Predicting the duration of reach-to-grasp movements to objects with asymmetric contact surfaces

Abstract

The duration of reach-to-grasp movements is influenced by the size of the contact surfaces, such that grasping objects with smaller contact surface areas takes longer. But what is the influence of asymmetric contact surfaces? In Experiment 1a, participants reached-to-lift wooden blocks off a table top, with the contact locations for the thumb and index finger varying in surface size. The time taken to lift the block was driven primarily by the thumb contact surface, which showed a larger effect size for the dependent variable of movement duration than the index finger’s contact surface. In Experiment 1b participants reached-to-grasp (but not lift) the blocks. The same effect was found with duration being largely driven by contact surface size for the thumb. Experiment 2 tested whether this finding generalised to movements towards conical frusta grasped in a different plane mounted off the table top. Experiment 2 showed that movement duration again was dictated primarily by the size of the thumb’s contact surface. The thumb contact surface was the visible surface in experiments 1 and 2 so Experiment 3 explored grasping when the index finger’s contact surface was visible (participants grasped the frusta with the index finger at the top). An interaction between thumb and finger surface size was now found to determine movement duration. These findings provide the first empirical report of the impact of asymmetric contact surfaces on prehension, and may have implications for scientists who wish to model reach-to-grasp behaviours.

A review of grasping as the movements of digits in space

“…Dashed line indicates the best linear fit. B: combining movements of finger and thumb in grasping an asymmetric object (Experiments 1a, 2, and 3 from Coats et al 2018). Four different distances were used (not indicated; longer distances have higher difficulty).…”

“…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).…”

“…1) can predict the speedaccuracy relationship in grasping. Recently, Coats et al (2018) performed an experiment that is very well-suited to test whether this is the case. They varied the size of the contact surface that was available for the thumb independently of that for the index finger (both 1, 2, or 3 cm wide) and also varied movement distance (10, 30, and 50 cm in their Experiment 1a and 10, 20, and 30 cm in their Experiments 2 and 3).…”

“…According to the digit-in-space framework, grasping can be regarded as two digits of the same hand moving to targets that are on a single object. We, therefore, plotted the results of the three experiments of Coats et al (2018) as a function of the digit difficulty (Fig. 6B).…”

“…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).…”