Smith MA and Shadmehr R (2005) Intact ability to learn internal
models of arm dynamics in Huntington's disease but not cerebellar degeneration,
Journal of Neurophysiology, 93:2809-2821.
Abstract: Two different compensatory
mechanisms are engaged when the nervous system senses errors during a reaching
movement. First, on-line feedback control mechanisms produce in-flight
corrections to reduce errors in the on-going movement. Second, these errors
modify the internal model with which the motor plan is transformed into motor
commands for the subsequent movements. What are the neural mechanisms of these
compensatory systems? In a previous study we reported that while online error
correction was disturbed in patients with Huntington's disease (HD), it was
largely intact in patients with cerebellar degeneration. Here we altered
dynamics of reaching and studied the effect of error in one trial on the motor
commands that initiated the subsequent trial. We observed that in patients with
cerebellar degeneration, motor commands changed from trial-to-trial by an
amount that was comparable to control subjects. However, these changes were
random and were uninformed by the error in the preceding trial. In contrast,
the change in motor commands of HD patients was strongly related to the error
in the preceding trial. This error dependent change had a sensitivity that was
comparable to healthy controls. As a result, HD patients exhibited no
significant deficits in adapting to novel arm dynamics while cerebellar
subjects were profoundly impaired. These results demonstrate a double
dissociation between on-line and trial-to-trial error correction suggesting
that these compensatory mechanisms have distinct neural bases that can be
differentially affected by disease.
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