The production of syllables in stuttering adults under normal and altered auditory feedback

Altered Auditory Feedback (AAF) is a powerful instrument that is able to reduce the frequency of stuttering episodes, although the rationales for this effect are still unknown (Bloodstein & Bernstein-Ratner, 2008). The alterations in stutterers' speech do not only concern the motor aspect but also imply the sensory-motor loop (Hickok et alii, 2011; Namasivayam & van Lieshout, 2011). It is also known that stutterers' fluent speech is affected by the intrinsic characteristics of the speech units, as the frequency of occurrence and the articulatory complexity of the syllables, among others (Huink et alii, 2004; Howell et alii, 2006; Smith et alii, 2010). The present study describes the influence of the AAF on the production of different types of syllables varying for frequency and complexity in four Italian adult females who use to stutter (AWS). They differ in stuttering severity according to SSI-3 (Riley, 1994), ranging from moderate to very severe. We want to study: o the influence of the articulatory complexity (Howell et alii, 2006) and/or frequency of occurrence in the syllables (Stella & Job, 2000), without and with AAF, by means of transcription based quantitative-qualitative analyses of disfluencies and errors; o the influence of articulatory complexity, frequency of occurrence and feedback condition on the fluent speech of AWS, in terms of acoustic duration of syllables and intra-syllabic coarticulation of CV syllables (C= voiced plosives, V= [i]; [u]; [a]), according to "Locus of Equation" method (Sussman et alii, 2010). Subjects repeated each target syllable nine times, immediately after a recorded voice, under normal auditory feedback (NAF) and AAF (combining a delayed auditory feedback of 60 ms with a 40% reduction of the original F0). Results show that both higher articulatory complexity and lower frequency of occurrence of syllables increased the number of errors and stuttering episodes only for two subjects, the other two being fluent in both the auditory feedback conditions. For the subjects who stuttered, AAF improved fluency, in terms of a reduction in errors and dysfluencies. As for the influence of the AAF, with respect to NAF, on the intrasyllabic coarticulation degree of fluent syllables, we found opposite effects according to stuttering severity: while severe AWS, which benefitted most from AAF, showed a lower degree of coarticulation, moderate AWS, which benefitted less from AAF, showed a higher degree of coarticulation. These results suggest that, from a theoretical point of view, two general strategies promoting fluency could exist: the reduction of the speech rate (speech variations in the temporal dimension) and the reduction of the coarticulation (speech variations in the frequency dimension). They could be independent in principle, but more often they interact in variable ways (Namasivayam & van Lieshout, 2011). Every stuttering subject could adopt one of them, or a peculiar combination, perhaps according to the degree of severity.