The effect of transcranial magnetic (TMS) stimulation on reaction time and motor cortex excitability in premovement period was investigated. Single and paired-pulse TMS were applied to the motor cortex at different delays after a visual command for index finger abduction. Motor evoked potentials (MEPs) recorded from the right first dorsal interosseous were analysed.

The MEPs in response to single pulse TMS were augmented in a period of 90 - 100 ms before the voluntary myoelectrical activity (EMG) onset; MEP amplitude was gradually increased in premovement period. In paired-pulse TMS - 13 ms, the MEP augmentation was smaller but started earlier before the EMG, and a gradual increase of MEP amplitudes was not evident. In case of 3 ms paired-pulse TMS, the expected intracortical inhibition (ICI) was evident only when TMS preceded the voluntary EMG by an interval of more than 60 ms, but at shorter intervals, rather some MEP augmentation was observed.

The augmentation of MEPs in the premovement period was more pronounced in single pulse TMS. The parallel action of intracortical facilitation (ICF) and ICI as well as different dynamic behaviour of ICF and premovement facilitation may explain the above mentioned effects.This leads to an extended description of the rather subtle TMS influence on RT.