Motor Task Difficulty and Brain Activity: An Investigation of Goal-Directed Reciprocal Aiming Using Positron Emission Tomography (PET). Winstein, Carolee J., Scott T. Grafton, Patricia S. Pohl. Departments of Biokinesiology and Neurology and the PET Imaging Science Center, University of Southern California, Los Angeles, CA 90033.
APStracts 3:0249N, 1996.
ABSTRACT
Differences in the kinematics and in the pattern of relative regional cerebral blood flow (rCBF) during unimanual reciprocal aiming using a Fitts' task with three difficulty conditions and two aiming types were investigated in healthy right-handers with video-based movement analysis and positron emission tomography (PET). Movement time (MT) and kinematic characteristics were analyzed together with the magnitude of CBF to identify areas of brain activity proportionate to task and movement variables. Differences in rCBF between task conditions were determined by ANOVA with planned comparisons of means using group mean weighted linear contrasts. Data were first analyzed for the group; then, individual subject differences were related to each subjects' anatomy by magnetic resonance imaging. Significant differences in rCBF during aiming compared to no-movement were found in a mosaic of well known cortical and sub-cortical areas. As task difficulty increased, rCBF increased in areas associated with the planning of complex movements requiring visuomotor processing. These included bilateral occipital, left inferior parietal, left dorsal cingulate--caudal SMA-proper and right dorsal premotor areas. Analyses of individual subject differences revealed a correspondence between the spatial extent of rCBF changes with task difficulty and the individuals' MTs. As task difficulty decreased, significant increases in rCBF were evident in right anterior cerebellum, left middle occipital gyrus, and right ventral premotor areas. Functionally, these areas are associated with aiming where the motor execution demands are high and precise trajectory planning is minimal. A functional dissociation resulted between larger (limb transport) or smaller (endpoint targeting) type amplitude/target width aiming conditions. Areas with significantly greater rCBF for targeting were the left motor cortex, left intraparietal sulcus, and left caudate. In contrast, those areas with greater rCBF associated with limb transport included bilateral occipital lingual gyri and right anterior cerebellum. Numerous theoretical explanations for the speed/accuracy tradeoffs of rapid aiming movements have been proposed since the original information-theory of Fitts (1954). This is the first report to relate the predictable variations in motor control under changing task constraints with the functional anatomy of these goal-directed movements. Differences in unimanual aiming task difficulty lead to dissociable activation of cortical-sub-cortical networks. Further, these data suggest that when more precise targeting is required, independent of task difficulty, a motor cortical-subcortical loop is activated. This supports the role of motor cortex in controlling movement direction based on population encoding.

Received 19 April 1996; accepted in final form 1 November 1996.
APS Manuscript Number J326-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 31 December 1996