Neural Dynamics and Determinants of Mnemonic Representations

Abstract

This dissertation explored the dynamics of memory representations across different phases and the lifespan, introducing novel methods and insights into cortical shifts during memory processes. Chapter 2 presents trial-level representational similarity analysis (tRSA), a methodological advance that improves detection of neural representations at the single-trial level. Unlike classical RSA, tRSA accounts for trial-level variability, offering a refined approach to detecting representational strength in fMRI studies where trial count variability can introduce noise that hampers within- and between- participant statistical inference. Chapter 3 applied tRSA to investigate shifts in memory representations between encoding and retrieval (RERS). Objects were represented in posterior occipitotemporal cortex during encoding but shifted to anterior parietal cortex during retrieval, consistent with prior research. Additionally, an encoding-retrieval flip was observed that supports the view that RERS may occur because of shifts in attentional focus. Specifically, parietal representations, which may fluctuate with internal attention, were detrimental to memory performance during encoding, but support memory during retrieval. Chapter 4 examined age-related changes in representations, revealing age-related reductions in visual representational strength in posterior occipitotemporal cortex. Mediation and moderation analyses revealed that the decline in visual representation strength is driven by declining structural connectivity of visual representation regions. Older adults also exhibited increases in semantic representations in anterior brain regions and, notably, perceptual memory performance correlated with stronger semantic representations, supporting the compensatory role of semantics in anterior cortical areas. Moreover, older adults with preserved anterior connectivity exhibited stronger semantic representations, highlighting that intact structural integrity is critical for facilitating the compensation. Together, these findings provide a comprehensive view dynamic memory representation. This work affords a foundation for future research to probe how representations dynamically shift with memory phase, changes in structural integrity, and compensatory processes.