Reduced intracortical inhibition and facilitation in the primary motor tongue representation of adults who stutter

ObjectiveWe aimed at detecting neurophysiological changes, in the primary motor tongue representation in adults with persistent stuttering.MethodsUsing transcranial magnetic stimulation in 12 patients and 14 controls, we examined motor threshold, motor-evoked potential (MEP) input–output curve, short-term intracortical inhibition (SICI) and intracortical facilitation (ICF), based on eight trials per conditioning-test interval.ResultsIn controls inhibition of the MEP-amplitude at short inter-stimulus intervals (ISI) and facilitation of the MEP-amplitude at long ISIs was evident. Patients showed an inhibition at ISI 3 ms and weaker non-significant inhibition at ISI 2 ms; this delay of inhibitory activity was especially prominent in the right hemisphere. Facilitation was reduced at ISI 10 and 15 ms in patients. Furthermore, MEP input–output curve was steeper in patients. Motor thresholds did not differ between groups.ConclusionsIn persistent stuttering intracortical excitability of the primary motor tongue representation is altered with a deviant time course for inhibitory activity in the right hemisphere and reduced paired-pulse facilitation.SignificanceThese results specify changes in intracortical networks possibly mediated by altered GABAergic regulations in persistent stuttering. Thus, a better understanding of pathomechanisms and a potential role in understanding pharmacological treatment responses emerge by using transcranial magnetic stimulation.

► We study intracortical excitability of a speech muscle in persistent stuttering.
► Patients show an impaired facilitation in response to intracortical conditioning.
► Patients show an increased corticobulbar energization to voluntary contraction.
► Our results reveal mechanisms of abnormal motor cortical activity in stuttering.
► Future treatments targeting neuromodulation in stuttering might be monitored by TMS.