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Our memories are stored simultaneously in short-term and long-term memory
The results of a recent study by Japanese researchers could revolutionize the understanding of short-term and long-term memory. Previously, the assumption was that experiences are first stored in the short-term memory and then transferred to the long-term memory. In their current study, the scientists conclude that storage occurs simultaneously in long-term and short-term memory.
The ability to remember experiences long after they happen has a significant impact on our behavior and personalities. Scientists from the RIKEN-MIT Center for Neural Circuit Genetics (CNCG) have investigated how information transfer into long-term memory works and that their current study results have revolutionized the previous understanding of the storage of information. The results of the study were published in the journal "Sience Translational Medicine".
Memories are stored simultaneously, according to a recent study by Japanese researchers in short-term memory and long-term memory. (Image: Sergey Nivens / fotolia.com) 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 the RIKEN-MIT CNCG, studied how memory formation works on mice. 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 retrieving the memory, the researchers explain.
Engram cells in long-term memory
The episodic memories stored in the engram cells of the hippocampus are known to be short-lived. To date, science has suggested that permanent memories gradually form over time, such as new engram cells and neuronal connections in the cerebral cortex, explain the Japanese researchers. However, the current series of experiments have shown that this assumption is only partially correct. "We discovered the existence of cortical engram cells, but it turned out that they do not form gradually over time, but form at the same time as the first memory in the hippocampus," study author Takashi Kitamura reports.
Cortical memory created on the first day
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 to various brain areas during conditioning (memory formation) or during memory recall over a period of three weeks. Long-term retrieval was only impossible if information transfer was blocked directly during conditioning, the researchers report. "That 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.
Activatable Engram Cells Sign for existing memory
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 prompted the mice to show behavior that suggested their recalled experience - a hallmark of engram cells," the researchers 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 do so when the cells are silent, the experts further explain. The cortical engram cells could be activated in this way, but only if more than a week had passed since conditioning. By that time, the engram cells in the hippocampus had already lost their memories, researchers said.
Maturation process of several days
The fact that the engram cells were already formed on the first day, but could only be activated much later, according to the scientists indicates that "it took time for them to mature and to switch from silent engram cells to active ones "Further tests have shown that this maturation process is influenced by the engram cells for several days. The team also demonstrated a link between engram cells for positive and negative emotional memories in the hippocampus and 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)