| Patients with Alzheimer's Disease (AD) and patients with Semantic Dementia (SD) both exhibit impairments on explicit tasks of semantic memory such as picture naming and category fluency. The deficits in both patient groups have been attributed to a degradation of the central semantic network. An alternative explanation for the semantic memory deficits in AD is that the ability to consciously retrieve items from the semantic network is impaired. The present study used both implicit and explicit tests to evaluate the semantic networks of both patient groups and dissociate contrasting explanations for the observed deficits in AD patients. Experiment 1 used an implicit lexical-decision priming task to examine the integrity of the underlying semantic network. Semantic priming effects were evaluated for three types of semantic relationships (attributes, category coordinates, and category superordinates) and compared to lexical associative priming. Experiment 2 then tested the patients with two sets of semantic questions (direct and indirect) to evaluate the explicit retention and processing of semantic attributes. The AD (n=11) and SD (n=11) patient groups were matched for age, education, level of dementia and impairment on four explicit semantic memory tasks. Healthy controls (n=22) were matched to the patients on age and level of education. In Experiment 1, the Healthy controls showed significant priming across all conditions. In contrast, AD patients showed normal category superordinate priming, and significant (although somewhat reduced) category coordinate priming, but no attribute priming. SD patients showed no priming effect for any semantic relationship. All three groups showed significant lexical associative priming. In Experiment 2, the Healthy controls performed well on both direct and indirect semantic questions, while SD patients were impaired on both question types. AD patients performed normally on indirect semantic questions, but showed a significant impairment on direct semantic questions. The findings from these two experiments suggest that SD patients do indeed have substantial degradation of semantic memory. In contrast, AD patients appear to have impaired explicit retrieval from a partially degraded semantic network. |
