Our memories are stored simultaneously in short-term and long-term memory

New understanding of long-term and short-term memory
The results of a current study by Japanese researchers could revolutionize the previous understanding of short-term and long-term memory. So far, the assumption was that experiences are first stored in short-term memory and then transferred to long-term memory. In their current study, the scientists come to the conclusion that storage takes place simultaneously in long-term and short-term memory.

The ability to remember experiences long after they happen has a major impact on our behavior and personalities. Scientists at the RIKEN-MIT Center for Neural Circuit Genetics (CNCG) have investigated how information transfer into long-term memory works and, with their current study results, revolutionized the previous understanding of how information is stored. The results of the study were published in the scientific journal "Sience Translational Medicine".

Engram cells store the memories
The short-term storage of experiences is created in the brain region of the hippocampus. Here, the research team led by Susumu Tonegawa, director of RIKEN-MIT CNCG, used mice to investigate how memory formation works. They marked the neurons that represent the memory of an event when it is stored in the hippocampus. These types of neurons are called engram cells and their activation - either naturally or by optogenetic stimulation with colored light - is the basis for recalling the memory, the researchers explain.

Engram cells in long-term memory
The episodic memories that are stored in the engram cells of the hippocampus are known to be short-lived. To date, science has assumed that permanent memories like new engram cells and neuronal connections gradually form in the cortex over time, the Japanese researchers explain. However, the current series of experiments has shown that this assumption is only partially correct. "We discovered the existence of cortical engram cells, but it turned out that they do not gradually form over time, but form at the same time as the first memory in the hippocampus," says study author Takashi Kitamura.

Cortical memory created on the first day
In order to determine which areas in the cortex (cerebral cortex) were important for the formation of long-term memory, the researchers blocked the input of signals into different areas of the brain during conditioning (formation of the memory) or during memory recall over a period of three weeks. Long-term access was only possible if the information transfer was blocked directly during conditioning, the scientists report. "This was surprising because it indicated that cortical memory was probably created on the first day and not gradually, as previously thought," said study leader Tonegawa.

Engram cells that can be activated
In a next step, the scientists identified the engram cells in the prefrontal cortex and excited them with blue light using light-sensitive ion channels. "As with previous studies in the hippocampus, this caused the mice to show behavior that indicated their remembered experience - a hallmark of engram cells," the scientists report. By definition, the animals should be able to remember an event when engram cells respond to the signal and should not be able to remember when the cells are silent, the experts explain. The cortical engram cells could be activated in this way, but only if more than a week had passed since conditioning. By then, the engram cells in the hippocampus had already lost their memories, the researchers continued.

Maturation process of several days
According to the scientists, the fact that the engram cells were formed on the first day, but could only be activated much later, “indicates that it took time for them to mature and switch from silent engram cells to active ones “Further tests have shown that this maturation process is influenced by the engram cells over several days. The team was also able to demonstrate a connection between engram cells for positive and negative emotional memories in the hippocampus and the frontal cortex with another part of the brain, the amygdala. The storage of memories in our brain seems to be completely different than previously thought. (fp)

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