Abstract
Electrophysiological studies have shown that cortico-spinal excitability increases during motor preparation period in reaction time (RT) paradigms. However, there is a line of contradictory evidence with transcranial magnetic stimulation (TMS) that its excitability is reduced during the preparation period. In these studies, subjects can predict the TMS timings. Thus, we investigated how predictability of TMS timing affects cortico-spinal excitability. A single-pulse TMS was delivered to the hand section of the left motor cortex while seven right-handed subjects relaxed their hand in a flexed position. We prepared three conditions: (i) in the semi-PREDICTABLE condition, two visual stimuli at 500 ms interval were presented and the TMS was delivered at random timing during the interval; (ii) in the PREDICTABLE condition, the TMS was provided only at 500 ms after the first stimulus; (iii) in the UNPREDICTABLE condition, no visual cue preceded the TMS. We recorded motor evoked potentials (MEPs) from wrist flexor and extensor muscles. We found a significant reduction of MEP amplitude in the flexor muscles in both the PREDICTABLE and semi-PREDICTABLE conditions, but not in the UNPREDICTABLE condition. These results showed that predictability of TMS per se, without preparing motor outputs, can reduce cortico-spinal excitability.
