DELAY IN SIMPLE REACTION-TIME AFTER FOCAL TRANSCRANIAL MAGNETIC STIMULATION OF THE HUMAN BRAIN OCCURS AT THE FINAL MOTOR OUTPUT STAGE

It is known that the execution of the motor response in a simple react ion time (RT) task can be delayed by transcranial magnetic stimulation (TMS). This paper is aimed at determining the site of action where th e delay in RT occurs. A delay in RT was obtained only at those TMS sit es over the motor cortex contralateral to the responding hand, which p roduced also a muscle twitch in the responding hand. The delay in RT c ovaried with the TMS intensity and increased the closer the time of TM S approached the expected time of reaction onset. Visual and auditory go-signals yielded similar delays in RT, but only when TMS was applied about 40 ms later for the visual go-signal, corresponding to the moda lity specific difference in RT control values. TMS of the supplementar y motor area (SMA) immediately prior to the expected time of reaction onset produced no delay in RT. Spinal excitability as tested by F wave s showed a pre-movement facilitation in the control. trials which cont inued seemingly undisturbed during the period of RT delay after TMS. I t can be concluded that the delay in RT is not due to SMA stimulation or spinal inhibition but depends on effective stimulation of neural el ements in the motor cortex which are active very late in the process o f movement release from the final motor output stage.