Abstract
Previous research by our group has shown that grasping a 3D object elicits a time-dependent adherence to Weber's law whereas grasping a 2D object produces a time-independent adherence to the law's psychophysical properties (Holmes et al., 2012: J Vis). Such results indicate that unitary relative information mediates 2D grasping whereas relative and absolute visual cues mediate the early and late stages, respectively, of 3D grasping. Interestingly, some work focusing on a late occurring kinematic marker (i.e., peak grip aperture) has shown that decoupling the spatial relations between stimulus and response (i.e., pantomiming) renders aperture specification via relative visual information (Goodale et al., 1994: Neuropsychologia; Westwood et al., 2000: Exp Brain Res). As such, the present study sought to determine whether pantomimed actions elicit a time-independent or time-dependent use of relative visual information. Participants grasped and pantomimed grasping differently sized (20, 30, 40 and 50 mm) 2- and 3D objects. Importantly, we computed just-noticeable-difference (JND) scores as within-participants standard deviations in grip aperture at decile increments of grasping time and interpreted linear scaling of JNDs to object size as extant adherence to Weber's law. As expected, JNDs for 3D grasping elicited a time-dependent scaling to object size consist with our group's earlier work. In turn, JNDs for 2D grasping as well as 2- and 3D pantomiming scaled to target size throughout the response. Thus, results support the position that decoupling stimulus and response for 2- and 3D objects renders aperture shaping via unitary and relative visual information.Acknowledgments: Supported by NSERC