Contact points during multidigit grasping of geometric objects

Abstract

We investigated the choice of contact points during multidigit grasping of different objects. In Experiment 1, cylinders were grasped and lifted. Participants were either instructed as to the number of fingers they should use, ranging from a two-finger grasp to a five-finger grasp, or could grasp with their preferred number of fingers. We found a strong relationship between the position of the fingertips on the object and the number of fingers used. In general, variability in the choice of contact points was low within- as well as between participants. The virtual finger, defined as the geometric mean position of fingers opposing the thumb, was in almost perfect opposition to the thumb, suggesting the formation of a functional unit using all contributing fingers in the grasp. In Experiment 2, four more complex shapes (rectangle, hexagon, pentagon, curved object) were grasped. Although we found some moderate between-participant variability in the choice of contact points, the within-participant variability was again remarkably low. In both experiments, participants showed a strong preference to use four or five fingers during grasping when left with free choice. Taken together, our findings suggest a preplanning of the grasping movement and that grasping results from a coordinated interplay between the fingers contributing to the grasp that cannot be understood as independent digit movements.

Neural bases of hand synergies

“…More generally, the core idea of optimal control is that behavior reflects the optimum of some cost function (Todorov and Jordan, 2002; Todorov, 2004). In the context of grasping, this general framework has been used to explain both how a set of contact positions (Friedman and Flash, 2007; Lukos et al, 2007, 2008; Ciocarlie et al, 2009; Fu et al, 2010, 2011; Craje et al, 2011; Gilster et al, 2012) and contact forces (Hershkovitz et al, 1997; Chalfoun et al, 2004; Pataky et al, 2004; Baud-Bovy et al, 2005; Niu et al, 2009; Terekhov et al, 2010) are selected. However, a problem with this framework is that it does not explain how the optimal solution might be actually computed in a biologically plausible manner.…”

Learned Manipulation at Unconstrained Contacts Does Not Transfer across Hands

“…Several recent studies have shown that when such digit placement constraints are removed, subjects actively modulate contact points as a function of object properties such as mass distribution [14] or shape [15], [16], as well as planned manipulation [17], [18]. Most importantly, it has been shown that digit placement and forces are not independent and that their trial-to-trial covariation suggests the existence of high-level representations of learned manipulation task, i.e., the net torque that has to be generated for any combination of digit force and position [19], [20].…”

Human-inspired selection of grasp hypotheses for execution on a humanoid robot

“…Notably, the choice of fingertip contact points has been studied in most cases only for two-finger grasps that involved the thumb and index finger. This contrasts with the recent finding that the default grasp of humans seems to involve all five fingers [25]. In general, the small variability of contact points and the preshaping of the hand in early phases of the prehension movement indicate a preplanning in the choice of contact points [26].…”