Inhibitory interference to abandonment of voluntary finger movement by single-pulse transcranial magnetic stimulation

Brain function dynamics related to an inhibitory interference in voluntary motor abandonment was investigated with single-pulse transcranial magnetic stimulation (TMS) and electroencephalogram (EEG). As the voluntary motor movement, a point-to-point reaching movement of the right index-finger was conducted. The starting time of the movement was indicated with the clock making one revolution for 4 s. The time the clock hand passed the 9 o´clock position was defined as a go-signal. In the go trials, the subject was instructed to start the movement at the timing of the go signal. In some trials, called as pre-stop trials, a stop signal was presented with red LED illumination -100 ms from the timing of the go-signal. The go-trials and pre-stop trials were randomly performed in the series of the trials. In all trials, TMS or sham-TMS were conducted. TMS was delivered with a round coil on the subject´s head at various timings. Sham-TMS trials were with a click sound of TMS produced by another coil located near the head without the brain stimulation. In the sham-TMS trials of the pre-stop trials, the subject was able to prevent the finger movement. However, the TMS conducted at -150, -100 or -50 ms from the go-signal induced the involuntary finger movement in the pre-stop trials. We also measured brain potentials in the sham-TMS and TMS trials. The potential at Fz electrode showed a large positive peak in the sham-TMS trials of the pre-stop trials, whereas the potentials at the same latency were attenuated in the TMS trials of the pre-stop trials. These results indicated that the single-pulse TMS applied around the stop-signal in the reaching finger movement could intervene in the brain function of the voluntary motor abandonment conducted at medial frontal cortex.