Auditory-motor integration influences on speech motor control and fluency: A comparison of normally fluent speakers and people who stutter

The overall purpose of this dissertation was to identify the effects of delayed auditory feedback (DAF) on speech motor stability and speech fluency in normally fluent adults (PWNS) and to compare them with effects on adults who exhibit persistent stuttering. If the auditory processing influences differently the two groups, it might shed light on the basic role of auditory-to-motor integration in speech production. Differences in auditory feedback dependency were also expected between normally fluent individuals who are highly susceptible to DAF and normally fluent individuals who are minimally affected by DAF. Three studies were conducted to (I) subgroup 62 normally fluent males and females based on their responses to DAF during spontaneous conversational speaking and reading tasks, (II) compare responses to DAF between 15 normally fluent adults who showed high stuttering-like disfluencies (SLD) under DAF (High-SLD group), 15 normally fluent adults who showed low SLD under DAF (Low-SLD group), and 15 adults who stutter, and (III) compare the specific effects of DAF on speech motor stability and movement patterns (lower-lip movement) in 11 normally fluent adults versus 11 adults who stutter.;Study I revealed that normally fluent participants, as a group, increased the amount of SLD and speech errors (SE), and decreased articulation rate (AR) when speaking under DAF relative to speaking under amplified non-delayed condition (aNAF). Sex had a limited differential effect on other disfluencies (OD) and AR whereas task effects were seen in OD, SE, and AR. Neither sex nor task, however, did influence SLD. Three subgroups, Low, Middle, and High Responders were identified. There were clear differences between the Low and High Responders in that the Low Responders exhibited smaller numbers of SLD and SE as well as faster AR than the High Responders. The Middle Responders revealed task dependency, showing different responses than those exhibited by the Low and High Responders.;The results of Study II showed that people who stutter did not reduce SLD counts under DAF condition, a finding that deviates from reports of previous studies. PWS also exhibited higher mean number of SLD than the normally fluent Low-SLD group but similar to that of normally fluent High-SLD group. For all groups, the number of SE was higher and AR was slower under DAF than aNAF. PWS showed larger individual variability in responses to aNAF compared to natural feedback conditions in that some PWS reduced SLD under aNAF condition.;Study III revealed significant difference between PWS and PWNS in speech kinematic stabilities, showing that PWS exhibited higher spatio-temporal index (i.e., lower speech motor stability) than PWNS across auditory feedback conditions in four utterance stimuli. PWS, however, yielded higher speech movement stability under aNAF and 25ms-DAF in longer stimuli compared to natural feedback condition. There was no group difference in movement displacement and velocity for opening-closing sequential movements but the patterns were different in each group. PWNS showed lower displacement and velocity under aNAF and DAF compared to natural feedback condition for both opening and closing movements whereas PWS showed higher displacement under DAF than aNAF condition and higher velocity under nNAF than 25ms-DAF. Normally fluent people showed shorter movement duration and faster articulation rate than PWS across auditory feedback and stimuli.;This dissertation study has demonstrated different influences of auditory feedback on speech motor control in normally fluent adults and adults who stutter. In both groups, individual variability was obvious. In normally fluent adults, diverse responses to DAF could be explained with different auditory feedback dependency for ongoing speech articulatory production. PWS were more dependent on auditory feedback for speech production, and auditory-to-motor integration under aNAF and short delay feedback conditions helped them to improve speech motor stability.