Dynamic Invariance in Goal-Directed Aiming

One of the most robust empirical observations in the domain of movement behavior is that the time necessary to successfully complete a goal-directed action varies as a function of the relative precision requirements. Fitts’s (1954) aimed movement paradigm elegantly captures the essence of goal directedness, defining the gap between the desired situation and the current situation by a target distance (D) and the goal tolerance by a target width (W). With these two independent variables measured along a single dimension, it allows experimental control of the task difficulty operationalized through an index of difficulty most commonly defined as ID = log2(2D/W). Within this framework, the robust empirical observation alluded to previously takes the form of a monotonic relation between movement time (MT) and task difficulty known as Fitts’s Law. Because the kinematics of movement offer a window into the processes underlying behavioral organization, we set out to explore the changes in movement pattern that underlie the changes in movement duration brought about by increasing task difficulty. The results of the first phase of the research program developed, summarized in this contribution, revealed the existence of an invariant behavioral dynamics, providing intriguing new perspectives on the mechanisms underlying perceptuomotor control. ECOLOGICAL PSYCHOLOGY, 16(1), 55–60 Copyright © 2004, Lawrence Erlbaum Associates, Inc.