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Hippocampal "time cells" bridge the gap in memory for discontiguous events.
Neuron. 2011 Aug 25; 71(4):737-49
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04 Oct 2011
New Finding
DOI: 10.3410/f.13328963.14692063
This exciting paper offers a first glimpse into how the brain represents time, place and behavioral events in a coordinated manner. One remarkable discovery is that many hippocampal pyramidal cells (PCs) discharge not only at specific locations (e.g. place cells), but also at specific times, i.e. they...Continue reading
This exciting paper offers a first glimpse into how the brain represents time, place and behavioral events in a coordinated manner. One remarkable discovery is that many hippocampal pyramidal cells (PCs) discharge not only at specific locations (e.g. place cells), but also at specific times, i.e. they code for 'when' or even simultaneously for 'when' as well as 'where'.
The authors recorded from multiple hippocampal PCs while a rat was trained in a difficult task. It first sampled and identified one of two objects (A versus B); then, following a 10-second delay in a second empty chamber, the rat entered a third chamber where it encountered one of two odors (OD1 versus OD2). If A was followed by OD1 or B followed by OD2 (both learned object/odor matches), the rat had to dig for a food reward at the odorant site. If, however, A was followed by OD2 or B by OD1 (non-matching object/odors), then the rat had to dig at a different site. The rat therefore had to remember, for 10 seconds, whether object A or B had been presented in order to make the correct decision after discriminating odor OD1 from OD2. The key finding is that a subset of PCs discharged at specific times during the 10-second delay period, thereby demonstrating that hippocampal PC ensembles can directly encode duration; cells encoding duration during the object and odor sampling periods were also found. In addition, some of these duration encoding PCs were also sensitive to event sequences, i.e. these cells responded differently depending on which object the rat initially encountered, and whether a matching or non-matching odor was subsequently sampled. Taken together with the vast literature on hippocampal place cells, this paper demonstrates that hippocampus cell ensembles can encode spatiotemporal sequences, i.e. what has occurred when and where. As all excellent research, this study raises many fascinating questions. I wonder about how the hippocampal target networks use the vast amounts of information encoded in the behaviorally modulated spatiotemporal firing patterns of the PC ensembles. More specifically, how do these spatiotemporal firing patterns relate to the prominent rhythms (theta, gamma) exhibited by the hippocampus and associated brain regions. Clearly, this paper is only the beginning of exploration in this fascinating area of research.
The authors recorded from multiple hippocampal PCs while a rat was trained in a difficult task. It first sampled and identified one of two objects (A versus B); then, following a 10-second delay in a second empty chamber, the rat entered a third chamber where it encountered one of two odors (OD1 versus OD2). If A was followed by OD1 or B followed by OD2 (both learned object/odor matches), the rat had to dig for a food reward at the odorant site. If, however, A was followed by OD2 or B by OD1 (non-matching object/odors), then the rat had to dig at a different site. The rat therefore had to remember, for 10 seconds, whether object A or B had been presented in order to make the correct decision after discriminating odor OD1 from OD2. The key finding is that a subset of PCs discharged at specific times during the 10-second delay period, thereby demonstrating that hippocampal PC ensembles can directly encode duration; cells encoding duration during the object and odor sampling periods were also found. In addition, some of these duration encoding PCs were also sensitive to event sequences, i.e. these cells responded differently depending on which object the rat initially encountered, and whether a matching or non-matching odor was subsequently sampled. Taken together with the vast literature on hippocampal place cells, this paper demonstrates that hippocampus cell ensembles can encode spatiotemporal sequences, i.e. what has occurred when and where. As all excellent research, this study raises many fascinating questions. I wonder about how the hippocampal target networks use the vast amounts of information encoded in the behaviorally modulated spatiotemporal firing patterns of the PC ensembles. More specifically, how do these spatiotemporal firing patterns relate to the prominent rhythms (theta, gamma) exhibited by the hippocampus and associated brain regions. Clearly, this paper is only the beginning of exploration in this fascinating area of research.
Disclosures
None declared
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Maler L: F1000Prime Recommendation of [MacDonald CJ et al., Neuron 2011, 71(4):737-49]. In F1000Prime, 04 Oct 2011; DOI: 10.3410/f.13328963.14692063. F1000Prime.com/13328963#eval14692063
28 Oct 2011
New Finding
DOI: 10.3410/f.13328963.14726641
This is an important study providing evidence for a plausible neural mechanism in the hippocampus for supporting memories that include a temporal context and associating events with intervening temporal gaps.
