Patients started to complete experimental tasks in the gait laboratory 3–5 days after electrode implantation. All anti-parkinsonian medication was stopped at least 12 h, and stimulation was stopped 2 h before all recordings. Motor tasks were conducted under three conditions: no-stimulation, high-frequency stimulation (HFS, 130 Hz), and low-frequency stimulation (LFS, 60 Hz). The no-stimulation condition was always tested first, with the order of HFS and LFS being randomly counterbalanced across patients (HFS first in nine patients, LFS first in seven patients). A 30–60 min wash-in period was set to prepare patients for the upcoming tasks conducted in stimulation conditions. All subjects were blinded to their stimulation parameters during the experiment. We used a portable analogue stimulator (T901, Pins Medical) to deliver square biphasic pulses in a bipolar configuration. Stimulation bandwidth was always set to 60 μs. Stimulation voltage was optimized according to the patient’s feedback on motor improvement and the results of simplified motor test batteries.
