Comparison of representational similarity of episodic memory traces at encoding and retrieval in young and older healthy adults

Healthy aging is classically associated with changes in episodic memory performance and related brain activity. We investigated here if lower performance in older is related to less similar neuronal traces between the stages of encoding and retrieval of information. 53 young (age: 23.64 + 3.07) and 63 healthy older (age: 66.24 + 4.4) subjects underwent an episodic memory task for object pictures in an fMRI scanner, with incidental encoding and yes-no recognition procedure. Representational Similarity Analyses (RSA; a Multivariate Pattern Analyses approach exploring neuronal activity by employing voxel-by-voxel variability) was employed to assess pattern similarity activity between age groups and encoding/recognition conditions. Encoding-Retrieval Similarity maps were computed for each participant at the item level (comparison of encoding and retrieval for a given item) and at the set level (comparison of each item to the average of all remaining items). Behavioral results indicates lower performance on response accuracy and reaction time in the older group (p<0.05). At the brain level, larger encoding-retrieval similarity at the item level is observed in occipital areas bilaterally and left fusiform gyrus for the young group by comparison to the older one (p<0.001 corrected at cluster level). Since the item level measures the specific reactivation of individual pictures, these results can be interpreted as less specific episodic memory traces for visual characteristics of objects in healthy aging.