This study shows that hippocampal neurons reliably fire in distinct temporal epochs during a...Continue reading
This study shows that hippocampal neurons reliably fire in distinct temporal epochs during a...Continue reading
This is an important study providing evidence for a plausible neural mechanism in the hippocampus for supporting memories that include a temporal context and associating events with intervening temporal gaps.
This study shows that hippocampal neurons reliably fire in distinct temporal epochs during a delay period (up to 20s) in a two-item sequence task. The authors called these neurons ‘time cells’. Notably, the time cells showed robust modulation by time even after the statistical removal of non-temporal events. Remarkably, the time cells share many of the same characteristics identified in place cells. For example, the cells showed remapping in their firing patterns following salient changes in the temporal parameters of the task. However, just as place cell activity can be modulated by non-spatial variables, the firing of time cells was modulated by non-temporal variables. These putative time cells are proposed to be the neural substrate for the representation of time, and, together with the well-studied place cells of the hippocampus, may compose a neural substrate that supports the formation of spatiotemporal contexts in memory.
Place cells have long been hypothesized to support memories that include a spatial context. Here, the demonstration of hippocampal cells that, as an ensemble, can represent a timeline relative to salient behavioral events (e.g. presentation of an object cue) is also consistent with the hypothesis that the hippocampus also supports memories that have a temporal context. Importantly, a major theoretical function of the hippocampus is to support episodic memory, which is a capacity thought to include a spatial and/or temporal context with a specific event.
While this study provides evidence of neural activity in the hippocampus consistent with temporal response fields, it will ultimately be important to examine the relationship of these time cells to behavioral tasks that place a demand on memory for time.
This study shows that hippocampal neurons reliably fire in distinct temporal epochs during a delay period (up to 20s) in a two-item sequence task. The authors called these neurons ‘time cells’. Notably, the time cells showed robust modulation by time even after the statistical removal of non-temporal events. Remarkably, the time cells share many of the same characteristics identified in place cells. For example, the cells showed remapping in their firing patterns following salient changes in the temporal parameters of the task. However, just as place cell activity can be modulated by non-spatial variables, the firing of time cells was modulated by non-temporal variables. These putative time cells are proposed to be the neural substrate for the representation of time, and, together with the well-studied place cells of the hippocampus, may compose a neural substrate that supports the formation of spatiotemporal contexts in memory.
Place cells have long been hypothesized to support memories that include a spatial context. Here, the demonstration of hippocampal cells that, as an ensemble, can represent a timeline relative to salient behavioral events (e.g. presentation of an object cue) is also consistent with the hypothesis that the hippocampus also supports memories that have a temporal context. Importantly, a major theoretical function of the hippocampus is to support episodic memory, which is a capacity thought to include a spatial and/or temporal context with a specific event.
While this study provides evidence of neural activity in the hippocampus consistent with temporal response fields, it will ultimately be important to examine the relationship of these time cells to behavioral tasks that place a demand on memory for time.
Disclosures
None declared
Stark C, Mattfeld A and Allen T: F1000Prime Recommendation of [MacDonald CJ et al., Neuron 2011, 71(4):737-49]. In F1000Prime, 28 Oct 2011; DOI: 10.3410/f.13328963.14726641. F1000Prime.com/13328963#eval14726641
F1000Prime Recommendations, Dissents and Comments for [MacDonald CJ et al., Neuron 2011, 71(4):737-49]. In F1000Prime, 20 Jun 2013; F1000Prime.com/13328963
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The hippocampus is critical to remembering the flow of events in distinct experiences and, in doing so, bridges temporal gaps between discontiguous events. Here, we report a robust hippocampal representation of sequence memories, highlighted by "time cells" that encode successive moments during an empty temporal gap between the key events, while also encoding location and ongoing behavior. Furthermore, just as most place cells "remap" when a salient spatial cue is altered, most time cells form qualitatively different representations ("retime") when the main temporal parameter is altered. Hippocampal neurons also differentially encode the key events and disambiguate different event sequences to compose unique, temporally organized representations of specific experiences. These findings suggest that hippocampal neural ensembles segment temporally organized memories much the same as they represent locations of important events in spatially defined environments.
Copyright © 2011 Elsevier Inc. All rights reserved.
DOI: 10.1016/j.neuron.2011.07.012
PMID: 21867888
