Effects of internal globus pallidus stimulation on motor cortex excitability

Background: Deep brain stimulation is a promising treatment for PD, but its physiologic effects and mechanisms of action remain poorly understood. Magnetic stimulation studies have revealed abnormalities in several different excitatory and inhibitory circuits in the motor cortex in PD. Methods: The physiologic effects of internal globus pallidus (GPi) stimulation in seven patients with PD and seven age-matched healthy volunteers were studied. The stimulators were set at the optimal parameters (ON), at half the optimal amplitude (Half-Amp), or switched off (OFF) in random order. Patients were taking their usual medications. Magnetic stimulation was applied to the motor cortex, and motor evoked potentials (MEP) were recorded from the contralateral first dorsal interosseous muscle. Several excitatory and inhibitory pathways that have been found to be abnormal in PD were tested. Results: The motor threshold (MT), MEP recruitment curve (stimulus intensities from 100 to 150% of MT), short and long interval intracortical inhibition, and intracortical facilitation were similar in the three stimulator conditions tested both at rest and during voluntary contraction. The silent period (SP) was longer in the OFF and Half-Amp conditions than in normal control subjects. In the stimulator ON condition, the SP was significantly reduced compared with the OFF condition and became similar to that in normal control subjects. Conclusions: GPi stimulation while on dopaminergic medications reduced the SP following magnetic stimulation but did not change corticospinal excitability or other measures of intracortical inhibition and facilitation. The reduction of SP may be related to the antidyskinetic and levodopa-blocking effects of ventral GPi stimulation.