Leçons de niveau 17

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Objectifs[modifier | modifier le wikicode]

Vue d'ensemble des types basiques de mémoire.
  • Apprendre comment les expériences sensorielles sont encodées et stockées dans le cerveau.
  • Apprendre les différents types de mémoires et les mécanismes qui les sous-tend.
  • Apprendre comment, une fois formés, les souvenirs sont rappelés à chaque niveau différent.

Types de mémoire[modifier | modifier le wikicode]

  • La mémoire déclarative consiste à se rappeler d'items spécifiques. Elle peut se diviser en mémoire sémantique, qui est la capacité à rappeler les faits (ex. : George Washington était le premier président américain) et la mémoire épisodique, qui est la capacité à rappeler des expériences ou événements spécifiques (ex. : l'été dernier, j'ai visité le Mont Vernon et j'en ai appris beaucoup sur George Washington). Les souvenirs épisodiques traitant exclusivement de soi-même sont aussi regroupés dans la mémoire autobiographique. Les souvenirs déclaratifs de tous types sont stockés dans le cervelet.
  • La mémoire procédurale consiste principalement en un rappel implicite (par opposition à conscient) de certaines aptitudes motrices. C'est implicite car on peut, par exemple, savoir faire du vélo sans pouvoir consciemment indiquer tous les mouvements nécessaires pour le faire. Les souvenirs procéduraux, une fois consolidés, sont généralement stockés dans le cervelet.

Mémoire de travail et à court-terme[modifier | modifier le wikicode]

La mémoire de travail, qui peut durer de quelques secondes à plusieurs minutes, peut être généralisée comme un « rafraîchissement » actif et constant de l'expérience. Les circuits cérébraux impliqués dans l'expression de cette expérience restent dans une boucle d'activité constante, gardant ainsi cette expérience à l'esprit. À titre d'exemple, imaginez qu'une image doit être conservée dans la mémoire de travail. Alors que cette image est encore en train d'être visualisée, le circuit cérébral le plus actif part du lobe occipital où les stimuli visuels sont d'abord traités, va dans les aires du lobe pariétal où cette information visuelle est convertie en objets cohérents et unifiés (à savoir la vue des roues noires et de la peinture rouge vive, ainsi qu'une échelle et des tuyaux sont liés ​​ensemble dans la perception visuelle d'un camion de pompier). Lorsque l'image n'est plus présente mais qu'elle est rappelée activement par la mémoire de travail, cette « voie » occipitale vers pariétale est activée en boucle, de sorte que l'image reste consciemment notée.

Consolidation de la mémoire et mémoire à long-terme[modifier | modifier le wikicode]

Modèle:Annotated image/sandbox Long term memory, which lasts between minutes up to an entire lifetime, is slightly more complex, as it involves a period of consolidation and a series of crucial neurochemical changes. The first stage is memory consolidation, which takes place in the hippocampus.

The hippocampus is the essential location for almost all memory consolidation, although it is important to note that most long-term memories are not stored there. The cellular mechanism responsible for memory formation is long term potentiation (LTP). To start, the information to be encoded is represented by a network of connections between neurons. The strength of these connections can vary or change over time, and it is this principle that allows LTP to occur. Synchronized firing among neurons in a network in response to being presented repeatedly with some stimulus strengthens the connections among those neurons. LTP is simply the neurochemical process of strengthening these connections (through the strengthening of the synapses between neurons). Thus, on a rudimentary level, the information memories contain is stored in the relative strength of the connections between a neurons in a network. A network whose connections are very strong can be thought of as a strong or vivid memory, while those with weaker connections require more synchronized activation in order to become a lasting memory (which gives rise to the old cliche that practice (or repetition) makes perfect).

The next stage of long-term memory is a period of consolidation where the networks strengthened in the hippocampus are 'moved' to their final storage destinations in the cortex. In the case of complex memories, each portion of that memory experience is theorized to be stored in the cortical region associated with that experience modality. So when remembering some event such as riding the train, the sounds one remembers hearing on the train ride are stored in auditory areas, the sights in visual areas, etc.

Memory retrieval[modifier | modifier le wikicode]

Memory networks can be fully or partially activated by associated stimuli, such as this picture of Friedrich Nietzsche

The basis of memory recall is a re-creation of that memory. As shown by the discussion of LTP in long-term memory, memories are stored as patterns of neural network activation across the brain, such that those networks encode the information of the event (information as in the sights, sounds, details, etc.. Once some sort of stimulus causes a part of one of these memory networks to become activated, the strength of the connections between members of that network cause the entire network to become activated too, thereby recalling the memory experience. In contrast to this full re-creation of the memory experience generated by complete activation of the memory network, partial activation of a network can lead to unconscious recognition or presque vu.

An example of recognition would be seeing a picture of Friedrich Nietzsche, then from that remembering his name, some basic facts about his life, etc. In this example, the picture acts as the stimulus which can crudely be seen as activating some part of your 'Friedrich Nietzsche' network. If the strength of the connections in this network are strong, then this stimulus will cause the whole network to reactivate, thus accounting for your ability to recall facts about Nietzsche, such as what books he wrote, when he lived, etc. If these connections are not as strong, there may only be partial activation of the Nietzsche network, leading to moments where the person's name is on the 'tip of your tongue', and the only way to access it is to try to activate more of the network. For example, just seeing Nietzsch's face may not be enough to trigger recall of his name, but add to that some vague recollection of 19th century philosophers, something about the word Übermensch, and then suddenly there is enough of the 'Nietzsche network' activated to be able to retrieve his name.

A secondary point which is worth mentioning is that because the brain is a dynamic, ever-changing system, memories are prone to being altered during recall. The reason behind this is that the memory network is being activated at the same time other networks are being activated by the present world. For example, you may recall a favorite camping trip you went on as a child at the same time you are happening to eat a graham cracker. If a neuron from the 'camping trip network' should form a strong connection during this time to a neuron associated with experiencing the graham cracker, then part of the experience of eating a graham cracker may accidentally fuse into the memory's network. So potentially years later when recalling that same camping trip, at the end you may distinctly remember eating graham crackers over the campfire, when in fact this never happened.