Semantic memory and Alzheimer's disease: a normative, neuropsychological and fmri study

Abstract

The main aim of this doctoral thesis was to investigate the organization of semantic memory, and its degradation in patients with Alzheimer’s disease (AD). This thesis includes normative, neuropsychological and functional-Magnetic-Resonance-Imaging (fMRI) investigations, in order to provide novel methodological tools and empirical evidence concerning this important issue. The normative study involves the construction and standardization of two new batteries of semantic memory tests, concerning concrete and abstract concepts respectively, on two large samples of healthy subjects. These tools entail a number of advantages over the existing alternatives, in that they control for several variables that are known to influence subjects’ performance. These two batteries were then used to investigate three different and crucial aspects of semantic memory impairments in Alzheimer’s disease, namely, 1) the semantic degradation at the feature level and its relationship with picture naming performance, 2) the presence of the Living/Non Living dissociation, and 3) the status of the abstract conceptual domain. 1) A different pattern of deterioration for different types of distinctive features (with respect to the shared ones) emerged in relation to the performance obtained in a picture naming task. It can be assumed that not all the distinctive features of a concept have the same importance in leading to a confusion between close concepts, and consequently to a picture naming deficit in AD. 2) A genuine category-specific effect, with a worse performance on Living items compared to Non Living ones, could be only observed in few individual patients, after taking in account all the methodological confounds. 3) Contrary to the few evidence reported in literature, not all AD patients show an impairment for abstract words, at least at the early stages of disease. Interestingly, a selective sparing of the emotion words arose as well. The fMRI study was aimed at adding further evidence to the neural bases of semantic memory, and particularly to the issue of category-specificity. Stimuli carefully controlled for several variables were employed and healthy elderly individuals were recruited for this study. A greater activation for Non Living (compared to Living) is reported in the inferior parietal lobule, region associated with the manipulability of the objects.