The brain localization of motor sequence learning was studied in normal subjects with positron emission tomography. Subjects performed a serial reaction time (SRT) task by responding to a series of stimuli that occurred at four different spatial positions. The stimulus locations were either determined randomly or according to a 6-element sequence that cycled continuously. The SRT task was performed under two conditions. With attentional interference from a secondary counting task there was no development of awareness of the sequence. Learning-related increases of cerebral blood flow were located in contralateral motor effector areas including motor cortex, supplementary motor area, and putamen, consistent with the hypothesis that nondeclarative motor learning occurs in cerebral areas that control limb movements. Additional cortical sites included the rostral prefrontal cortex and parietal cortex. The SRT learning task was then repeated with a new sequence and no attentional interference. In this condition, 7 of 12 subjects developed awareness of the sequence. Learning-related blood flow increases were present in right dorsolateral prefrontal cortex, right premotor cortex, right ventral putamen, and biparieto-occipital cortex. The right dorsolateral prefrontal and parietal areas have been previously implicated in spatial working memory and right prefrontal cortex is also implicated in retrieval tasks of verbal episodic memory. Awareness of the sequence at the end of learning was associated with greater activity in bilateral parietal, superior temporal, and right premotor cortex. Motor learning can take place in different cerebral areas, contingent on the attentional demands of the task.

The authors theorize that 2 neurocognitive sequence-learning systems can be distinguished in serial reaction time experiments, one dorsal (parietal and supplementary motor cortex) and the other ventral (temporal and lateral prefrontal cortex). Dorsal system learning is implicit and associates noncategorized stimuli within dimensional modules. Ventral system learning can be implicit or explicit It also allows associating events across dimensions and therefore is the basis of cross-task integration or interference, depending on degree of cross-task correlation of signals. Accordingly, lack of correlation rather than limited capacity is responsible for dual-task effects on learning. The theory is relevant to issues of attentional effects on learning; the representational basis of complex, sequential skills; hippocampal-versus basal ganglia-based learning; procedural versus declarative memory; and implicit versus explicit memory.

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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.

E. H. Schumacher, T. L. Seymour, J. M. Glass, D. E. Kieras, and D. E. Meyer (2001) reported that dual-task costs are minimal when participants are practiced and give the 2 tasks equal emphasis. The present research examined whether such findings are compatible with the operation of an efficient response selection bottleneck. Participants trained until they were able to perform both tasks simultaneously without interference. Novel stimulus pairs produced no reaction time costs, arguing against the development of compound stimulus-response associations (Experiment 1). Manipulating the relative onsets (Experiments 2 and 4) and durations (Experiments 3 and 4) of response selection processes did not lead to dual-task costs. The results indicate that the 2 tasks did not share a bottleneck after practice.

Whereas the human right hemisphere is active during execution of contralateral hand movements, the left hemisphere is engaged for both contra- and ipsilateral movements, at least for right-handed subjects. Whether this asymmetry is also found during motor learning remains unknown. Implicit sequence learning by the nondominant left hand was examined with the serial reaction time (SRT) task during functional brain imaging. As learning progressed, increases in brain activity were observed in left lateral premotor cortex (PMC) and bilaterally in supplementary motor areas (SMA), with the increase significantly greater in the left hemisphere. The left SMA site was similar to one previously identified with right-hand learning, suggesting that this region is critical for representing a sequence independent of effector. Learning with the left hand also recruited a widespread set of temporal and frontal regions, suggesting that motor skill learning with the nondominant hand develops within both cognitive and motor-related functional networks. After skill acquisition, subjects performed the SRT task with their right hands, and sequence transfer was tested with the original and a mirror-ordered sequence. With the original sequence, the stimulus sequence and series of response locations remained unchanged, but the finger movements were different. With the mirror-ordered sequence, the response sequence involved finger movements homologous to those used during training. Performance of the original and mirror sequence by the right hand was significantly better than with random stimuli. Mirror transformation of the sequence by the right hand was associated with a marked increase in regional activity in the left motor cortex, consistent with a role for sequential transformation at this level of the motor output pathway.

Positron emission tomography was used to identify neural systems involved in the acquisition and expression of sequential movements produced by different effectors. Subjects were tested on the serial reaction time task under implicit learning conditions. In the initial acquisition phase, subjects responded to the stimuli with keypresses using the four fingers of the right hand. During this phase, the stimuli followed a fixed sequence for one group of subjects (group A) and were randomly selected for another group (group B). In the transfer phase, arm movements were used to press keys on a substantially larger keyboard, and for both groups, the stimuli followed the sequence. Behavioral indices provided clear evidence of learning during the acquisition phase for group A and transfer when switched to the large keyboard. Sequence acquisition was associated with learning-related increases in regional cerebral blood flow (rCBF) in a network of areas in the contralateral left hemisphere, including sensorimotor cortex, supplementary motor area, and rostral inferior parietal cortex. After transfer, activity in inferior parietal cortex remained high, suggesting that this area had encoded the sequence at an abstract level independent of the particular effectors used to perform the task. In contrast, activity in sensorimotor cortex shifted to a more dorsal locus, consistent with motor cortex somatotopy. Thus, activity here was effector-specific. An increase in rCBF was also observed in the cingulate motor area at transfer, suggesting a role linking the abstract sequential representations with the task-relevant effector system. These results highlight a network of areas involved in sequence encoding and retrieval.

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