Considerable progress has been made on this project in the past reporting year. The first part of this project involved the role of the motor parts of the frontal lobe in skill learning, particularly the transfer of skills learned with one hand to the other hand. In collaboration with Leonardo Cohen and his colleagues in NINDS, we found with functional brain imaging that the supplementary motor area (SMA) and its principal thalamic nucleus have more activity when a skill transfers well compared to when it transfers poorly. Furthermore, using repetitive transcranial magnetic stimulation during skill learning, we found that disrupting neural processing in the SMA blocks such transfer. These findings, reported in the journal Current Biology, provide direct evidence for an SMA-based mechanism that supports intermanual transfer of motor skill learning. Further work, accepted for publication in the Journal of Neuroscience, studied the timing of this contribution relative to movement. In this aspect of the project, people learned a 12-item sequence, practiced with their right hand. The SMA's contribution to the transfer of this skill was blocked only when repetitive transcranial magnetic stimulation occurred during the period between movements, when the memory of a prior movement contributes to the encoding of specific sequences. These results provide insight into frontal lobe contributions to procedural knowledge. In collaboration with a psychologist from the University of Virginia, Daniel Willingham, we also reviewed the field's current understanding about the learning of such sequential-movement skills. In this article for the New Encyclopedia of Neuroscience, we explained that the motor system learns how the body interacts with the world and uses this knowledge to plan movements and produce the forces needed to reach targets. It does so, in part, by learning to correct both previous and ongoing errors. The motor parts of the frontal lobe, in concert with the cerebellum, correct errors made on previous movements, and they act in concert with the basal ganglia to correct ongoing movements. We also made progress in understanding the development of the frontal lobe, both in relation to other cortical areas and in relation to its evolutionary history. In collaboration with Judith Rapoport, W. Philip Shaw and their colleagues in the Child Psychiatry Branch of NIMH, we described regional development of cortical thickness. We found that the regional patterning of cortical growth aligned with aspects of architectonic maps, putting these brain maps in a novel, developmental perspective. Polysensory and high-order areas of cortex, which are the most complex areas in terms of their laminar structure, had the most complex developmental patterns, as well. Structurally less complex cortical regions, including many limbic areas, showed simpler growth patterns. From a comparative perspective, many of the areas with relatively simple developmental patterns have clearly identified homologues in all mammalian brains and thus likely evolved in early mammals. By contrast, all of the regions innovated or dramatically expanded in primates, such as the granular prefrontal cortex and high-order sensory areas such as the inferotemporal cortex, have complex developmental trajectories. In collaboration with a neuroimager at the University of California (Los Angeles), Russel Poldrak, we wrote a second article for the New Encyclopedia of Neuroscience. This review advanced the idea that the basal ganglia resolves the selection demands that confront behaving organisms. This function can be viewed as an aspect of error reduction, as described above for basal ganglia, and it requires correctly prioritizing, scheduling, planning, and sequencing behaviors, as well as controlling the way in which context elicits the correct behavior for a given circumstance. Finally, work on this project included a new hypothesis about the function of the prefrontal areas, in the context of its evolutionary history, to be published in Trends in Neuroscience (TINS). Contrary to some views of the frontal lobe, the largest part of the primate prefrontal cortex has no homologue in other mammals. My TINS article proposes that the newly evolved prefrontal cortex encodes, represents and stores knowledge about behaviors, including its consequences. This project is coming to a conclusion some time during Fiscal Year 2009.
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