Grasping at laws: Speed-accuracy trade-offs in manual prehension.

Abstract

Most of human performance is subject to speed-accuracy trade-offs. For spatially constrained aiming, the trade-off is often said to take the specific form of Fitts' law, in which movement duration is predicted from a single factor combining target distance and target size. However, efforts to extend this law to the three-dimensional context of reaching to grasp (prehension) have had limited success. We suggest that there are potentially confounding influences in standard grasping, and we introduce a novel task to regularize the direction of approach and to eliminate the influences of nearby surfaces. In six participants, we examined speed-accuracy trade-offs for prehension, manipulating the depth (in the plane of the reach), height (orthogonal to the reach), and width (the grasped dimension) of the target object independently. We obtained lawful relationships that were consistent at the group and individual levels. It took longer to reach for more distant objects, and more time was allowed when placing the fingers on a contact surface smaller in either depth or height. More time was taken to grasp wider objects, but only beyond a critical width that varied between individuals. These speed-accuracy trade-offs showed substantial departures from Fitts' law, and were well described by a two-factor model in which reach distance and object size have separate influences on movement duration. We discuss empirical and theoretical reasons for preferring a two-factor model, and we propose that this may represent the most general form of speed-accuracy trade-off, not only for grasping but also for other spatially constrained aiming tasks. (PsycINFO Database Record

A review of grasping as the movements of digits in space

“…According to the model that implements independent-digit control by minimum-jerk movements (Smeets and Brenner 1999), a larger maximum grip aperture to obtain better precision should occur earlier in the movement. This effect of precision on the timing of maximum grip aperture has indeed been reported (McIntosh et al 2018;Zaal and Bootsma 1993).…”

“…In addition to the data of Fig. 6B involving unequal contact surfaces (Coats et al 2018), we included 4 studies on speed-accuracy trade-off in grasping in which the constraints for both digits were the same: 2 classic ones (Bootsma et al 1994;Marteniuk et al 1990) and 2 recent ones (Hoffmann et al 2019;McIntosh et al 2018).…”

“…A first constraint is related to the tolerance of the selection of grasping points: the contact surface area. For a given object size, it has been found that a larger contact surface results in a larger maximum grip aperture (Bootsma et al 1994;McIntosh et al 2018). Why does the size of the contact surface affect grip aperture?…”

“…We replotted the results of Fig. 6B (Coats et al 2018) and added the results of two other recent studies on the effect of contact surface size on the movement time in grasping (Hoffmann et al 2019;McIntosh et al 2018) as well as adding the results of two classic studies (Bootsma et al 1994;Marteniuk et al 1990). The fits were again reasonably good (R 2 ϭ 0.92, 0.93, 0.90, 0.81, and 0.68), with quite a large variation in the rate of information transfer (5.5, 10.0, 11.2, 8.8, and 18.5 bits/s).…”

Some binocular advantages for planning reach, but not grasp, components of prehension

“…We found an overall correlation between shorter movement times and wider PGAs in this condition. This relationship suggests a speed–accuracy trade-off (e.g., Wing et al 1986 ; McIntosh et al 2018 ), whereby the faster-moving participants—perhaps in an effort to grasp the more challenging object before their memorial representation of it had substantially degraded—may have built an extra safety margin into their PGA which contributed to the more marked effect we observed. Consistent with this possibility, post hoc analyses revealed a significant correlation between shorter movement durations and wider peak grips when our subjects binocularly planned to grasp the smaller (Spearman’s = 0.54, p = 0.014), but not the larger ( = 0.09, p = 0.7), object in the absence of visual feedback.…”