The Role of Visual Feedback in Movement Control in Individuals with Multiple Sclerosis
In individuals with MS, upper extremity motor impairments, including intention tremor, can arise around the endpoint of a movement; when endpoint accuracy is not emphasized, tremor is reduced. We previously examined tremor using a series of continuous, compensatory tracking tasks. Our results suggest that individuals with MS exhibit significant changes in sensory feedback (lengthened visual delay) and prediction mechanisms (inaccurate estimation of visual delay and limb dynamics) when stabilizing the hand about an endpoint.
Use a continuous pursuit tracking task to examine whether previous results can be extended to a task where trajectory, and not endpoint accuracy, is emphasized.
Seven subjects with MS (ages 25-68, 5 women, 6 right-handed), and seven control subjects (ages 25-61, 5 women, 7 right-handed) completed several continuous compensatory and pursuit tracking tasks (0-10Hz, band-limited white noise). During the tasks, subjects used a 1-D robotic manipulandum controlled by flexion/extension of the right elbow to control a cursor on a computer monitor. Subjects were asked to visually track a target by placing the cursor on the target as quickly and accurately as possible. Subjects’ performance was fit to a dual-feedback model of sensorimotor control to characterize differences in control between tasks.
During pursuit tracking, MS subjects exhibited a frequency response that was consistent with that of control subjects. However, visual gains for MS subjects were significantly lower during pursuit (vs. compensatory) tracking, and lower than the visual gains of control subjects for both tasks (Controls: 0.45±0.20 vs. 0.67±0.24 [t(6) = 1.95; p = 0.071]; MS: 0.11±0.15 vs. 0.40±0.24 [t(6) = -2.70; p = 0.036]). In MS subjects who exhibited tremor on the day of testing (N = 4), mismatches between actual and internal estimates of inertia (t(3) = -4.90; p = 0.0163) and viscosity (t(3) = -4.73; p = 0.0178) were also significantly lower during pursuit tracking. Mismatches in stiffness did not decrease in pursuit tracking (t(3) = -1.23; p = 0.31), however, the correlation with tremor severity observed during compensatory tracking was not present. Control subjects had no significant mismatches (p>.05).
Results suggest that subjects with MS take advantage of the reduced need for visual feedback during pursuit tracking and reduce tremor by significantly down-weighting visual gain. The task dependence of the mismatch between actual and predicted limb inertia, viscosity, and stiffness argues against a central impairment. This is consistent with simulations suggesting that mismatches in limb dynamics can be compensatory in the presence of a delay mismatch. Together, these results support the interpretation that an inability to compensate for increased visual feedback delays may be the primary cause of intention tremor in persons with MS.