MOTOR TASK-DIFFICULTY AND BRAIN ACTIVITY - INVESTIGATION OF GOAL-DIRECTED RECIPROCAL AIMING USING POSITRON EMISSION TOMOGRAPHY

Differences in the kinematics and pattern of relative regional cerebra l blood flow (rCBF) during goal-directed arm aiming were investigated with the use of a Fitts continuous aiming paradigm with three difficul ty conditions (index of difficulty, ID) and two aiming types (transpor t vs. targeting) in six healthy right-handed young participants with t he use of video-based movement trajectory analysis and positron emissi on tomography. Movement time and kinematic characteristics were analyz ed together with the magnitude of cerebral blood flow to identify area s of brain activity proportionate to task and movement variables. Sign ificant differences in rCBF between task conditions were determined by analysis of variance with planned comparisons of means with the use o f group mean weighted linear contrasts. Data were first analyzed for t he group. Then individual subject differences for the movement versus no movement and task difficulty comparisons were related to each indiv idual subjects' anatomy by magnetic resonance imaging. Significant dif ferences in rCBF during reciprocal aiming compared with no-movement co nditions were found in a mosaic of well-known cortical and subcortical areas associated with the planning and execution of goal-directed mov ements. These included cortical areas in the left sensorimotor, dorsal premotor, and ventral premotor cortices, caudal supplementary motor a rea (SMA) proper, and parietal cortex, and subcortical areas in the le ft putamen, globus pallidus, red nucleus, thalamus, and anterior cereb ellum. As aiming task difficulty (ID) increased, rCBF increased in are as associated with the planning of more complex movements requiring gr eater visuomotor processing. These included bilateral occipital, left inferior parietal, and left dorsal cingulate cortices-caudal SMA prope r and right dorsal premotor area. These same areas showed significant increases or decreases, respectively, when contrast means were compare d with the use of movement time or relative acceleration time, respect ively, as the weighting factor. Analysis of individual subject differe nces revealed a correspondence between the spatial extent of rCBF chan ges as a function of task ID and the individuals' movement times. As t ask ID decreased, significant increases in rCBF were evident in the ri ght anterior cerebellum, left middle occipital gyrus, and right ventra l premotor area. Functionally, these areas are associated with aiming conditions in which the motor execution demands are high (i.e., coordi nation of rapid reversals) and precise trajectory planning is minimal. These same areas showed significant increases or decreases, respectiv ely, when contrast means were compared with the use of movement time o r relative acceleration time, respectively, as the weighting factor. A functional dissociation resulted from the weighted linear contrasts b etween larger (limb transport) or smaller (endpoint targeting) type am plitude/target width aiming conditions. Areas with significantly great er rCBF for targeting were the left motor cortex, left intraparietal s ulcus, and left caudate. In contrast, those areas with greater rCBF as sociated with limb transport included bilateral occipital lingual gyri and the right anterior cerebellum. Various theoretical explanations f or the speed/accuracy tradeoffs of rapid aiming movements have been pr oposed since the original information theory hypothesis of Fitts. This is the first report to relate the predictable variations in motor con trol under changing task constraints with the functional anatomy of th ese rapid goal-directed aiming movements. Differences in unimanual aim ing task difficulty lead to dissociable activation of cortical-subcort ical networks. Further, these data suggest that when more precise targ eting is required, independent of task difficulty, a cortical-subcorti cal loop composed of the contralateral motor cortex, intraparietal sul cus, and caudate is activated. This is consistent with the role of mot or cortex for controlling direction of movement on the basis of popula tion encoding.