Engram (neuropsychology)

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An engram is a unit of cognitive information imprinted in a physical substance, theorized to be the means by which memories are stored [1] as biophysical or biochemical [2] changes in the brain or other biological tissue, in response to external stimuli.

Contents

Demonstrating the existence of, and the exact mechanism and location of, neurologically defined engrams has been a focus of persistent research for many decades. [3]

History

The term "engram" was coined by memory researcher Richard Semon in reference to the physical substrate of memory in the organism. Semon warned, however: "In animals, during the evolutionary process, one organic system—the nervous system—has become specialised for the reception and transmission of stimuli. No monopoly of this function by the nervous system, however, can be deduced from this specialisation, not even in its highest state of evolution, as in Man." [4] One of the first ventures on identifying the location of a memory in the brain was undertaken by Karl S. Lashley who removed portions of the brain in rodents. In Lashley's experiments, rats were trained to run through a maze and then tissue was removed from their cerebral cortex. Increasing the amount of tissue removed increased the degradation of memory, but more remarkably, where the tissue was removed from made no difference. His search thus proved unsuccessful, and his conclusion – that memory is diffusely distributed in the brain – became widely influential. [5] However, today we appreciate that memory is not completely but only largely distributed in the brain; this, together with its dynamic nature, makes engrams challenging to identify using traditional scientific methods. [5] [6]

Later, David A. McCormick and Richard F. Thompson sought the engram in the cerebellum, rather than the cerebral cortex. They used classical conditioning of the eyelid response in rabbits in search of the engram. They puffed air upon the cornea of the eye and paired it with a tone. After a number of experiences associating it with a tone, the rabbits became conditioned to blink when they heard the tone even without a puff. One region that David A. McCormick studied was the lateral interpositus nucleus (LIP). He found that recordings of neurons in this nucleus revealed activity that mirrored the learning, stimulation of the nucleus elicited the learned response, and lesion of this brain region abolished the response. [7] Other members of the Thompson group found that when the interpositus nucleus was deactivated chemically, the conditioned response disappeared; when re-activated, they responded again, demonstrating that the LIP is a key element of the engram for this response. [8] This approach, targeting the cerebellum, though successful, examines only basic, automatic responses, which virtually all animals possess. However, engrams of specific types of memory are found in the subsystems mediating that learning process and as such solely engrams of simple conditioning are associated with the LIP but not, for instance, engrams of semantic memory.

Overview

Neuroscience acknowledges the existence of many types of memory and their physical location within the brain is likely to be dependent on the respective system mediating the encoding of this memory. [9] Such brain parts as the cerebellum, striatum, cerebral cortex, hippocampus, and amygdala are thought to play an important role in memory. For example, the hippocampus is believed to be involved in spatial and declarative memory, as well as consolidating short-term into long-term memory.

Studies have shown that declarative memories move between the limbic system, deep within the brain, and the outer, cortical regions. These are distinct from the mechanisms of the more primitive cerebellum, which dominates in the blinking response and receives the input of auditory information directly. It does not need to "reach out" to other brain structures for assistance in forming some memories of simple association.

An MIT study found that behavior based on high-level cognition, such as the expression of a specific memory, can be generated in a mammal by highly specific physical activation of a specific small subpopulation of brain cells. By reactivating these cells by physical means in mice, such as shining light on neurons affected by optogenetics, a long-term fear-related memory appears to be recalled. [10]

Another study used optogenetics and chemogenetics to control neuronal activity in animals encoding and recalling the memory of a spatial context to investigate how the brain determines the lifetime of memories. The results found by the researchers have defined a role for specific hippocampal inhibitory cells (somatostatin expressing cells) in restricting the number of neurons involved in the storage of spatial information and limiting the duration of the associated memory. [11]

In 2016, an MIT study found that memory loss in early stages of Alzheimer's disease could be reversed by strengthening specific memory engram cell connections in the brains of Alzheimer mouse models. [12]

See also

Related Research Articles

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<span class="mw-page-title-main">Amygdala</span> Each of two small structures deep within the temporal lobe of complex vertebrates

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<span class="mw-page-title-main">Long-term potentiation</span> Persistent strengthening of synapses based on recent patterns of activity

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<span class="mw-page-title-main">Fear conditioning</span> Behavioral paradigm in which organisms learn to predict aversive events

