Biology:Hippocampal replay

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Short description: Phenomenon observed in several animals

Hippocampal replay is a phenomenon observed in rats, mice,[1] cats, rabbits,[2] songbirds[3] and monkeys.[4] During sleep or awake rest, replay refers to the re-occurrence of a sequence of cell activations that also occurred during activity, but the replay has a much faster time scale. It may be in the same order, or in reverse. Cases were also found where a sequence of activations occurs before the actual activity, but it is still the same sequence. This is called preplay.

The phenomenon has mostly been observed in the hippocampus, a brain region associated with memory and spatial navigation. Specifically, the cells that exhibit this behavior are place cells, characterized by reliably increasing their activity when the animal is in a certain location in space. During navigation, the place cells fire in a sequence according to the path of the animal. In a replay instance, the cells are activated as if in response to the same spatial path, but at a much faster rate than the animal actually moved in.

Background

Place cell activity was already well established when the first study explored this phenomenon in 1989.[5] They showed that neural activity of single place cells during sleep resembled the activity during the awake state. This activity was greater than that of other cells and this study was only the first step towards understanding replay. Subsequent studies showed that large groups of cells also demonstrated this type of increased activity during sleep. In addition, it was discovered that the order of activity of place cells was also replicated during sleep.[6] Firing sequences of three and more neurons observed in the hippocampus during locomotion were shown to recur selectively during subsequent slow-wave sleep more likely than during the preceding sleep, and the sequence replay was compressed during high frequency oscillations.[7] These high frequency field oscillations called ripples were observed in the sleep state and later shown to play a causal role in memory consolidation.[8][9]

The next step was the discovery of replay during the awake state. In 1999, ten years after the initial discovery, neural recordings in the awake state were also shown to have replay activity.[10] It is considerably more difficult to detect this activity in the awake state and several methods including Bayesian decoding have been used to quantify replay events that occur during short wave ripples.[11] Recent advances include finding that replay can occur in reverse[12] and that it has also been found to occur in different environments.[13] The role of replay in memory consolidation in these different conditions and environments is still being explored and several theories attempt to answer this question.

Location and behavioral state

Replay can occur in several different behavioral, physiological, and environmental conditions. The first distinction between awake and sleep states may represent different roles in memory consolidation. In the sleep state, the ripple events and place cell activity similar to that of the activity in the environment define the replay events. In the sleep state, there is also a distinction between REM (rapid eye movement) and SWS (slow wave sleep) which has implications for replay events.[14][15] During SWS the place cells fire in a sequential order indicating replay and possibly indicate memory consolidation. However, during REM sleep where dreams occur in humans, replay events also occurred suggesting a possible role for place cells in dreams.

In the awake state the same activity occurs, however it is more difficult to detect and the animal must be in a resting state. Lastly, there are many environments for replay events in the awake animal. The length of the track can be short or long and still be replayed by a population of place cells.[16] In addition, replay of a single environment can occur when the animal is in that environment or in different environments.[17][18] This may show that consolidation of memory is a persistent process that may occur in several different types of environments and behavioral conditions. The robustness of the replay events indicates the importance of this process.

Preplay

As mentioned above, the sequential activation of hippocampal place cells according to their place fields may occur during rest periods before the animal is actually traversing the activated path, even if the animal has never experienced it before.[19] This suggests that hippocampal activation during rest may have a function not only in memory consolidation and retrieval, but also in planning: it contributes to the organization of the network for improving the encoding of future events.

Sensory cue for activation

Sensory stimuli can induce replay events or enhance the replay:[20] in the awake state, replay often begins from the current location and continue either forward or backward in time, and nearby locations are more likely to be the place fields of neurons exhibiting replay than far away locations. This is like cued memory retrieval, where a sensory input triggers retrieval of similar or relevant memories. The cue may even trigger a replay in a different environment, if the place cells cued represent a location in a different environment in addition to the current location of the animal.

