H215O will be intravenously injected in the arm contralateral to the hand movement. PET measurements will be performed with an EXACT HR+ tomograph (CTI/Siemens, Knoxville, TN, USA) allowing the simultaneous 3D acquisition of 63 transaxial slices. Spatial resolution after reconstruction reached 4.5 and 4.1 mm in the transaxial and axial direction, respectively {Bendriem, 1996 #39}(19). To measure rCBF, 300 MBq of H215O will be administered for each 80-second emission scan. To allow complete decay of injected tracer activity, image acquisitions will be performed 10 minutes apart.