The surgical procedure involved the bilateral implantation of deep brain stimulation electrodes each with four contacts (model 3389, Medtronic, Minneapolis, MN, USA). In one patient implantation was carried out unilaterally. In London, the intended target for the lowermost contact (contact zero) was determined on the stereotactic axial T2-weighted MRI scan lying at the level of the uppermost part of the mammillothalamic tract, which corresponds to the ventral part of the STN as visualised on the MRI (Hariz et al., 2003, Foltynie et al., 2011). An immediate post-implantation stereotactic T2-weighted MRI was performed to verify electrode location (see Supplementary Fig. S1). Electrodes were connected to an accessory kit, typically both connectors being tunnelled to the left temporoparietal area and externalised through the frontal region. For more details see Litvak et al. (2011a). In Oxford, the targeting was the same but was performed on a pre-operative volume-based MRI using T2-weighted and FLAIR sequences to target the dorsolateral STN at the level of the red nucleus with advancement of the electrode 3–5 mm caudally. The MRI was fused to an intraoperative stereotactic CT head scan to obtain the coordinates and a post-operative check CT scan was also fused to the pre-operative MRI.
