No modulatory effect of rhythmic visual alpha/gamma stimulation in a bistable motion illusion

Abstract

The motion quartet, also called stroboscopic alternative motion (SAM), is a bistable apparent motion paradigm where dots are perceived as moving either horizontally or vertically. These two percepts are known to switch randomly depending on both alpha (8 - 12 Hz) and gamma (30 - 80 Hz) oscillations within and between hemispheres, respectively. However, whether these oscillations play a causal role in triggering perceptual switches remains unknown. Here, we aimed to entrain brain oscillations in the visual system by presenting flickering stimuli. Two LEDs were flickering either at 10 Hz, 40 Hz or corresponding jittered stimulation frequencies on the left and right sides of the visual field during the presentation of a motion quartet. In our control condition (no stimulation), we show that interhemispheric gamma connectivity (spectral coherence) between the visual cortices of both hemispheres is important for sensory feature integration. On the other hand, alpha power did not predict perceptual switches for the motion quartet. Rhythmic visual stimulation with 10 Hz and 40 Hz evoked resonance-like neural responses but did not alter visual perception. Thus, although sensory stimuli seem to entrain ongoing brain rhythms, the effect on human behavior differs from the principles observed for endogenous brain oscillations.