Address for correspondence: J. Hore, Department of Physiology, University of Western Ontario, London, Ontario N6A 5C1, Canada. Voice: 519-661-3817; fax 519-661-3827.
jon.hore{at}fmd.uwo.ca
Ann. N.Y. Acad. Sci. 978: 1-5 (2002).
Although there is agreement that an important sign of cerebellar
dysfunction is disorder in timing of movement, it appears that
authors who study different behaviors mean different things
when they use the term "timing," and that the underlying mechanisms
are likely to be different. For overarm throwing, skilled throwers
can time ball release with a precision of less than 7 ms, whereas
cerebellar subjects show a large variability of 50 ms or more
in this timing. Furthermore, cerebellar patients show a larger
variability in the amplitude of finger opening which could either
reflect a disorder in force, or result indirectly from the increased
variability in timing. To determine whether timing and force
of finger opening were dependent variables, the time of ball
release was plotted against the amplitude of finger opening.
In control subjects these two parameters were related, with
early (mistimed) throws having smaller finger amplitudes. However,
in cerebellar subjects the increased variability in finger amplitude
could not be accounted for by the increased variability in timing.
Similarly, the increased timing windows could not be explained
by disorder in force at the fingers. It is concluded that the
abnormal finger opening that occurs in cerebellar patients when
making overarm throws results from increased variability in
both the timing and force of finger extension. Whether the increased
variability in timing is a disorder in triggering finger opening
at the right moment, or is due to a failure to combine finger
opening appropriately with the hand trajectory, remains to be
determined.
