Grasping the Müller-Lyer illusion: not a change in perceived length.

Peak grip aperture has often been used to quantify the influence of illusions on judgments of size for action. However, a larger peak grip aperture need not mean that the object looks larger. It could also mean that it was grasped more carefully. These two possibilities can be distinguished on the basis of the velocity of grip closure just before contact. We let people grasp a bar that was placed on the shaft of a Müller-Lyer figure. The Müller-Lyer figure influenced the peak grip aperture. It did not influence the velocity of grip closure in the way that one would expect if size were mis-perceived. Thus there is no reason to assume that the perceived size guides the way that we reach and grasp an object.

The relation between force and movement when grasping an object with a precision grip.

When reaching out for objects, the digits' paths curve so that they approach their positions of contact moving more or less perpendicularly to the local surface orientation. This increases the accuracy of positioning the digits and ensures that any forces exerted at contact are nearly perpendicular to the surface, so that friction will prevent the digits from slipping along the surface. When lifting the object a similar force perpendicular to the surface is needed to prevent the object from slipping from one's fingers. In order to determine whether these two issues are dealt with simultaneously we let subjects pick up a cube from three different starting positions and measured the digits' movements and forces from before contact until the moment the cube started moving. The impact force was low. After impact, the digits spent about 200 ms in contact with the surface of the cube before the latter started to move. The digits first decelerated, and then they gradually built up the grip- and lift forces to move the cube upwards. We found no direct relationship between the control of the reaching movement towards the object and the force applied at the surface of the object to pick it up. We conclude that the reaching and lifting movements are quite independent.

Effects of the Ebbinghaus figure on grasping are not only due to misjudged size.

It is not evident how the small effects of the flankers of the Ebbinghaus figure on peak grip aperture (PGA) should be interpreted. One interpretation is that the flankers influence the estimated size, which in turn influences the grasp. If this interpretation is correct, then only the size-dependent aspects of the grasping movement should depend on the spatial positions of the flankers. An alternative interpretation is that the effect on grip aperture is caused by a change in judgement of the required precision, in which case various aspects of the grasping movement could be influenced by the size and position of the flankers. We presented subjects with a display consisting of a central disk surrounded by four large or small flankers. The array of circular flankers could be rotated by 45 degrees . There were two tasks: to reproduce the perceived size of the central disk, and to grasp the central disk. As in other studies, the reproduced size and the PGA were both influenced by the size of the flankers. The effect on reproduced size settings was independent of the flankers' spatial position. Nevertheless, the flankers' position did influence the final grip aperture and the grip orientation at PGA and at movement offset. Because the flankers changed more than only the PGA, we conclude that the effect of the flankers on prehension cannot only be because of misjudgement of the size of the central disk.

Impact forces cannot explain the one-target advantage in rapid aimed hand movements.

A pointing movement is executed faster when a subject is allowed to stop at the first target than when the subject has to proceed to a second target ("one-target advantage"). Our hypothesis was that this is because the impact at the target helps to stop the finger when the finger does not have to proceed to a second target. This hypothesis would predict that the horizontal force at contact with the first target should be larger when there is only one-target. Modelling smooth movements with larger forces at contact using a minimum-jerk model, shows that the peak velocity is slightly higher and it occurs later during the movement when there is only one target. Although the one-target advantage was present in our experiment, the horizontal force at contact in the one-target condition was not larger than in the two-target condition. The time of the maximum velocity did not differ, but the maximum velocity was higher in the one-target condition. Thus our hypothesis is rejected, favouring a non-mechanical explanation of the one-target advantage.

The influence of obstacles on the speed of grasping.

The movement time of a reach-to-grasp movement increases when obstacles are placed close to the target object. We investigated whether this increase can best be explained by limits on the grip aperture or by limits on the paths of the individual digits. In our experiment subjects were instructed to pick up an object with their index finger and thumb. There was an obstacle at either side of the object. The increase in movement time when either obstacle was placed closer to the object was best described by a model in which the movement amplitude and the distance between each obstacle and the target object are independent factors. We conclude that the way that obstacles influence the movement time in reach-to-grasp movements is determined by the extent to which they limit the digits' paths.

Independent control of the digits predicts an apparent hierarchy of visuomotor channels in grasping.

If an object changes position at the onset of a reach-to-grasp movement, both the transport speed and the grip aperture are adjusted. If the object changes in size at the onset, only the grip aperture is adjusted. This combination of results has been interpreted as being the consequence of a hierarchical relationship between visuomotor channels for transport and grip. We argue that our alternative view on grasping can account for the observed behaviour without making new assumptions. In our view, grasping consists of smooth (minimal jerk) movements of each digit to a target position on the object. The digits' target positions change, both when object position and when object size change. A model in which the individual digits move smoothly to these new positions yields the same behaviour as is observed experimentally.