Representation of semantic and spatial knowledge in Alzheimer's patients: implications for models of preserved learning in amnesia.

Patients with Alzheimer's disease may exhibit severe, but variable patterns of cognitive deficits which can complicate and confound investigations of memory. Three patients are described with comparable levels of intellectual and memory abilities as assessed by the Wechsler scales. One patient (W) had a relatively circumscribed impairment of word-finding ability concurrent with intact visuospatial and constructional skill. Another patient (C) showed the opposite profile of abilities, while the third patient (G) exhibited deficits in both domains. All three patients were severely and globally amnesic when tested with traditional recall and recognition procedures. Patient W was able to demonstrate preserved semantic knowledge on a superordinate and category level but not for specific object attributes. Patient C demonstrated a remarkably preserved ability to generate and copy complex, meaningful, but not meaningless material. Corresponding profiles of preserved learning were revealed, but only under conditions that limited the demands on encoding and retrieval processes. Models were offered to account for these contrasting patterns of impairment. In addition, the possibility that medial temporal structures contribute to the ability to consciously reconstruct prior experiences was discussed.