Motor skill learning in patients with cerebellar degeneration
Introduction
Numerous studies have suggested that the cerebellum and its associated brain stem circuitry plays a role in adapting motor behavior in order to master novel motor tasks. The architecture of the cerebellar cortex, with two distinct inputs regulating the activity of the single output neuron, the cerebellar Purkinje cell, is considered an ideal substrate for implementing learning capabilities in the nervous system 1, 9, 15. There is ample experimental evidence that the cerebellum plays an important role in certain apparently simple learning processes. Lesions of the vestibulocerebellum and its target nuclei in the brain stem reduce the ability of the animal to modify the gain of simple reflex behavior, such as the vestibulo-ocular reflex 14, 22. Visual–motor adaptation to rescaled visual inputs is impaired in patients with cerebellar lesions 6, 23. The cerebellum appears to be involved in the adaptation of single joint movements to novel loads [7]or to changes in gain of a visual–motor task [4]. In addition, cerebellar structures support other forms of simple motor learning, such as the acquisition of classically conditioned responses 19, 21. It is not yet known whether or not the pathways that support such simple forms of motor learning can also mediate more complex forms of motor learning. Experimental evidence bearing on this issue is scarce.
[18]compared the performance of patients with cerebellar degeneration with that of normal subjects who were attempting to improve their speed in tracing a polygon. Whether a true increase in motor capacity occurred is unclear because both groups increased their speed only at the cost of decreasing their accuracy. This trade-off of two associated movement variables [5]represents an alteration of motor behavior that we have called motor adaptation learning 12, 18. However, in the trade-off of speed for accuracy, no new capabilities, or skills, were necessarily added to the repertoire of available motor programs. To emphasize this attribute, we operationally defined motor skill learning as the improvement of one or more naturally antagonistic movement performance variables (e.g. speed, accuracy, energy savings) without the degradation of one or more others. In a second task of [18], the subjects also traced a polygon, but while observing their hand in a mirror. The normal subjects had an improvement in both speed and accuracy, but the patients with cerebellar cortical atrophy were deficient, indicating an abnormality of complex motor skill learning. Among several components of that task, however, was a requirement to perform a difficult visual–motor adaptation that may have prevented detection of a component of skill learning.
In the present experiments, we studied the improvement in accuracy of planar multijoint arm movements as a model of complex motor skill learning in patients with cerebellar dysfunction and in healthy subjects. To prevent a trade-off of the associated movement variables of speed and accuracy, we evaluated changes in movement accuracy at constant movement times. We also evaluated the acquisition of skill under different dynamic conditions (slow movements vs. fast movements) and assessed the retention of the learned motor skill over time.
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Patients and methods
We studied ten patients with cerebellar cortical atrophy (CCA), aged 55±13 years (mean±SD), and nine patients with olivopontocerebellar atrophy (OPCA), aged 46±17 years. Sixteen healthy subjects, aged 43±14 years, served as control subjects. The patients' clinical characteristics are summarized in Table 1. The diagnosis was CCA if patients had only upper or lower limb ataxia and/or midline ataxia, dysarthria and cerebellar oculomotor dysfunction without radiological evidence of additional
Experimental day 1
Both the normal subjects and the patients were able to adjust their arm trajectories in order to nearly connect the via points when performing slow arm movements (Fig. 1). Initially, the normal subjects' and patients' movements showed large total spatial errors, of the order of 10.8±5.5 cm in the twelve normal subjects and 16.6±5.3 cm in the sixteen patients (P=0.01, Wilcoxon). With practice, however, their trajectories became more accurate (Fig. 2).
The initial ln(spatial error), a, tended to
Discussion
Our results show that although cerebellar patients exhibited poorer performances than normal subjects, at slow speeds, they were able to improve their complex motor skill to the same relative extent, if not at the same rate, as normal subjects. They also were able to retain a substantial amount of this acquired skill for at least 24 h. On the other hand, the patients clearly showed relative difficulty in improving their skill at high speed. We may try to understand these findings in terms of,
Acknowledgements
The authors wish to thank Dr T. Zeffiro for technical assistance with the data acquisition, and Ms B.J. Hessie for skillful editing. Dr Topka was supported by the Deutsche Forschungsgemeinschaft, Germany.
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