Abstract: PURPOSE: Rhythmic brain activity has been proposed to structure neural information processing, with rhythms of different frequencies playing distinct roles. Here we investigated whether short rhythmic bursts of left or right parietal transcranial magnetic stimulation (TMS) at beta frequency (20 Hz) can affect visual size perception, which is indexed by the magnitude of two classic visual size illusions (i.e., the Ebbinghaus and the Ponzo illusions).
METHODS: On each trial, rhythmic TMS was applied over left or right (superior parietal lobule) SPL in a train of five pulses at beta frequency. Immediately after the last pulse of stimulation train, a size illusion configuration was presented at the screen center. Participants were required to adjust the size of a comparison stimulus (i.e., a circle for the Ebbinghaus illusion and a bar for the Ponzo illusion) to match that of the target stimulus without time limit. The vertex was selected as a control site for disruption of behavioral performance due to non-specific TMS effect.
RESULTS: Short rhythmic bursts of right-parietal TMS at beta frequency causally strengthened the magnitudes of both the Ebbinghaus and the Ponzo illusions relative to control stimulation, whereas left-parietal stimulation had a negligible effect on the illusion effects.
CONCLUSIONS: These findings provide clear evidence that parietal beta rhythm is actively involved in shaping visual size perception, supporting the causal contribution of parietal cortex to the processing of visual size illusions, in a hemisphere-asymmetric manner.

Key words: TMS, Ebbinghaus illusion, Ponzo illusion, SPL