Probing the neural dynamics of mnemonic representations in humans

Abstract

Memories are not stored as static engrams, but as dynamic representations affected by processes occurring after initial encoding or even consolidation. How the modulation of memory traces after their formation is reflected in the neural activity during subsequent retrieval is currently not well understood. Using fMRI in 27 healthy human participants, we probed how neural representations of associative memories are dynamically modulated by two behavioral techniques that can either strengthen or weaken memories after encoding. Behaviorally, we demonstrated that, after an initial delay of 24 hours, associative memories can still be strengthened or weakened by repeated retrieval or suppression, respectively. Neurally, we show that repeated retrieval dynamically reduced activity amplitude in ventral visual cortex and hippocampus, but enhanced the distinctiveness of activity patterns in the ventral visual cortex. Critically, a larger reduction of activity amplitude in the ventral visual cortex associated with larger enhancement of distinctiveness of activity patterns in the same region. In contrast, repeated memory suppression was associated with reduced lateral prefrontal activity, but relative intact activity patterns. These results reveal dynamic adaptations of mnemonic representations in the human brains and how retrieval-related activity amplitude and distinctiveness of activation patterns change as a function of strengthening or weakening.