Grasping and representational momentum

Observers tend to misremember the stopping point of a change in the direction of the change. We investigated whether this representational momentum effect is reflected in grasping. To do this, we presented 14 subjects with a sequence of 3 still images in which a pair of household pliers was seen to open or close in 1 cm steps. In a visual task, subjects indicated whether a fourth, probe image differed from the stopping point of the sequence. In a grasping task, subjects reached out and closed a virtual version of the third pair of pliers, just after the image disappeared. The subjects' thumb and index finger were attached to robot arms which allowed us to provide haptic feedback and to measure the movement of the digits. When grasping identical versions of the third pliers, subjects opened their digits wider if the width of the pliers had been increasing compared to when the width had been decreasing. This is consistent with representational momentum. The direction of change (decreasing or increasing pliers width) had an effect equivalent to 4 mm physical width variation in grasping. In the visual task, the pliers that were perceived as equal to the third ones tended to have a larger opening width when the width had been increasing than when it had been decreasing. This is also consistent with representational momentum. However, the visual effect was only significant for subjects who did the visual task first (p<.01). For these subjects, the size of the effect was 1 mm pliers width. Although subjects were asked to grasp the third pliers and got the appropriate haptic feedback, they apparently extrapolated the opening or closing of the pliers. This grasping effect does not appear to be directly related to the visual representational momentum effect, because the impact of direction was larger and more reliable in grasping than in perception and the effects were not correlated between subjects