Abstract
Deaf late signers provide a unique perspective on the impact of impoverished early language exposure on the neurobiology of language: insights that cannot be gained from research with hearing people alone. Here we contrast the effect of age of sign language acquisition in hearing and congenitally deaf adults to examine the potential impact of impoverished early language exposure on the neural systems supporting a language learnt later in life. We collected fMRI data from deaf and hearing proficient users (N = 52) of British Sign Language (BSL), who learnt BSL either early (native) or late (after the age of 15 years) whilst they watched BSL sentences or strings of meaningless nonsense signs.There was a main effect of age of sign language acquisition (late > early) across deaf and hearing signers in the occipital segment of the left intraparietal sulcus. This finding suggests that late learners of sign language may rely on visual processing more than early learners, when processing both linguistic and nonsense sign input – regardless of hearing status. Region-of-interest analyses in the posterior superior temporal cortices (STC) showed an effect of age of sign language acquisition that was specific to deaf signers. In the left posterior STC, activation in response to signed sentences was greater in deaf early signers than deaf late signers. Importantly, responses in the left posterior STC in hearing early and late signers did not differ, and were similar to those observed in deaf early signers. These data lend further support to the argument that robust early language experience, whether signed or spoken, is necessary for left posterior STC to show a ‘native-like’ response to a later learnt language.
Highlights
In the language literature, ‘age of language acquisition’ effects are typically discussed in the context of second language learning, with the assumption that a robust first language has been established
We investigated whether the effects of age of sign language acquisition differed between deaf and hearing signers, who differ in early language exposure
We found an effect of age of sign language acquisition that was common to both deaf and hearing signers in the occipital segment of left intraparietal sulcus where activation was greater for late than early signers
Summary
In the language literature, ‘age of language acquisition’ effects are typically discussed in the context of second language learning, with the assumption that a robust first language has been established This situation applies in the vast majority of hearing individuals: only in situations of extreme neglect or severe disability might hearing children not develop a spoken language. 90-95% of deaf children are born to hearing parents For these individuals, the first language is a spoken language, established on the basis of impoverished auditory input and visual speech (lipreading), which can only afford limited access to the speech signal since many articulators are invisible. We contrast the impact of age of sign language acquisition in deaf and hearing adults This allows a unique perspective on the impact of impoverished early language exposure on the neural systems supporting a language learnt later in life
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