Oscillatory mechanisms underlying the enhancement of visual motion perception by multisensory congruency

Multisensory interactions shape every day perception and stimuli in one modality can enhance perception in another even when not being directly task relevant. While the underlying neural principles are slowly becoming evident, most work has focused on transient stimuli and little is known about those mechanisms underlying audio-visual motion processing. We studied the facilitation of visual motion perception by auxiliary sounds, i.e. sounds that by themselves do not provide the specific evidence required for the perceptual task at hand. In our experiment human observers became significantly better at detecting visual random dot motion when this was accompanied by auxiliary acoustic motion rather than stationary sounds. EEG measurements revealed that both auditory and visual motion modulated low frequency oscillations over the respective sensory cortices. Using single trial decoding we quantified those oscillatory signatures permitting the discrimination of visual motion similar to the subject's task. This revealed visual motion-related signatures in low (1-4 Hz) and alpha (8-12 Hz) bands that were significantly enhanced during congruent compared to disparate audio-visual conditions. Importantly, the auditory enhancement of these oscillatory signatures was predictive of the perceptual multisensory facilitation. These findings emphasise the importance of slow and alpha rhythms for perception in a multisensory context and suggest that acoustic motion can enhance visual perception by means of attention or priming-related mechanisms that are reflected in rhythmic activity over parieto-occipital regions.