Effects of a secondary task on the accuracy of single aiming movements.

Recently, Schmidt, Zelaznik, and Frank and Schmidt, Zelaznik, Hawkins, Frank, and Quinn have demonstrated that in rapid, single aiming movements, variability in the movement's kinetic requirements resulted in variability in the movement's amplitude. This new explanation of the speed-accuracy trade-off in motor control, however, does not predict or explain inaccuracy for slower movements (greater than 200 msec). In the two experiments reported, we demonstrate that the Schmidt et al. model can predict variability in slow aiming movements if attention is occupied with an additional task. Subjects were required to perform single aiming movements in either 500 (Experiment 1) or 200 (Experiment 2) msec. In both experiments, the movement amplitude (30, 45, 60, and 75 cm in Experiment 1, and 10, 20, and 30 cm in Experiment 2) and the probability of an auditory probe-reaction time (RT) task were manipulated. Results indicated that only when the movement time (MT) was 500 msec did the probe-RT task change the relationship between the effective target width and the movement's average velocity. This result extended the scope of the Schmidt et al. model to movements with a duration greater than 200 msec. In addition, it seems as though slow movements are controlled by attention-demanding mental processes.