On the Relation Between Object Shape and Grasping Kinematics

Abstract

Despite the many studies on the visual control of grasping, little is known about how and when small variations in shape affect grasping kinematics. In the present study we asked subjects to grasp elliptical cylinders that were placed 30 and 60 cm in front of them. The cylinders' aspect ratio was varied systematically between 0.4 and 1.6, and their orientation was varied in steps of 30 degrees. Subjects picked up all noncircular cylinders with a hand orientation that approximately coincided with one of the principal axes. The probability of selecting a given principal axis was the highest when its orientation was equal to the preferred orientation for picking up a circular cylinder at the same location. The maximum grip aperture was scaled to the length of the selected principal axis, but the maximum grip aperture was also larger when the length of the axis orthogonal to the grip axis was longer than that of the grip axis. The correlation between the grip aperture--or the hand orientation--at a given instant, and its final value, increased monotonically with the traversed distance. The final hand orientation could already be inferred from its value after 30% of the movement distance with a reliability that explains 50% of the variance. For the final grip aperture, this was only so after 80% of the movement distance. The results indicate that the perceived shape of the cylinder is used for selecting appropriate grasping locations before or early in the movement and that the grip aperture and orientation are gradually attuned to these locations during the movement.

Grasping reveals visual misjudgements of shape

“…The methods were very similar to those of our previous study (Cuijpers et al 2004) so that we could directly compare the results. To compare the data of the two experiments, we also applied the new analyses developed for the present data to the data of the previous study.…”

“…For comparison, the results of our previous study (Cuijpers et al 2004) are indicated by the thin symbols in Fig. 9.…”

“…It turned out that these subjects behaved very similarly to the others in most respects. Most subjects were familiar with the task because they had also participated in our previous study (Cuijpers et al 2004). Except for the authors EB and JS, the subjects were unaware of the specific questions that we wanted to answer.…”

“…If we separate the peaks at the minima between them (at 39°and 133°), we find probabilities of 39 and 61% for choosing the major and minor axis, respectively. For comparison, we also included the results that we obtained in our previous study when the cylinders were placed at waist height (Cuijpers et al 2004; dashed curve). The peaks in that study appear at very similar locations (À3°and 85°) but they are taller and narrower.…”

“…We previously examined how an object's shape influences the preshaping of the hand and the selection of grip locations (Cuijpers et al 2004). Subjects grasped elliptical cylinders that were placed at waist height.…”