Does changing Fitts' index of difficulty evoke transitions in movement dynamics?

  • Huys Raoul
  • Knol Hester
  • Sleimen-Malkoun Rita
  • Temprado Jean-Jacques
  • Jirsa Viktor


Background: The inverse relationship between movement speed and accuracy in goal-directed aiming is mostly investigated using the classic Fitts' paradigm. According to Fitts' law, movement time scales linearly with a single quantity, the index of difficulty (ID), which quantifies task difficulty through the quotient of target width and distance. Fitts' law remains silent, however, on how ID affects the dynamic and kinematic patterns (i.e., perceptual-motor system's organization) in goal-directed aiming, a question that is still partially answered only. Methods: Therefore, we here investigated the Fitts' task performed in a discrete as well as a cyclic task under seven IDs obtained either by scaling target width under constant amplitude or by scaling target distance under constant target width. Results: Under all experimental conditions Fitts' law approximately held. However, qualitative and quantitative dynamic as well as kinematic differences for a given ID were found in how the different task variants were performed. That is, while ID predicted movement time, its value in predicting movement organization appeared to be limited. Conclusion: We conclude that a complete description of Fitts' law has yet to be achieved and speculate that the pertinence of the index of difficulty in studying the dynamics underlying goal-directed aiming may have to be reconsidered.