Exploring the domain specificity and the neural correlates of memory unawareness in Alzheimer's disease

Patients with Alzheimer's disease (AD) are less accurate than controls to predict their episodic performance, but they are as accurate as controls to predict their semantic performance. However, the dissociation between episodic and semantic metamemory had never been tested directly in the same patients. This study aimed to explore the dissociation between episodic and semantic metamemory in AD using the feeling-of-knowing paradigm. In addition, we investigated the link between memory awareness and resting-state cerebral glucose metabolism and gray matter density, in episodic and semantic tasks independently. Data from 50 patients with AD were compared to data from 30 healthy controls. Results showed that patients with AD had more difficulties to predict their recognition in the episodic task than in the semantic task, while this difference was smaller in controls. However, this dissociation was only shown with a measure of absolute accuracy, but not with a measure of relative accuracy. Lack of awareness in the episodic task was associated with hypometabolism in right frontoparietal areas in patients with AD, while semantic metamemory was associated with gray matter integrity in the left angular gyrus. The consequence of metacognitive bias and memory status on metamemory judgments are discussed.