Goal-directed grasping: Haptic and visual percepts of object size influence early but not late aperture shaping

Abstract

Previous work has shown that visually and memory-guided grasping yields a time-dependent adherence to Weber's law. In particular, aperture variability (i.e., just-noticeable-difference scores: JNDs) during the early, but not late, stages of a response increases with the size of a to-be-grasped target object. The present study examined whether JND/object size scaling is specifically related to the visual properties of a target object. Participants grasped and manually estimated the size of target objects in visual and haptic conditions. In the visual condition, participants were provided a visual preview of the target object and then grasped, or manually estimated, the same target object without visual feedback. In the haptic condition, participants held an appropriately sized object in their non-grasping (i.e., left) limb for a preview and then grasped, or manually estimated, a target object without vision. As expected, a robust JND/object size scaling was observed for visual and haptic manual estimation tasks (i.e., Weber's law). Moreover, visual and haptic grasping tasks showed a JND/object size scaling on par to the manual estimation task from 20 through 60% of grasping time but not during the later stages of the response (i.e., > 60%). Thus, results show that visually and haptically defined information related to object size elicits a time-dependent adherence to the psychophysical principles of Weber's law.

Acknowledgments: This work was supported by NSERC and NSERC-USRA