Spatial cognition in Alzheimer's disease: subtypes of global-local impairment

This study investigated whether subgroups of AD patients exhibit different patterns of impairment in analyzing global (configural) and local (detail) features of complex visual stimuli. A High Spatial AD subgroup (i.e., patients with better block constructions than naming) and a High Verbal AD subgroup (i.e., patients with better naming than block constructions) were impaired in analyzing both global and local forms. As predicted, however, the High Spatial AD patients exhibited greater impairment in analyzing the local forms than the High Verbal AD patients and normal controls. In contrast, the High Verbal AD patients exhibited greater impairment in analyzing the global forms than the High Spatial AD patients and normal controls. There was a striking separation of the subgroups: Using the local-global difference score, the hit rate for classifying these patients into the two subgroups was 91%. Robust correlations were found between the AD patients' ability to construct global and local forms and their scores on traditional visuospatial and verbal tests, respectively. The findings suggest that it may be misleading to subdivide AD patients using a verbal/spatial dichotomy, because even those AD patients who appear to have relative strengths on traditional visuospatial tests are likely to exhibit a primary impairment in analyzing local features of complex visual stimuli. The results underscore the importance of a process (qualitative) approach to neuropsychological assessment for a more valid understanding of the behavioral subtypes of Alzheimer's disease.