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 Parkinsonʹs disease. Results: On 9/11 sides tested, single stimuli elicited a negative potential with latency of 3 ms which was largest over the frontal region. Its short chronaxie (50 μs) 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 5 ms (6/11 sides) and 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.