Long-term associative memory and spatial pattern separation impairments in individuals with subjective cognitive decline: A neuropsychological and medial temporal lobe subregions volumetric analysis

Subjective cognitive decline (SCD) has been proposed as a potential preclinical stage of Alzheimer's disease (AD) dementia. This study aimed to investigate whether objective impairments could be identified in SCD on highly demanding memory tasks and their possible associations with medial temporal lobe (MTL) volumes. The performance of 31 individuals with SCD and 29 healthy elderly with no worries of cognitive decline (HC) was compared on two experimental tasks assessing respectively face–name-occupation associative memory and spatial pattern separation. The diagnostic power of these tests in classifying cognitive status was assessed. In addition, a sub-group of 20 SCD and 19 HC underwent a 3T-MRI. Volumes of individual hippocampal subfields and surrounding cortices within the MTL were calculated and compared between the two groups. Finally, possible associations between brain volumes and performance on experimental tasks were evaluated. While traditional neuropsychological tests showed no significant between-group differences, SCDs obtained significantly lower scores than HCs on experimental tasks. These measures also correctly classified group membership with good overall accuracy. Volumetric data revealed significant between-group differences in specific hippocampal subfields (particularly CA1 and dentate gyrus) and surrounding cortices (particularly entorhinal and perirhinal cortices). Furthermore, lower scores on experimental tasks significantly correlated with reduced volumes in specific MTL sub-regions (particularly CA1 and perirhinal cortices). These findings provide the first evidence in SCD of an association between objective memory impairments in associative memory and spatial pattern separation and volume reductions in specific MTL sub-regions known to be primarily vulnerable to AD neuropathology.