UC San Diego

One of the most challenging questions in linguistics is why the ability to acquire language declines with age. The critical period hypothesis, which claims that language acquisition is driven by brain maturation, is widely accepted despite a lack of evidence to support it. Because most children experience language from birth, the relationship between brain growth and early language experience is poorly understood. This dissertation describes the language acquisition and the neural language processing in three deaf individuals (cases) who were cut off from nearly all language until adolescence; they could not hear spoken language and, due to anomalies in their upbringing, did not experience sign language until adolescence when they became immersed in American Sign Language (ASL). These developmental circumstances allow us to investigate the effects of first language acquisition begun in adolescence, and test the critical period hypothesis from a unique perspective. The first part of the dissertation focuses on the cases' language following one to two years of ASL use. Their language is remarkably similar to that of young children: their lexicons are biased towards nouns, and their utterances are short and simple. The second part of the dissertation explores the link between the age onset of language acquisition and the neural representation of sign meaning using anatomically constrained magnetoencephalography. Chapter 3 demonstrates that under ideal developmental circumstances, when language is available from birth, the neural processing of sign in deaf participants is highly similar to the processing of speech in hearing participants. However, in subsequent studies with the cases (Chapter 4), we observe atypical neural activation patterns, which diverge significantly from those associated with native sign or spoken language learning. These results indicate that early language experience is crucial in establishing canonical neural language processing patterns. The atypical neural activation patterns we find in the cases may be associated with the slowed rate of language development we observe in follow-up language studies. Our findings provide some of the initial direct evidence in support the critical period hypothesis and have important theoretical and practical implications