Spontaneous changes in the prestimulus alpha power predict both objective and subjective aspects of perception-evidence from 4 paradigms and 471 experimental sessions.

Whether a given stimulus will be perceived depends not only on its physical intensity but also by the functional state of the visual system, particularly power of the spontaneous electroencephalography (EEG) alpha-band activity (7 to 14 Hz) at the stimulus onset. The mechanism behind this effect remains a matter of debate, as some studies reported that low alpha power improves perceptual sensitivity, but others showed alpha power affects only the subjective measures (criterion and confidence/awareness ratings) and thus concluded it is related with non-specific cortical excitability. To address this controversy, we analyzed data from four experimental paradigms in which different types of stimuli-from Gabor patches, through face images, to real-world scenes-were presented to participants at a perceptual threshold. Single-trial analyses revealed that the prestimulus alpha power predicted not only subjective visibility but also objective identification accuracy. Therefore, our results are incompatible with the view that spontaneous alpha activity defines nonspecific, general cortical excitability only, and rather indicates its role in defining perceptual sensitivity and improving signal-to-noise ratio.