Human corticospinal excitability evaluated with transcranial magnetic stimulation during different reaction time paradigms

The aim of this study was to evaluate corticospinal excitability of both he mispheres during the reaction time (RT) using transcranial magnetic stimula tion (TMS), Nine right-handed subjects performed right and left thumb exten sions in simple (SRT), choice (CRT) and go/no-go auditory RT paradigms, TMS , inducing motor-evoked potentials (MEPs) simultaneously in the extensor po llicis brevis muscles bilaterally, was applied at different latencies from the tone, For all paradigms, MEP amplitudes on the side of movement increas ed progressively in the 80-120 ms before EMG onset, while the resting side showed inhibition. The inhibition was significantly more pronounced for rig ht than for left thumb movements. For the left SRT, significant facilitatio n occurred on the right after EMG onset, Initial bilateral facilitation occ urred in SRT trials with slow RT, After no-go tones, bilateral inhibition o ccurred at a time corresponding to the mean RT to go tones. The timing of t he corticospinal rise in excitability on the side of movement was independe nt of task difficulty and RT, This suggests that corticospinal activation i s, to some extent, in series and not in parallel with stimulus processing a nd response selection. Corticospinal inhibition on the side not to be moved implies that suppression of movement is an active process. This inhibition is more efficient for right- than for left-side movements in right-handed subjects, possibly because of left hemispheric dominance for movement.