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Colloquium on Neuroimaging of Human Brain Function
programs or sets of programs in a way different from naive performance (8, 29). In this light, the idea that unskilled and skilled performance of a task in some sense represents performance of “different tasks” in a neurobiological sense maps on to our intuitions. The idea that different brain regions might then be involved does not seem so far-fetched.
Other conceptual distinctions can be made within this framework. In one scenario, both “scaffolding” and “storage” areas are active in parallel at all stages of learning, and what switches with practice is the balance of activity between the pathways. In the other scenario, one set of areas is essentially or exclusively active early, and when the task is overlearned passes the activity necessary to the performance of the task to other areas. In neither of our experiments does the activity in the “scaffolding regions” ever completely disappear, seeming more consistent with the parallel activity idea and the notions that control gradually shifts from scaffolding to storage areas and that both areas may contribute to the task, especially during intermediate skill levels. However, in neither experiment is the task truly overlearned (task performance does not reach asymptote for an extended period of time with only 10 min of practice). On the other hand, some evidence does exist in the verbal learning case for the complete transfer idea. In that case, simple reading (the overlearned control) actually appears to inhibit some of the verb generation “scaffolding” areas. At this point, either type of explanation seems plausible, and it may be that each is most relevant for different learning tasks.
Another interesting issue is the identification of specific processes represented in the scaffolding and storage regions. For this issue as well, many alternative explanations exist. For example, left frontal opercular activation at or near that seen in the verb generation task has been variously attributed to episodic encoding (30), (lexical) retrieval (31), semantic processing (12, 32, 33), willed generation (34), working memory for verbal material (35, 36), high-level phonological processing (17, 37–39), etc. On the storage side, the insular activation in the verbal learning task, and the SMA activation in the maze-learning task, may represent regions where specific information for the performance of the task is stored. SMA might be a likely candidate for the storage of the sequential (40, 41) and/or temporal aspects of a motor sequence (22, 23). Alternatively, SMA and insula may represent regions controlling access to that information, which might be stored elsewhere.
We believe that the observations of change in functional anatomy through practice provide an interesting foundation for understanding processing distinctions in learning. As can be seen from the incomplete outline of remaining issues, there is still much to be learned about the functional anatomy of skill learning.
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