Infants’ Motor Synchronization to Speech

Speech is quasi-rhythmic, with key acoustic events, such as syllables, occurring at semi-regular intervals (about 4.5 syllables/second) across languages. Adults’ ability to align their speech motor productions to this rate, known as sensorimotor synchronization to speech, is linked to the auditory-motor coupling of the brain, as well as their performance on cognitive tasks. When does the ability emerge and does it play a role during early language development? This question was addressed by measuring infants’ non-nutritive sucking (NNS) while presented with syllable streams at two relevant rates (3 Hz and 4.5 Hz) at 2 - 4 months of age (N = 24; 9 females). NNS is an innate oromotor response produced by infants in the absence of receiving nutrition, which typically consists of periods of rhythmic behavior (bursts) and pauses. We evaluated whether infants showed tempo-flexibility, i.e., whether they adjusted NNS from their spontaneous frequency to better align with the frequency of the presented stimuli. We also explored phase synchronization, i.e., the degree of synchronization in time between NNS and the stimuli. Further, we assessed the correlation between these synchronization measures and communicative development at 12 months of age. NNS bursts became significantly more organized around 3 Hz when exposed to the 4.5 Hz speech stimuli compared to baseline, where NNS averaged around 1-2 Hz. Interestingly, infants also showed some phase synchronization to the 4.5 Hz, but not the 3 Hz stimulus. Although, phase synchronization to the 4.5 Hz stimulus was still relatively low and did not follow the same bimodal distribution, i.e. of “high” and “low” auditory-motor synchronizers to speech, that has previously been observed for adults (Assaneo et al., 2019). Neither of our measures of sensorimotor synchronization was significantly correlated with language outcomes at 12 months of age. Infants display some flexibility and sensorimotor synchronization in their NNS response towards the speech stimuli, which suggests that the auditory-motor coupling is operational in a rudimentary form from as early as 2 to 4 months of age.