One source of information present in a displaced visual array concerns the veridical directions of objects from the observer (Rock 1966: chaps. 2–4). Normally, when engaging in forward locomotion, the perceived radial direction of an object straight ahead of the observer’s body remains constant while the perceived radial directions of objects to either side undergo constant change. This pattern also obtains when the observer wears prisms that displace the retinal image to side. Hence, “an object seen through prisms which retains the same radial direction as we approach must be seen to be moving in toward the sagittal plane” (Rock 1966: 105). On Rock’s view, at least some forms of adaptation to ORDs can be explained by our ability to detect and exploit such invariant sources of spatial informational in locomotion-generated patterns of optic flow.