The tactics and timing of smooth corrections in human arm movements

Background. Human arm movements frequently need 'on-line' correction due to object movement and intrinsic inaccuracy in motor control. This study addressed four outstanding questions regarding how the nervous system adjusts arm movements in response to target displacement. A double-step paradigm was used with participants required to move to a target that changed its starting position (46cm along the midline) 10ms after movement initiation on 50% of trials (perturbation condition). The data from the trials where the target did not jump were compared to identical trials from a subsequent condition where the target was never displaced (non-perturbation). Results. We discovered differences in the spatial path between conditions (presumed to be strategic) but the temporal characteristics of the movement were unaltered. It was found that the system could make smooth continuous corrections (defined as an absence of any visible inflection on the movement trajectory) in the perturbation trials. A study of the inter-trial variability across time suggested that the system implemented corrections in the deceleration phase of the movement (circa 230ms) although the system is capable of modifying a trajectory as early as 110ms. It was found that movement time to a displaced target was a function of both total distance travelled and the displacement distance. Conclusions.These findings demonstrate strategic responses to statistically probable object movement. These strategies allow the system to respond smoothly to object movement occurring even after movement initiation. The corrections took place in the deceleration phase of the movement with the modifications impacting upon movement duration.