||Induction of speech fluency by using transcranial direct current electrical stimulation and delayed auditory feedback
Stuttering is a speech fluency disorder characterized by word or part-word repetition, prolongation, and silent blocks. Past brain imaging studies have reported functional anomalies in speech-related regions such as overactivity of the right inferior frontal gyrus (IFG) and low activity of the bilateral superior temporal gyrus (STG) and structural abnormalities in these regions in people with stuttering (PWS). Moreover, it is also known that stuttering may be improved by delayed auditory feedback (DAF), suggesting that PWS may have abnormality in the auditory feedback function. However, the detailed mechanism of the occurrence of stuttering is still unknown, and imaging studies alone are unable to show the causal relationship between the occurrence of stuttering and the functional abnormality of speech-related regions. In addition, it is known that the improvement of stuttering due to changes in altered auditory feedback is greatly different among PWS. Based on previous literature on stuttering, in this study, we hypothesized that the cause of the stuttering is in the speech-related motor area and that abnormal function of the auditory cortex including auditory feedback is a secondary factor that exacerbates stuttering. We conducted a behavioral experiment to read aloud under the influence of transcranial direct current stimulation (tDCS) and DAF. First, in Experiment 1, in order to investigate the causal relationship between speech-related area activity and speech disfluency of PWS, the bilateral IFG and STG were stimulated with tDCS, and reading-aloud task was carried out under the condition that the activity of these brain regions were changed. When the bilateral IFG and left STG were stimulated, a significant change in stuttering frequency was observed as compared with the non-stimulated (sham stimulation) condition. In particular, the degree of this change was the largest when we stimulated the right IFG. The stuttering frequency increased with anodic stimulation, and on the contrary, it decreased with cathodic stimulation. Moreover, only under conditions of the right IFG stimulations, a significant positive correlation was observed between the change of stuttering frequency and the severity of stuttering. These results demonstrated that the activity of the right IFG greatly affects speech disfluency of stuttering. Next, in Experiment 2, in order to investigate the influence of auditory feedback on the speech disfluency of PWS, the reading-aloud task was performed under the condition that the auditory feedback was changed by the DAF. In this experiment, there was a significant negative correlation between the exacerbation of speech disfluency by DAF and severity of stuttering. Moreover, in PWS whose level of speech disfluency in the reading-aloud task was in the normal range, the DAF rather increased speech disfluency. Regarding the speech speed, all stutterers showed a decrease in speech rate under the DAF conditions. These findings suggested that the influence of altered auditory feedback on the speech disfluency of PWS is related to the severity of stuttering, and that improvement of stuttering by DAF is not caused only by a decrease in speech rate. Based on the above results and reviewing previous literature, I propose that, in stuttering, the speech error occurs because there is abnormality in the motion prediction signal due to the altered activity of the speech-related regions. Furthermore, as a compensation for the abnormal prediction signal, the speech motor control system becomes heavily dependent on the auditory feedback in PWS. I further propose that there is a possibility that stuttering is caused mainly by activity abnormality in the speech-related motor area including the right IFG and that, as secondary factors, abnormalities in the auditory cortex and those of the auditory feedback make the stuttering more severe.
首都大学東京, 2017-03-25, 修士（言語科学）