Unconstrained visually-guided grasping is not precision grip

In everyday life our hands serve as versatile tools, interacting with objects in numerous ways. Yet most research on visually-guided grasping focuses on precision grips, constraining participants to use only their thumb and index finger. We asked whether insights gained from precision grip studies extend to situations in which participants are free to grasp objects however they chose. To test this, we used a subset of 3D objects from a recent study investigating how participants select precision grips on multi-material (brass and wood) objects [Klein, Maiello et al 2020]. Twenty participants grasped these objects while we tracked their hand movements using a Qualisys passive marker motion capture system. In a first, unconstrained grasping session, participants were free to grasp the stimulus objects however they wanted. In a second, precision grip session, participants were required to grasp the objects using only thumb and index finger. We find that in unconstrained sessions participants rarely employed two-digit precision grips, which accounted for only 9.5% of unconstrained trials (p<.001), and the average position of the digits on the objects differed significantly between precision and unconstrained sessions (p=.019). Nevertheless, in both precision grip and unconstrained sessions participants shifted their grasps towards the objects’ center of mass to minimize grip torque (p=.023). Our data thus confirmed the influence of object visual material appearance—previously observed in precision grip experiments—and extended this result to unconstrained grasping. Additionally, upon closer inspection we found that the position of the thumb and index finger on the stimulus objects did not significantly differ between precision and unconstrained sessions (p=.218), suggesting that the remaining fingers primarily provided a support function. Thus, while participants may rarely spontaneously choose two-digit grasps, previous insights gained from precision gip experiments may still extend to natural, unconstrained grasping behaviours.