Single-Session Cerebellar Transcranial Direct Current Stimulation Affects Postural Control Learning and Cerebellar Brain Inhibition in Healthy Individuals.

Cerebellar transcranial direct current stimulation (ctDCS) modulates cerebellar activity and postural control. However, the effects of ctDCS on postural control learning and the mechanisms associated with these effects remain unclear. To examine the effects of single-session ctDCS on postural control learning and cerebellar brain inhibition (CBI) of the primary motor cortex in healthy individuals. In this triple-blind, sham-controlled study, 36 participants were allocated randomly to one of three groups: (1) anodal ctDCS group, (2) cathodal ctDCS group, and (3) sham ctDCS group. ctDCS (2 mA) was applied to the cerebellar brain for 20 min prior to six blocks of standing postural control training (each block consisted of five trials of a 30-s tracking task). CBI and corticospinal excitability of the tibialis anterior muscle were assessed at baseline, immediately after, 1 day after, and 7 days after training. Skill acquisition following training was significantly reduced in both the anodal and cathodal ctDCS groups compared with the sham ctDCS group. Changes in performance measured 1 day after and 7 days after training did not differ among the groups. In the anodal ctDCS group, CBI significantly increased after training, whereas corticospinal excitability decreased. Anodal ctDCS-induced CBI changes were correlated with the learning formation of postural control (r = 0.55, P = 0.04). Single-session anodal and cathodal ctDCS could suppress the skill acquisition of postural control in healthy individuals. The CBI changes induced by anodal ctDCS may affect the learning process of postural control.

Interindividual Variability of Lower-Limb Motor Cortical Plasticity Induced by Theta Burst Stimulation.

Theta burst stimulation (TBS) has been used as a tool to induce synaptic plasticity and improve neurological disorders. However, there is high interindividual variability in the magnitude of the plastic changes observed after TBS, which hinders its clinical applications. The electric field induced by transcranial magnetic stimulation (TMS) is strongly affected by the depth of the stimulated brain region. Therefore, it is possible that the variability in the response to TBS over the lower-limb motor cortex is different for the hand area. This study investigated the variability of TBS-induced synaptic plasticity in the lower-limb motor cortex, for intermittent TBS (iTBS), continuous TBS (cTBS), and sham iTBS, in 48 healthy young participants. The motor cortical and intracortical excitability of the tibialis anterior was tested before and after TBS using TMS. The results showed that iTBS had facilitatory effects on motor cortex excitability and intracortical inhibition, whereas cTBS exerted opposite effects. Twenty-seven percent of individuals exhibited enhanced motor cortical plasticity after iTBS, whereas 63% of participants showed enhanced plasticity after cTBS. In addition, the amount of TBS-induced plasticity was correlated with the intracortical excitability and the variability of the motor evoked potential prior to TBS. Our study demonstrated the high variability of the iTBS-induced lower-limb motor cortical plasticity, which was affected by the sensitivity of intracortical interneuronal circuits. These findings provide further insights into the variation of the response to TBS according to the anatomy of the stimulated brain region and the excitability of the intracortical circuit.