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<span class="mw-page-title-main">Nucleus accumbens</span> Region of the basal forebrain

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<span class="mw-page-title-main">Susumu Tonegawa</span> Japanese scientist (born 1939)

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<span class="mw-page-title-main">Reward system</span> Group of neural structures responsible for motivation and desire

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<span class="mw-page-title-main">Medial dorsal nucleus</span>

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<span class="mw-page-title-main">Synaptic gating</span>

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<span class="mw-page-title-main">Basolateral amygdala</span> The lateral, basal, and accessory-basal nuclei of the amygdala

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<span class="mw-page-title-main">Hippocampus anatomy</span>

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<span class="mw-page-title-main">Granule cell</span> Type of neuron with a very small cell body

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References

  1. ↑ Liu, Xu; Ramirez, Steve; Pang, Petti T.; Puryear, Corey B.; Govindarajan, Arvind; Deisseroth, Karl; Tonegawa, Susumu (22 March 2012). "Optogenetic stimulation of a hippocampal engram activates fear memory recall". Nature. 484 (7394): 381–385. Bibcode:2012Natur.484..381L. doi:10.1038/nature11028. PMC   3331914 . PMID   22441246.
  2. ↑ Ryan, T. J.; Roy, D. S.; Pignatelli, M.; Arons, A.; Tonegawa, S. (28 May 2015). "Engram cells retain memory under retrograde amnesia". Science. 348 (6238): 1007–1013. Bibcode:2015Sci...348.1007R. doi:10.1126/science.aaa5542. PMC   5583719 . PMID   26023136.
  3. ↑ Levy, Adam (14 January 2021). "Memory, the mystery". Knowable Magazine. doi:10.1146/knowable-011421-3 . Retrieved 25 March 2022.
  4. ↑ Semon, Richard (1921). "Chapter II. Engraphic Action of Stimuli on the Individual". The Mneme. London: George Allen & Unwin. p. 24; trans by Louis Simon.{{cite book}}: CS1 maint: postscript (link)
  5. 1 2 Sa, Josselyn; S, Köhler; Pw, Frankland (September 2015). "Finding the Engram". Nature Reviews. Neuroscience. 16 (9): 521–534. doi:10.1038/nrn4000. PMID   26289572. S2CID   205511443 . Retrieved 2020-06-02.
  6. ↑ Bruce, Darryl (1 December 2001). "Fifty Years Since Lashley's In Search of the Engram: Refutations and Conjectures". Journal of the History of the Neurosciences. 10 (3): 308–318. doi:10.1076/jhin.10.3.308.9086. PMID   11770197. S2CID   27180078.
  7. ↑ McCormick, David; Thompson, Richard (20 Jan 1984). "Cerebellum: Essential Involvement in the Classically Conditioned Eyelid Response". Science. 222: 296–299. doi:10.1126/science.6701513.
  8. ↑ James W. Kalat, Biological Psychology p. 392–393
  9. ↑ Gerrig and Zimbardo (2005) Psychology and Life (17th edition: International edition)
  10. ↑ Costandi, Mo (6 April 2012). "Light brings back bad memories". The Guardian.
  11. ↑ Stefanelli, Thomas; Bertollini, Cristina; Lüscher, Christian; Muller, Dominique; Mendez, Pablo (March 2016). "Hippocampal Somatostatin Interneurons Control the Size of Neuronal Memory Ensembles". Neuron. 89 (5): 1074–1085. doi: 10.1016/j.neuron.2016.01.024 . PMID   26875623.
  12. ↑ Roy, Dheeraj S.; Arons, Autumn; Mitchell, Teryn I.; Pignatelli, Michele; Ryan, Tomás J.; Tonegawa, Susumu (March 2016). "Memory retrieval by activating engram cells in mouse models of early Alzheimer's disease". Nature. 531 (7595): 508–512. Bibcode:2016Natur.531..508R. doi:10.1038/nature17172. PMC   4847731 . PMID   26982728.
  13. ↑ Garner, Aleena R.; Rowland, David C.; Hwang, Sang Youl; Baumgaertel, Karsten; Roth, Bryan L.; Kentros, Cliff; Mayford, Mark (2012-03-23). "Generation of a synthetic memory trace". Science. 335 (6075): 1513–1516. doi:10.1126/science.1214985. ISSN   1095-9203. PMC   3956300 . PMID   22442487.

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