Two senses, one rhythm: Pre- and post-stimulus neural states predict perceived audio-visual synchrony

How does the brain integrate or segregate multiple sensory inputs? In our study, we used magnetoencephalography to comprehensively investigate local and network dynamics before and after the presentation of sequences of beeps and flashes that are sometimes perceived as synchronous or asynchronous. Frequency-tagging and spectral analysis revealed that under identical physical conditions, subjective perceptual fusion of multisensory signals elicits stronger evoked sensory responses in both visual and auditory cortices. Crucially, perceptual fusion is preceded by locally reduced pre-stimulus alpha activity, as well as enhanced functional connectivity between auditory and visual cortices within the pre-stimulus gamma activity. We also show that pre-stimulus neuronal states in the occipital cortex predict post-stimulus sensory responses to auditory sequences in the temporal cortex, but not the opposite. The current findings provide evidence that the perceptual fusion of multisensory events relate to a complex interplay between local and inter-sensory neuronal states before and after stimulus presentation. Significance StatementHow does the brain blend information from multiple sensory systems into one holistic perception? We developed a novel frequency-tagging paradigm in Magnetoencephalography (MEG) to robustly estimate the sensory responses to a series of flashes and beeps in early sensory areas, and to demonstrate how these responses are shaped by ongoing fluctuations in local, and long-range network activity. By combining the steady-state method with spectral, and functional connectivity analysis of ongoing oscillations in source space, the current study provides novel insights into the relationship between internal brain states and the way we perceive multisensory events.