In recent years, we have developed experimental procedures adopted from studies of normal perception in order to investigate relative impairments in components of spatial cognition in focal and diffuse brain-damaged patients. This methodology has enabled us to isolate striking dissociation in visuospatial components in subgroups of patients with Alzheimer's disease (AD). For instance, we found that """"""""High Spatial"""""""" AD patients (i.e., patients with better block constructions than naming on clinical tests) were not free from spatial impairment. These patients exhibited a pronounced deficit in perceiving visual details (i.e., local elements) even though their perception of global shapes was considerably better. In contrast, we found that """"""""High Verbal"""""""" AD patients (i.e., patients with better naming than block constructions) were impaired in perceiving global shapes in the face of considerably better perception of visual details. An """"""""Equal"""""""" AD subgroup (i.e., patients with equivalent impairments in naming and block constructions) exhibited similar levels of dysfunction in both global and local perception (Massman et al., in press). We propose to extend these studies by investigating (1) neurocognitive mechanisms that may underlie the dissociations of visual perception in AD subgroups (e.g., the AD subgroups may differ in their ability to perceive different spatial frequencies); 2) longitudinal changes in the visual-perceptual and visual memory deficits in AD subgroups; 3) relationships between our measures of spatial cognition and neuroanatomical changes as assessed by MRI morphometric indices and, when available, postmortem neuropathological tests among AD subgroups who display differential spatial deficits (e.g., in terms of types of naming errors). These studies may lead to (1) a more accurate characterization of differential spatial deficits in AD subgroups; (2) future development of more sensitive clinical tests for early detection of perceptual deficits in AD patients; and (3) a better understanding of brain mechanisms related to spatial cognition.
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