Abstract
How alpha rhythms influence visual perception is an open question for more than a century. For example, it was proposed that two stimuli falling within a cycle are integrated into a single percept. However, temporal integration can last longer than the window of an alpha cycle. Here, we analyzed electroencephalography recordings (EEG) during the sequential metacontrast paradigm (SQM), where information is integrated across space and time for almost half a second. In the SQM, participants discriminate the offset of a central line followed by a stream of flanking lines. Surprisingly, the offset is visible at the flanking lines even though they are not offset. When a second line in the stream has an offset in the opposite direction, the two offsets integrate: only a small offset is perceived, which is either in accordance with the first or the second offset. Integration of the offsets is mandatory and unconscious. Using linear discriminant analysis, we first isolated electrodes carrying information about the reported offset in the stream. We then showed that pre-stimulus alpha activity at these electrodes is predictive of the reported percept: increases in alpha power led to more frequent reports of the first offset, decreases to more frequent reports of the second offset. In contrast, we found no effect of phase. Since the two offsets integrate before conscious perception is reached, these results suggest that alpha power affects the relative weighting of individual features during unconscious processing. We argue that the use of paradigms with long-lasting integration helps to disentangle the effects of power and phase and to better understand how alpha rhythms modulate perception.