Research on infant vocal development has provided notable insights into vocal interaction with caregivers, elucidating growth in foundations for language through parental elicitation and reaction to vocalizations. A role for infant vocalizations produced endogenously, potentially providing raw material for interaction and a basis for growth in the vocal capacity itself, has received less attention. We report that in laboratory recordings of infants and their parents, the bulk of infant speech-like vocalizations, or "protophones", were directed toward no one and instead appeared to be generated endogenously, mostly in exploration of vocal abilities. The tendency to predominantly produce protophones without directing them to others occurred both during periods when parents were instructed to interact with their infants and during periods when parents were occupied with an interviewer, with the infants in the room. The results emphasize the infant as an agent in vocal learning, even when not interacting socially and suggest an enhanced perspective on foundations for vocal language.

Human infant vocalization is viewed as a critical foundation for vocal learning and language. All apes share distress sounds (shrieks and cries) and laughter. Another vocal type, speech-like sounds, common in human infants, is rare but not absent in other apes. These three vocal types form a basis for especially informative cross-species comparisons. To make such comparisons possible we need empirical research documenting the frequency of occurrence of all three. The present work provides a comprehensive portrayal of these three vocal types in the human infant from longitudinal research in various circumstances of recording. Recently, the predominant vocalizations of the human infant have been shown to be speech-like sounds, or ‘protophones’, including both canonical and non-canonical babbling. The research shows that protophones outnumber cries by a factor of at least five based on data from random-sampling of all-day recordings across the first year. The present work expands on the prior reports, showing the protophones vastly outnumber both cry and laughter in both all-day and laboratory recordings in various circumstances. The data provide new evidence of the predominance of protophones in the infant vocal landscape and illuminate their role in human vocal learning and the origin of language. This article is part of the theme issue ‘Vocal learning in animals and humans’.

27 Research on infant vocal development is focused primarily on vocal interaction with caregivers, 28 where it appears to be largely assumed that infants vocalize mostly for the purpose of interaction. 29 A survey of both parents and non-parents indicated that public opinion conformed to the 30 expectation that infant vocalization is mostly socially interactive. However, we report that in 31 laboratory recordings of infants and their parents, the bulk of infant speech-like vocalizations 32 ("protophones") were directed toward no one, and instead appeared to be generated 33 endogenously in exploration of vocal abilities. The tendency to produce protophones without 34 directing them to others occurred both during periods when parents were instructed to interact 35 with their infants and during periods when parents were occupied with an interviewer, with the 36 infants in the room. The results emphasize the infant as an agent in vocal learning, not as a 37 passive recipient of vocal input.38 Keywords: Speech development 1 , Social interaction 2 , Illocutionary force 3 , Prelinguistic 39 communication 4 , Origin of language 5 , Language development 6 , Evolutionary-development 7 42 The study of vocal development has been dominated by the expectation that infants primarily 43 vocalize in a speech-like manner when they are in social engagement, an expectation suggesting 44 social interaction drives prelinguistic vocal development (1-6). Granted, social learning is 45 required in order for infants to acquire the language-specific syllables and phonemic elements 46 and the largely arbitrary pairings of words with meanings in languages. Thus, there can be no 47 doubt that social interaction plays a critical role in infant vocal learning and language 48 acquisition. Surprisingly, however, we know little about the extent to which infants actually 49 engage in directed vocal interaction using the speech-like sounds or "protophones" of infancy 50 (which include both canonical babbling and precanonical speech precursors in accord with the 51 terminology of Oller, 2000), as opposed to simply vocalizing playfully or exploratorily. The 52 proportion of infant protophones that are socially-directed has, to our knowledge, never been 53 previously quantified, so the extent to which infant protophone production may be primarily 54 endogenous rather than social is unknown.55 Even so, infant vocalization, especially in the context of social interaction, has been researched 56 for half a century (8-13). A social feedback loop has been posited to exist in infant and child 57 vocalization, and that loop has been thought to promote contingent infant vocalizations with 58 respect to caregiver vocalizations (14-17). Experimental studies in the still-face paradigm (18) 59 have shown that by 5-6 months of age, infants increase the rate of protophone production when 60 the parent disengages from an ongoing vocal interaction (19,20), suggesting infants by that age 61 seek to repair broken interactions with increased vocalization. SOCIAL AND NON...

A growing body of research emphasizes both endogenous and social motivations in human vocal development. Our own efforts seek to establish an evolutionary and developmental perspective on the existence and usage of speech-like vocalizations (“protophones”) in the first year of life. We evaluated the relative occurrence of protophones in 40 typically developing infants across the second-half year based on longitudinal all-day recordings. Infants showed strong endogenous motivation to vocalize, producing vastly more protophones during independent vocal exploration and play than during vocal turn taking. Both periods of vocal play and periods of turn-taking corresponded to elevated levels of the most advanced protophones (canonical babbling) relative to periods without vocal play or without turn-taking. Notably, periods of turn taking showed even more canonical babbling than periods of vocal play. We conclude that endogenous motivation drives infants’ tendencies to explore and display a great number of speech-like vocalizations, but that social interaction drives the production of the most speech-like forms. The results inform our previously published proposal that the human infant has been naturally selected to explore protophone production and that the exploratory inclination in our hominin ancestors formed a foundation for language.

There are many theories surrounding infant imitation; however, there is no research to our knowledge evaluating the reliability of listener perception of vocal imitation in prelinguistic infants. This paper evaluates intra- and inter-rater judgments on the degree of “imitativeness” in utterances of infants below 12 months of age. 18 listeners were presented audio segments selected from naturalistic recordings to represent in each case a parent vocal model followed by an infant utterance ranging from low to high degrees of imitativeness. The naturalistic data suggested vocal imitation occurred rarely across the first year, but strong intra- and inter-rater correlations were found for judgments of imitativeness. Our results suggest salience of the infant's vocal imitation despite its rare occurrence as well as active perception by listeners of the imitative signal. We discuss infant vocal imitation as a potential signal of well-being as perceived by caregivers.