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.
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