Actions obey Weber's law when precision is evaluated at goal attainment

Abstract

Webers law, the psychophysical principle stating that the just noticeable difference (JND), which is inversely related to sensory precision, increases proportionally with the magnitude of a stimulus, impacts all domains of human perception, but it is violated in visually guided grasping actions. The underlying reasons for this dissociation between perception and action are still debated, and various hypotheses have been put forward, including a different coding of visual size information for perception and action, the use of positional information to guide grasping, biomechanical factors, or sensorimotor calibration. To contrast these hypotheses, we investigated Webers law in a new action task, the two-finger pointing task. Participants reached and touched two targets positioned at different distances apart by using their index finger and thumb. Consistent with Webers law, we found that the standard deviation (SD) of the final inter-finger separation, serving as a measure analogous to the JND, increased with larger inter-target distances. These findings suggest that, when considering measures strongly related with the attainment of the action goal, Webers law is regularly at play.