Abstract
Previous work by our group has shown that visually derived grasping yields a dynamic adherence to the psychophysical principles of Weber’s law (Heath et al. 2011: Neurosci Lett; Holmes et al. 2011: Vis Res). In particular, aperture variability (i.e., just-noticeable-difference scores: JND) during the early - but not late - stages of aperture shaping increases with the size of a to-be-grasped target object. This ‘dynamic’ adherence was interpreted to evince that the early kinematic parameterization of a response is mediated via relative visual information and that later control is subserved via absolute visual information. The goal of the present study was to determine whether early JND/object size scaling similarly characterizes aperture trajectories when object size is defined haptically. Participants were provided a haptic preview of object size (i.e., 20, 30, 40 50 and 60 mm) by holding an appropriately sized target object with their non-grasping (i.e., left) limb. Following the preview, participants were cued to either manually estimate (i.e., perceptual task) or grasp (i.e., motor task) the target object, which was located 450 mm distal to a common start location. Importantly, responses in the motor task were performed with (no-delay) and without (i.e., delay) online haptic feedback, and for all tasks vision was occluded. As expected, manual estimations elicited a robust JND/object size scaling (i.e., Weber’s law). For the motor task, both conditions showed an early scaling of JNDs to object size on par to the perceptual task; however, aperture shaping later in the response (> 50% of grasping time) did not. These results indicate that the time-dependent scaling of grip aperture to Weber’s law represents a polysensory representation of object size. That is, vision and haptics provide relative and absolute information to support goal-directed actions.
Meeting abstract presented at VSS 2012