Rees, GE; Romei, V; (2015) Individual differences in alpha frequency drive crossmodal illusory perception. Current Biology , 25 (2) pp. 231-235. 10.1016/j.cub.2014.11.034.

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Abstract

Perception routinely integrates inputs from different senses. Stimulus temporal proximity critically determines whether or not these inputs are bound together. Despite the temporal window of integration being a widely accepted notion, its neurophysiological substrate remains unclear. Many types of common audio-visual interactions occur within a time window of -100ms [1-5]. For example, in the sound- induced double-flash illusion, when two beeps are presented within -100ms together with one flash, a second illusory flash is often perceived [2]. Due to their intrinsic rhythmic nature, brain oscillations are one candidate mechanism for gating the temporal window of integration. Interestingly, occipital alpha-band oscillations cycle on average every -100ms with peak frequencies ranging between 8-14Hz (i.e. 120-60ms cycle). Moreover, presenting a brief tone can phase-reset such oscillations in visual cortex [6, 7]. Based on these observations, we hypothesized that the duration of each alpha cycle might provide the temporal unit to bind audio-visual events. Here we first recorded EEG while participants performed the sound-induced double-flash illusion task [4] and found positive correlation between individual alpha-frequency (IAF) peak and the size of the temporal window of the illusion. Participants then performed the same task while receiving occipital transcranial alternating current stimulation (tACS), to modulate oscillatory activity [8] either at their IAF or at off-peak alpha-frequencies (IAF±2Hz). Compared to IAF tACS, IAF-2Hz and IAF+2Hz tACS respectively enlarged and shrunk the temporal window of illusion, suggesting that alpha oscillations might represent the temporal unit of visual processing that cyclically gates perception and the neurophysiological substrate promoting audio-visual interactions.

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