Abstract
At the core of Gibson's ecological psychology is the notion of invariant—specifically, an invariant pattern in the changing energy flux. Among the invariants identified to date are texture gradients, horizon ratio, optic flow, and tau. Gibson considered his discovery of the occluding edge the most radical because observers can perceive the layout of the environment (both unprojected and projected surfaces); and the accretion and deletion of optical texture, or dynamic occlusion, at the occluding edge resulting from observer movement produces an invariant pattern. Here I present the results of an experiment demonstrating diminished sensitivity to dynamic occlusion in Alzheimer's disease (AD) that led to their reduced capacity to recover 3D shape from motion. Young controls, age-matched elderly controls, and AD patients participated in the study. Participants watched computer simulations of an object, depicted as either transparent or opaque, rotating about the vertical axis against a background rendered in random dot texture. Young controls were most accurate, followed by elderly controls and AD patients. Both control groups identified opaque objects better than transparent objects, but AD patients identified both objects equally poorly. These results demonstrate the facilitating effect of the dynamic occlusion invariant to recover 3D shape from motion, the capacity of which is severely impaired in AD.