When we live a new experience, the memory of that event is stored in a neural circuit that connects various parts of the hippocampus and other brain structures. Each group of neurons can store different aspects of memory, such as the location of the event and even the emotions associated with it.
Until now, neuroscientists studying memory believed that when we remember, our brain recreates the same hippocampal circuitry that was activated when memory was originally formed. However, now neuroscientists at the Massachusetts Institute of Technology (MIT) have shown, for the first time, that bringing back a memory requires a ‘detour’ circuit that branches out of the original memory circuit. The results have been published by the journal Cell.
“This study addresses one of the most fundamental questions in brain research, which is how memories are formed and retrieved , and provides unexpected evidence: there are different circuits for memory retrieval and formation, ” according to Susumu Tonegawa , lead author of the study, in a note issued by MIT.
This memory circuit has never been seen before in vertebrate animals, although a study published in 2016 found a similar circuit in the worm Caenorhabditis elegans .
The hippocampus is divided into several regions, each with different functions related to memory, and most of them have been studied extensively by scientists. However, a small area called the subiculum has been, until now, an unknown.
Tonegawa’s lab set out to investigate this region using mice genetically engineered so that their subicular neurons could be turned on or off using light.
The researchers used this method to monitor memory cells during a fear conditioning event – that is, they emitted a mild electrical shock when the mouse was in a particular room. The memory of this discharge, therefore, must instill fear when they returned to the same room.
Previous research showed that the encoding of these memories involved cells in a part of the hippocampus called CA1, which then transmits information to another brain structure, the entorhinal cortex, via the subiculum. However, the function of the subiculum was unknown in this process.
At each location, small subsets of neurons fire, forming memory fingerprints . “The circuits that are involved in the formation of these memory fingerprints were thought to be the same as the circuits involved in the reactivation of these cells that occurs during the memory retrieval process,” says Tonegawa.
In a group of mice, the MIT team inhibited neurons in the subiculum after fear conditioning occurred. These mice did not show the usual fear response, showing that their ability to remember had been impaired.
The experiment is a proof that the deviation circuit in which the subiculum is involved is necessary for remembering, but not for the formation of new memories. In addition, other experiments revealed that in the direct circuit of CA1 to the entorhinal cortex, the opposite occurs: it is not necessary for remembering, but it is necessary for memory formation.
References:
Roy, DS, Kitamura, T., Okuyama, T., Ogawa, SK, Sun, C., Obata, Y., Yoshiki, A., Tonegawa, S. Distinct Neural Circuits for the Formation and Retrieval of Episodic Memories. Cell. 170: 1-13 (2017).
