Does planning a different trajectory influence the choice of grasping points?

Abstract

We examined whether the movement path is considered when selecting the positions at which the digits will contact the object's surface (grasping points). Subjects grasped objects of different heights but with the same radius at various locations on a table. At some locations, one digit crossed to the side of the object opposite of where it started. In doing so, it moved over a short object whereas it curved around a tall object. This resulted in very different paths for different objects. Importantly, the selection of grasping points was unaffected. That subjects do not appear to consider the path when selecting grasping points suggests that the grasping points are selected before planning the movements towards those points.

Fig. 1

Different hand paths for objects placed at the same location. Subjects move over the sphere (a), whereas they curve around the cylinder (b)

Fig. 2

Schematic top view of the set-up. The nine possible object locations are indicated by circles. The two different starting positions are shown as blue and red discs. The yellow area indicates the horizontal grip orientation

Fig. 3

Average hand trajectories in experiment 1 (mean of 5 repetitions by each of the 10 participants for each of the 36 conditions) as seen from above (a) and from the side (b)

Fig. 4

Average maximal deviations of the digits from a straight line (mean and standard deviations across subjects) in experiment 1. Data for objects placed at the near right location. Values for each digit when reaching towards the two different objects from the two starting points

Fig. 5

The significant interactions of experiment 1. a Starting position by object location interaction for grip orientation (the inset shows the extremely small starting position by object type interaction). b Starting position by object location interaction for movement time. c Object type by object location interaction for movement time. d Starting position by object location interaction for maximal grip aperture

Fig. 6

Average hand trajectories in experiment 2 (mean of 5 repetitions by each of the 9 participants for each of the 16 conditions) as seen from above (a) and from the side (b)

Fig. 7

Average maximal deviations of the digits from a straight line (mean and standard deviations across subjects) in experiment 2. Data for objects placed at the near right location. Values for each digit when reaching towards the four different objects from the two starting points

Fig. 8

Top view of average hand trajectories in experiment 2. Movements were selected on the basis of whether the hand moved over or around the object. Despite the large differences in hand trajectories, the grasping points are very similar

Fig. 9

The significant interactions of experiment 2. a Starting position by object location interaction for grip orientation, b object type by object location interaction for movement time, and c starting position by object location interaction for maximal grip aperture