Disorders of visual and spatial perception in the early stage of Alzheimer's disease.

A battery of visual and spatial perception tests (VOSP) has been administered to a sample of 25 mild Alzheimer's disease (AD) patients and 25 age- and education-matched controls in order to assess visuospatial skills in the early phase of the disease. Among visual object perception tests, AD patients were impaired only in the task where they had to name the stimulus (Silhouettes task). The score in this task significantly correlated with the Boston Naming Test score (r = .73; p < 0.0001). The performance on the Object decision task (which does not require naming), figure-ground analysis, and identification of degraded stimuli was unimpaired. Similarly there were no significant differences for any of the spatial perception tests. The results showing that the performances on most measures of visual and spatial perception is unimpaired, indicate that the "early" perceptual processes, as well as the access to structural knowledge, are not defective in early AD. On the other hand, AD patients are impaired at the level of semantic access and name retrieval. The finding of an highly significant correlation among tests requiring visual access to structural and semantic knowledge suggests that a rigid separation between perceptual and semantic mechanisms may be unwarranted.