Dissociating episodic from semantic access mode by mutual information measures: evidence from aging and Alzheimer's disease.

Re-thinking the semantic vs. episodic distinction with new experimental paradigms, we have designed a simple classification task to assess episodic and semantic access modes to memory for famous faces. The task requires to post 54 cards into nine mail boxes arranged in a 3x3 (nationality by field of activity) array, allowing for a quantitative analysis of the distribution of their responses, in particular of their classification errors. By using an information theoretical approach, we have developed an index of the concentration of errors, i.e. the metric content index. High levels of metric content indicate strong dependence of the classification performance on perceived relations among the set of stimuli, and therefore a preferred semantic access mode. We have found (1) a significant effect of age on the metric content, indicative of a shift from episodic to semantic access in older subjects (Experiment 1); (2) a significant correlation between the metric content and relevant measures assessing episodic and semantic retrieval mode in the Remember (R)/Know (K) paradigm introduced by Tulving [Tulving, E. 1985. Memory and consciousness. Can. Psychol. 26, 1-12] (Experiment 2); (3) a significant increase in metric content in early Alzheimer's disease patients compared to normal controls, consistent with their specific impairment in episodic access (Experiment 3).