Patients who are included in this study will have four extra study visits at the University Hospital Brussels. The first assessment will take place during the week prior to surgery, the second minimal one month after definitive implantation of the stimulator. The assessments consist of filling in the Pittsburgh Sleep Quality Index (PSQI) and the Pain Catastrophizing Scale (PCS) and recording brain activity before and during the pain protocol. Additionally, patients will wear an Actiwatch for measuring sleep patterns and fill in a VAS diary (back and leg pain separately), one month before the baseline assessment and one month before the second assessment.