Independent Influences of Movement Distance and Visual Distance on Fitts' Law

Abstract

Fitts Law is among a handful of psychophysical laws. However, one of the fundamental variables of Fitts Law - the movement distance, D - confounds two variables: the physical distance the effector has to move to reach a goal, and the visually perceived distance to that goal. While these two quantities are functionally equivalent in typical motor behaviors, decoupling them might improve our understanding of Fitts Law. Here we leveraged the phenomenon of visuomotor gain adaptation to de-confound movement and visual distance during goal-directed upper-limb movements. We found that movement distance and visual distance can influence movement times, supporting a variant of Fitts Law that considers both. The weighting of movement versus visual distance was modified by restricting movement range (emphasizing vision) or degrading visual feedback (emphasizing proprioception). These results reflect a multisensory integration process in the computation of expected movement difficulty, perhaps during early stages of motor planning.