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Episodic memories are comprised of details of “where” and “when”; spatial and temporal relations, respectively. However, evidence from behavioural, neuropsychological, and neuroimaging studies has provided mixed interpretations about how memories for spatial and temporal relations are organised—they may be hierarchical, fully interactive, or independent. In the current study, we examined the interaction of memory for spatial and temporal relations. Using explicit reports and eye-tracking, we assessed younger and older adults’ memory for spatial and temporal relations of objects that were presented singly across time in unique spatial locations. Explicit change detection of spatial relations was affected by a change in temporal relations, but explicit change detection of temporal relations was not affected by a change in spatial relations. Younger and older adults showed eye movement evidence of incidental memory for temporal relations, but only younger adults showed eye movement evidence of incidental memory for spatial relations. Together, these findings point towards a hierarchical organisation of relational memory. The implications of these findings are discussed in the context of the neural mechanisms that may support such a hierarchical organisation of memory. [ABSTRACT FROM PUBLISHER]

Over 50 years of research has revealed a critical role for the hippocampus in the formation of long-term declarative memories. More recent evidence has specified the functions of the hippocampus as the binding and comparison of memory representations that may be used under shorter, as well as longer, delays (Olsen, Moses, Riggs, & Ryan, 2012). Hippocampal neural oscillations (e.g., theta rhythm) have been studied extensively in animals; however, the oscillations that underlie binding, comparison, and their relationship to memory performance remain to be fully explored in humans. Here magnetoencepha-lography was used to examine theta oscillations within the hippocampus and cortex to address this critical gap in the literature. The task consisted of (a) an encoding phase in which participants had to integrate the relative spatial positions among 3 sequentially presented objects, (b) a delay phase, and (c) a test phase in which all study objects were presented simultaneously in novel locations, and participants had to indicate whether the relative positions had changed. Theta power in the hippocampus and medial prefrontal cortex (PFC) increased across encoding and delay periods during which binding and maintenance processes dominate, while comparison of spatial relations at test was associated with greater theta power in right lateral PFC and intraparietal sulcus for manipulated versus intact trials. Critically, relational memory was positively related to hippocampal theta power increases across the encoding period. These findings provide novel evidence for the role of hippocampal theta in the incremental formation and retention of relations across space and time. [ABSTRACT FROM AUTHOR]