On-line control of rapid aiming movements: Unexpected target perturbations and movement kinematics

Two experiments were conducted to assess the use of on-line visual information during rapid goal-directed aiming movements. In both experiments, participants were required to complete a discrete aiming movement to a single target on a graphics tablet. In Experiment I, during 76% of the experimental trials, the target width remained constant throughout the movement. The remaining 24% of the trials were evenly divided between two target perturbations in which the width of the target unexpectedly increased or decreased in size upon movement initiation. During Experiment 2, the spatial location of the single target was perturbed to a new location closer to, or further away from the original target position. The proportion of perturbation trials remained constant across experiments. The results indicated that peak velocity was determined prior to movement initiation in order to meet the speed-accuracy demands of the original target width or movement amplitude. In contrast, during deceleration, participants modified their movement trajectories to account for a perturbation in target width or movement amplitude. These data suggest that on-line monitoring of visual information can be used to modify the latter half of a movement trajectory.