Abstract
Previous work (Holmes et al. 2013: J Mot Behav) has shown that actions entailing dissociable spatial relations between stimulus and response are mediated via relative visual information. In particular, grasping to an area offset from a veridical target renders aperture shaping that adheres to the psychophysical principles of Weber’s law. The goal of the present study was to determine whether the aforementioned results are attributed to: 1. the dissociation between the target and the grasping response or, 2. the absence of a veridical (i.e., graspable) target at movement offset. To accomplish our objective, we employed open-loop grasping responses to differently sized (20, 30, 40 and 50 mm) target objects in conditions wherein: 1. the to-be-grasped object remained present throughout the response (i.e., veridical grasping) and, 2. the to-be-grasped object was removed prior to movement onset (i.e., pantomime-grasping). The different conditions were performed in separate blocks. Thus, the critical between-condition difference was premovement knowledge relating to whether a graspable object would be present at movement offset. As expected, results for the veridical grasping condition showed that late aperture shaping violated Weber’s law: a result demonstrating the use of absolute visual information. In contrast, aperture trajectories for the pantomime-grasping condition showed an early through late adherence to Weber’s law. Such results demonstrate that the absence of a veridical target represents a pivotal factor in the mediation of aperture trajectories via relative visual information. Moreover, our results indicate that pantomime-grasping does not represent a proxy for understanding the visual control of grasping.Acknowledgments: Supported by NSERC.