The patients will be randomized to receive one of 3 treatment arms : rTMS of the motor cortex, rTMS of the prefrontal cortex, or placebo (sham stimulation) of the motor or prefrontal cortex. The study will be double blind, eg the patient and the investigator will not know the nature of treatment. the stimulation protocol will consist of an induction phasse of 5 daily sessions then a maintenance phase of several sessions : 3 sessions a week apart, 3 sessions a fortnight apart, and 3 sessions 3 weeks apart.