 Regional cerebral blood flow rCBF will be measured on a Siemens ECAT HR camera. Attenuation-corrected data are reconstructed into 63 slices, with a resulting resolution of 4.5 mm full-width-at-half-maximum. Fifteen seconds before each scan, 7 mCi of H215O is administrated by bolus injection. Data are collected during 80 s. All rCBF examinations will be performed at rest during sleep induced by premedication 4 mgkg of pentobarbital. We have previously reported that this premedication has no effects on absolute rCBF values and regional distribution. 