The target as an obstacle: Grasping an object at different heights.

Humans use a stereotypical movement pattern to grasp a target object. What is the cause of this stereotypical pattern? One of the possible factors is that the target object is considered an obstacle at positions other than the envisioned goal positions for the digits: while each digit aims for a goal position on the target object, they avoid other positions on the target object even if these positions do not obstruct the movement. According to this hypothesis, the maximum grip aperture will be higher if the risk of colliding with the target object is larger. Based on this hypothesis, we made a set of two unique predictions for grasping a vertically oriented cuboid at its sides at different heights. For cuboids of the same height, the maximum grip aperture will be smaller when grasped higher. For cuboids whose height varies with grip height, the maximum grip aperture will be larger when grasped higher. Both predicted relations were experimentally confirmed. This result supports the idea that considering the target object as an obstacle at positions other than the envisioned goal positions for the digits is underlying the stereotypical movement patterns in grasping. The goal positions of the digits thus influence the maximum grip aperture even if the distance between the goal positions on the target object does not change.

The influence of target object shape on maximum grip aperture in human grasping movements.

The shape of a target object could influence maximum grip aperture in human grasping movements in several different ways. Maximum grip aperture could be influenced by the required precision of digit placement, by the aim to avoid colliding with the wrong parts of the target objects, by the mass of the target objects, or by (mis)judging the width or the volume of the target objects. To examine the influence of these five factors, we asked subjects to grasp five differently shaped target objects with the same maximal width, height and depth and compared their maximum grip aperture with what one would expect for each of the five factors. The five target objects, a cube, a three-dimensional plus sign, a rectangular block, a cylinder and a sphere, were all grasped with the same final grip aperture. The experimentally observed maximum grip apertures correlated poorly with the maximum grip apertures that were expected on the basis of the required precision, the actual mass, the perceived width and the perceived volume. They correlated much better with the maximum grip apertures that were expected on the basis of avoiding unintended collisions with the target object. We propose that the influence of target object shape on maximum grip aperture might primarily be the result of the need to avoid colliding with the wrong parts of the target object.

The influence of object height on maximum grip aperture in empirical and modeled data.

During a grasping movement, the maximum grip aperture (MGA) is almost linearly scaled to the dimension of the target along which it is grasped. There is still a surprising uncertainty concerning the influence of the other target dimensions on the MGA. We asked healthy participants to grasp cuboids always along the object's width with their thumb and index finger. Independent from variations of object width, we systematically varied height and depth of these target objects. We found that taller objects were generally grasped with a larger MGA. At the same time, the slope of the regression of MGA on object width decreased with increasing target height. In contrast, we found no effect of varying target depth on the MGA. Simulating these movements with a grasping model in which the objective to avoid contact of the digits with the target object at positions other than the goal positions was implemented yielded larger effects of target height than of target depth on MGA. We concluded that MGA does not only depend on the dimension of the target object along which it is grasped. Furthermore, the effects of the other 2 dimensions are considerably different. This pattern of results can partially be explained by the aim to avoid contacting the target object at positions other than the goal positions.

Why does an obstacle just below the digits' paths not influence a grasping movement while an obstacle to the side of their paths does?

When we grasp objects in daily life, they are often surrounded by obstacles. To decrease the chance of colliding with an obstacle, people tend to move in a manner that does not bring body parts too near to the obstacle. However, in a previous study, when we compared moving above empty space and moving above an obstacle (a table), we did not find an effect of the obstacle on the height of the digit's paths despite the fact that the distance between the final positions of the digits and the obstacle was marginal. This lack of effect seems to be inconsistent with what we know about avoiding obstacles, because we would expect an increase in the height of the digits' paths when the obstacle is present. We consider four possible explanations for the lack of effect: that people changed movement speed rather than movement path, that the height component is not sensitive to obstacles that do not physically obstruct the movement, that obstacles below the starting position are not taken into account because the digits do not enter the space below the starting position, and that manipulable obstacles interfere with movement planning while a table does not. We found that from these four explanations only not taking obstacles placed below the starting position into account can be responsible for the lack of effect found in our previous study.

Gravity affects the vertical curvature in human grasping movements.

When humans make grasping movements their digits' paths are curved vertically. In a previous study the authors found that this curvature is largely caused by the local constraints at the start and end of the movement. Here the authors examined the contribution of gravity to the part of the curvature that was not explained by the local constraints. Subjects had to grasp a tealight (small cylinder) while sitting on a chair. The authors could rotate the whole setup, including the subject, relative to gravity, whereby the positions of the starting point and of the tealight relative to the subject did not change. They found differences between the paths that are consistent with a direct effect of gravity pulling the arm downward.

Why are the digits' paths curved vertically in human grasping movements?

When humans grasp an object off a table, their digits generally move higher than the line between their starting positions and the positions at which they end on the target object, so that the digits' paths are curved when viewed from the side. We hypothesized that this curvature is caused by limitations imposed by the environment. We distinguish between local constraints that act only at the very beginning or the very end of the movement, and global constraints that act during the movement. In order to find out whether the table causes this vertical curvature by acting as a global constraint, we compared grasping a target object positioned on a table with the same task without the table. The presence of the table did not affect the vertical curvature. To find out whether constraints at the beginning and end of the movement cause the vertical curvature, we manipulated the constraints locally at those positions by letting the subject start with his digits either above or below the end of a rod and by attaching the target object either to the top or to the bottom of another rod. The local constraints at the start of the movement largely explain the vertically curved shape of the digits' paths.

Grasping kinematics from the perspective of the individual digits: a modelling study.

Grasping is a prototype of human motor coordination. Nevertheless, it is not known what determines the typical movement patterns of grasping. One way to approach this issue is by building models. We developed a model based on the movements of the individual digits. In our model the following objectives were taken into account for each digit: move smoothly to the preselected goal position on the object without hitting other surfaces, arrive at about the same time as the other digit and never move too far from the other digit. These objectives were implemented by regarding the tips of the digits as point masses with a spring between them, each attracted to its goal position and repelled from objects' surfaces. Their movements were damped. Using a single set of parameters, our model can reproduce a wider variety of experimental findings than any previous model of grasping. Apart from reproducing known effects (even the angles under which digits approach trapezoidal objects' surfaces, which no other model can explain), our model predicted that the increase in maximum grip aperture with object size should be greater for blocks than for cylinders. A survey of the literature shows that this is indeed how humans behave. The model can also adequately predict how single digit pointing movements are made. This supports the idea that grasping kinematics follow from the movements of the individual digits.