On-line visual control of grasping movements

Abstract

Even though it is recognized that vision plays an important role in grasping movements, it is not yet fully understood how the visual feedback of the hand contributes to the on-line control. Visual feedback could be used to shape the posture of the hand and fingers, to adjust the trajectory of the moving hand, or a combination of both. Here, we used a dynamic perturbation method that altered the position of the visual feedback relative to the actual position of the thumb and index finger to virtually increase or decrease the visually sensed grip aperture. Subjects grasped objects in a virtual 3D environment with haptic feedback and with visual feedback provided by small virtual spheres anchored to the their unseen fingertips. We found that the effects of the visually perturbed grip aperture arose preeminently late in the movement when the hand was in the object's proximity. The on-line visual feedback assisted both the scaling of the grip aperture to properly conform it to the object's dimension and the transport of the hand to correctly position the digits on the object's surface. However, the extent of these compensatory adjustments was contingent on the viewing geometry. The visual control of the actual grip aperture was mainly observed when the final grasp axis orientation was approximately perpendicular to the viewing direction. On the contrary, when the final grasp axis was aligned with the viewing direction, the visual control was predominantly concerned with the guidance of the digit toward the visible final contact point.

A review of grasping as the movements of digits in space

“…The two frameworks each suggest that a certain kind of visual feedback is used to guide grasping: the locations of the digits (digit-in-space framework) or the grip aperture and hand position (visuomotor-channel framework). To evaluate which of the two kinds of information is used, we will interpret the findings of an experiment that manipulated the reliability of visual information (Volcic and Domini 2016).…”

“…Therefore, one can expect a lower gain of online adjustments for the index finger than for the thumb. This prediction was tested experimentally by Volcic and Domini (2016) by scaling the visual feedback about the distance between the digits (i.e., about grip aperture). When grasping in the frontal plane with both digits continuously in view, both digits responded equally to the manipulated feedback.…”

A margin for error in grasping: hand pre-shaping takes into account task-dependent changes in the probability of errors

“…Moreover, although our study is framed in terms of the relationship between hand opening and object size, this does not imply that grip aperture is necessarily an explicitly controlled component of grasping movements (Jeannerod 1984, 1988). Other possibilities exist, including that grip apertures are merely an emergent property of a control process that operates on the separate digits (Smeets and Brenner 1999; see also; Volcic and Domini 2016). In our view, the concept of a margin for error applies regardless of the specific control mode (although implementation would differ), because the individual digits still need to approach the object in a way that reflects uncertainty about the position of its surfaces if the probability of errors is to be controlled.…”

Viewing geometry determines the contribution of binocular vision to the online control of grasping

“…Our study was not designed to distinguish between different accounts of the specific “control mode” via which visual information is used in visual grasp control; whether grasp aperture, or the separation between digits and target locations is explicitly controlled, for example (Jeannerod 1984 ; Smeets and Brenner 1999 ; Volcic and Domini 2016 ). This factor could in principle interact with our manipulation of viewing angle.…”

“…1 b are arbitrary, because they depend on the simulation parameters chosen. And this situation is of course much simpler than real movements, where (i) reach trajectories are typically curved above the table surface, rather than being precisely parallel to it, and (ii) both thumb and finger (and/or grip aperture) must be explicitly controlled (Jeannerod 1984 ; Smeets and Brenner 1999 ; Volcic and Domini 2016 ). Nonetheless, it allows relative comparisons to be made about how binocular and monocular retinal signals to object-digit separation vary in informativeness as the direction of separation varies with respect to the line of sight.…”