Visually guided grasping movements directed to real, 3D objects are characterized by a distinguishable trajectory pattern that evades the influence of Weber's law, a basic principle of perception. Conversely, grasping trajectories directed to 2D line drawings of objects adhere to Weber's law. It can be argued, therefore, that during 2D grasping, the visuomotor system fails at operating in analytic mode and is intruded by irrelevant perceptual information. Here, we explored the visual and tactile cues that enable such analytic processing during grasping. In Experiment 1, we compared grasping directed to 3D objects with grasping directed to 2D object photos. Grasping directed to photos adhered to Weber's law, suggesting that richness in visual detail does not contribute to analytic processing. In Experiment 2, we tested whether the visual presentation of 3D objects could support analytic processing even when only partial object-specific tactile information is provided. Surprisingly, grasping could be performed in an analytic fashion, violating Weber's law. In Experiment 3, participants were denied of any haptic feedback at the end of the movement and grasping trajectories again showed adherence to Weber's law. Taken together, the findings suggest that the presentation of real objects combined with indirect haptic information at the end of the movement is sufficient to allow analytic processing during grasp.

Virtual-reality and telerobotic devices simulate local motor control of virtual objects within computerized environments. Here, we explored grasping kinematics within a virtual environment and tested whether, as in normal 3D grasping, trajectories in the virtual environment are performed analytically, violating Weber's law with respect to object's size. Participants were asked to grasp a series of 2D objects using a haptic system, which projected their movements to a virtual space presented on a computer screen. The apparatus also provided object-specific haptic information upon "touching" the edges of the virtual targets. The results showed that grasping movements performed within the virtual environment did not produce the typical analytical trajectory pattern obtained during 3D grasping. Unlike as in 3D grasping, grasping trajectories in the virtual environment adhered to Weber's law, which indicates relative resolution in size processing. In addition, the trajectory patterns differed from typical trajectories obtained during 3D grasping, with longer times to complete the movement, and with maximum grip apertures appearing relatively early in the movement. The results suggest that grasping movements within a virtual environment could differ from those performed in real space, and are subjected to irrelevant effects of perceptual information. Such atypical pattern of visuomotor control may be mediated by the lack of complete transparency between the interface and the virtual environment in terms of the provided visual and haptic feedback. Possible implications of the findings to movement control within robotic and virtual environments are further discussed.

Grasping movements directed toward real objects are typically unaffected by irrelevant aspects of the object and its surroundings, indicating that such interactions are based on analytic processing of object shape and size. However, recent findings show that grasping directed toward two-dimensional (2D) objects is subjected to perceptually mediated effects of relative shape and size. It is unclear however, whether context-dependent processing-a hallmark of visual perception-affects 2D grasping in the same fashion. Here, we explored this possibility by comparing the influence of a newly discovered contextual effect on 2D and on 3D grasping. According to the range of standard effect (RSE), the perceptual resolution for a stimulus depends on the range of the other stimuli presented within the same session, with higher resolution obtained under narrow compared to wide context range. In two experiments, participants were asked to grasp 3D and 2D objects embedded in a wide or a narrow range. The results showed that, unlike 3D grasping, which was immune to contextual information, the resolution during 2D grasping was significantly modulated by the range of the irrelevant context. The findings suggest that visuomotor control directed to 2D objects is intruded by irrelevant perceptual information, making it context-dependent.

No abstract

No abstract