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Producing speech is a highly complex task, involving the integration of sensory and linguistic information, with the precise, high-speed, co-ordination of muscles controlling breathing and the movement of the vocal folds and articulators. In spite of this complexity, producing fluent speech - moving smoothly from one speech sound to the next - can appear effortless. Speech fluency is highly socially valued, and the personal and societal costs of living with a disorder of fluency, such as developmental stuttering, are considerable. The outcomes of behavioural therapies to increase fluency are limited, however, especially for those seeking treatment in adulthood. The overarching aim of this thesis was to investigate how anodal transcranial direct current stimulation (A-TDCS) can be used to increase speech fluency, with a particular focus on the potential application to developmental stuttering. A-TDCS is a noninvasive brain stimulation technique that can enhance the effects of motor, speech, and language training. First, in a series of single-session experiments in typically fluent speakers, I demonstrated that applying A-TDCS over the left IFC increased speech motor learning relative to a sham control, but did not improve consolidation of this learning (chapter 2). Furthermore, I found that neither increasing stimulation intensity from 1 mA to 2 mA, nor changing from a unihemispheric to a bihemispheric configuration, had an additional effect on learning. Next, in single-session study with adults who stutter, I assessed the feasibility of using A-TDCS to improve fluency (chapter 3). Fluency was temporarily induced, by speaking in unison with another person, but the concurrent application of 1-mA unihemispheric A-TDCS over left inferior frontal cortex did not significantly prolong this fluency. Nevertheless, a trend towards stuttering reduction gave some indication that fluency might be increased using a multiple-session approach. Furthermore, I gained a number of important insights from these single-session studies, which I used to inform the design of the final multiple-session trial. In this final study, I completed a randomised controlled trial in 30 adult males with moderate to severe stuttering. Participants were randomized to receive either 1-mA A-TDCS or sham stimulation over left inferior frontal cortex combined with temporary fluency inducing behavioural techniques, for 20 minutes a day over 5 days (chapter 4). A-TDCS significantly reduced disfluency for at least 5 weeks following this intervention. The effect was specific to the speech impairment of development stuttering, as measures of the psycho-social consequences of stuttering were not modulated by A-TDCS. The findings of these studies offer significant promise for the future application of non-invasive stimulation as an adjunctive therapy for adults who stutter. In the concluding chapter, I discuss the important implications of my findings for the future use of this technique.

This dissertation is comprised of four studies investigating the hypothesis that adults and children who stutter differ from their same-age fluent peers in the neuroanatomy and neurophysiology underlying auditory speech processing. It has been consistently reported that adults who stutter demonstrate unique functional neural activation patterns during speech production, including reduced auditory activation, relative to nonstutterers. The extent to which these functional differences are accompanied by abnormal morphology of the brain in stutterers is unclear. The first study in this dissertation examined the neuroanatomical differences in speech-related cortex between adults who do and do not stutter using magnetic resonance imaging and voxel-based morphometry analyses. Adults who stutter were found to have localized grey matter volume increases in auditory and motor speech related cortex. The second study extended this line of research to children who stutter, who were found to have localized grey matter volume decreases in motor speech related cortex. Together, these studies suggest an abnormal trajectory of regional grey matter development in motor speech cortex of people who stutter. The last two studies investigated the mechanism underlying the repeated findings of reduced auditory activation during speech in people who stutter in more detail. Magnetoencephalography was used to investigate the hypothesis that people who stutter have increased speech induced suppression of early evoked auditory responses. Adults and children who stutter demonstrated typical levels of speech induced suppression relative to fluent peers. However, adults and children who stutter showed differences from peers in the timing of cortical auditory responses. Taken together, the studies demonstrate structural and functional abnormalities in brain regions related to auditory processing and point to the possibility that people who stutter have difficulty forming the neural representations of speech sounds necessary for fluent speech production.

This dissertation is comprised of four studies investigating the hypothesis that adults and children who stutter differ from their same-age fluent peers in the neuroanatomy and neurophysiology underlying auditory speech processing. It has been consistently reported that adults who stutter demonstrate unique functional neural activation patterns during speech production, including reduced auditory activation, relative to nonstutterers. The extent to which these functional differences are accompanied by abnormal morphology of the brain in stutterers is unclear. The first study in this dissertation examined the neuroanatomical differences in speech-related cortex between adults who do and do not stutter using magnetic resonance imaging and voxel-based morphometry analyses. Adults who stutter were found to have localized grey matter volume increases in auditory and motor speech related cortex. The second study extended this line of research to children who stutter, who were found to have localized grey matter volume decreases in motor speech related cortex. Together, these studies suggest an abnormal trajectory of regional grey matter development in motor speech cortex of people who stutter. The last two studies investigated the mechanism underlying the repeated findings of reduced auditory activation during speech in people who stutter in more detail. Magnetoencephalography was used to investigate the hypothesis that people who stutter have increased speech induced suppression of early evoked auditory responses. Adults and children who stutter demonstrated typical levels of speech induced suppression relative to fluent peers. However, adults and children who stutter showed differences from peers in the timing of cortical auditory responses. Taken together, the studies demonstrate structural and functional abnormalities in brain regions related to auditory processing and point to the possibility that people who stutter have difficulty forming the neural representations of speech sounds necessary for fluent speech production.

No description available.

No description available.