The Reciprocal Nature of Fitts' Law in Space and Time.

Human movement takes place in both space and time so that measures of movement accuracy in space are made with respect to time, and vice-versa providing a foundation to the proposal of the complementarity of spatial and temporal error in aiming movements. We examined this hypothesis in both the standard Fitts and Peterson discrete movement speed-accuracy protocol that requires moving to stop within a fixed spatial target (distance (D) with target bandwidth (W_D) in an emergent movement time (T) and, also in the reciprocal and novel space-time protocol introduced here that required moving for a fixed temporal target goal T with bandwidth of W_T with an emergent D. Experiment 1 examined a range of D conditions (45, 100, 180, 280, and 405 mm with bandwidth W_D ±5 mm) within the Fitts' Law discrete spatial accuracy protocol to provide compatible spacetime boundary conditions for the reciprocal spacetime protocol in Experiment 2 that examined the effect of target time (T - 250, 460, 670, and 880 ms each with bandwidth W_T ±50 ms) on the emergent D. The findings showed that the spatial and temporal error profiles in Experiment 2 were consistent with exchanging D and T in Fitts' Law Equation. This provides evidence for the reciprocal nature of the spacetime error functions in Fitts' type movement aiming protocols and is compatible with the reciprocal profiles of spatial and temporal errors in other classes of movement aiming tasks.