PUB - Publikationen an der Universität Bielefeld

In postdiction, the last stimulus of a sequence changes the perception of the preceding stimuli. Postdiction has been reported in all sensory modalities but its neural underpinnings remain poorly understood. In the rabbit illusion, a sequence of non-equidistant stimuli presented isochronously is perceived as equidistantly spaced. This illusion might be driven by an internal prior favoring a constant-speed motion. Here, we hypothesized that pre-stimulus alpha oscillations (8 – 12 Hz), known to correlate with perceptual expectations and biases, would reflect the degree to which perceptual reports are influenced by a constant-speed prior. Human participants were presented with ambiguous visual sequences while being recorded simultaneously with MEG and EEG: the same sequences yielded an illusory perception in about half the trials, allowing contrasting brain responses elicited by identical sequences causing distinct percepts. As a proxy of an individual’s prior, we used the percentage of perceived illusion and the detection criterion, assuming that a strong constant-speed prior would result in a higher rate of illusory percepts. We found that high fronto-parietal alpha power was associated with perceiving the sequence according to the individual’s prior: participants with high susceptibility to the illusion would report the illusion, while participants with low susceptibility would report the veridical sequence. Additionally, we found that pre-stimulus alpha phase in occipito-parietal regions dissociated illusion from no-illusion trials. We interpret our results as suggesting that alpha power reflects an individual’s constant-speed prior whereas alpha phase modulates sensory uncertainty.