J Neurophysiol 98:3516-3524.
abstract
Humans routinely make movements to targets that have different accuracy
requirements in different directions. Examples extend from everyday
occurrences such as grasping the handle of a coffee cup to the more
refined instance of a surgeon positioning a scalpel. The attainment
of accuracy in situations such as these might be related to the
nervous system's capacity to regulate the limb's resistance to
displacement, or impedance. To test this idea, subjects made
movements from random starting locations to targets that had
shape-dependent accuracy requirements. We used a robotic device to
assess both limb impedance and patterns of movement variability just
as the subject reached the target. We show that impedance increases
in directions where required accuracy is high. Independent of target
shape, patterns of limb stiffness are seen
to predict spatial patterns of movement variability. The nervous
system is thus seen to modulate limb impedance in entirely
predictable environments to aid in the attainment of reaching
accuracy.
Address for reprint requests and other correspondence: D. J. Ostry, Department of Psychology, McGill University, 1205 Dr. Penfield Ave., Montreal, QC, Canada H3A 1B1 (E-mail: ostry@motion.psych.mcgill.ca)