October 16, 1996

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A region of the brain believed important for maintaining the calibration between visual and motor systems necessary for accurate eye-hand coordination has been identified in human subjects, report researchers from Emory University and the University of California, Los Angeles, in today's Nature.

The subjects were attempting to recalibrate their reaching to compensate for visual displacements that resulted from viewing the world through prism goggles. The brain region was located within a specialized area of posterior parietal cortex (area PEG) in the hemisphere opposite the reaching hand.

"Presumably, recalibration of eye-hand coordination takes place continuously throughout our lives, as our limbs and visual systems grow and age, and as we learn to deal with commonplace sensory distortions such as using a mirror to guide certain movements or adjusting to the minor spatial distortions caused by a new pair of glasses," says senior author Garrett E. Alexander, M.D., Ph.D., professor of Neurology at the Emory University School of Medicine.

The process of recalibration associated with adapting to prisms was pinpointed using positron emission tomography (PET) at the Emory PET Center, to image patterns of brain activation (reflected by changes in regional blood flow) that occurred as subjects performed two behavioral tasks. In one task, subjects attempted to reach to visual targets viewed through displacing prisms. In the other, they made reaching movements identical to those of the prism task (including error corrections caused by displacing the target while the subject was in midreach), but there were no prisms to distort their vision. Subtracting the second set of images from the first showed the net effects of the recalibration (prism adaptation) process, and revealed the selective activation of area PEG. This region of posterior parietal cortex is known to contain both visual and somatosensory maps, making it a plausible candidate for subserving the process of visuomotor recalibration.

Moreover, patients in whom this region has been damaged by strokes or tumors show impairments of hand-eye coordination when they attempt to make visually guided reaches with the opposite hand.

Dr. Alexander's collaborators include Dottie M. Clower, B.A., graduate student in the Neurosciences program and Department of Neurology at Emory; John M. Hoffman, M.D., associate professor, Departments of Neurology and Radiology at Emory; John R. Votaw, Ph.D., assistant professor, and Tracy L. Faber, Ph.D., assistant professors, Department of Radiology at Emory; and Roger P. Woods, M.D., assistant professor, Division of Brain Mapping, UCLA School of Medicine.

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