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

Fitts’ Law is one among a small number of psychophysical laws. However, a fundamental variable in Fitts’ Law – the movement distance, D – confounds two quantities: 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 everyday motor behavior, decoupling them might improve our understanding of the factors that shape speed-accuracy tradeoffs. Here we leveraged the phenomenon of visuomotor gain adaptation to de-confound movement and visual distance during goal-directed reaching. 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 and degrading visual feedback. These results may reflect the role of sensory context in 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 must move that 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 may be related to the relevance of 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.