Parietal TMS and binocular rivalry: stimulus and timing dependence resolve contradictory findings

Binocular rivalry occurs when dissimilar images are presented to corresponding retinal locations in the two eyes: Rather than a combined image being perceived, one image dominates awareness while the other is suppressed, and dominance switches periodically. This bistable phenomenon is useful for investigating conscious awareness, as sensory stimuli remain constant while awareness changes.
Neuroimaging studies have shown that activity in the right anterior superior parietal lobule (SPL) is time-locked to perceptual switches in rivalry. Further transcranial magnetic stimulation (TMS) studies established this region’s causal role in rivalry by showing that stimulating it altered switch rates.
However, the effects found in different studies were contradictory: TMS either shortened or lengthened rivalry durations. Here, we replicate both findings and show under which conditions each occurs.
Participants underwent parietal and vertex (control) TMS both before viewing binocular rivalry (offline) and while viewing rivalry (online). Each combination of stimulation location and timing was delivered in a separate session; session order was randomised. In each session, dominance durations were compared with a baseline measured before stimulation.
In Experiment 1, the rivalrous stimuli were orthogonal gratings. In Experiment 2, the stimuli were pictures of a face and a house. In both experiments, offline parietal (but not vertex) stimulation shortened dominance durations compared to baseline, as in previous studies. Neither experiment found an effect of online stimulation when comparing stimulation to baseline, but in Experiment 2, direct comparison of online-parietal and online-vertex dominance durations (as done in a previous study using similar face/house stimuli) showed lengthening under parietal stimulation. The differential effects of offline and online stimulation are consistent with offline stimulation inhibiting neural activity and online stimulation increasing noise; our findings thus support the view that right anterior SPL mediates maintenance of current expectations about ambiguous stimuli rather than generating switches. The finding that online TMS lengthens dominance for face/house but not grating stimuli demonstrates a previously unreported stimulus-dependence for the effect of TMS in rivalry, suggesting that such stimulation may alter interactions between stimulus-specific regions and a parietal selection mechanism.