Enhancing speech fluency using transcranial direct current stimulation

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.

Keywords: transcranial current stimulation
randomised controlled trial
speech disorders
developmental stuttering
Advisor: Mottonen, R
Watkins, K