Grasping and hitting moving objects

Abstract

Some experimental evidence suggests that grasping should be regarded as independent control of the thumb and the index finger (digit control hypothesis). To investigate this further, we compared how the tips of the thumb and the index finger moved in space when grasping spheres to how they moved when they were hitting the sphere using only one digit. In order to make the tasks comparable, we designed the experiment in such a way that subjects contacted the spheres in about the same way in the hitting task as when grasping it. According to the digit control hypothesis, the two tasks should yield similar digit trajectories in space. People hit and grasped stationary and moving spheres. We compared the similarity of the digits' trajectories across the two tasks by evaluating the time courses of the paths of the average of the thumb and the index finger. These paths were more similar across tasks than across sphere motion, supporting the notion that grasping is not controlled fundamentally differently than hitting.

Fig. 1

Set-up and task a Top view of a single trial: the subject moved her hand from the starting position towards a sphere that was either stationary at one of three positions indicated by the three arrows or moving at a speed of 1 m/s from right to left along the horizontal track. b Three different tasks: depending on the instruction given before the trial started, the subject hit the sphere away from him or herself with the thumb (left), hit it towards him or herself with the index finger (middle) or grasped it between thumb and index finger. All three tasks were performed with both stationary and moving spheres

Fig. 2

Average path: to obtain the average path, we averaged the movement paths of the thumb and the index finger. We hypothesize that in hitting (solid lines) the thumb and the index finger might curve out more than in grasping (dashed lines) because in the former case, there is less influence of the tissue connecting the digits. However, because this influence is likely to be symmetrical, the average paths in grasping and hitting are predicted to be very similar

Fig. 3

Top view of the trajectories of the thumb and the index finger, averaged over subjects

Fig. 4

Movement time (left), peak velocity (middle) and time to peak velocity as a percentage of movement time (right) for grasping the stationary sphere (Gs), grasping the moving sphere (Gm), hitting the stationary sphere (Hs) and hitting the moving sphere (Hm)

Fig. 5

Lateral velocity as a function of time to contact. Data are the averages per subject averaged over subjects. Because we align the data on time to contact and there is variability in the movement times both within and between subject, the beginning of the average velocity is not very reliable. We only show the average from the moment that at least 2 subjects have performed at least 5 trials. Target motion clearly shifts the time of the peak velocity further to the end of the movement

Fig. 6

Average paths per condition for each subject. The average of the thumb and finger is plotted as a function of the percentage of the total movement path. If the thumb curves out further than the index finger, this value is negative. The bar graph shows the average (mean ± SE) absolute differences between the conditions (differences between the curves), ^† = one-sided t test not significant (the large difference is in the opposite direction of the hypothesis), ns = two-sided t test not significant. As hypothesized, moving towards a moving and a stationary target are not more similar than grasping and hitting (on the contrary, the left pair of bars is lower than the right pair suggesting that grasping and hitting are more similar than moving towards a moving and a stationary target) and the effect of target motion the average paths is similar for grasping as for hitting (the heights of the right two bars do not differ significantly). The mean difference between the individual subjects’ average paths is shown as an open bar

Fig. 7

Top view of the trajectories of the thumb and the index finger averaged over subjects

Fig. 8

Lateral velocity as function of time to contact. See Fig. 5 for further details. Reversing the direction of the sphere’s motion compared to Experiment 1 (from left to right rather than from right to left) reduced the difference between the velocity profiles for static and moving spheres

Fig. 9

Average differences between the conditions. Grasping and hitting differ as much from each other as moving towards a moving and a stationary target, and the effect of target motion the average paths is the same for grasping as for hitting. Values shown are means ± SE, ^† = one-sided t test not significant, ns = two-sided t test not significant. The mean difference between the individual subjects’ average paths is shown as an open bar

Fig. 10

Top view of the trajectories of the thumb and the index finger, averaged over subjects

Fig. 11

Lateral velocity as a function of time to contact. See Fig. 5 for further details

Fig. 12

Average differences between the conditions. Grasping and hitting differ as much from each other as moving towards a moving and a stationary target and the effect of target motion on the average paths is the same for grasping as for hitting. Values shown are means ± SE, ^† = one-sided t test not significant, ns = two-sided t test not significant. The mean difference between the individual subjects’ average paths is shown as an open bar