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.
Public Significance You will automatically slow your movement when picking up a needle five inches away versus a handkerchief three inches away. This fact is elegantly formalized by Fitts’ Law, which mathematically relates movement duration to movement difficulty. However, one of the fundamental variables in the law – the distance of a planned movement – is ambiguous: Is it the actual distance the hand has to move that shapes planning and biases movement duration, or is it the visually perceived distance? We decoupled these variables, finding that Fitts’ Law is shaped by both quantities, and that the influence of one versus the other is related to the relevance of proprioceptive versus visual information. We believe our “addendum” to Fitts’ Law is timely, as everyday motor behavior has become increasingly enmeshed with virtual environments that abstract our movements into digital realities.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
AUTHOR EMAILS
aalfawa1{at}jhu.edu
samuel.mcdougle{at}yale.edu
AUTHOR CONTRIBUTIONS
NA and SDM contributed to conceptualization, data curation, formal analysis, methodology, software, visualization, and writing. NA performed the investigation. SDM contributed supervision and funding acquisition.
