Grasping a shape with uncertain location

Successful grasp planning requires an appropriate finger placement for which object geometry and location need to be known. Here we investigate how position uncertainty and shape influence the selection of a two-finger pinch grasp. Elliptical cylinders were stereoscopically presented in rapid succession. The position of each cylinder was randomly selected using two orthogonal Gaussian distribution whose orientation changed at each trial. The axes of the elliptical base were aligned with these orthogonal directions. Participants reported the grasp they deemed more likely to be successful. In randomized trials the variance of the Gaussian distributions and the shape of the cylinders varied. Results show an interaction between position uncertainty and shape resulting in a combination of two strategies; (1)grasp is aligned with the direction of maximum position uncertainty and (2) grasp is aligned with the minor axis of the elliptical base. In conditions where ellipses are aligned with the maximum uncertainty, there is a trade-off between the two strategies that depends on magnitude and uniformity of position variability and on cylinder’s eccentricity. Overall, participants seem to maximize the chance of actually reaching the objects while also trying to pinch them along the minor axis (ie, as in a stable grasp).