On perceived synchrony—neural dynamics of audiovisual illusions and suppressions

Whenever temporally incongruent audiovisual sequences are presented, the perceived flash rate follows the physical flutter rate. Increasing the auditory flutter rate increases the perceived flicker rate (visual illusions). Likewise, decreasing the flutter rate decreases the perceived flicker rate (visual suppressions). Here, we investigated the electrophysiological correlates of this perceptual phenomenon. Two sequences of visual flashes and auditory beeps were presented either synchronously (both visual flashes (F) and auditory beeps (B) at 3 or 5 Hz, respectively) or asynchronously at different rates (3F5B or 5F3B). Event-related potentials were acquired, while subjects reported the perceived number of flashes (response options: 3, 4, and 5). During asynchronous trials, subjects' flash counts were significantly higher when the flutter rate exceeded the flicker rate (i.e. visual illusions occurred); and lower flutter rate was below the flicker rate (i.e. visual suppressions occurred). Differential brain responses for reported illusions and suppressions (incorrect flash counts) vs. no-illusions/suppressions (correct flash counts) were found over parieto-occipital sites, followed by slow modulations over frontal and occipital areas. Importantly, the modulation over occipital electrodes starting around 500 ms had an inverse polarity for illusions vs. suppressions. These results provide evidence that both sound-induced visual illusions and suppressions are mediated by an interplay of distributed brain regions, in the attempt to fuse asynchronous audiovisual stimuli into a synchronous percept.