Neural correlates of familiarity across time scales and their involvement in explicit memory decisions

Familiarity is a type of memory signal that can support recognition of prior occurrences without retrieval of associated contextual information. It is typically probed with respect to recent laboratory exposure in recognition-memory studies involving human participants. This line of work has revealed several neural correlates including event-related potentials (ERPs) and blood-oxygenation-level-dependent (BOLD) activity in several regions. However, few studies have examined familiarity accumulated outside of laboratory settings through lifetime experience. Hence, it is currently unclear whether similar neural correlates are involved. The fluency-attribution framework decomposes familiarity judgement into automatic and decision-related processes. Since recent and lifetime familiarity are phenomenologically and experimentally dissociable for meaningful stimuli, another question is whether certain neural correlates track both types of familiarity regardless of task relevance --- as a marker of automaticity, and whether they can be distinguished from other neural correlates that are decision-related. To answer these questions, I conducted an ERP and an fMRI study using a common paradigm in which degree of recent and lifetime familiarity could be compared in both task-relevant and -irrelevant conditions. In Chapter 2, I focused on ERP (FN400/N400 and LPC) responses and found that the LPC tracked both lifetime and recent familiarity when they were relevant to the task, while the N400 tracked both types of familiarity regardless of task relevance. The FN400 was sensitive only to task-relevant recent familiarity. In Chapter 3, I focused on BOLD activity in PrC and found that the left PrC tracked both types of familiarity regardless of task-relevance, while a set of frontoparietal regions tracked only task-relevant familiarity. In Chapter 4, I attempted to further delineate the decision-related neural correlates in familiarity judgement by combining the fMRI data collected in Chapter 3 with drift-diffusion modelling (DDM). A model comparison procedure showed that familiarity effects in medial frontal regions were most strongly involved in decision-making, followed by PrC, then by medial parietal regions. Overall, these results revealed temporally (ERP) and spatially (fMRI) distinct neural correlates corresponding to the automatic and decision-related processes in both recent and lifetime familiarity judgement. Furthermore, a hierarchy exists among the decision-related neural correlates