Visual identification and spatial location in Alzheimer's disease

Twelve patients with Alzheimer's disease (AD) and 15 healthy elderly control subjects were shown sets of luminance-defined letters, texture-defined letters, luminance-defined squares, and texture-defined squares. They were asked to name the letters or point to the target square on each page. The stimuli were graded into four levels of difficulty based on the amount of contrast between the figure and the background. Performance was measured in terms of the maximum level of difficulty at which the participant correctly identified or located the three figures. Contrary to expectations, no significant difference was found between the performance of AD patients and control subjects on texture discrimination tasks vs. luminance discrimination tasks. However, results indicate that AD patients are impaired in performing a task requiring them to locate a texture-defined target of known shape in a noisy background field. By contrast, AD patients show no significant deficit in a task requiring them to locate a texture-defined shape in a known location. This argues that the observed deficit in the location task is not due to a failure in the system that discriminates target texture from background texture (since both location and identification tasks require the same textural discriminations), but rather to an impairment of the system responsible for "finding things" (i.e., locating known targets at unknown locations). This observation suggests that AD patients may suffer selective damage to the dorsal "Where" pathway, which is responsible for localizing objects in space.