Visual selection of multi-digit contact surfaces for objects of varying mass

Humans use vision to select where and how to grasp objects. How this is accomplished remains unexplored in many respects. For example, previous research has often focused on the visual selection of digit contact points (e.g., during precision-grip grasping). Yet, the whole surface of our fingers and palm may come into contact with objects during natural, unconstrained grasping. Here, we investigated how finger contact areas varied when participants grasped objects of different materials. In a first experiment, we asked 6 participants to freely grasp and lift wood (50g) and brass (700g) bars (2.5 cm grip width) while wearing a data glove. We recorded the hand pose (joint angles of each finger) selected by participants to grasp the objects. Participants employed distinct hand postures: they selected a precision grip when grasping light wooden objects, but employed multiple digits when grasping heavy brass objects (p<.05). In a second experiment, we asked 5 participants to grasp a single bar (2.5 cm grip width) with either a precision grip or a multi-digit grasp. We coated the stimulus with thermochromic paint⁠—which changes colour at body temperature—allowing us to estimate the hand contact regions on the objects. As expected, total contact area increased when participants used multi-digit grasps (p<.01). Interestingly however, the average contact area across fingers decreased with multi-digit grasps (p<.05), possibly because contact surfaces were distributed across multiple fingers, thus requiring smaller grip forces per finger to lift the object. Therefore, when visually selecting how to grasp heavier objects, participants increased the number of digits they employed to increase the total grip surface area while maintaining relatively low grip forces. Our findings demonstrate how humans modify their grasping behaviours to achieve comfortable and effective grasps as perceived shape and material properties vary.