Intracortical Excitability in Healthy Human Subjects After Tongue-training

Tongue dysfunction often has serious implications for a patient's general and oral health. Training of specific muscles causes short-term plastic changes in corticomotor pathways and paired-pulse trans-cranial magnetic stimulation (ppTMS) can be used to assess intracortical inhibitory (ICI) and facilitatory (ICF) networks. However, such training-related effects have not been described in detail for cranial muscles.

Objectives: This study examined short-term changes in ICI and ICF in tongue motor-cortex after a standardized tongue-training-task.

Methods: Thirteen healthy volunteers (6 men, 7 women) were tested before and after 30 min of tongue-training using ppTMS. ppTMS were applied to the “hot-spot” for the tongue motor-cortex and motor-evoked potentials (MEPs) were recorded from tongue muscles. The ppTMS protocol consisted of 8 single stimuli, and 32 paired-pulse stimuli with four different interstimulus intervals (ISI): 2, 3, 10, and 15ms in randomized order. The first dorsal interosseous muscle (FDI) served as control. The success-rate for the tongue-training task was compared between the first 3 min and the last 3 min. Normalized data were analyzed with repeated measurement analyses of variance and correlation analyses were performed between changes in success-rate and changes in MEP, ICI, and ICF.

Results: In tongue motor-cortex, no statistically significant ICI (P>0.466) but clear ICF was detected before and after training (P<0.031). There were no significant effects of training on single-MEPs or ICF in tongue or FDI motor-cortex (P>0.151). The success-rate improved significantly from 16+2% to 26+2% during training (P<0.001) and changes in success-rate were positively correlated to changes in single-MEP-amplitude (P<0.014) but not to ICI/ICF (P>0.194).

Conclusion: This first study of ICI/ICF after a tongue-training-task showed no changes in intracortical inhibitory or facilitatory networks. However, there was an association between task performance and changes in corticomotor excitability assessed by single-TMS supporting previous observations of short-term plasticity in corticomotor control of tongue muscles.