Contribution of acoustic analysis to the detection of vocoid epenthesis in apraxia of speech and other motor speech disorders

Background: Vocoid epenthesis within consonant clusters has been claimed to contribute to the diagnosis of apraxia of speech. In clinical practice, the clinicians often doubt about the correct production of clusters as the C-C transition may be minimally disrupted. Aims: To demonstrate the value of acoustic analysis in clinical practice as a reliable complement to perceptive judgment. Methods & Procedures: We compared the acoustic signature and the perceptive detection of vocoid epentheses in unvoiced consonant clusters within pseudo-words produced by 40 participants presenting different subtypes of motor speech disorders (including apraxia of speech (AoS) and dysarthria) and matched neurotypical controls. Outcomes & Results: The results indicate that vocoid epenthesis was acoustically visible in 3 out of 10 participants with AoS, and in one out of 30 participants with dysarthria. One-quarter of these vocoid epentheses was not detected via auditory perception by expert listeners (speech and language therapists) who also made false detections. Conclusions: The current results indicate that vocoid epenthesis is not systematic at least in mild AoS. Moreover, an important proportion is misdetected by ear, even by expert clinicians, meaning that visualisation of the acoustic signal can be of precious help.