Dissociation between the detection and perception of motion in Alzheimer's disease.

To better understand the damage to the motion pathway that occurs in senile dementia of the Alzheimer type (SDAT), we developed a system to assess separately the conscious perception and unconscious detection of motion in patients with SDAT. Motion perception thresholds were significantly elevated in SDAT (n = 9) compared with controls (n = 12), but motion detection thresholds were normal. This dissociation between the perception and detection of motion in early SDAT parallels histologic evidence of a disconnection between primary and association visual cortices. This disconnection may underlie the severe visual perception deficits seen in SDAT.