Large perspective changes yield perception of metric shape that allows accurate feedforward reaches-to-grasp and it persists after the optic flow has stopped!

Lee et al. (Percept Psychophys 70:1032-1046, 2008a) investigated whether visual perception of metric shape could be calibrated when used to guide feedforward reaches-to-grasp. It could not. Seated participants viewed target objects (elliptical cylinders) in normal lighting using stereo vision and free head movements that allowed small (a parts per thousand 10A degrees) perspective changes. The authors concluded that poor perception of metric shape was the reason reaches-to-grasp should be visually guided online. However, Bingham and Lind (Percept Psychophys 70:524-540, 2008) showed that large perspective changes (a parts per thousand yen45A degrees) yield good perception of metric shape. So, now we repeated the Lee et al.'s study with the addition of information from large perspective changes. The results were accurate feedforward reaches-to-grasp reflecting accurate perception of both metric shape and metric size. Large perspective changes occur when one locomotes into a workspace in which reaches-to-grasp are subsequently performed. Does the resulting perception of metric shape persist after the large perspective changes have ceased? Experiments 2 and 3 tested reaches-to-grasp with delays (Exp. 2, 5-s delay; Exp. 3, a parts per thousand 16-s delay) and multiple objects to be grasped after a single viewing. Perception of metric shape and metric size persisted yielding accurate reaches-to-grasp. We advocate the study of nested actions using a dynamic approach to perception/action.