Perception of movement and shape in Alzheimer's disease.

Alzheimer's disease is the most prevalent cause of abnormal cognitive decline in older adults and commonly affects visual function. Recent evidence suggests that Alzheimer's disease impairs the processing of visual motion, but these conclusions are based on conflicting results in the few cases studied, and the processing of complex motion images has not been investigated. In the present study of motion processing in Alzheimer's disease we assessed visual functions in 63 adults: 41 with mild to moderate Alzheimer's disease (mean age 72.3 years) and 22 without dementia (mean age 71.7 years). Processing of motion cues was tested with computer animation sequences known as random-dot cinematograms, which resemble the stimuli used to define motion processing deficits in primates with lesions of cortical area MT. Results showed that participants with Alzheimer's disease required significantly greater thresholds for perceiving shapes defined by motion cues compared with participants without dementia (P = 0.0005). There were no significant differences between the two groups (P < 0.05) in static visual acuity, static spatial contrast sensitivity and, surprisingly, dynamic visual acuity, which was normal, and motion direction discrimination, which was relatively spared. We conclude that mild to moderate Alzheimer's disease has significant effects on the perception of structure from motion with relative sparing of motion direction discrimination. We cannot exclude a contribution by retinal pathology, but retinal dysfunction alone cannot explain the pattern of defects we observed. The complex motion image processing deficit we identified is likely to have a cerebral basis and has the potential to affect navigation and the recognition of objects in relative motion, as encountered during walking and automobile driving.