![]() This representation thus constitutes a manifold with both temporal dimension and a stimulus-coding dimension that could support working memory. The time cells we observed were stimulus specific meaning that they provide not only information about timing, but also conjunctively code what and when information. We observed time cells in the macaque lPFC during a working memory task. Time cells, which activate sequentially provide analogous support for the dimension of time. Place cells in the hippocampus and other brain regions provide basis functions to support the dimension of physical space. These findings suggest that working memory includes a compressed timeline of what happened when, consistent with longstanding cognitive theories of human memory. The encoding of the elapsed time became less precise as the sample stimulus receded into the past. These sequences of neurons encoded both stimulus identity and the elapsed time. Each sample stimulus triggered a consistent sequence of neurons, with each neuron in the sequence firing during a circumscribed period of time. We analyzed single-unit recordings from the lateral prefrontal cortex (lPFC) of two macaque monkeys during performance of a delayed-match-to-category task. Previous studies of the neural underpinnings of working memory have focused on sustained firing, which can account for maintenance of the stimulus identity, but not for representation of the elapsed time. Hippocampal activity could reflect a scale-invariant spatiotemporal context as suggested by theories of memory from cognitive psychology.Ĭognitive theories suggest that working memory maintains not only the identity of recently-presented stimuli but also a sense of the elapsed time since the stimuli were presented. Consistent sequences in the hippocampus are observed over a wide range of time scales, from seconds to minutes. ![]() ![]() ![]() Computational measures of consistency show reliable sequences within experimental trials at the scale of seconds as one would expect from time cells or place cells during the trial, as well as across experimental trials on the scale of minutes within a recording session. This paper studies the consistency of contextual drift in three chronic calcium imaging recordings from the hippocampus CA1 region in mice. It was thought that these slow dynamics are caused by stochastic drift or a continuous change in the representation of the episode, rather than consistent sequences unfolding over minutes. For longer timescales, firing of hippocampal neurons also changes slowly over minutes within experimental sessions. Neurons in the hippocampus fire in consistent sequence over the timescale of seconds during the delay period of some memory experiments. We also discuss a range of related issues including group and individual variation in contextual processing and different types of context, such as temporal and source. We review the evidence for each of these predictions. If true, this should manifest in subjects’ behavior as a tendency for items experienced nearby in time to cluster together during recall (i.e., a contiguity effect) and in neurophysiological recordings as a temporally graded pattern of reinstatement. Most critically, they predict that during memory search successfully recalling one item should trigger reinstatement of the state of context associated with that item. Specifically, they predict that the brain should maintain a representation that changes gradually during study and that new events should become associated with this context representation. When applied to laboratory tasks such as free recall, these models make specific testable predictions about not only recall behavior but also to the neurophysiology of memory search. They further assume that memory search proceeds by using the context representation as a cue and that recalled memories reinstate their own context which then forms part of the cue for the next recall. They assume that an internal context representation updates, or drifts, whenever new events are experienced and that these events form associations to this representation. ![]() Retrieved context models specify a set of computational mechanisms that place context at the very heart of episodic memory processes. There is less agreement on the nature and centrality of this role. There is broad agreement that context plays a role in episodic memory. ![]()
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