Audiovisual synchrony improves motion discrimination via enhanced connectivity between early visual and auditory areas.

Audiovisual synchrony enables integration of dynamic visual and auditory signals into a more robust and reliable multisensory percept. In this fMRI study, we investigated the neural mechanisms by which audiovisual synchrony facilitates shape and motion discrimination under degraded visual conditions. Subjects were presented with visual patterns that were rotated by discrete increments at irregular and unpredictable intervals while partially obscured by a dynamic noise mask. On synchronous trials, each rotation coincided with an auditory click. On asynchronous trials, clicks were noncoincident with the rotational movements (but with identical temporal statistics). Subjects discriminated shape or rotational motion profile of the partially hidden visual stimuli. Regardless of task context, synchronous signals increased activations bilaterally in (1) calcarine sulcus (CaS) extending into ventral occipitotemporal cortex and (2) Heschl's gyrus extending into planum temporale (HG/PT) compared with asynchronous signals. Adjacent to these automatic synchrony effects, synchrony-induced activations in lateral occipital (LO) regions were amplified bilaterally during shape discrimination and in the right posterior superior temporal sulcus (pSTS) during motion discrimination. Subjects' synchrony-induced benefits in motion discrimination significantly predicted blood oxygenation level-dependent synchrony effects in V5/hMT+. According to dynamic causal modeling, audiovisual synchrony increased connectivity between CaS and HG/PT bidirectionally, whereas shape and motion tasks increased forwards connectivity from CaS to LO or to pSTS, respectively. To increase the salience of partially obscured moving objects, audiovisual synchrony may amplify visual activations by increasing the connectivity between low level visual and auditory areas. These automatic synchrony-induced response amplifications may then be gated to higher order areas according to behavioral relevance and task context.