What are dormant memories

"Wait a minute, I've already been here!" - How the brain creates local memories

Neuroscientists from Tübingen have succeeded in activating dormant memory cells in rats. Using targeted weak electrical impulses, they were able to induce previously inactive cells in the hippocampus to recognize the location of the impulse administration.

The hippocampus is responsible for memory in rodents as well as humans. The study by the research team at the Werner Reichardt Center for Integrative Neurosciences (CIN) at the University of Tübingen therefore provides information on how memories are formed in our brain. The results have now been published in the journal Current Biology.

Memory is one of the most important functions of our brain. With his help, we will not only be able to tell our grandchildren one day about our youth. It is indispensable, especially for everyday processes.

It is constantly and immediately active when we experience something: When we get to know someone, we recognize them again after hours or days. And anyone who visits the perfume department, the personnel office or the toilet in a strange building for the first time usually finds the exit again without difficulty.

The memory not only “thinks” constantly, it also creates new memories particularly quickly, usually during the first interaction. This is because certain memory cells are directly responsible for each person, for each place and probably also for many other concepts. One type of these neurons, the granule cells, is located in the hippocampus, a central region of the brain.

When memory concepts such as "my living room" or "Angela Merkel" are activated - for example by entering the living room or looking at a photograph of the Chancellor - a small number of responsible granule cells react with electrical impulses. The vast majority of granule cells, on the other hand, remain inactive.

So far it has not been clear by which mechanism individual memory cells are assigned to a specific memory - especially since the vast majority of granule cells normally rest and seem to have no function. The Tübingen research team headed by Dr. Andrea Burgalossi now looked into the question of whether dormant granule cells can be "woken up" under certain circumstances.

Your guess: granule cells can become active memory cells through electrical impulses. To test the hypothesis, they placed hair-thin microelectrodes in the dentate gyrus - an area in the hippocampus that contains local memory - of rats, through which they could send weak electrical impulses to individual granule cells.

The rats ran freely through a simple maze. At a certain location within the labyrinth, individual granule cells were stimulated by microelectrodes with weak electrical impulses (in the nanoampere range). Using the same electrode, the researchers then measured the activity of the treated granule cells.

The result: If the rats came back to the place in the labyrinth where the impulse had previously been administered, the stimulated granule cells now fired on their own. The impulse had stimulated the dentate gyrus to form a memory of the location in the affected granule cells.

Burgalossi and his research group also found that the duration and type of impulses administered play a major role. They lead to a more stable memory of place when they are done in accordance with the natural theta vibration of the brain, a build-up and breakdown of electrical potential that occurs about four to twelve times a second.

Another finding could be just as significant: rats that received the impulse when entering the labyrinth for the first time reacted much more strongly to the induced memory than rats that already knew their way around the labyrinth beforehand. Apparently, memory cells are activated more easily when the brain has to process new information.

The new insights into memory formation shed light on one of the most important brain functions. There is still much to be done before such fundamental findings as the present ones can contribute to the development of treatments for memory disorders (for example in Alzheimer's, Parkinson's or dementia) - but they are an indispensable first step on the way.

Maria Diamantaki, Markus Frey, Patricia Preston-Ferrer, Andrea Burgalossi: Priming Spatial Activity by Single-Cell Stimulation in the Dentate Gyrus of Freely-Moving Rats. Current Biology (in press). 4th February 2016.

Dr. Andrea Burgalossi
Werner Reichardt Center for Integrative Neurosciences (CIN)
andrea.burgalossi [at] cin.uni-tuebingen.de

Press contact CIN:
Dr. Paul Töbelmann
Science communication
Werner Reichardt Center for Integrative Neurosciences (CIN)
Tel .: +49 7071 29-89108
paul.toebelmann [at] cin.uni-tuebingen.de


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