PubMed1996
A subject lacking proprioceptive and tactile sensibility below the neck and a group of control subjects performed sequences of periodic finger taps involving a pattern of accentuation. The required intertap interval was 700 ms. In some situations, the taps were synchronized with the clicks of a metronome. Feedback conditions were manipulated by either allowing or not allowing the subjects to hear the taps and see their finger movements. We recorded the trajectory of the subjects' finger displacement in the vertical plane, and the force and moment of contact of the finger with the response key. The control subjects achieved precise timing of the finger taps by trading off downstroke onset for movement duration, e.g., they initiated shorter-duration tapping movements with a delay. This strategy did not vary depending on task demands (e.g., synchronization) or feedback conditions. The deafferented patient produced intertap intervals on average close to the required value. However, his tap timing was characterized by increased variability and severe distortion (lengthening) after the accentuated tap, regardless of feedback conditions. He did not manifest the compensatory strategy whereby, in control subjects, movement onset was adjusted to movement duration. Thus, such a strategy in controls seems to depend on intact proprioceptive and/or tactile information from the moving limb. Upon withdrawal of visual and acoustic feedback, the deafferented subject increased the force of the taps and the amplitude of tapping movements; his mean synchronization error with the metronome also increased. However, he did not lose correct phasing between the taps and the clicks of the metronome. These findings suggest that, under normal circumstances, sequential movements are timed by an internal timekeeper which paces sensory consequences relating to the occurrence of behaviorally important events (e.g., finger taps), and not the onset of the movements eliciting those events. In the synchronization task, the timekeeper may be phase locked to the periodic acoustic stimuli by direct entrainment. Feedback information may be needed, however, for keeping any synchronization error as small as possible.