Attention and stimulus characteristics determine the locus of motor-sequence encoding. A PET study.

PET revealed the effects of stimulus characteristics on the neural substrate of motor learning. Right-handed subjects performed a serial reaction time task with colour-coded stimuli to eliminate the potential for learned eye-movements. The task was performed with the right hand under two different conditions. In one condition, subjects simultaneously performed a distractor task. Although they did show behavioural evidence of learning, they were not explicitly aware of the stimulus-response sequence. In the second condition, there was no distractor task, and seven out of the 11 subjects then became explicitly aware of the stimulus sequence. Metabolic correlates of learning were distinct in the two conditions. When learning was implicit under dual-task conditions, learning-related changes were observed in left motor and supplementary motor cortex as well as in the putamen. These regions are similar to those observed in a previous study in which the stimuli were cued by spatial position. Under single-task conditions, metabolic changes were found in the right prefrontal cortex and premotor cortex, as well as in the temporal lobe. A similar shift to the right hemisphere was observed in the spatial study during single-task learning. However, explicit learning of the task with colour stimuli activated more ventral regions. The areas supporting motor-sequence learning are contingent on both stimulus properties and attentional constraints.