Motor cortex excitability changes preceding voluntary muscle activity in simple reaction time task

The effect of transcranial magnetic stimulation (TMS) on reaction time (RT) and motor cortex excitability in the premovement period was investigated. Single and paired-pulse TMS with 3 and 13 ms interstimulus intervals (ISI) were applied to the left motor cortex at different delays after a visual command for isometric right hand index finger abduction. Motor evoked potentials (MEPs) recorded from the right first dorsal interosseous (FDI) were analysed to assess cortex excitability. The MEPs in response to single pulse TMS were gradually increased in the premovement period in general, but strongly augmented in a period of 90-100 ms before the voluntary myoelectrical activity (EMG) onset. In paired-pulse TMS (13 ms ISI), the MEP augmentation was smaller but started earlier before the EMG burst, and a gradual increase of MEP amplitudes was not evident. In case of 3 ms ISI, 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. Generally, the augmentation of MEPs in the premovement period was more pronounced in single pulse TMS. Most strikingly, a dead band period without MEP occurrences was observed within an interval of 30-50 ms before the voluntary EMG. In conclusion, parallel action of intracortical facilitation (ICF) and ICI as well as different dynamic behaviour of ICF and pre movement facilitation may explain the earlier mentioned effects. Moreover, this leads to an extended description of the rather subtle TMS influence on RT.