Rhythmic beta-frequency TMS over human right parietal cortex strengthens visual size illusions.

Rhythmic brain activity has been proposed to structure visual processing. Here we investigated the causal contributions of parietal beta oscillations to context-dependent visual size perception, which is indicated by the classic Ebbinghaus and Ponzo illusions. On each trial, rhythmic transcranial magnetic stimulation (TMS) was applied over the left or right superior parietal lobule in a train of five pulses at beta frequency (20 Hz). Immediately after the last pulse of the stimulation train, participants were presented with the illusory configuration, and performed a size-matching task. The results revealed that right parietal stimulation significantly increased the magnitudes of both size illusions relative to control vertex stimulation, whereas the illusion effects were unaffected with left parietal stimulation. Moreover, the stimulation effect was not observed with right parietal TMS at theta frequency (5 Hz). The findings clearly demonstrate the functional relevance of beta oscillations for the implementation of cognitive performance, supporting the causal contribution of parietal cortex to the processing of visual size illusions.