An ERP study of audiovisual simultaneity perception

Abstract

The aim of this study was to examine relation between conscious perception of temporal relation between the elements of an audiovisual pair and the dynamics of accompanying neural activity. This was done by using a simultaneity judgment task and EEG event-related potentials (ERP). During Experiment 1 the pairs of 10 ms white-noise bursts and flashes were used. On presenting each pair subjects pressed one of two buttons to indicate their synchrony. Values of stimulus onset asynchrony (SOA) were based on individual estimates of simultaneity thresholds (50∕50 probability of either response). They were estimated prior to EEG measurement using interleaved staircase involving both sound-first and flash-first stimulus pairs. Experiment 2 had the identical setup, except subjects indicated if audio–visual pair began simultaneously (termination was synchronous). ERP waveforms were time-locked to the second stimulus in the pair. Effects of synchrony perception were studied by comparing ERPs in trials that were judged as simultaneous and non-simultaneous. Subjects were divided into two subgroups with similar SOA values. In both experiments at about 200 ms after the second stimulus onset a stronger ERP wave positivity for trials judged as non-simultaneous was observed in parieto-central sites. This effect was observed for both sound-first and video-first pairs and for both SOA subgroups. The results demonstrate that the perception of temporal relations between multimodal stimuli with identical physical parameters is reflected in localized ERP differences. Given their localization in the posterior parietal regions, these differences may be viewed as correlates of conscious perception of temporal integration vs. separation of audiovisual stimuli.