Characterization of stimulus response curves obtained with transcranial magnetic stimulation from bilateral anterior digastric muscles in healthy subjects.

Purpose: The purpose of the study was to describe measurements of stimulus-response curves in the anterior digastric muscle (ADM) bilaterally following transcranial magnetic stimulation (TMS) to the right and left hemispheres. The first dorsal interosseous muscle (FDI) was the control muscle.

Materials and methods: The subjects were 20 healthy young adults. Test sessions determined motor thresholds (MT) and stimulus-response curves (1.0, 1.2, 1.4, 1.6 × MT) from either the FDI or ADM following TMS to left and right hemispheres using the double cone coil. Bilateral recordings of MEPs in the left and right ADM allowed us to generate stimulus response curves following ipsilateral and contralateral TMS.

Results: Intraclass correlation coefficients (ICC) for MEP amplitudes from ipsilateral and contralateral ADMs were >0.60 at motor threshold (MT) and >0.90 at stimulus intensities above MT. There was a linear increase in MEP amplitudes across stimulus intensities for the FDI following contralateral TMS, while MEP amplitudes from the ADM following contralateral and ipsilateral TMS increased linearly across stimulus intensities [F_{(3, 57) [Muscle × Recording Site × Stim Intensity]} = 33.57; p < 0.05]; ${(\eta }_p^2$ = 0.64). The slopes of the stimulus-response curve of the contralateral FDI was greater than the slopes of the stimulus response curves of the ipsilateral and contralateral ADM (p < 0.05).

Conclusions: The current study provided insights on the methodology for recording stimulus response curves in the ADM with TMS. These findings may translate into a valid, reliable, and relevant clinical outcome to study the pathophysiology of the corticobulbar motor system.