The spatio-temporal profile of multisensory integration

Abstract

Task-irrelevant visual stimuli can enhance auditory perception. However, while there is some neurophysiological evidence for mechanisms that underlie the phenomenon, the neural basis of visually-induced effects on auditory perception remains unknown. Combining fMRI and EEG with psychophysical measurements in two independent studies, we identified the neural underpinnings and temporal dynamics of visual-induced auditory enhancement. Lower- and higher-intensity sounds were paired with a non-informative visual stimulus while participants performed an auditory detection task. Behaviourally, visual co-stimulation enhanced auditory sensitivity. Using fMRI, enhanced BOLD-signals were observed in primary auditory cortex for low-intensity audiovisual stimuli which scaled with subject-specific enhancement in perceptual sensitivity. Concordantly, a modulation of event-related potentials could already be observed over frontal electrodes at an early latency (30-80 ms), which again scaled with subject-specific behavioural benefits. Later modulations starting around 280 ms, i.e. in the time range of the P3, did not fit this pattern of brain-behaviour correspondence. Hence, the latency of the corresponding fMRI-EEG brain-behaviour modulation points at an early interplay of visual and auditory signals in low-level auditory cortex, potentially mediated by crosstalk at the level of the thalamus. However, fMRI-signals in primary auditory cortex, auditory thalamus and the P50 for higher-intensity auditory stimuli were also elevated by visual co-stimulation (in the absence of any behavioural effect) suggesting a general, intensity-independent integration mechanism. We propose that this automatic interaction occurs at the level of the thalamus and might signify a first step of audiovisual interplay necessary for visually-induced perceptual enhancement of auditory perception.

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