Differences in memory for what, where, and when components of recently-formed episodes

Abstract

An integral feature of human memory is the ability to recall past events. What distinguishes such episodic memory from associative and semantic memories is the joint encoding and retrieval of "what," "where," and "when" (WWW) of events. Here, we investigated whether the WWW components of episodes are retrieved with equal fidelity. Using a novel task where human participants were probed on the WWW components of a recently-viewed synthetic movie, we found fundamental differences in mnemonic accuracy between these components. The memory of "when" had the lowest accuracy and was most severely influenced by primacy and recency. Further, the memory of "when" and "where" were most susceptible to interference due to changes in memory load. These findings suggest that episodes are not stored and retrieved as a coherent whole. Rather, memory components preserve a degree of independence, suggesting that remembering coherent episodes is an active reconstruction process.Episodic memories store past events that shape how we remember our lives. Each episodic engram unites disparate streams from our sensory cortices into a combined representation 1 . At minimum, episodic memories bind "what", "where", and "when" (WWW) components into these engrams 2,3 . Therefore, studies in animals 4-8 and humans 1,2,[9][10][11] have focused on these key components when probing the presence of-and brain structures responsible for-episodic memory.However, little is known about the strength of association and interactions between the WWW components of episodic memory 12-14 . Put more simply: are the what, where and when components of episodic memory encoded and recalled with equal fidelity? One possibility is that an episodic engram is a holistic representation in which the WWW components are inseparably intertwined. In this case, encoding and retrieval of each component should invariably be at the same level as the other components. Alternatively, the WWW components may be stored separately, and re-joined post-hoc during retrieval to synthesize a coherent memory of a past event. Between these two extreme hypotheses, there is a spectrum in which the what, where, and when components of an event are encapsulated with various degrees of separability in an engram.A handful of studies have investigated this separability between the components of episodes 1,12-19 . An early study that tasked humans to remember series of consonants that differed in either spatial (where) or temporal (when) order found limited differences in recall accuracy once phonemic coding (saying the letters to remember them) was accounted for 12 . A more recent study attempting to probe the separability of spatial vs. temporal memory utilized objects with little resemblance to