Electrophysiological indices of language processing in infants at risk for ASD
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Behavioral symptoms of autism spectrum disorder (ASD) begin to emerge around 12 months of age and are preceded by subtle differences in how infants process and interact with the world (Elsabbagh & Johnson, 2010). Similar atypical behavioral patterns and markers of brain organization (`endophenotypes') are present in infants at risk for ASD (HRA) due to their family history, regardless of whether they ultimately develop the disorder. Possible endophenotypes of ASD were investigated through four studies that examined event-related potentials (ERPs) to speech and language in HRA and low-risk control (LRC) infants as part of a larger, longitudinal project. Chapter 2 examined ERPs to language-specific phonemes at 6, 9, and 12 months (n=59 at 6mo, 77 at 9mo, and 70 at 12mo) and found that HRA infants were not delayed in phonemic perceptual narrowing yet exhibited atypical hemispheric lateralization of ERPs at 9 and 12 months. Chapter 3 explored these findings further in a sample with known developmental outcome (n=60 at 6mo, 75 at 9mo, and 72 at 12mo) in order to understand how these ERPs differ between infants who ultimately develop ASD and infants who do not. Chapter 4 examined responses to repeated speech stimuli at 9 months (n=95). HRA infants exhibited atypically large ERPs to repeated speech, and this pattern was associated with better later language ability. Finally, Chapter 5 examined ERPs to words at 18 and 24 months (n=41 at 18mo, 52 at 24mo) and found evidence for atypical topography of responses to known versus unknown words, particularly at 18 months. These findings provide evidence that in HRA infants, even those who do not develop ASD, neural processing of linguistic stimuli is altered during infancy and toddlerhood. The results from Chapter 4 suggest that at least some of the differences seen in HRA infants who do not develop ASD may reflect beneficial, rather than disordered, processing. Overall, these results contribute to growing evidence that familial risk for ASD is associated with atypical processing of speech and language during infancy. Future work should continue to investigate more closely the implications of atypical neural processing for infants' later development.