The Section on Neurophysiology studies two major, related problems: the specialized functions of the frontal cortex and the neural network underlying symbolically guided actions. Much of our research involves role of the frontal cortex in the mapping of symbols to actions, which, although not always recognized as an important issue in mental health research, comprises a fundamental feature of mental life. Diseases such as schizophrenia, attention deficit-hyperactivity disorder, obsessive-compulsive disorder, and others may result from inappropriate selection and control of actions. The work on this project has shown that context-appropriate action depends upon the proper function of specific parts of the frontal cortex and specific additional parts of the brain. Our research has been directed at understanding symbolically guided actions in health, so that physicians, therapists, and other health care specialists can one day improve treatments for people who make inappropriate decisions. An important aspect of our work focuses on what is called symbolic mapping behavior. In symbolic mapping, the choice of an action to be made depends on the behavioral context provided by a symbol. This is the basis for learning the meaning of most words, for learning to associate that meaning with the motor programs necessary to generate speech and language, and for the wide variety of symbol-guided behavior that underlies much of our higher-order behavior generally. In the past year, the highlights of our research include: (1) demonstrating an important physiological distinction between premotor cortex and prefrontal cortex. We found neurons that encode the relative location of objects. Those in prefrontal cortex reflect whether the monkey is fixating the leftmost or rightmost of two objects. Cells in dorsal premotor cortex, on the other hand, do not reflect that kind of relative location, but instead encode the leftmost or rightmost target of future reaching movements or attention; (2) showing that cells in dorsolateral prefrontal cortex encode spatial memory in much smaller numbers and with much weaker representational reliability than those encoding spatial attention; (3) the first empirical test of the theory that cortex and basal ganglia function in distributed modular architectures, commonly known as 'loops'. Support for this theory was obtained by comparing learning-related changes in neuronal activity as monkeys solved novel symbolic guidance problems; and (4) demonstrating that the hippocampal system functions more generally in symbolically guided action than previously believed. Lesion of the fornix, a major fiber pathway of the hippocampal system, causes deficit in symbolically guided actions even when neither the symbol nor the action involves spatial information processing. The deficit was long-lasting and significantly impaired one-trial learning.
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