We found evidence of disrupted language network activity in children with ASD, consistent with previous investigations of language processing in autism (17
). Previous fMRI findings of word segmentation in healthy adults (32
) revealed a consistent pattern of decreasing cortical activity within fronto-temporal-parietal networks as the number of cues to word boundaries increased from the R (minimal statistical cues) to the U (strong statistical cues) and S (strong statistical cues + prosodic cues) conditions. The TD children in this study demonstrated the same pattern of more focal activity as speech parsing cues increased, a pattern supported by direct comparisons between conditions. Specifically, we found that recruitment of dorsolateral frontal cortices, including middle frontal gyrus, decreased with increasing cues to word boundaries. These findings further support a role for the middle frontal gyrus, previously shown to be important for phonological and sequential processing (43
), in language learning. In contrast, the ASD children showed more similar activation profiles across conditions, involving primarily bilateral temporal cortices, without evidence for frontal involvement. Direct comparisons between conditions in this group revealed some greater activity for the R than the U condition in right and left dorsolateral prefrontal cortices but no regions of greater activity for the U compared with the S conditions. These results suggest that ASD children might be less sensitive to the implicit cues that guide word segmentation during language acquisition, particularly when both statistical and prosodic cues (e.g., stress) are present. This is noteworthy because infants as young as 8 months of age have been shown to weight prosodic cues more heavily than statistical regularities to identify word boundaries (39
As in our prior studies with neurotypical populations (32
), we further characterized the neural correlates of online word segmentation by examining changes in activity that occurred as a function of exposure to the speech streams (i.e., as transitional probabilities are computed as stimuli repeat over time). The TD children showed increased recruitment of regions subserving statistical learning (i.e., basal ganglia) (33
) and language processing (i.e., left SMG) (see Gitelman et al.
] for review) while listening to the speech streams containing high transitional probabilities and prosodic cues, but no significant signal increases while listening to the speech stream containing minimal cues to word boundaries. In contrast, children with ASD did not demonstrate significant signal increases over time for any speech stream. A between-group comparison indicated that the difference for the two artificial language conditions was reliable between groups, further suggesting that the ASD brain might fail to capitalize on two important cues to word boundaries (i.e., transitional probabilities and prosodic cues).
Next, we investigated whether this failure to rely on statistical and speech cues to guide word segmentation might be related to the linguistic and communicative impairments that are hallmark features of autism. To address this question we examined the relationship between communicative impairments in our sample of children with ASD (as indexed by the ADI-R communication subscale) and signal increases during exposure to the two artificial languages (↑U+↑S). We found a significant negative correlation between a participant's communicative deficits and signal increases in the left putamen and IPL during exposure to the artificial language streams, such that children with less impairment showed greater signal increases in these areas. Importantly, the regions where activity was found to correlate with the degree of communicative impairments are the same as those showing significant learning-related increases during the artificial language conditions in TD children. The ADI-R communication subscale taps into atypical development of communicative skills and captures delayed language abilities. The negative correlation observed between this reliable index of atypical development and signal increases in regions shown to be involved in word segmentation supports the hypothesis that involvement of these structures during word segmentation is related to the development of language and communicative skills. Behavioral research has shown that the ability to correctly segment words from continuous speech is predictive of future language development (28
). Our results further indicate a critical role of word segmentation abilities for normative language development while also implicating a corticostriatal network as the neural substrate of this fundamental process.
We hypothesized, on the basis of the persistent impairments in higher-order syntactic abilities in individuals with ASD (2
) and the demonstrated link between statistical learning and grammar acquisition (46
), that differences in the neural substrate of statistical learning might underlie the abnormal development of language in children with ASD, a population characterized by language impairment. Our findings indicate significant abnormalities in the neural architecture subserving language-related learning in ASD children and relate communicative impairments observed in this population to decreased sensitivity to the statistical and speech cues available in the language input. To elucidate the underlying causes of these abnormalities, further studies are warranted to determine whether longer exposures to similar speech streams would result in significant behavioral learning and to characterize the functioning of this corticostriatal network in ASD. An additional limitation of the current study is that our ASD and TD participants were matched on verbal IQ. Although this controls a potential confound in the interpretation of our findings, future studies should include individuals with autism who continue to demonstrate language impairments. Furthermore, greater characterization of the linguistic abilities of individuals with ASD (i.e., central auditory processing) and their relation to word segmentation abilities would be valuable in future studies. Interestingly, genetic investigations of language ability in ASD have identified a variant that is associated with the disorder (47
) and is expressed in fronto-temporal and striatal regions of the developing brain (49
). Thus, it might be particularly informative to examine the role that this genetic variant plays in establishing aberrant patterns of corticostriatal connectivity in ASD as well as in other developmental disorders characterized by marked linguistic impairments, such as Williams syndrome and specific language impairment.