Interaction with cortex

Several studies are beginning to understand that replay may not only occur in the hippocampus. Replay has been linked to coherent activity in cortical regions of the default mode network.[21] After learning, similar replay events occurred in both the thalamus and cortex.[22] In addition, the visual cortex showed population activity that was both coordinated into discrete time regions and that it occurred simultaneously with activity in the hippocampus.[23] Concurrent re-activation in the hippocampus and cortex may demonstrate that memory consolidation requires cortical input/output to maintain a memory. This hypothesis fits well with the idea that the cortex plays an integral role in memory retrieval after consolidation occurs. Therefore, hippocampal replay may play the role of information transfer between the hippocampus and cortex, yet this idea requires further verification.

Function in memory

Hippocampal replay in the awake state has been implicated, though not demonstrated, to correlate with performance in navigation tasks after the replay event.[24] Replay may play a role in consolidation of memories related to spatial location,[25] although a clear causal relationship between replay and memory consolidation is still unproven. Furthermore, replay also seems to be related to memory retrieval: it is activated by cues that also trigger memory retrieval, and in situations that require it, such as planning a trajectory based on the consequences of past choices. This relationship is also still only correlative, but there are studies showing evidence for the necessity of replay in successful memory retrieval.[26]

Replay is likely to participate in both the consolidation of memories and in building a cognitive map. Replay is also likely to play an important role in generating and maintaining a value map, which is a proposed variation of cognitive map in which memories are reinforced according to their values. [27]

In studies done to monitor the patterns of the hippocampal replay of events happening in the minds of the rats, it was discovered that they rely heavily on long term re-playable memories for their daily lives. To determine if the animals could really retain the memories scientists set up a series of episodic events (i.e. a series of tunnels, a maze, a track full of twists and turns) to visualize the neuron activity as the rat completes its round around the new surroundings. The chemogenetic activating drug clozapine N-oxide (CNO), but not vehicle, reversibly impairs episodic memory replay in rats previously injected bilaterally in the hippocampus with a recombinant viral vector containing an inhibitory designer receptor exclusively activated by a designer drug (DREADD; AAV8-hSyn-hM4Di-mCherry). By contrast, two non-episodic memory assessments are unaffected by CNO, showing selectivity of this hippocampal-dependent impairment.[28]

