Time Course of Plasticity in Corticomotor Control of Tongue Muscles

Objectives: We have recently characterized the cortical control of the human tongue musculature and its modification by a 7-day tongue protrusion task. The aim now was to determine the time course following a 1-h training task. Methods: The corticomotor excitability was assessed by changes in EMG activity elicited in the tongue musculature by transcortical magnetic stimulation (TMS). Eleven healthy subjects participated. Stimulus-response curves were generated from the motor evoked potentials (MEPs) recorded in the tongue musculature and, from the first dorsal interosseos (FDI) muscle as a control, at four time points: at baseline, immediately after the tongue-protrusion task, 1 day after and 1 week after. The tongue-training task required each subject to protrude the tongue onto a force transducer placed in front of the subject, and consisted of a relax-protrude-hold-relax cycle lasting 12.5 s with 1 N as the target at the hold phase. The subjects repeated this task for a total of 60 min. Results: All subjects could successfully perform the task. The threshold for evoking MEPs by TMS in the tongue musculature was significantly decreased after the tongue-protrusion task compared with baseline and the 1-week follow-up (P < 0.001). The amplitude of the MEPs in the tongue musculature was significantly increased at higher intensities of TMS after the tongue-protrusion task and further increased 1 day after and the returned back to baseline values at 1-week follow-up (ANOVAs: P = 0.005). No significant effect of the training on MEPs in the FDI was observed (P = 0.493). Conclusion: The findings further document that a specific plasticity of the corticomotor excitability related to tongue muscle control readily can be induced when humans perform a novel tongue task for only 1 h and that the maximum changes occur after 1 day and returns to baseline values within 1 week.