Good times for multisensory integration: Effects of the precision of temporal synchrony as revealed by gamma-band oscillations.

The synchronous occurrence of the unisensory components of a multisensory stimulus contributes to their successful merging into a coherent perceptual representation. Oscillatory gamma-band responses (GBRs, 30-80 Hz) have been linked to feature integration mechanisms and to multisensory processing, suggesting they may also be sensitive to the temporal alignment of multisensory stimulus components. Here we examined the effects on early oscillatory GBR brain activity of varying the precision of the temporal synchrony of the unisensory components of an audio-visual stimulus. Audio-visual stimuli were presented with stimulus onset asynchronies ranging from -125 to +125 ms. Randomized streams of auditory (A), visual (V), and audio-visual (AV) stimuli were presented centrally while subjects attended to either the auditory or visual modality to detect occasional targets. GBRs to auditory and visual components of multisensory AV stimuli were extracted for five subranges of asynchrony (e.g., A preceded by V by 100+/-25 ms, by 50+/-25 ms, etc.) and compared with GBRs to unisensory control stimuli. Robust multisensory interactions were observed in the early GBRs when the auditory and visual stimuli were presented with the closest synchrony. These effects were found over medial-frontal brain areas after 30-80 ms and over occipital brain areas after 60-120 ms. A second integration effect, possibly reflecting the perceptual separation of the two sensory inputs, was found over occipital areas when auditory inputs preceded visual by 100+/-25 ms. No significant interactions were observed for the other subranges of asynchrony. These results show that the precision of temporal synchrony can have an impact on early cross-modal interactions in human cortex.