Objective: The aim of the present study was to investigate the qualitative characteristics of semantic impairment in patients with mild to moderate Alzheimer's disease (AD). In particular, we wanted to verify if subordinate and distinctive concept features are affected earlier than superordinate or shared ones and whether sensory features are more vulnerable than nonsensory ones. Also, we investigated if feature correlation and level of feature occurrence in concept description (dominance) influence the resilience of concept features to degenerative damage. Methods: We administered a feature-listing task to nine mild and 10 moderate AD patients and 19 matched healthy controls. The concepts utilized were taken from a normative database, which allowed quantifying semantic indexes of the concepts representation, that is, feature type, distinctiveness, correlation, and dominance. Results: Data confirmed that although semantic knowledge of concepts is overall impoverished in AD patients, (a) superordinate knowledge is better preserved than other types of knowledge, (b) nonsensory information is more preserved than sensory information, and (c) highly shared features are less affected than distinctive ones. Furthermore, patients' feature production was influenced by dominance and by age of acquisition. Conclusions: These data are in line with theories postulating that among the dimensions along which feature-based semantic representations may vary, those relative to type and distinctiveness are most sensitive to the differential effect of AD. Results also showed a preserved implicit knowledge about how informative a given feature is in these patients and highlighted the importance of early acquisition of concept knowledge for resilience to degenerative damage.
