Loss of sensitivity to dynamic occlusion in patients with Alzheimer's disease

The current study examined whether diminished sensitivity to dynamic occlusion in Alzheimer's disease (AD) contributes to 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. Six geometric solids were graphically simulated, each rendered in three texture densities, against three different levels of background texture densities. Participants identified the displayed object by pointing to the matching wooden object. Young controls were most accurate (79%), followed by elderly controls (61%) and AD patients (43%). Both control groups identified opaque objects better than transparent objects, but AD patients identified both objects equally poorly. These results demonstrate that dynamic occlusion (i.e., accretion and deletion of optical texture at the occluding edge) facilitates recovery of 3D shape from motion but such capacity is severely impaired in AD. The current results suggest the need for more research into dynamic occlusion, not only as source of information to recover 3D shape from motion, but also as a visual deficit in AD.