Brain, Vol 121, Issue 2 357-372, Copyright © 1998 by Oxford University Press
BL Day, PD Thompson, AE Harding and CD Marsden
The effects of vision on spatial and temporal characteristics of free
unrestrained reaching movements of the arm were examined in 17 patients
with ataxic syndromes due to degenerative disease of the cerebellum and its
connections. Subjects were required to reach out and touch a visually
presented target either in the dark or with the target and their finger
visible. Overall, patients had prolonged reaction times and their movements
were performed slower than normal. The spatial paths described by their
fingertips were more circuitous, being of greater length than normal, a
characteristic that was uninfluenced by visual conditions. Ataxic movements
were less accurate than normal in two ways. First, there was greater
spatial variability between repeat paths to the same target. The increased
variability was present very early in the movement trajectory and at that
stage was not influenced by visual feedback. Secondly, there were large
constant errors at the end of movement, but only when moving in darkness.
Patients with Friedreich's ataxia as well as those with intrinsic
cerebellar degeneration showed the above abnormalities, although there were
some quantitative differences between the two groups. We suggest these
spatial errors arise because the cerebellum contributes either directly or
indirectly to preparatory motor processes which, based on limb
proprioceptive and retinal information, compute the pattern of muscle
activity required to launch the limb accurately towards a target. Patients
were largely successful at using visual guidance to make midflight
adjustments to their movements in order to improve accuracy. This
manifested as a reduction in spatial variability between repeat paths as
the target was approached and a reduction in constant error. However, the
visual correction mechanism did not appear normal. Under visual guidance,
the end-phase of movement was often prolonged and characterized by
excessive deviations or direction changes in the path. These deviations may
be the expression of a visual guidance system producing corrections which
themselves contain error requiring further correction. Thus, this process
may be abnormal for the same reason that the initial pattern of muscle
activity is misjudged.
ARTICLES
Influence of vision on upper limb reaching movements in patients with cerebellar ataxia
MRC Human Movement and Balance Unit, Institute of Neurology, London, UK.
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