Practice does not lead to changes in movement trajectories in Fitts' Law tasks


According to the prevailing theory to explain Fitts's Law (Meyer et al., 1988), performers need to optimize the speed of the primary submovement so that secondary submovements are not required on a large proportion of the trials. Abrams and Pratt (1993; Pratt & Abrams, 1966) show that in a Fitts task of moderate difficulty that over practice performers lengthen the duration of the primary submovement and decrease the duration of the secondary submovement. In the presently reported study the role of submovements and practice was re-examined. There were two Fitts tasks, a low ID and a high ID task which were practice over 150 trials each. Instead of using what is termed an indirect movement task (controlling a joystick to move a computer cursor) participants moved a hand-held stylus from a home position to acquire a target of width, W, and D units of distance. Utilization of submovements was assessed by noting the number of times that acceleration changed sign and by computing normalized mean squared jerk. There were no effects of practice on the low or high ID Fitts task. We interpreted this finding by theorizing that in the performance of direct visual-motor tasks by adults, that the optimized sub-movement strategy has already been learned because the mapping of hand motion to visual changes are highly compatible. In indirect Fitts tasks this mapping has to be learned so that performers learn to minimize secondary submovements as a consequence of learning a visual-motor mapping.