Auditory modulation during speech planning in stuttering and nonstuttering individuals
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Stuttering is associated with atypical structural and functional connectivity among sensorimotor brain areas. However, it remains entirely unknown which specific mechanisms of sensorimotor control are affected by these neurological differences. In the program of research described here, I used a novel experimental paradigm and electroencephalographic (EEG) recordings to study motor-to-sensory interactions during speech movement planning in stuttering versus nonstuttering speakers. Experiment 1 investigated whether stuttering adults are deficient in modulating the auditory system prior to speech initiation. Auditory modulation was examined by recording auditory evoked potentials in response to probe tones presented during movement planning in a delayed-response speaking condition as compared with no-speaking control conditions. Findings indicated that stuttering speakers did not show the modulation of auditory processing (reflected in reduced amplitude of the N1 component) that was observed in nonstuttering speakers. This finding raised the question whether stuttering individuals have problems specifically with generating or evaluating a planning-related efference copy signal that can be used to predict upcoming self-generated sensory inputs or, more generally, with using any available information to make sensory predictions. In Experiment 2, probe tones were therefore delivered while participants anticipated either self-producing speech or hearing their own pre-recorded speech played back and in a control condition without auditory input. Results showed that auditory modulation differed between stuttering and normally fluent adults in both conditions with predictable auditory input. Experiment 3 was designed to start exploring the functional significance of pre-speech auditory modulation in general, and the functional implications of stuttering speakers' lack of modulation. Participants in this experiment completed a sensorimotor adaptation task with formant-shifted auditory feedback, and the results served to estimate each speaker's reliance on auditory feedback. In a separate session, pre-speech auditory modulation was again assessed by means of probe tones, but this time N1 modulation relative to a no-speaking control condition was quantified both in a condition that allowed typical reliance on auditory feedback (non-delayed auditory feedback; NAF) and in a condition that did not allow reliance on auditory feedback (delayed auditory feedback; DAF). Results revealed that (a) stuttering speakers showed only limited adaptation to formant-shifted auditory feedback; (b) for nonstuttering speakers, DAF caused the amount of pre-speech auditory modulation to be reduced whereas for stuttering speakers, DAF enhanced pre-speech auditory modulation; and (c) across the two groups, there was a relationship between the effect of DAF on pre-speech auditory modulation and reliance on auditory feedback during the adaptation task. These studies demonstrate that stuttering individuals have difficulties with using auditory predictions--both those related to active movement planning and those related to input that is not a consequence of one's own actions--to prime this sensory system with critical importance for speech production. Moreover, stuttering individuals showed not only a lack of modulation of the auditory system under normal speaking conditions (NAF) but also a lower reliance on auditory feedback as revealed here during a sensorimotor adaptation task with formant-shifted auditory feedback. Overall, findings suggest that stuttering is associated with deficits in auditory-motor integration, and that the auditory system may be not appropriately modulated for its role in online feedback control during speech production. I speculate that the inability to use predictive information for appropriately priming task-relevant sensory systems for their role in monitoring articulatory movements may lead to unnecessary and disruptive attempts at correcting ongoing movements. These maladaptive "repairs" may contribute to the fluency breakdowns that form the primary symptoms of stuttering.
- Speech