Pre‐stimulus alpha‐band power and phase fluctuations originate from different neural sources and exert distinct impact on stimulus‐evoked responses

Ongoing oscillatory neural activity before stimulus onset influences subsequent visual perception. Specifically, both the power and the phase of oscillations in the alpha-frequency band (9-13 Hz) have been reported to predict the detection of visual stimuli. Up to now, the functional mechanisms underlying pre-stimulus power and phase effects on upcoming visual percepts are debated. Here, we used magnetoencephalography recordings together with a near-threshold visual detection task to investigate the neural generators of pre-stimulus power and phase and their impact on subsequent visual-evoked responses. Pre-stimulus alpha-band power and phase opposition effects were found consistent with previous reports. Source localization suggested clearly distinct neural generators for these pre-stimulus effects: Power effects were mainly found in occipital-temporal regions, whereas phase effects also involved prefrontal areas. In order to be functionally relevant, the pre-stimulus correlates should influence post-stimulus processing. Using a trial-sorting approach, we observed that only pre-stimulus power modulated the Hits versus Misses difference in the evoked response, a well-established post-stimulus neural correlate of near-threshold perception, such that trials with stronger pre-stimulus power effect showed greater post-stimulus difference. By contrast, no influence of pre-stimulus phase effects were found. In sum, our study shows distinct generators for two pre-stimulus neural patterns predicting visual perception, and that only alpha power impacts the post-stimulus correlate of conscious access. This underlines the functional relevance of prestimulus alpha power on perceptual awareness, while questioning the role of alpha phase.