References

  1. ↑ "Reactivation, replay, and preplay: how it might all fit together". Neural Plasticity 2011: 1–11. 2011. doi:10.1155/2011/203462. PMID 21918724. 
  2. ↑ "Hippocampal ripple-contingent training accelerates trace eyeblink conditioning and retards extinction in rabbits". The Journal of Neuroscience 30 (34): 11486–92. August 2010. doi:10.1523/JNEUROSCI.2165-10.2010. PMID 20739570. 
  3. ↑ "Song replay during sleep and computational rules for sensorimotor vocal learning". Science 290 (5492): 812–6. October 2000. doi:10.1126/science.290.5492.812. PMID 11052946. Bibcode: 2000Sci...290..812D. 
  4. ↑ "EEG sharp waves and sparse ensemble unit activity in the macaque hippocampus". Journal of Neurophysiology 98 (2): 898–910. August 2007. doi:10.1152/jn.00401.2007. PMID 17522177. 
  5. ↑ "Influences of hippocampal place cell firing in the awake state on the activity of these cells during subsequent sleep episodes". The Journal of Neuroscience 9 (8): 2907–18. August 1989. doi:10.1523/JNEUROSCI.09-08-02907.1989. PMID 2769370. 
  6. ↑ "Replay of neuronal firing sequences in rat hippocampus during sleep following spatial experience". Science 271 (5257): 1870–3. March 1996. doi:10.1126/science.271.5257.1870. PMID 8596957. Bibcode: 1996Sci...271.1870S. 
  7. ↑ "Replay and time compression of recurring spike sequences in the hippocampus". The Journal of Neuroscience 19 (21): 9497–507. November 1999. doi:10.1523/JNEUROSCI.19-21-09497.1999. PMID 10531452. 
  8. ↑ "Sustained increase in hippocampal sharp-wave ripple activity during slow-wave sleep after learning". Learning & Memory 15 (4): 222–8. April 2008. doi:10.1101/lm.726008. PMID 18385477. 
  9. ↑ "Selective suppression of hippocampal ripples impairs spatial memory". Nature Neuroscience 12 (10): 1222–3. October 2009. doi:10.1038/nn.2384. PMID 19749750. 
  10. ↑ "Reactivation of hippocampal cell assemblies: effects of behavioral state, experience, and EEG dynamics". The Journal of Neuroscience 19 (10): 4090–101. May 1999. doi:10.1523/JNEUROSCI.19-10-04090.1999. PMID 10234037. 
  11. ↑ "Hippocampal replay of extended experience". Neuron 63 (4): 497–507. August 2009. doi:10.1016/j.neuron.2009.07.027. PMID 19709631. 
  12. ↑ "Awake replay of remote experiences in the hippocampus". Nature Neuroscience 12 (7): 913–8. July 2009. doi:10.1038/nn.2344. PMID 19525943. 
  13. ↑ "Hippocampal replay in the awake state: a potential substrate for memory consolidation and retrieval". Nature Neuroscience 14 (2): 147–53. February 2011. doi:10.1038/nn.2732. PMID 21270783. 
  14. ↑ "Temporally structured replay of awake hippocampal ensemble activity during rapid eye movement sleep". Neuron 29 (1): 145–56. January 2001. doi:10.1016/S0896-6273(01)00186-6. PMID 11182087. 
  15. ↑ "Memory of sequential experience in the hippocampus during slow wave sleep". Neuron 36 (6): 1183–94. December 2002. doi:10.1016/s0896-6273(02)01096-6. PMID 12495631. 
  16. ↑ "Hippocampal replay of extended experience". Neuron 63 (4): 497–507. August 2009. doi:10.1016/j.neuron.2009.07.027. PMID 19709631. 
  17. ↑ "Forward and reverse hippocampal place-cell sequences during ripples". Nature Neuroscience 10 (10): 1241–2. October 2007. doi:10.1038/nn1961. PMID 17828259. 
  18. ↑ "Awake replay of remote experiences in the hippocampus". Nature Neuroscience 12 (7): 913–8. July 2009. doi:10.1038/nn.2344. PMID 19525943. 
  19. ↑ "Preplay of future place cell sequences by hippocampal cellular assemblies". Nature 469 (7330): 397–401. January 2011. doi:10.1038/nature09633. PMID 21179088. Bibcode: 2011Natur.469..397D. 
  20. ↑ "Hippocampal replay in the awake state: a potential substrate for memory consolidation and retrieval". Nature Neuroscience 14 (2): 147–53. February 2011. doi:10.1038/nn.2732. PMID 21270783. 
  21. ↑ "Replay bursts in humans coincide with activation of the default mode and parietal alpha networks". Neuron 109 (5): 882–893. March 2021. doi:10.1016/j.neuron.2020.12.007. PMID 33357412. 
  22. ↑ "Long-lasting novelty-induced neuronal reverberation during slow-wave sleep in multiple forebrain areas". PLOS Biology 2 (1): E24. January 2004. doi:10.1371/journal.pbio.0020024. PMID 14737198. 
  23. ↑ "Coordinated memory replay in the visual cortex and hippocampus during sleep". Nature Neuroscience 10 (1): 100–7. January 2007. doi:10.1038/nn1825. PMID 17173043. 
  24. ↑ "Hippocampal replay in the awake state: a potential substrate for memory consolidation and retrieval". Nature Neuroscience 14 (2): 147–53. February 2011. doi:10.1038/nn.2732. PMID 21270783. 
  25. ↑ "A dual role for hippocampal replay". Neuron 65 (5): 582–4. March 2010. doi:10.1016/j.neuron.2010.02.022. PMID 20223195. 
  26. ↑ "The reorganization and reactivation of hippocampal maps predict spatial memory performance". Nature Neuroscience 13 (8): 995–1002. August 2010. doi:10.1038/nn.2599. PMID 20639874. 
  27. ↑ "Distinct effects of reward and navigation history on hippocampal forward and reverse replays". Proceedings of the National Academy of Sciences of the United States of America 117 (1): 689–697. December 23, 2019. doi:10.1073/pnas.1912533117. PMID 31871185. 
  28. ↑ "Replay of Episodic Memories in the Rat". Current Biology 28 (10): 1628–1634.e7. May 2018. doi:10.1016/j.cub.2018.04.006. PMID 29754898. 



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