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Human motor control and adaptation in cerebellar ataxia

People with damage to the cerebellum exhibit the neurological symptom of ataxia (poor movement coordination). As a result, their movements are generally characterized by directional errors, dysmetria (under- or over-shooting), and increased variability. This project seeks to improve our understanding of the role of the cerebellum in motor control of the arm through behavioral studies with the KINARM exoskeleton system (BKIN Technologies, Inc.). A better understanding of cerebellar function can inspire new rehabilitation therapies for this patient population.

Cerebellar motor learning: Effect of environment dynamics vs. error size

Cerebellar damage has been shown to impair sensorimotor adaptation, the learning of predictable perturbations via an error-based process. We studied the effect of specific dynamic perturbations (clockwise vs. counter-clockwise force field) and error size (abruptly vs. gradually introduced forces) on motor learning of a reaching movement in patients with cerebellar damage. We also examined the coordinate system (hand vs. joint space) in which learning generalized to different movements.

We were surprised to find that error size did not affect patients' adaptation (as was previously shown); instead, the direction of the force field determined their ability to learn. Patients performed best when the force field helped compensate for their arm dynamics during the reach. Our results and analysis suggest that that patients use a different mechanism than controls to improve their performance in the presence of forces that help counteract their movement biases.


T. L. Gibo
, S. E. Criscimagna-Hemminger, A. M. Okamura, and A. J. Bastian.
Cerebellar motor learning: Are environment dynamics more important than error size?
Journal of Neurophysiology, 110(2):322-333, 2013.

T. L. Gibo, A. M. Okamura, and A. J. Bastian.
Do cerebellar patients generalize after abrupt or gradual motor learning?
Program No. 825.13, 2011 Neuroscience Meeting Planner, Washington, DC: Society for Neuroscience, 2011. Online.

A. M. Hadjiosif, S. E. Criscimagna-Hemminger, T. L. Gibo, A. M. Okamura, R. Shadmehr, and M. A. Smith. Cerebellar damage reduces the stability of motor memories. Translational and Computational Motor Control, 2014

Modulation of arm stiffness during postural maintenance

Impedance control allows us to interact with our environment during postural and movement tasks, adjusting the mechanical behavior of our limbs to account for instability and noise. Young, healthy individuals have been shown to selectively control the endpoint stiffness of their arms via co-contraction, depending on the direction of environmental disturbances. Impedance modulation is hypothesized to be distinct from the central nervous system controller used for joint torque coordination. While the cerebellum is thought to be involved in joint torque control, the regions of the brain involved in impedance control are less understood.

We had subjects perform a static postural maintenance task under two types of directional force perturbations. We found an effect of both cerebellar damage and age on the ability of subjects to modify their arm stiffness between the two perturbation conditions. Thus, the cerebellum may be involved in optimally modulating arm impedance, although this appears to be affected by other factors such as age. Additionally, the cerebellar patients' intact ability to alter their limb stiffness during the postural task (albeit less than young, healthy controls) improved their movement performance in a subsequent tracing task. This transfer of stiffness control from a static to movement task may be a strategy that can be used to compensate for their motor deficits.

T. L. Gibo, A. J. Bastian, and A. M. Okamura.
Cerebellar ataxia impairs modulation of arm stiffness during postural maintenance.
Journal of Neurophysiology, 110(7):1611-1620, 2013.

T. L. Gibo, A. J. Bastian, and A. M. Okamura.
Effect of age on stiffness modulation during postural maintenance of the arm.
13th International Conference on Rehabilitation Robotics, 2013.