Neurophysiological effects of stimulation through electrodes in the human subthalamic nucleus

The effects of stimulation through macroelectrodes implanted in the subthal amic nucleus (STN) were studied in 14 patients with parkinsonism, Single st imuli delivered directly to the STN electrodes with an external stimulator modulated voluntary electromyography (EMG) of contralateral muscles in most patients. A short-latency facilitation ('peak') was attributed to the acti vation of the corticospinal system. A longer latency inhibition ('dip'), of ten preceded or followed by facilitations, appeared to arise from the activ ation of large-diameter fibres running parallel to the electrode and to be transmitted through the motor cortex. It is possible that the dip could res ult from the inhibition of thalamocortical neurons. With high-frequency sti mulation (similar to 100 Hz) the peaks occurred at the stimulus frequency; the dips became confluent and outlasted the duration of the stimulus train. There was no evidence that high-frequency stimulation produced 'blocking', The studies were repeated in 12 patients a mean of 5.8 months after implan tation of the stimulator. The same short-latency effects were obtained, The y were present on 7 out of 23 sides at the settings in use and on the major ity of sides if the stimulus intensity was slightly increased. There was no clear relationship between these short-latency effects and the patients' o verall clinical improvement; the effects may result from the spread of curr ent to large-fibre systems near the STN, In five patients, high-frequency s timulation on one side immediately reduced tremor in the contralateral limb s. This effect arose from the activation of large-diameter fibres and, like the dip, had about the same threshold at each of the contacts. Frequencies as low as 70 Hz were sufficient. We conclude that the control of tremor by STN stimulation is due to the activation of a large-fibre system.