This study aimed to examine the effects of non-periodic repetitive finger movements on short-latency somatosensory evoked potentials. Eleven healthy adults (age, 24.0±2.8 years) were included in the study. The motor task was right index finger metacarpophalangeal joint flexion and extension, performed in time with a sound signal. For task 1, the signal was sounded at a regular interval of 1,000 ms. For task 2 the signal was sounded at random intervals of 750 ms, 1,000 ms, and 1,250 ms. N20, P25 amplitude was significantly lower during task 2 than during at rest. Gating of non-periodic repetitive movements occurred in the 3b area and higher somatosensory area.

View full abstract

Previous studies on transcranial magnetic stimulation (TMS) demonstrated that excitability of the ipsilateral primary motor cortex (M1) increases during unilateral compared with pseudo goal-directed finger movement. However, it remains unclear whether unilateral goal-directed movement modulates short (SICI) and long (LICI) interval intracortical inhibition circuits in the ipsilateral M1. The aim of the present study was to investigate whether SICI and LICI in the ipsilateral M1 are affected by execution of unilateral goal-directed finger movement. Ten healthy right-handed subjects performed four finger tapping tasks with the right index finger. The tasks included the following: (1) maintaining the finger at rest, as a control; (2) self-paced simple tapping on the desk; (3) typing real words using a virtual laser keyboard; and (4) typing pseudo words. During each of the tasks, TMS was delivered to evoke a motor evoked potential (MEP) from the left first dorsal interosseous (FDI) muscle. The test stimulus (TS) intensity was adjusted to elicit an MEP of approximately 1 mV peak-to-peak amplitude in the left FDI. For paired-pulse TMS paradigms, the conditioning stimulus (CS) intensity was set to 80% of the resting motor threshold for SICI. To assess LICI, the intensity of CS was set identical to that of the TS. Interstimulus intervals (ISI) between both CS and TS were fixed at 3 and 100 ms for SICI and LICI, respectively. EMG activity in the right FDI was similar during all finger tapping tasks. At an ISI of 100 ms, MEPs were significantly increased during the real words typing task compared with both control conditions and the pseudo words typing task. However, with single pulse TMS and paired-pulses with an ISI of 3 ms, MEPs remained unchanged during all tasks. These results indicate that LICI was significantly decreased during the real words typing task. Previous TMS studies reported that LICI in the contralateral M1 decreases during precision grip compared with simple finger abduction. Therefore, our findings suggest that performing unilateral goal-directed movement decrease not only LICI in the contralateral M1, but also that of the ipsilateral M1 via the transcallosal pathway.

View full abstract