RCBF in three deferent stimulations settings will be compared. PET will be performed before and one hour after changes in the activity status of the pacemakers. Four weeks prior to the first scan, patients are informed to switch both neurostimulator off. PET will be performed with the pacemaker settings: OFF/OFF (Stimulation status of left/right pacemaker), On/Off and On/On. The order of the On/OFF and On/On period will be random. Unilateral stimulation will be performed with the pacemaker there has the best efficacy, determined prior to study enrolment. The interval between these three pacemaker settings is four weeks. Four weeks prior to each PET scan the patients fill in bowel habit diaries, bowel habit questionnaires and quality of life assessments.