Purpose: To demonstrate the presence of a CNS timekeeper for an over-learned repetitive voluntary movement (pencil shading), and to learn if the timekeeper is influenced by changes in sensory feedback.
Methods: Self-paced pencil shading; fast, maximally-fast, and slow hand waving, as well as enhanced physiologic tremor (EPT) were recorded on 3 separate occasions with a surface-mounted accelerometer placed on the hand in 9 normal volunteers. Variation in inter-trial peak frequency was calculated. Shading and EPT were also recorded with and without visual masking in 9 normals and in 2 deafferented patients. Variation in intra-trial beat-to-beat intervals, a measure of movement regularity, was calculated.
Results: Shading and maximally-fast waving displayed preferred frequencies with no more variability in peak frequency between trials than did EPT, while slow and fast waving had significant inter-trial variability. Variation in beat-to-beat intervals for the shading task was less in controls than for EPT, and less in controls than for the patients in both the masked and unmasked conditions. In addition, in the masked condition, pencil shading by the patients was performed with much higher amplitude and lower frequency than by the controls.
Conclusions: These data support the hypothesis that certain repetitive voluntary movements, such as pencil shading, are paced by central timekeepers that are influenced by changes in sensory feedback.