The "What" versus the "Where" System in Alzheimer's Disease

Progress Updates

A study was undertaken to examining the characteristics of visual dysfunction in Alzheimer's disease (AD). To date, research on psychological impairments in AD has emphasized memory loss, whereas far less research has focused on perceptual impairments. Visual symptoms, however, are prevalent in AD, and vary widely among patients in their characteristics and degree of severity. To sort through the multitude of visual symptoms reported to accompany AD, a comparison was made between the degree of impairment related to visual object recognition and that related to vísuospatial relations. These two basic functions, object recognition and visuospatial ability are known to be processed in different areas of the neocortex, and to a large degree independently. It was recently reported that AD has a greater impact on those areas of the brain that process object recognition as compared to spatial relations, although both areas are affected. This pattern of pathology in the brain suggests that a separation may occur in visual symptoms between object recognition and visuospatial abilities, whereby object recognition is more severely impaired. Our research tested this hypothesis.

The perceptual abilities of 21 patients with mild to moderate AD were compared to 21 elderly control subjects. Alt subjects received eight standardized tests of visual perception, four of which tested object recognition abilities, and four of which examined visuospatial abilities. All tests minimized the involvement of memory and other cognitive functions, and were mainly influenced by perceptual abilities. Further, all subjects received a thorough eye examination to rule out possible effects of eye disease on the perceptual tests.

The results indicated that AD patients were significantly impaired on all test. Further, AD patients demonstrated a greater degree of impairment on the four tests of object recognition compared to the four tests of visuospatial abilities. This finding confirms our hypothesis and is consistent with the pattern of pathology found in the brain of AD patients. We also found however, that some AD patients were not impaired on
some visual tests, and performed similarly to the control subjects. Further analyses are now underway to analyze the pattern of impairment in individual patients.