FRONTAL AND PARIETAL NETWORKS FOR CONDITIONAL MOTOR-LEARNING - A POSITRON EMISSION TOMOGRAPHY STUDY

Studies on nonhuman primates show that the premotor (PM) and prefronta l (PF) areas are necessary for the arbitrary mapping of a set of stimu li onto a set of responses. However, positron emission tomography (PET ) measurements of regional cerebral blood flow (rCBF) in human subject s have failed to reveal the predicted rCBF changes during such behavio r. We therefore studied rCBF while subjects learned two arbitrary mapp ing tasks. In the conditional motor task, visual stimuli instructed wh ich of four directions to move a joystick (with the right, dominant ha nd). In the evaluation task, subjects moved the joystick in a predeter mined direction to report whether an arrow pointed in the direction as sociated with a given stimulus. For both tasks there were three rules: for the nonspatial rule, the pattern within each stimulus determined the correct direction; for the spatial rule, the location of the stimu lus did so; and for the fixed-response rule, movement direction was co nstant regardless of the pattern or its location. For the nonspatial r ule, performance of the evaluation task led to a learning-related incr ease in rCBF in a caudal and ventral part of the premotor cortex (PMvc , area 6), bilaterally, as well as in the putamen and a cingulate moto r area (CM, area 24) of the left hemisphere. Decreases in rCBF were ob served in several areas: the left ventro-orbital prefrontal cortex (PF v, area 47/12), the left lateral cerebellar hemisphere, and, in the ri ght hemisphere, a dorsal and rostral aspect of PM (PMdr, area 6), dors al PF (PFd, area 9), and the posterior parietal cortex (area 39/40). D uring performance of the conditional motor task, there was only a decr ease in the parietal area. For the spatial rule, no rCBF change reache d significance for the evaluation task, but in the conditional motor t ask, a ventral and rostral premotor region (PMvr, area 6), the dorsola teral prefrontal cortex (PFdl, area 46), and the posterior parietal co rtex (area 39/40) showed decreasing rCBF during learning, all in the r ight hemisphere. These data confirm the predicted rCBF changes in prem otor and prefrontal areas during arbitrary mapping tasks and suggest t hat a broad frontoparietal network may show decreased synaptic activit y as arbitrary rules become more familiar.