Continuous theta-burst stimulation over the primary somatosensory cortex modulates interhemispheric inhibition

One mechanism thought to mediate hand and upper limb control across motor cortices is called interhemispheric inhibition (IHI). Somatosensory cortices are important in the motor control of the hand, although the neural mechanisms by which somatic loci act are not fully understood. In the present study, we study the possibility that the primary somatosensory cortex (SI) influences IHI as one mechanism to modulate hand control. IHI from the motor cortices was measured before and after continuous theta-burst stimulation (cTBS) was delivered over the left-hemisphere SI. IHI was evoked using paired-pulse transcranial magnetic stimulation and measured using electromyography electrodes over the first dorsal interosseous muscles of both hands at short (10 ms) and long (40 ms) intervals to evoke short interval IHI and long interval IHI, respectively. Measures were taken before and for up to 1 h after 600 pulse cTBS was delivered over SI. Results indicate that cTBS over SI increases short interval IHI in the left hand (i.e. ipsilateral to cTBS) for 45-60 min after stimulation. These results indicate that SI is indeed able to modify IHI, and this is therefore one neural mechanism by which SI may influence hand control.