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Scientists link unrelated memories for the first time

For the first time researchers have found a way to create an artificial link between unrelated memories using optogenetics. This discovery by Japanese researchers could lead to new treatment strategies for people with posttraumatic stress disorder and other psychiatric conditions.

The investigators genetically modified certain brain cell populations in mice, causing them to associate two unrelated memories, one of a foot shock and the other of exploring a safe environment.

Senior study author Kaoru Inokuchi of the University of Toyama in Japan pointed to past studies that have found that specific neurons, which are activated during the learning process are reactivated when attempts are made later to recall what we learned. All that is required for successful retrieval of that memory is reactivation of these specific neurons.

“These findings show that information associated with an experience is encoded in a specific cell ensemble, which gets selected and activated during a corresponding event,” explain the authors.

Further, artificial reactivation of a certain brain cell ensemble connected to a pre-existing memory can modify the addition of a new memory. This leads to the creation of false memories.

In the study mice were exposed to a “fear-learning paradigm,” with one group of mice being placed in a cylinder-shaped enclosure for six minutes and another group minutes in a cube-shaped enclosure for the same time.

After spending 30 minutes outside their enclosures, both groups were then placed in the cube-shaped enclosure and were immediately given a foot shock. Two days later, the mice were re-exposed to one of the two enclosures. As expected the mice re-exposed to the cube-shaped enclosure which came with the foot shock exhibited greater fear than those that were re-exposed to the cylinder-shaped enclosure.

Then applying a method called “optogenetics” which involves genetically modifying certain brain cell ensembles so they express light-sensitive proteins that control the activity of brain cells, the mice’s memory of the foot shock were reactivated in the cube-shaped enclosure and the safe environment of the cylinder-shaped enclosure – memories that were not linked.

When the team used the optogenetic method to stimulate brain cell ensembles in both the hippocampus and the amygdala they found the mice spent more time in a scared state when re-exposed to the cylinder-shaped enclosure than when the two brain regions were stimulated separately or not at all.

These results indicate that simultaneous activation of certain brain cell clusters can create artificial links between unrelated memories causing long-term behavioral changes. These findings could spawn new treatment interventions for individuals suffering from psychological conditions triggered by traumatic events.

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