Atypicalities in vocal behavior have been consistently associated with autism since the earliest descriptions of the condition. Many studies analyzing speech development in autism have focused on investigating differences in intonation and rhythm associated specifically with prosodic deficits that seem to be characteristic of ASD. In our previous research, we showed that prosodic deficits, in the form of elevated and more variable fundamental frequency contours, may appear within the first year of life in infants at risk of autism, accompanied by atypical patterns of vocal interaction. However, it is not yet clear whether differences in prosody are specifically linked to deficits in social engagement, as our data suggest, or might instead simply be due to more general problems with motor control seen in ASD. Since fundamental frequency is only one property of the speech signal, characteristics of other voice source parameters might reveal a clearer picture about the origin of speech disorders in autism.
The goal of this study is to analyze developmental profiles of voice quality measures in the first two years of life in infants at risk of autism. By comparing measures that characterize different aspects of glottal function, we test the hypothesis that vocal atypicalities in high-risk infants reflect a core prosodic deficit specific to autism, as opposed to a more general deficit in phonatory function arising from motor impairments comorbid with ASD.
As part of an ongoing study, speech data were collected from 4 low-risk infants with no family history of autism and 4 high-risk infants with older siblings already diagnosed with autism. Day-long audio recordings of each child were made with a digital audio recording device (LENA Foundation) sent to participating families by mail and worn by the child for the entire day of the recording. The recording sessions began at 2 months and continued at monthly intervals for 12 months. Non-cry vocalizations for each child were extracted from each recording by hand labeling. Standard measures of voice quality including the fundamental frequency, open quotient, return quotient and speed quotient were estimated on a frame-by-frame basis for all segmented speech files. Developmental changes in all of our measures were quantified by using Functional Data Analysis to align the resulting densely sampled longitudinal profiles across individuals and derive growth charts of vocal development. Clinical assessments were carried out at 24 months to determine outcome.
In our high-risk sample, two infants were diagnosed with broader autism phenotype and two had non-autistic language delays; all low-risk infants were found to be typically developing. Voice quality profiles showed that fundamental frequency was elevated and more variable across BAP and LD infants, diverging from TD infants at around 9 months. No significant differences are currently observed in our other measures.
Our results indicate that atypical vocal behavior in infants at risk of autism begins to develop towards the end of the first year of life, and may present as specific differences in intonational prosody rather than properties reflecting more general problems in phonatory control.
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