Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Episodic Memory Decisions Induce Lingering Mnemonic Biases



Science 27 July 2012: Vol. 337 no. 6093 pp. 485-487

Memory’s Penumbra: Episodic Memory Decisions Induce Lingering Mnemonic Biases

Katherine Duncan, Arhanti Sadanand, Lila Davachi

1Department of Psychology, Columbia University, New York, NY 10027, USA.

2Department of Psychology, New York University, New York, NY 10003, USA.

3Center for Neural Science, New York University, New York, NY 10003, USA.


How do we decide if the people we meet and the things we see are familiar or new? If something is new, we need to encode it as a memory distinct from already stored episodes, using a process known as pattern separation.   If familiar, it can be used to reactivate a previously stored memory, by a process known as pattern completion. To orchestrate these conflicting processes, current models propose that the episodic memory system uses environmental cues to establish processing biases that favor either pattern separation during encoding    or pattern completion during retrieval. To assess this theory, we measured how people’s memory formation and decisions are influenced by their recent engagement in episodic encoding and retrieval. We found that the recent encoding of novel objects improved subsequent identification of subtle changes, a task thought to rely on pattern separation. Conversely, recent retrieval of old objects increased the subsequent integration of stored information into new memories, a process thought to rely on pattern completion. These experiments provide behavioral evidence that episodic encoding and retrieval evoke lingering biases that influence subsequent mnemonic processing.

The often effortless way in which we can encode the present and remember the past belies the complexity of the underlying processes, however. Neuroscience research shows that both encoding new memories and retrieving old ones depend on the same specific brain region, the hippocampus, but computational models propose that encoding and retrieval are differentially supported by computationally incompatible network processes. Specifically, episodic encoding is thought to rely on pattern separation, a process that makes overlapping representations more distinct, whereas retrieval is thought to depend on pattern completion, a process that increases overlap by reactivating related memory traces. Because a process that emphasizes overlap cannot simultaneously operate on the same representation as one that deemphasizes it, a potential resolution to the paradox is that the hippocampus can establish processing biases that favor either pattern separation or completion, depending on the current context. In fact, neurocomputational models have long hypothesized that neuromodulatory systems may dynamically bias  hippocampal processing toward either pattern completion or separation.

The crux of our approach lies in the relatively slow action of neuromodulators in the hippocampus. If switching between  pattern completion and separation biases is, in fact, mediated by hippocampal neuromodulatory input, it follows that a processing bias should linger in time and, thus, influence subsequent mnemonic processing. To test this, we presented participants with pictures of novel and familiar objects and asked them to make old/new recognition decisions. According to models, detecting novelty should bias the memory system toward pattern separation to support distinctive encoding of the new information, whereas recognizing that a stimulus was previously experienced should induce a pattern completion bias that supports retrieval of stored representations.

The results provide behavioral evidence that episodic encoding and retrieval can evoke biases that influence subsequent mnemonic processing, a phenomenon that computational models predict to be the consequence of a tension between  episodic encoding and retrieval operations. The results provide evidence that memory decisions can exert a temporally extended bias on subsequent computational processes thought to support encoding and retrieval — namely pattern separation and completion.

Although there is agreement across several hippocampal memory models that encoding and retrieval should be temporally segregated, the time required to switch between these processing biases varies widely across models from a few hundred milliseconds to 10 s. The current results suggest that the influence of an earlier memory decision  decays after a few seconds, consistent with the theoretical time scale of acetylcholine modulation in the hippocampus. The results shed light on fundamental computational issues of memory encoding and retrieval and highlight that our ongoing processing of the world is influenced by other preceding cognitive operations.

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