Anticipating translating versus transforming objects: Visual perception and grasping

Observers tend to misremember the stopping point of a movement or change as being further forward in the direction of movement or change, a phenomenon known as representational momentum. Recent findings suggest that this anticipation-like effect is stronger for changes in position (object translation) than for changes in shape or size (object transformation). Here, we asked subjects to judge the final distance between two spheres after this distance had been increasing or decreasing. The spheres were two separate translating objects or were connected to form a single transforming object (a dumbbell). Subjects performed a perceptual judgment task and a motor task in which they grasped the final objects. For grasping, the subject's thumb and index finger were attached to two robot arms (PHANToM ™) which provided haptic feedback and allowed us to measure the maximum grip aperture. Results from the perceptual task showed that subjects always remembered the final distance between the spheres as larger when that distance had been increasing compared with that when it had been decreasing, regardless of stimulus type. However, for the dumbbells, (a) the effect of transformation direction was reduced, and (b) there was a stronger bias to remember the final distance as smaller than it actually was. For grasping, only the dumbbells produced an anticipation-like effect (with subjects opening their fingers wider if the dumbbells were growing compared to when they were shrinking), even though the grasp locations were identical for both types of stimuli. Overall, while anticipation-like effects can be observed in both perception and grasping, these two effects were modulated in different ways by our stimulus manipulations and, when they did appear together, were uncorrelated for individual subjects