Transcranial direct current stimulation of SMA modulates anticipatory postural adjustments without affecting the primary movement

Recent works provide evidences that anticipatory postural adjustments (APAs) are programmed with the prime mover recruitment as a shared posturo-focal command. However the ability of the CNS to adjust APAs to changes in the postural context implies that the postural and voluntary components should take different pathways before reaching the representation of single muscles in the primary motor cortex. Here we test if such bifurcation takes place at the level of the supplementary motor area (SMA). TDCS was applied over the SMA in 14 subjects, who produced a brisk index-finger flexion. This activity is preceded by inhibitory APAs, carved in the tonic activity of Biceps Brachii and Anterior Deltoid, and by an excitatory APA in Triceps Brachii. Subjects performed a series of 30 flexions before, during and after 20min of tDCS in CATHODAL, ANODAL or SHAM configuration. The inhibitory APA in Biceps and the excitatory APA in Triceps were both greater in ANODAL than in SHAM and CATHODAL configurations, while no difference was found among the latter two (ANODAL vs. SHAM: biceps +26.5%, triceps +66%; ANODAL vs. CATHODAL: biceps +20.5%, triceps: +63.4%; for both muscles, ANOVA p<0.02, Tukey p<0.05). Instead, the APA in anterior deltoid was unchanged in all configurations. No changes were observed in prime mover recruitment and index-finger kinematics. Results show that the SMA is involved in modulating APAs amplitude. Moreover, the differential effect of tDCS observed on postural and voluntary commands suggests that these two components of the motor program are already separated before entering SMA.