Center or side: biases in selecting grasp points on small bars

Abstract

Choosing appropriate grasp points is necessary for successfully interacting with objects in our environment. We brought two possible determinants of grasp point selection into conflict: the attempt to grasp an object near its center of mass to minimize torque and ensure stability and the attempt to minimize movement distance. We let our participants grasp two elongated objects of different mass and surface friction that were approached from different distances to both sides of the object. Maximizing stability predicts grasp points close to the object's center, while minimizing movement costs predicts a bias of the grasp axis toward the side at which the movement started. We found smaller deviations from the center of mass for the smooth and heavy object, presumably because the larger torques and more slippery surface for the heavy object increase the chance of unwanted object rotation. However, our right-handed participants tended to grasp the objects to the right of the center of mass, irrespective of where the movement started. The rightward bias persisted when vision was removed once the hand was half way to the object. It was reduced when the required precision was increased. Starting the movement above the object eliminated the bias. Grasping with the left hand, participants tended to grasp the object to the left of its center. Thus, the selected grasp points seem to reflect a compromise between maximizing stability by grasping near the center of mass and grasping on the side of the acting hand, perhaps to increase visibility of the object.

Predicting precision grip grasp locations on three-dimensional objects

“…Spatial Biases The spatial biases we observe are consistent with participants attempting to increase object visibility(25, 27), and our data also replicate the finding that these biases are reduced when object weight increases(19, 25).…”

“…It has been proposed that these may arise from an attempt to minimize energy expenditures through shorter reach movements(24). However, Paulun et al (25) have shown that these biases may in fact arise from participants attempting to optimize object visibility. While our current dataset was not designed to untangle these competing hypotheses, re-analyzing published data (19, 27) confirms that object visibility—not reach length—is most likely responsible for the biases.…”

“…We reasoned that grasps are selected to minimize costs associated with instability and discomfort. Accordingly, we implemented a model that combines five factors computed from the object’s shape, mass distribution, and orientation: (i) force closure(14), (ii) torque(15–19) (iii) natural grasp axis(16, 20–22), (iv) natural grasp aperture for precision grip(23) and (v) visibility(24, 25). We find that the model predicts human grasp patterns strikingly well.…”

The Visual Perception of Material Properties Affects Motor Planning in Prehension: An Analysis of Temporal and Spatial Components of Lifting Cups

“…Moreover, we found stimulus weight and rated heaviness based on the post-transportation ratings, influence the peak velocity during the transportation phase, in which filled cups (i.e., heavier) were moved significantly slower than empty (i.e., lighter) cups. In this respect, our results are in line with previous findings demonstrating longer planning phase of movements for objects made of heavy materials (e.g., brass) (Weir et al, 1991;Fleming et al, 2002;Paulun et al, 2014Paulun et al, , 2016. The filled cups were heavier than the empty cups because they comprised of an additional material, water, which is heavier than the paper cups alone.…”

A review of grasping as the movements of digits in space

“…Large torques can make the object slip if not compensated for by increasing the grip force. A third factor is the visibility of the object: grasping points are chosen such that the grasping hand moves in a way that maximizes the visibility of the object, so the right hand will grasp more to the right than the left hand (Paulun et al 2014). A fourth factor is the comfort of the configuration, a term used to describe the subjective preference for a certain grip configuration when more grip configurations are possible (Rosenbaum et al 1990).…”