The visual coding of grip aperture shows an early but not late adherence to Weber's law

An exemplar metric of goal-directed grasping (i.e., peak grip aperture) has been shown to be refractory to a psychophysical principle governing visuoperceptual estimations of object size (i.e., Weber's law). This dissociation suggests that vision for action and vision for perception are mediated by absolute and relative visual information, respectively. The present investigation examined whether aperture shaping elicits a unitary or process-dependent violation of Weber's law. Participants grasped differently sized objects (20, 30, 40, 50 and 60 mm of width) and just noticeable difference (JND) scores related to grip aperture were computed at the time of peak grip aperture as well as at normalized deciles of the response (i.e., 10-90% of grasping time). JNDs during the early and middle stages of the trajectory scaled to object size whereas values late in the trajectory (>50% of grasping time and including the time of peak grip aperture) did not. Thus, results show an early, but not late, adherence to Weber's law and indicate that movement planning and movement control are supported via relative and absolute visual information, respectively.