real-time with the steady-state VEP. Vision Research, 37(17), 2401–2408. https://doi.org/10.1016/S0042-6989(97)00045-X