Observer movement and size constancy

It is commonly assumed that size constancy-invariance of perceived size of objects as they change retinal size because of changes in distance-depends solely on retinal stimulation and vergence, but on no other action-related signals. Distance to an object can change through displacement of either the observer or the object. The common assumption predicts that the two types of displacement should lead to the same degree of size constancy. We measured size constancy while observers viewed stationary stimuli at different distances. Changes in distance between trials were either actively produced by the observer or generated by real or simulated object displacement, with retinal stimulation held constant across the movement conditions. Responses were always closer to perfect constancy for observer than for object movement. Thus, size constancy is enhanced by information from observer displacement, and, more generally, processes thought to be purely perceptual may have unexpected components related to action.