Gradual changes in monocular neural signals during long dominance periods in binocular rivalry

One striking feature of binocular rivalry is the apparently random duration of dominance periods, in which one eye’s stimulus is exclusively perceived. These durations vary stochastically, with sequential periods often having very different lengths. However, the neural bases of the variations in duration remain unexamined in humans. Here we test whether patterns of neural activity differ between longer vs shorter dominance periods, using frequency-tagged steady-state visual evoked potentials (SSVEP). Eighty-four participants were presented with orthogonal (± 45°) flickering gray scale gratings, one to each eye. During twelve 2-minute runs, participants indicated a percept of “tilt left”, “tilt right”, or a mixed percept with button presses, while frequency-tagged EEG data were collected. We extracted amplitudes of the EEG signal at dominant and suppressed stimulus frequencies (with eyes tagged with 14.4 and 18.0 Hz). These SSVEP amplitudes were then compared between dominance periods of different durations, within participants. For all durations, we observed increasing amplitude at the suppressed stimulus frequency and decreasing amplitude at the dominant stimulus frequency throughout the period. These changes were rapid at the end of all periods, where the amplitude of the suppressed signal became greater than that for the dominant, presumably reflecting the perceptual switch ending the period. Critically, during the beginning and middle of periods, longer duration periods were characterized by more gradual increases in the amplitude of the suppressed signal and more gradual decreases of the dominant signal as compared to shorter duration periods. This effect was highly significant as tested by regression models examining effects of dominance duration on SSVEP amplitudes at different timepoints. Our results are consistent with models of rivalry in which noisy signals accumulate over the dominance period, with perceptual switches occuring after a threshold is reached, and accumulation is more rapid in some periods than others.