A New View on Grasping

Abstract

Reaching out for an object is often described as consisting of two components that are based on different visual information. Information about the object's position and orientation guides the hand to the object, while information about the object's shape and size determines how the fingers move relative to the thumb to grasp it. We propose an alternative description, which consists of determining suitable positions on the object—on the basis of its shape, surface roughness, and so on—and then moving one's thumb and fingers more or less independently to these positions. We modeled this description using a minimum-jerk approach, whereby the finger and thumb approach their respective target positions approximately orthogonally to the surface. Our model predicts how experimental variables such as object size, movement speed, fragility, and required accuracy will influence the timing and size of the maximum aperture of the hand. An extensive review of experimental studies on grasping showed that the predicted influences correspond to human behavior.

Grasping reveals visual misjudgements of shape

“…For grasps to the variable axis, we obtain a gain of 0.57±0.03, which is significantly larger than zero (P<0.001). This value lies above the range of 0.2À0.5 obtained when correlating with the final grip aperture, but it is still smaller than other experimentally observed values (Smeets and Brenner 1999). Comparing the two preceding analyses, we see that the maximum grip aperture scales better with the length of the nearest principal axis than with the final grip aperture [if we compare the mean gains of the two analyses, which we generated based on the pooled data of all subjects, we obtain: t(2605)=4.372, P<0.001].…”

“…We find gains varying from 0.2 to 0.5 for different subjects (averaged across subjects the gain is 0.36±0.03 and the average R is 0.53±0.05). These values are exceptionally small compared to the range that is theoretically predicted and experimentally observed (Smeets and Brenner 1999). If we apply the same analysis to the data from our previous study, we find that the gains for grasping cylinders at waist height range between 0.5 and 0.8 (the average gain is 0.66±0.03 and the average R is 0.80±0.02).…”

“…Typically, a linear relationship is found between maximum grip aperture and object size with a gain of about 0.7À0.8 (Smeets and Brenner 1999). For our elliptical cylinders the 'object size' depends on the grip orientation at the time of contact.…”

“…Similarly, misperceiving an object's size can influence grasping (Franz et al 2003), but the evidence is still under debate (Aglioti et al 1995;Smeets and Brenner 1999). Neurological evidence that perceiving shapes veridically is not crucial for grasping them correctly has been taken as a support for grasping having access to different, veridical, visual information about the object (Goodale et al 1994;Goodale and Milner 1992).…”

“…Since different aspects of the visual information do not have to be mutually consistent (Smeets et al 2002), any difference between performance on two tasks could be explained by assuming that different aspects of the visual information are used for the two tasks. For example, if grasping is accomplished by moving the fingertips to suitable locations on the target's surface then the distance between these points is irrelevant so that an illusion of size should have no effect (Smeets and Brenner 1999). Matters become more complicated if more than one aspect is relevant for the task.…”