We performed cortical stimulation of primary motor cortex via subdural grid electrodes in 3 patients who had been admitted to the epilepsy monitoring unit as part of their routine presurgical cortical mapping.

Bipolar stimulation at variable frequencies of 1Hz, 5Hz, 10Hz, 20Hz and 50Hz was used. At each frequency of stimulation, the current intensity was increased from 1mA to a maximum of 15mA.

In all patients, the extremity involved in the clonic movement was monitored via surface EMG electrodes, with one pair placed on agonists and antagonists each.

1) The EMG bursts in agonists and antagonists are always synchronized with comparable amplitudes and durations. 

2) The latency to first motor response decreases with increasing current intensity.

3) Increasing the intensity at the same frequency of stimulation converts clonic movement into a sustained tonic contraction.