Similarities between digits' movements in grasping, touching and pushing

Abstract

In order to find out whether the movements of single digits are controlled in a special way when grasping, we compared the movements of the digits when grasping an object with their movements in comparable single-digit tasks: pushing or lightly tapping the same object at the same place. The movements of the digits in grasping were very similar to the movements in the single-digit tasks. To determine to what extent the hand transport and grip formation in grasping emerges from a synchronised motion of individual digits, we combined movements of finger and thumb in the single-digit tasks to obtain hypothetical transport and grip components. We found a larger peak grip aperture earlier in the movement for the single-digit tasks. The timing of peak grip aperture depended in the same way on its size for all tasks. Furthermore, the deviations from a straight line of the transport component differed considerably between subjects, but were remarkably similar across tasks. These results support the idea that grasping should be regarded as consisting of moving the digits, rather than transporting the hand and shaping the grip.

Fig. 1

Top view of the experiment. The two thin lines indicate the position to which the subject had to push the cube. The dashed lines indicate how the cube was aligned to the subject. The thick curves are hypothetical paths of index finger and thumb

Fig. 2

Example trajectories. Top view of subject S6’s movement paths. Thin curves represent single trials; dark thick curves represent average paths. The movements of thumb (continuous lines), index finger (dotted lines) and cube (dashed lines) are indicated for grasping the object (left panel, grey curves), and for pushing the object with the thumb (green curves, central panel) and index finger (magenta curves, right panel)

Fig. 3

Overview of the active digits’ paths when performing the three tasks. A top view ( left column) and a side view (right column) is given for each subject (S1–S8). The colours and styles of the curves indicate the task: continuous black: grasping; dotted red: touch with thumb; dotted blue: touch with finger; dashed green: push with thumb; dashed magenta: push with finger

Fig. 4

Finger–thumb differences in the digits paths. The finger–thumb difference (the lateral deviation in the average of the active finger and thumb) is plotted as a function of the percentage of the movement to the cube. For grasping, the finger–thumb difference is equivalent to the lateral deviation of the transport component. If the thumb curves farther out than the finger, this measure will be positive

Fig. 5

Grip aperture in the three tasks. In all tasks, grip aperture is defined as the distance between the average path of the active index finger and active thumb at corresponding positions on their paths. For touching and pushing, such apertures obviously never occurred in a single trial. a The distance between the digits as a function of time. b, c The relation between peak grip aperture and the position (B) or time (C) of its occurrence. Different tasks are indicated by different symbols; different subjects by different colours (colour coding as in Fig. 4). Continuous line: straight line fit to the data of all tasks; dashed curve: minimum-jerk prediction (no fit parameters)

Fig. 6

Variability in the digits’ end positions in the five tasks. The standard deviation parallel to the cube’s surface (the square root of the sum of the variances in the forward and vertical direction). The variability in the active digit’s position is the same for the two digits in the three tasks and is much smaller than that of the passive digit in touching and pointing. Error bars indicate the standard error of the mean (across subjects)