ABSTRACT AND EFFECTOR-SPECIFIC REPRESENTATIONS OF MOTOR SEQUENCES IDENTIFIED WITH PET

Positron emission tomography was used to identify neural systems invol ved in the acquisition and expression of sequential movements produced by different effecters. Subjects were tested on the serial reaction t ime task under implicit learning conditions. In the initial acquisitio n phase, subjects responded to the stimuli with keypresses using the f our fingers of the right hand. During this phase, the stimuli followed a fixed sequence for one group of subjects (group A) and were randoml y selected for another group (group B). In the transfer phase, arm mov ements were used to press keys on a substantially larger keyboard, and for both groups, the stimuli followed the sequence. Behavioral indice s provided clear evidence of learning during the acquisition phase for group A and transfer when switched to the large keyboard. Sequence ac quisition was associated with learning-related increases in regional c erebral blood flow (rCBF) in a network of areas in the contralateral l eft hemisphere, including sensorimotor cortex, supplementary motor are a, and rostral inferior parietal cortex. After transfer, activity in i nferior parietal cortex remained high, suggesting that this area had e ncoded the sequence at an abstract level independent of the particular effecters used to perform the task. In contrast, activity in sensorim otor cortex shifted to a move dorsal locus, consistent with motor cort ex somatotopy. Thus, activity here was effector-specific. An increase in rCBF was also observed in the cingulate motor area at transfer, sug gesting a role linking the abstract sequential representations with th e task-relevant effector system. These results highlight a network of areas involved in sequence encoding and retrieval.