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 par-adigm 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 ex-plore the changes in movement pattern that underlie the changes in movement du-ration brought about by increasing task difficulty. The results of the first phase of the research program developed, summarized in this contribution, revealed the ex-istence of an invariant behavioral dynamics, providing intriguing new perspectives on the mechanisms underlying perceptuomotor control