 Randomisation between two treatment locations each programmed up to 3 milliamps in amplitude for 3 months: 1 caudal zona incerta and 2 dorsal zona incerta. This 6-month-long randomised phase is followed by 6 months of unblinded individualised empirically optimised settings programmed by a neurologist. Each of the three treatment periods ends with a full clinical, functional and quality of life assessment. 