The effect of the peristimulus alpha phase on visual perception through real-time phase-locked stimulus presentation.

The alpha phase has been theorized to reflect fluctuations in cortical excitability and thereby impose a cyclic influence on visual perception. Despite its appeal, this notion is not fully substantiated, as both supporting and opposing evidence has been recently reported. In contrast to previous research, this study examined the effect of the peristimulus instead of prestimulus phase on visual detection through a real-time phase-locked stimulus presentation approach. Specifically, we monitored phase data from magnetoencephalography (MEG) recordings over time, with a newly developed algorithm based on adaptive Kalman filtering. This information guided online presentations of masked stimuli that were phased-locked to different stages of the alpha cycle while healthy humans concurrently performed detection tasks. Behavioral evidence showed that the overall detection rate did not significantly vary according to the four predetermined peristimulus alpha phases. Nevertheless, the follow-up analyses highlighted that the phase at 90° relative to 180° likely enhanced detection. Corroborating neural parietal activity showed that early interaction between alpha phases and incoming stimuli orchestrated the neural representation of the hits and misses of the stimuli. This neural representation varied according to the phase and in turn shaped the behavioral outcomes. In addition to directly investigating to what extent fluctuations in perception can be ascribed to the alpha phases, this study suggests that phase-dependent perception is not as robust as previously presumed, and might also depend on how the stimuli are differentially processed as a result of a stimulus-phase interaction, in addition to reflecting alternations of the perceptual states between phases. Significance statement Alpha activity, a widely observed neural phenomenon, is postulated to be essential for organizing visual perception. However, our previous understanding of the functional relevance of the alpha phase is primarily inferred from the prestimulus or externally entrained phase. This study monitors real-time phase activity and presents stimuli that are phased-locked to different stages of the alpha cycle. This new approach allows us to investigate whether and how the alpha phases, during which stimuli are concurrently presented, directly lead to behavioral and neural changes in perception. Our evidence suggests that the extent to which the alpha phases affect perception depends on an early interaction between the phase and incoming stimuli, which is involved in shaping the perceptual fates of the stimuli.