The neural mechanisms of audiotactile binding depend on asynchrony

Asynchrony is a critical cue informing the brain whether sensory signals are caused by a common source and should be integrated or segregated. This psychophysics-electroencephalography (EEG) study investigated the influence of asynchrony on how the brain binds audiotactile (AT) signals to enable faster responses in a redundant target paradigm. Human participants actively responded (psychophysics) or passively attended (EEG) to noise bursts, "taps-to-the-face" and their AT combinations at seven AT asynchronies: 0, +/- 20, +/- 70 and +/- 500 ms. Behaviourally, observers were faster at detecting AT than unisensory stimuli within a temporal integration window: the redundant target effect was maximal for synchronous stimuli and declined within a <= 70 ms AT asynchrony. EEG revealed a cascade of AT interactions that relied on different neural mechanisms depending on AT asynchrony. At small (<= 20 ms) asynchronies, AT interactions arose for evoked response potentials (ERPs) at 110 ms and similar to 400 ms post-stimulus. Selectively at +/- 70 ms asynchronies, AT interactions were observed for the P200 ERP, theta-band inter-trial coherence (ITC) and power at similar to 200 ms post-stimulus. In conclusion, AT binding was mediated by distinct neural mechanisms depending on the asynchrony of the AT signals. Early AT interactions in ERPs and theta-band ITC and power were critical for the behavioural response facilitation within a <=+/- 70 ms temporal integration window.