Abstract
An essential feature of episodic memory is the ability to recall the multiple elements relating to one event from the multitude of elements relating to other, potentially similar events. Hippocampal pattern separation is thought to play a fundamental role in this process, by orthogonalizing the representations of overlapping events during encoding, to reduce interference between them during the process of pattern completion by which one or other is recalled. We introduce a new paradigm to test the hypothesis that similar memories, but not unrelated memories, are actively separated at encoding. Participants memorized events which were either unique or shared a common element with another event (paired “overlapping” events). We used a measure of dependency, originally devised to measure pattern completion, to quantify how much the probability of successfully retrieving associations from one event depends on successful retrieval of associations from the same event, an unrelated event or the overlapping event. In two experiments, we saw that within event retrievals were highly dependent, indicating pattern completion; retrievals from unrelated events were independent; and retrievals from overlapping events were antidependent (i.e., less than independent), indicating pattern separation. This suggests that representations of similar (overlapping) memories are actively separated, resulting in lowered dependency of retrieval performance between them, as would be predicted by the pattern separation account.
Highlights
High similarity of the neural patterns associated with different memories leads to decreased accuracy (Hopfield 1982; Treves and Rolls 1992)
The mnemonic similarity task (MST) requires participants to recognize whether a probe item is old, similar, or new, with pattern separation indexed as the successful identification of similar items (Kirwan and Stark 2007; Stark et al 2015)
Functional neuroimaging studies using the MST sought to solve the problem of explicit recall-to-reject by using incidental encoding and found greater activity in the dentate gyrus (DG)/ CA3 subregion of the hippocampus during presentation of a foil compared to presentation of a previously seen item (Bakker et al 2008; Lacy et al 2011)
Summary
High similarity of the neural patterns associated with different memories leads to decreased accuracy (Hopfield 1982; Treves and Rolls 1992). The mnemonic similarity task (MST) requires participants to recognize whether a probe item is old (i.e., previously seen, as some items are repeated), similar (a new item that is similar but not identical to a previously seen item), or new, with pattern separation indexed as the successful identification of similar items (Kirwan and Stark 2007; Stark et al 2015) The performance on this task, does not tell us whether the encoded representations of similar items are more separated than of dissimilar items, but instead may reflect how accurately the original items were encoded ( enabling detection of the novelty of similar new items). These tasks either did not provide a behavioral readout or did not look at memory for multielement events
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