Publication

A dynamical systems approach to synaptic consolidation and associations of concepts in hippocampus

Chiara Gastaldi
2021
Thèse EPFL
Résumé

In the present work, we approach two key aspects of memory formation: associative memory and synaptic consolidation. The storage of associative memory is commonly related to the medial temporal lobe in humans. Experimental evidence shows that the memories of objects, people or places are represented in this brain area by cell assemblies that respond selectively to single concepts. Neurons forming such assemblies are called concept cells. Associations between different concepts are linked to concept cells shared between assemblies: we refer to the number of shared neurons as the overlap between memory engrams. The respective assemblies of two associated concepts (e.g. Hillary and Bill Clinton) share more neurons compared to the assemblies of two unrelated concepts (e.g. Hillary Clinton and the Eiffel tower). In particular, three characteristics of assemblies of concept cells are important for this work: (a) they exhibit a very low mean activity (about 0.2% of neurons respond to each concept), (b) overlapping assemblies share about 4% of their cells, (c) non-overlapping assemblies share less then 1% of their cells. This implies that the association between two concepts induces a higher level of overlap between the relative memory engrams. In parallel, theoretical studies have shown that overlaps between memory engrams are fundamental in the process of free recall of sequences of words. These models assume that memory engrams have such a high mean activity that all assemblies are overlapping to a certain extent. Associative memory is traditionally modeled through attractor neural networks. Memory engrams are represented by binary patterns of active/silent neurons. While there is extensive literature on independent low-activity patterns, only a few studies can be found on correlated patterns. Extending the existing theory to include correlation is a key missing point to answer questions such as: How do shared neurons encode association? Why are 4% of neurons shared and not more? Using a mean-field approximation, we derive analytic equations for the network dynamics in the case of correlated patterns. Our results provide a theoretical framework that can explain the experimentally observed value of shared neurons. We find that for concepts represented by realistically sparse neural assemblies there are a minimal and a maximal fraction of shared neurons so that associations can be reliably coded. In the presence of a periodically modulated signal, such as hippocampal oscillations, chains of associations can be recalled analogously to theories of free recall of lists of memorized words. Finally, we compare the predicted number of concepts a neuron responds to with experimental data. We test different ways of constructing correlated patterns and confirmed the common opinion that information in the hippocampus is non-hierarchically organised. In the second part of the thesis, we propose a model of synaptic consolidation based on two coupled dynamical variables: the fast synaptic weights and a slow internal synaptic mechanism. In classical experiments, the consolidation of the synapse is related to the stimulation frequency and number of repetitions. We show that it is exactly the time scale separation between the dynamics of the two variables that determines which combination of stimulation amplitude and frequency are suitable to elicit synaptic consolidation.

À propos de ce résultat
Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.
Concepts associés (34)
Memory consolidation
Memory consolidation is a category of processes that stabilize a memory trace after its initial acquisition. A memory trace is a change in the nervous system caused by memorizing something. Consolidation is distinguished into two specific processes. The first, synaptic consolidation, which is thought to correspond to late-phase long-term potentiation, occurs on a small scale in the synaptic connections and neural circuits within the first few hours after learning.
Mémoire (psychologie)
thumb|350px|Les formes et fonctions de la mémoire en sciences. En psychologie, la mémoire est la faculté de l'esprit d'enregistrer, conserver et rappeler les expériences passées. Son investigation est réalisée par différentes disciplines : psychologie cognitive, neuropsychologie, et psychanalyse. thumb|Pyramide des cinq systèmes de mémoire. Le courant cognitiviste classique regroupe habituellement sous le terme de mémoire les processus dencodage, de stockage et de récupération des représentations mentales.
Mémoire à long terme
En psychologie cognitive, la mémoire à long terme (MLT) est la mémoire qui permet de retenir, de manière illimitée, une information sur des périodes de temps très longues (années). La notion de MLT est un concept utilisé dans les modèles de mémoire qui distinguent plusieurs sous-systèmes en fonction du type d'information mémorisé et de la durée de rétention. La mémoire à long terme s'oppose ainsi au registre sensoriel (ou mémoire sensorielle), à la mémoire à court terme et à la mémoire de travail.
Afficher plus
Publications associées (76)

Fear learning induces synaptic potentiation between engram neurons in the rat lateral amygdala

Henry Markram, Rodrigo de Campos Perin

The lateral amygdala (LA) encodes fear memories by potentiating sensory inputs associated with threats and, in the process, recruits 10-30% of its neurons per fear memory engram. However, how the local network within the LA processes this information and w ...
Nature Portfolio2024

A theory of memory consolidation and synaptic pruning in cortical circuits

Georgios Iatropoulos

Over the course of a lifetime, the human brain acquires an astonishing amount of semantic knowledge and autobiographical memories, often with an imprinting strong enough to allow detailed information to be recalled many years after the initial learning exp ...
EPFL2023

Noninvasive deep brain stimulation to modulate human behavior by means of transcranial temporal interference electrical stimulation

Elena Beanato

All functions we use in our everyday life depend on a complex interplay between both cortical and subcortical brain areas, communicating in between each others. When a region is affected by either an accident, aging or neurodegenerative diseases, the whole ...
EPFL2023
Afficher plus
MOOCs associés (32)
Simulation Neurocience
Learn how to digitally reconstruct a single neuron to better study the biological mechanisms of brain function, behaviour and disease.
Simulation Neurocience
Learn how to digitally reconstruct a single neuron to better study the biological mechanisms of brain function, behaviour and disease.
Simulation Neurocience
Learn how to digitally reconstruct a single neuron to better study the biological mechanisms of brain function, behaviour and disease.
Afficher plus

Graph Chatbot

Chattez avec Graph Search

Posez n’importe quelle question sur les cours, conférences, exercices, recherches, actualités, etc. de l’EPFL ou essayez les exemples de questions ci-dessous.

AVERTISSEMENT : Le chatbot Graph n'est pas programmé pour fournir des réponses explicites ou catégoriques à vos questions. Il transforme plutôt vos questions en demandes API qui sont distribuées aux différents services informatiques officiellement administrés par l'EPFL. Son but est uniquement de collecter et de recommander des références pertinentes à des contenus que vous pouvez explorer pour vous aider à répondre à vos questions.