How are life episodes stored in the brain?

Last update: 27/02/2023

Here is the adapted version of the answer I posted on Quora (French version) to the question “How are memories stored in the brain?” 4 years ago. As this question has come up again recently in my work in the cognitive modeling field, I thought it would be interesting to share it here with all the scientific references. This article is also available on medium.com

First, let’s give some context!

Cognitive modeling is a field of computer science concerned with modeling the cognitive functions that enable problem-solving and mental processing of information. This can concern modeling memory, reasoning, or attention.
Cognitive modeling is a field very close to artificial intelligence since a part of people in this field aims at creating AIs with the ability to adapt and understand the context, in order to have a successful interaction with the human involved. Cognitive modeling, despite recent works, does not concern only machine and deep learning. There are also cognitive models using Symbolic AI algorithms.
Working thus on the design of AI inspired by humans implies questioning how humans do it at the cognitive level, and thus asking questions such as the one we are addressing here

Cognitive modeling can also be very closely related to computational neuroscience, a field of research whose objective is to discover, understand and model the principles that govern neural development, the structure and physiology of the nervous system, and the emergence and functioning of associated cognitive abilities. This field may involve mathematical models and neural networks to propose artificial “models” of cerebral structures.

Life episodes in the brain

The brain is a set of cerebral structures that work in synergy to allow, among other things, each of us to use our own cognitive functions (memory, attention, language, executive functions, and visual-spatial functions) and to be who we are!

Memories are episodes in an individual’s life. It is therefore the episodic memory that comes into play.

Defined as the memory of personally experienced events in a specific spatiotemporal context (Tulving, 1983), episodic memory is a sub-part of declarative memory and is essentially “biographical”. Indeed, it allows us to link the memories of an individual’s life over time in order to form his or her identity. Consisting of information specific to individual experience, dated, localized, and sometimes emotionally connoted, it is opposed to semantic memory, the memory of facts and concepts (Tulving, 1972; Tulving, 1995).

With a specific brain substrate (internal temporal structures, hippocampus, and frontal cortex) (Insigrini, 2008), this memory system is characterized by information encoding, storage, and retrieval processes, which are essential for the formation, maintenance, and retrieval of representations of information perceived in this system (Ergis, 2008).

A key structure in the study of episodic memory, the hippocampus is closely related to the cortex and is present symmetrically in each hemisphere. In addition to memory, two other main functions characterize this brain structure: spatial navigation (O’Keefe, 1971) and behavioral inhibition (Gray, 2000)

The involvement of the hippocampus in episodic memory (Eichenbaum, 1993; Scoville, 1957; Squire, 2002) was initially demonstrated by the case of Henry Gustav Molaison (Scoville, 1957) (called the HM patient): the removal of the hippocampus to treat the patient’s epilepsy highlighted its importance in the process of acquiring new memories.Other patients with hippocampal lesions (observations of Scoville and Brenda Milner) have confirmed the importance of the hippocampus in episodic memory (Draganski, 2006).

The degeneration of the hippocampus is one of the signs of Alzheimer’s disease which manifests itself in the difficulty and then the impotence to acquire new memories for an individual.

Encoding

Encoding is the first step in the memorization process and is a process of information processing whose role is to process the characteristics of events (at the sensory-motor and emotional levels) and to transform them into a representation or memory trace. The latter, on a biological level, corresponds to a new configuration of neuronal activation.

It is notably during the encoding phase that individuals can put in place strategies for acquiring memories, such as the organization of information or mnemonics.

There are two types of encoding: the “superficial” and the “deep”. The first refers to rote learning, i.e. the exact reproduction of the original information. The second, more complex, consists of looking for associations with information already stored in order to relate common elements and distinguish those that are different.

