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Subjective impressions of speech delivery in Autism Spectrum Disorder (ASD) as monotonic or over-precise are widespread but still lack robust acoustic evidence. This study provides a detailed acoustic characterization of the specificities of speech in individuals with ASD using an extensive sample of speech data, from the production of narratives and from spontaneous conversation. Syllable-level analyses (30,843 tokens in total) were performed on audio recordings from two sub-tasks of the Autism Diagnostic Observation Schedule from 20 adults with ASD and 20 pairwise matched neuro-typical adults, providing acoustic measures of fundamental frequency, jitter, shimmer and the first three formants. The results suggest that participants with ASD display a greater articulatory stability in vowel production than neuro-typical participants, both in phonation and articulatory gestures.

This study examined whether the atypical speech style that is frequently reported in autistic adults is underpinned by an inflexible production of phonetic targets. In a first task, 20 male autistic adults and 20 neuro‐typicals had to read and produce native vowels. To assess the extent to which phonetic inflexibility is due to an overall fine‐grained control of phonetic behavior or to a lack of flexibility in the realization of one's phonological repertoire, the second task asked participants to reproduce artificial vowel‐like sounds. Results confirmed the presence of a greater articulatory stability in the production of native vowels in autistic adults. When instructed to imitate artificial vowel‐like sounds, the autistic group did not better approximate the targets' acoustic properties relative to neuro‐typicals but their performance at reproducing artificial vowels was less variable and influenced to a greater extent by the articulatory properties of their own vocalic space. These findings suggest that the greater articulatory stability observed in autistic adults arises from a lack of flexibility in the production of their own native vowels. The two phonetic tasks are devoid of any pragmatic constraint, which indicates that phonetic inflexibility in autism is partly independent of register selection. Lay Summary: Autistic and neuro‐typical adults took part in two tasks: one in which they produced vowels from French, their native tongue, and the other where they imitated unfamiliar vowels. Autistic adults displayed significantly less variation in their production of different French vowels. In imitating unfamiliar vowels, they were more influenced by the way they pronounce French vowels. These results suggest that the atypical speech style, frequently attested in autistic individuals, could stem from an unusually stable pronunciation of speech sounds. [ABSTRACT FROM AUTHOR]