Role of consonant bias in word learning in infants
In speech processing as well as in the world’s languages, there is bias towards consonants compared to vowels. Consonants are more suited to lexical processing, while vowels are better for syntax-related processes. However, it wasn’t known whether this ‘division of labour’ was a uniquely human attribute that plays a role in early syntax and lexical acquisition, or the result of the asymmetry present in the input. The Horizon 2020 SPIDE project addressed this question, exploring the developmental and evolutionary origins of an infant’s ability to assign specific functional roles to the different categories of sound comprising speech. Researchers conducted experiments on infants and rats to test the hypothesis that the disproportionate use of consonants compared to vowels is not a by-product of their physical differences. They also investigated whether it plays a significant role in early language acquisition and if it derives from more general biases shared with other species. A bias towards consonants Scientists used both behavioural and neuroimaging techniques to investigate the origins of the consonant/vowel asymmetry in language acquisition and evolution. “Our studies of infant behaviour showed that infants learning Spanish switch from an overall bias for vowels in familiar words when five months old, to favouring consonants at twelve months,” say project coordinator Prof. Juan Manuel Toro and Marie Curie grantee Dr Camillia Bouchon. The use of eye tracking revealed that younger infants respond more after vowel mispronunciations than after consonant mispronunciations. Interestingly, this pattern was reversed in older infants who responded more after consonant mispronunciations than after vowel mispronunciations. A comparative study using the same stimuli in laboratory rats demonstrated significantly greater sensitivity to a vowel mispronunciation than to a consonant mispronunciation. Results suggested that there is a transition from an acoustical strategy to a phonetic one during language learning in infants. “Vowels appear to be better for lexical recognition in both five months old human infants and in a non-human species, suggesting that the ability to target consonants preferentially in words may be a uniquely human ability,” explains Prof. Toro. Language development SPIDE has increased scientific understanding of language acquisition and how human predisposition for language evolved. According to Prof. Toro: “It also contributes to research on impaired language development, identifying the early signs of speech sound processing delays in babies and how exposure to more than one language may influence the use of consonants and vowels.” In addition, SPIDE showed that the ability to assign a particular linguistic role to consonants in words is not present in non-human animals such as laboratory rats, even when they are able to perceive and categorise consonant and vowel sounds and memorise word forms. This suggests that this ability is part of the uniquely human ability for language. The project's results will provide scientists with a better understanding of the developmental and evolutionary origins of consonant/vowel functional bias. “Our findings may act as a potential precursor of specific language impairments, like dyslexia and other language development related disorders, and help create suitable therapies,” Prof. Toro and Dr Bouchon conclude.
Keywords
SPIDE, language, consonant, vowel, infant, lexical, syntax