This last point explains why we tend to model the hippocampus (or one of its substructures) by associative memories (such as the Steinbuch model or the Hopfield model) which have the characteristic of being addressable by their content (if a part of the model is altered, it remains possible to retrieve a part of the information despite everything) and to learn the association between two elements (the word “Red” and the color red) (Beati, 2013)

Storage and consolidation

Occurring after encoding, storage is a process that allows the maintenance of encoded information in the appropriate brain structures. It is biologically translated by changes in brain activity and it impacts the cellular level (signal transduction between neurons and modification of brain plasticity) and systemic level (formation and maintenance of memory circuitry)(McGaugh, 2000). This second stage involves consolidation. This process, which is responsible for the reinforcement of memories, allows for the automatic unconscious repetition of information involving the cortex, to anchor the information sufficiently so that it can be retained for a long time. Active especially during sleep, consolidation participates in the restructuring of memories into long-term memory (Inostroza, 2013)

Retrieval

Once the information has been stored, it is the so-called retrieval process, which is active, controlled, and requires resources, that brings it back to consciousness. This process, also known as “recall” is defined as the access, selection, and reactivation or reconstruction of stored internal representations from cues (Buchanan, 2007; Tulving, 1976). Retrieval is considered optimized, according to the “principle of specificity of encoding” (Plancher, 2013) when the memory trace integrates both the content of the episode and its context. The latter can thus facilitate access to the representation later on.

Another element that facilitates recovery is repetition. Indeed, the more memory is recalled (brought back to consciousness), the more its memory trace is reinforced. As a result, it is retained longer.

In summary and in simplified form

It is the strengthening of neuronal connections in the brain that allows us to keep a memory! The hippocampus records during the day the episodes of life and during the sleep phases, it interacts with the amygdala (involved in the cognitive process of decision-making) to make the cortex learn the important episodes by repeating them several times.

References

• Beati, T. &. Alexandre F. (2013). How to combine learning of regularities and special cases? A bio-inspired model. At CAP’13, French Conference on Machine Learning.
• Buchanan, T. W. (2007). Retrieval of emotional memories. Psychology Bulletin, 133(5), 761
• Draganski, B, Gaser C, Kempermann G, Kuhn HG, Winkler J, et al. (2006). Temporal and Spatial Dynamics of Brain Structure Changes during Extensive Learning. The Journal of Neuroscience.
• Eichenbaum, H. (1993). Memory, Amnesia and the hippocampal system. MIT Press
• Ergis, AM, Eusop-Roussel E. (2008). Early episodic memory impairments in Alzheimer’s disease. Rev Neurol (Paris), 164 Suppl 3, , S96-S101.
• Gray, J. A., & McNaughton, N. (2000). The Neuropsychology of Anxiety: An Enquiry into the Functions of Septo-Hippocampal System. Oxford University Press.
• Inostroza, M., & Born, J. (2013). Sleep for preserving and transforming episodic memory. Annu Rev Neurosci.
• Insigrini, M, Taconnat L (2008). Episodic memory, frontal functioning, and aging. Rev Neurol (Paris), 164 Suppl 3, 91-95.
• McGaugh, J. L. (2000). Memory, a century of consolidation. Science, 287(5451), 248-251.
• O’Keefe, J., & Dostrovsky, J. (1971). The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely-moving rat. Brain research, 34(1), 171 -175.
• Plancher, G, Barra J, Orriols E, Piolino P. (2013). The influence of action on episodic memory, A virtual reality study. Q J Exp Pshychol (Hove), 895-909.
• Scoville, W. B., & Milner, B. (1957). Lost of Recent Memory after bilateral hippocampal lesions. Journal of neurology, neurosurgery and psychiatry, 11.
• Squire, L.R. & Schacter, D.L. (Eds.) (2002). The Neuropsychology of Memory (3rd Edition). New York: Guilford Press.
• Tulving, E. (1972). Episodic and semantic memory. In Organization of Memory. London: Academic, 381, e402.
• Tulving, E. (1976). The role of semantic memory in the storage and retrieval of episodic information. Psychology Bulletin, 19-25.
• Tulving, E. (1983). Element of episodic memory. Oxford, Oxford University Press.
• Tulving, E. (1995). Memory systems: Quo Vadis. (M. Press, Ed.) Gazzaniga MS (Ed.), The Cognitive Neurosciences, 839-847.