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The ‘social brain’, consisting of areas sensitive to social information, supposedly gates the mechanisms involved in human language learning. Early preverbal interactions are guided by ostensive signals, such as gaze patterns, which are coordinated across body, brain, and environment. However, little is known about how the infant brain processes social gaze in naturalistic interactions and how this relates to infant language development. During free-play of 9-month-olds with their mothers, we recorded hemodynamic cortical activity of ́social brain` areas (prefrontal cortex, temporo-parietal junctions) via fNIRS, and micro-coded mother's and infant's social gaze. Infants’ speech processing was assessed with a word segmentation task. Using joint recurrence quantification analysis, we examined the connection between infants’ ́social brain` activity and the temporal dynamics of social gaze at intrapersonal (i.e., infant's coordination, maternal coordination) and interpersonal (i.e., dyadic coupling) levels. Regression modeling revealed that intrapersonal dynamics in maternal social gaze (but not infant's coordination or dyadic coupling) coordinated significantly with infant's cortical activity. Moreover, recurrence quantification analysis revealed that intrapersonal maternal social gaze dynamics (in terms of entropy) were the best predictor of infants’ word segmentation. The findings support the importance of social interaction in language development, particularly highlighting maternal social gaze dynamics.

Rhythmicity characterizes both interpersonal synchrony and spoken language. Emotions and language are forms of interpersonal communication, which interact with each other throughout development. We investigated whether and how emotional synchrony between mothers and their 9-month-old infants relates to infants’ word segmentation as an early marker of language development. Twenty-six 9-month-old infants and their German-speaking mothers took part in the study. To measure emotional synchrony, we coded positive, neutral and negative emotional expressions of the mothers and their infants during a free play session. We then calculated the degree to which the mothers’ and their infants’ matching emotional expressions followed a predictable pattern. To measure word segmentation, we familiarized infants with auditory text passages and tested how long they looked at the screen while listening to familiar versus novel words. We found that higher levels of predictability (i.e. low entropy) during mother-infant interaction is associated with infants’ word segmentation performance. These findings suggest that individual differences in word segmentation relate to the complexity and predictability of emotional expressions during mother-infant interactions.

According to embodied cognition theories, concepts are represented in sensory and motor brain systems. These sensory-motor representations can give rise to priming effects. Action verbs (e.g., to push) presumably have a higher association with motor features, and sound verbs (e.g., to buzz) with auditory features. This study assesses whether motor and auditory primes differentially influence the lexical decision of action and sound verbs. Seventy-five healthy Russian-speaking adults participated in the experiment. An online lexical decision task with cross-modal priming was administered. Participants were presented with meaningless primes such as a video clip of a moving hand (motor prime), a bike bell sound (auditory prime), or a static video clip (neutral prime). Then they saw a verb (an action verb, a sound verb, or a pseudoverb) and had to report whether or not it was a real word by pressing a button on the keyboard. Linear mixed effect models indicated no effect of the prime type on response accuracy or speed for any verb type. However, intransitive verbs elicited less accurate responses than transitive and optional-transitive verbs overall, regardless of the prime type. Moreover, participants responded slower for pseudoverbs than for real verbs. The results do not suggest differential category specific effects for action- and soundrelated verbs in a lexical decision task. However, the results for intransitive verbs support the facilitation through complexity hypothesis. Our findings do not support embodied cognition theories, but await further replication. Recommendations and future directions are discussed.