Time course of premovement motor cortex excitability in simple reaction time paradigm

The aim of the present study was to investigate the effect of transcranial magnetic stimulation (TMS) on reaction time (RT) and on motor cortex excitability preceding voluntary movements. Single pulse and paired-pulse TMS with two interstimulus intervals (ISI) of 3 and 13 ms were applied to the left motor cortex at different delays after a visual go command for a voluntary isometric index finger abduction of the right (dominant) hand. The intensity of the testing and conditioning stimuli were 120% and 70% of motor threshold at rest, respectively. Changes of the parameters of motor evoked potentials (MEP) recorded from the first dorsal interosseous muscle (FDI) were used to assess motor cortex excitability. The MEN in response to single pulse TMS were augmented in a period around 80 ms before the voluntary myoelectrical activity (EMG) onset. In paired-pulse TMS at an ISI of 13 ms, the MEP augmentation was smaller but started earlier before the voluntary EMG. In case of ISI = 3 ms, the expected intracortical inhibition was evident only when TMS preceded the voluntary muscle activity by a premovement interval of more than 80 ms, but at shorter intervals, rather some MEP augmentation was observed. In all TMS experiments, a minimum interval between MEP onset and burst onset of the voluntary response was observed (dead band). Moreover, TMS influence on RT was accompanied by essential changes of RT distribution.