Potentials recorded at the scalp by stimulation near the human subthalamic nucleus

OBJECTIVE To record the potentials evoked at the scalp by stimulation through electrodes targeted at the human subthalamic nucleus (STN) and to determine whether the responsible pathways continue to be excited or become blocked with high frequency stimulation. METHODS We recorded the potentials evoked at the scalp in response to single and multiple stimuli delivered through STN contacts in 6 patients with Parkinson's disease. RESULTS On 9/11 sides tested, single stimuli elicited a negative potential with latency of approximately 3 ms which was largest over the frontal region. Its short chronaxie (50 micros) and refractory period imply that it arose from the activation of low threshold neural elements, possibly myelinated axons. This potential could follow at 100 Hz. This early potential was sometimes followed by later negative potentials at approximately 5 ms (6/11 sides) and approximately 8 ms (8/11 sides). The responsible neural elements had the same short chronaxie. These potentials were augmented by paired stimuli at separations of 2-7 ms and by trains of stimuli at 200 Hz. CONCLUSIONS Trains of stimuli delivered to the STN may excite low threshold neural elements which can transmit impulses at frequencies >100 Hz without blocking and which may produce postsynaptic facilitation at the cortex.