Mapping of cortical sites where transcranial magnetic stimulation results in delay of voluntary movement

Transcranial magnetic stimulation over the motor cortex has been shown to delay voluntary movement in man in a reaction time paradigm. In this study, we used a focal magnetic coil to stimulate 16 sites over the left hemisphere in 8 normal subjects. We measured the delay that occurred in the onset of a ballistic wrist flexion response and the size of the motor potentials evoked in the wrist flexors by stimulation at each site. The optimal site of stimulation to delay voluntary movement coincided with the optimal site to elicit an MEP in the agonist (flexor) muscle. However, MEPs were also obtained from some sites at which no delay in movement was induced.