Changes in the dynamics of tremor during goal-directed pointing

For successful performance of activities requiring a fine level of manipulative control and dexterity, precise control over the intrinsic oscillations (tremor) in each segment is essential. However, the question of how individuals control (minimize) their tremor during precise postural movements remains unresolved. The aim of this study was to investigate the changes observed in limb tremor during goal-directed postural pointing tasks. Seven subjects attempted to minimize limb tremor during a pointing task whereby progressively greater levels of accuracy were required. Subjects held a small lightweight laser pointer in their extended hand during all tasks, the goal being to maintain the laser emission within a specified target area. Frequency analysis showed that the tremor profile for the hand and index finger was characterized by two prominent frequency peaks, located between 2-4 and 8-12 Hz. When the accuracy requirement of the task increased, there was a significant increase in the amplitude of the 8-12 Hz peak for all segments. Analysis of the time series component of tremor revealed a similar trend with the root mean square (RMS) and approximate entropy (ApEn) of the finger tremor increasing as the accuracy requirement increased. This same pattern was not seen for hand tremor where a small but systematic decrease in both the tremor RMS and ApEn was observed. Overall, it would appear that subjects attempted to reduce tremor at the finger by exerting greater control over the hand (as evidenced by decreased tremor output and increased regularity in the tremor signal). Unfortunately, the consequence of this strategy was that the tremor in the distal effector actually increased. Changes in the tremor output observed as a result of defining an explicit external goal probably resulted from the enhanced visual information provided by the laser emission. However, it would appear that subjects were not able to utilize this feedback effectively to reduce their tremor during the targeting tasks.