In everyday life conversations, speakers often accompany verbal utterances with spontaneous gestures (beats) to emphasize certain parts of the utterance. Such affect is thought to be driven by the temporal alignment between visual (gestures) and auditory (speech) inputs. However, the brain mechanisms sustaining this ability remain unknown. We hypothesize that beats, usually preceding word onset by 200ms, drive the auditory system into an ‘optimal state’, enhancing its integration of ensuing acoustic information. In this study, we tested whether beat gestures reset the phase of ongoing neural oscillations and align neural activity into the high excitability phase at the arrival of the relevant auditory input. Electroencephalographic (EEG) recordings were acquired while participants watched a continuous, naturally recorded political discourse. The phase-locking index (PLI) was calculated from the EEG time locked to the onset of equivalent words appearing naturally in the discourse, sometimes preceded by beat gestures and sometimes not. We observed an increase in phase-locking at the delta-theta frequency range (2-6 Hz) from around 200 ms before word-onset to 200 ms post word-onset, when words were accompanied with a beat gesture compared to audio alone. Furthermore, this increase in phase-locking, most noticeable at fronto-central electrodes, was not accompanied by an increase in power in the same frequency range, confirming the oscillatory-based nature of this effect. These results suggest that beat gestures are used as robust predictive information capable to tune neural oscillations to the optimal phase for auditory integration of relevant parts of the discourse during natural speech processing.

Speech; Gestures; EEG; Oscillations