Summary Neural oscillations reflect fluctuations in the relative excitation/inhibition of neural systems 1–5 and are theorised to play a critical role in several canonical neural computations 6–9 and cognitive processes 10–14 . These theories have been supported by findings that detection of visual stimuli fluctuates with the phase of oscillations at the time of stimulus onset 15–23 . However, null results have emerged in studies seeking to demonstrate these effects in visual discrimination tasks 24–27 , raising questions about the generalisability of these phenomena to wider neural processes. Recently, we suggested that methodological limitations may mask effects of oscillation phase in higher-level sensory processing 28 . Thus, to test the generality of phasic influences requires a task that requires stimulus discrimination but depends on early sensory processing. Here, we examined the influence of oscillation phase in the visual tilt illusion, in which an oriented centre grating is perceived titled away from the orientation of a surround grating 29 . This illusion is produced by lateral inhibitory interactions in early visual processing 30–32 . We presented centre gratings at participants’ titrated subjective vertical angle and had participants report whether the grating appeared tilted leftward or rightward of vertical on each trial while measuring their brain activity with EEG. We observed a robust fluctuation in orientation perception across different phases of posterior alpha and theta oscillations, consistent with fluctuating illusion magnitude across the oscillatory cycle. These results confirm that oscillation phase affects complex processing involved in stimulus discrimination, consistent with their purported role in canonical computations that underpin cognition.

Effects of oscillation phase on discrimination performance in a visual tilt illusion