Investigating the neural basis of stuttering using transcranial magnetic stimulation: Preliminary case discussions

Abstract

Neuroimaging studies have found particular areas of the brain to be over- or under-active in persons who stutter (PWS) suggestive of atypical lateralization of speech and language processing. This study used single pulse transcranial magnetic stimulation (TMS) to investigate differences in left and right tongue motor cortex excitability in three adult male PWS compared with five adult control speakers (four males). TMS pulses were applied to the left and right side of the head over the tongue motor cortices. Measures of excitability included: active motor threshold (AMT, i.e. the amount of stimulation required to elicit a consistent motor evoked potential (MEP) in the tongue), and the y-intercept and slope of linear regressions derived from plots of electromyography contraction levels versus MEP amplitudes (input-output facilitation curves). The results are presented as a series of case discussions. The control group exhibited increased excitability for the left tongue motor cortex compared with the right based on y-intercept measures, although no inter-hemispheric excitability differences were found based on AMT or slope measures. A reversed pattern of excitability was found for the PWS who exhibited decreased and increased left and right tongue motor cortex excitability, respectively, based on the y-intercept and AMT measures compared with the control group. The results provide support for the suggestion of interhemispheric excitability disturbances in PWS and warrants further investigation using TMS techniques.