Vowel acoustic parameters in speech assessment and rehabilitation of minimally verbal and speech-motor-impaired autistic children: a narrative review.

Speech production difficulties in autism spectrum disorder (ASD) are heterogeneous and are not uniformly characterized by articulatory impairment across the spectrum. However, some autistic subgroups, particularly minimally verbal children, children with low expressive-language ability, and those with suspected co-occurring speech-motor difficulties, may show atypical vowel-acoustic patterns. Because vowel production can be quantified through measures such as formant frequencies, vowel-space area, duration, and variability, these parameters may offer useful objective information for characterizing speech impairment and informing rehabilitation planning in selected phenotypes. This narrative review critically examined the literature on vowel-acoustic characteristics in autistic children and their possible relevance to rehabilitation-oriented systems. Structured searches of PubMed, Scopus, and Web of Science were conducted for studies published up to September 2025. Evidence suggests that some autistic subgroups may exhibit reduced vowel distinctiveness, greater acoustic variability, and atypical temporal or dynamic speech features, although findings are heterogeneous and do not support a single uniform acoustic profile across ASD. Direct intervention evidence remains limited. Most rehabilitation-related studies are small, exploratory investigations, and the available literature is concentrated largely around Auditory-Motor Mapping Training (AMMT), an indirectly relevant speech-motor intervention that reports vowel-related outcomes but does not directly target vowel-acoustic parameters in isolation.Overall, vowel-acoustic measures are better regarded as candidate quantitative tools for subgroup characterization, monitoring, and rehabilitation planning rather than as universal biomarkers across ASD. Their translational potential is promising but remains insufficiently validated, requiring stronger longitudinal, mechanistic, and intervention research before broad clinical application can be justified.