The "hidden" semantic category dissociation in mild-moderate Alzheimer's disease patients

In patients manifesting mild-moderate Alzheimer's disease (AD), lexical semantic tasks are known to be influenced by several variables which should be adequately taken into account when studying semantic category dissociations. The following study provides indexes of three new variables (imageability (I), percentage of name agreement (pNA) and number of target alternatives (nTA)) and investigates their role in naming in a group of people with AD and in matched older adults controls. Forty young healthy participants rated I, pNA and nTA of 155 stimuli (including living and non-living items) from and sets. Forty-eight people with mild-moderate AD and 40 older adults were given the two naming tests and their naming ratings were analysed with a two-way ANOVA (two groupsxtwo categories) to assess category specificity and the effect of interaction. The influence of relevant concomitant variables in naming was measured using a multiple regression analysis. Semi-partial correlations were carried out to assess the independent contribution of each variable to naming. We found that living items were more imageable and had fewer lexical alternatives and higher name agreement than non-living items. We also found that controls significantly named better than AD patients (F=37.551, p<.001), whilst the two-way ANOVA showed no significant effect of category (F=.649, p=.423). Notably category effect emerged when assessing its independent contribution performing a semi-partial correlation (beta=-.278, p<.001) which kept the effect of relevant concomitant variables under control. Our results confirm that category dissociation does emerge in mild-moderate AD patients when the effect of relevant concomitant variables is adequately taken into account. The hypothesis that the highly correlated properties of items from biological categories may play a protective effect on living things, making them less prone to impairment in the early stages of AD, is discussed.