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

Neural oscillations reflect fluctuations in the relative excitation/inhibition of neural systems^1,2,3,4,5 and are theorized to play a critical role in canonical neural computations^6,7,8,9 and cognitive processes.^{10,11,12,13,14} These theories have been supported by findings that detection of visual stimuli fluctuates with the phase of oscillations prior to stimulus onset.^{15,16,17,18,19,20,21,22,23} However, null results have emerged in studies seeking to demonstrate these effects in visual discrimination tasks,^24,25,26,27 raising questions about the generalizability of these phenomena to wider neural processes. Recently, we suggested that methodological limitations may mask effects of phase in higher-level sensory processing.²⁸ To test the generality of phasic influences on perception requires a task that involves stimulus discrimination while also depending on early sensory processing. Here, we examined the influence of oscillation phase on the visual tilt illusion, in which a center grating has its perceived orientation biased away from the orientation of a surround grating²⁹ due to lateral inhibitory interactions in early visual processing.^30,31,32 We presented center gratings at participants' subjective vertical angle and had participants report whether the grating appeared tilted clockwise or counterclockwise from vertical on each trial while measuring their brain activity with electroencephalography (EEG). In addition to effects of alpha power and aperiodic slope, we observed robust associations between orientation perception and alpha and theta phase, consistent with fluctuating illusion magnitude across the oscillatory cycle. These results confirm that oscillation phase affects the complex processing involved in stimulus discrimination, consistent with its purported role in canonical computations that underpin cognition.