Increased excitability and reduced intracortical inhibition in the ipsilateral primary motor cortex during a fine-motor manipulation task.

The effects of a sensorimotor task on ipsilateral primary motor cortex (ipsi-M1) excitability mediated via the transcallosal pathway, including the changes in short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF), were examined in ten right-handed subjects. Transcranial magnetic stimulation (TMS) was delivered to evoke a motor evoked potential (MEP) from the first dorsal interosseous (FDI). The test-TMS intensity was adjusted to around 120% of the resting motor threshold (rMT). For the paired-pulse TMS paradigm, the conditioning-TMS intensity was set to 80% of the rMT, and the interstimulus interval was fixed at 3 ms for SICI and 12 ms for ICF. As a sensorimotor task, a fine-motor manipulation (FM) task (using chopsticks to pick up, transport, and release glass balls) was adopted. In addition, a pseudo-FM (pFM) task was also performed as a control task. These tasks were carried out using each hand separately. The MEPs evoked during the FM task were markedly increased compared with those evoked during the pFM task, and these effects were not dependent on the electromyographic activity of the FDI performing these tasks. SICI was significantly decreased during the FM task, indicating disinhibition of the ipsi-M1, and these effects were also noted when the subjects used their non-dominant hand. The present findings suggest that the differences between the effects of the FM and pFM tasks on ipsi-M1 excitability were caused by their property.