2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Episodic memory helps us remember "how to get to the library." For this, information must move from short-term memory to long-term memory. It is known that anatomically this requires the hippocampus and cerebral cortex. Conceptually, two scenarios are possible: first, short-term memory is encoded in the hippocampus, as the โlowerโ center of the brain, then it is transferred for long-term storage to the cerebral cortex and is erased from the hippocampus. The second option: episodic memory, even short-term, is simultaneously recorded in the hippocampus and in the cerebral cortex, and already in the latter is fixed over time. As the latest research published in Science shows, the second is most likely the correct option.
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Engram cells (red and green)
Information about which areas of the brain are needed for memory began to appear in the 1950s. For example, a patient with a damaged hippocampus could not form new memories, but retained old ones. From this it follows that the hippocampus is needed for education, but not for memory preservation. Further studies of patients with amnesia showed that the cerebral cortex is also needed to preserve memory.
The problem was how to trace, at the molecular level, the chain of events leading to the formation of memory. How to design an experiment is fairly straightforward, but technically challenging. Therefore, recent research from MIT reads as a guide to the latest methods of neurotechnology - here both optogenetics and calcium imaging in vivo, and labeling of cells based on their activity.
Using these methods, scientists first identified the engrams that appear in mice when they feel fear. Engrams are a "trace" in the brain, a neural circuit that occurs during the formation of memory. The mice were placed in a cell where they were electrocuted. In this case, an engram was formed in the brain, which includes certain neurons in the hippocampus, the prefrontal cortex and the amygdala (this region of the brain is responsible for emotions).
When the mice were placed back into the chamber every other day, the mice froze in fear, while only hippocampal neurons were activated from the engram, the neurons of the cortex were not excited. If fear is caused by an artificial (stronger) way - by activating the hippocampal neurons optogenetically, with the help of light - then the neurons of the cerebral cortex from the engram will also be excited. The conclusion from this is this: the first, short-term memories are stored not only in the hippocampus, but also in the cells of the cortex, but in the cortex they are still in an immature form (therefore, a powerful signal is needed to activate these memories).
Two weeks later, the neurons of the cortex matured, their anatomy and physiological properties changed, and the neurons from the hippocampal engram, on the contrary, fell silent. Now, in mice, freezing from fear at the sight of the "torture chamber", cells from the cortex engram were activated first of all, for this the hippocampus was not needed. But in the latter, the "remnants" of memory were nevertheless preserved: if the neurons of the hippocampus were activated, the entire neural chain of the engram was excited, including the cells of the cortex.
It turns out that memory is encoded in parallel in both the hippocampus and the cerebral cortex. Over time, the balance shifts towards the cells of the cortex - the signal becomes stronger, but weaker in the hippocampus. Unfortunately, current methods only allow you to track the development of the engrams for 20 days. Given that the hippocampus encodes very detailed features of memory, and the cortex retains general features of memory, it would be interesting to know if it is possible to remember (at least theoretically) what color the car was on the road on the way to the library 15 years ago.
