Task-relevant representational formats in multi-layered memory traces
During encoding, stimuli are embedded into memory traces that allow for their later retrieval. This process is selective, however, because not every aspect of our experiences can be remembered. In addition, post-encoding stages including consolidation are widely assumed to induce transformation processes of the memory trace. It is unclear, however, how selective the memory trace is, whether irrelevant information is completely removed during encoding and/or consolidation, and how this affects retrieval of either general (gist-like) or specific (perceptual) information. Here we show that memory traces consist of multi-layered representational spaces whose formats are flexibly strengthened or weakened during encoding and consolidation depending on task instructions, distinctly shaping their affordances for general or specific retrieval. In a series of behavioral experiments, participants first compared pairs of natural images on either two conceptual or two perceptual dimensions, leading them to incorporate the images into representational spaces defined by Euclidean distances. We found that distances in task-relevant but not irrelevant spaces affected memory strengths. Conceptual encoding benefitted general without impairing specific retrieval, suggesting that perceptual information remained in the memory trace even if it was task-irrelevant. By contrast, targeted memory reactivation (TMR) of conceptual encoding improved memory strength but deteriorated perceptual discrimination during retrieval, indicating that it weakened the accessibility of perceptual formats. Our results demonstrate the flexibility of representational formats that are incorporated into memory traces, and more generally show how the organization of information in representational spaces shapes human behavior.