We show using positron emission tomography in normal volunteers that for fo
ur tasks involving motor preparation, regional cerebral blood flow in the i
psilateral cerebellum is negatively correlated with reaction time. Each of
the tasks presented subjects with different amounts of advanced information
(from none to partial to full information) prior to a signal. to move one
of two possible fingers in one of two possible directions. The cerebellum w
as the only brain area that was correlated with reaction time in all the co
nditions. These results are compatible with the idea that the cerebellum pl
ays an important role in the preparation and initiation of motion.