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How is information transferred into long-term memory?

Scientists at the BCCN Berlin have found out how cellular learning processes contribute to long-term storage of information in the brain. (November 2005)

Every day our brain perceives countless impressions, only a small fraction of which are transferred into long-term memory. For a long time, scientists have been investigating the question of which neuronal mechanisms underlie the formation of memory. Depending on the level at which this question was asked, different answers were found. On the cellular level, it is already fairly well known how learning processes strengthen the contacts between neurons. Neuronal connections that are used more often are consolidated. Other scientists study the learning process in the context of the whole brain. They found that in order to save information from being forgotten, it has to be transferred from the working memory in the hippocampus to the brain's cortex, where it is stored on a long-term basis. For the first time, scientists around Uwe Heinemann, director of the Institute for Neurophysiology at the Johannes Müller Center for Physiology at the Charité and member of the Bernstein Center for Computational Neuroscience Berlin, have now established a connection between these two processes.

The hippocampus is the most important coordination center for recent impressions of any origin. It is here where impressions such as smell, touch, acoustic information and visual input are interlinked and stored on a short term basis. The nerve cells that underlie this function of the hippocampus are organized in neuronal assemblies that communicate through network waves (oscillations) and thereby transfer information.

2005_Heinemann Langzeitgedaechtnis_en

As the information capacity of the hippocampus is limited, and since our brain is exposed to a flood of new impressions every day, it tries to transfer information as fast as possible from working memory to long-term memory. According to the current state of knowledge this is done by transferring the information from the hippocampus to the brain’s cortex. In this process, short, synchronous oscillations in the frequency range around 200 Hertz occur, which are preferentially observed during sleep or in resting phases during daytime. Thus, good sleep improves your memory. Heinemann and his colleagues have intensively studied which cellular processes underlie the emergence of these high frequency oscillations.

Up to now, high frequency oscillations that lead to the formation of memory have only been observed in living animals. In preparations of the brain, so-called brain sections, the neural network is presumably not strong enough to generate such oscillations because important network components are missing. The scientists around Heinemann simulated learning in brain sections by applying repeated electrical stimuli that strengthen the connections between single nerve cells. This enhancement of neuronal contacts induced high frequency oscillations around 200 Hertz, just like those that are typically observed during information transfer to the brain’s cortex. In this way, for the first time, a connection was established between cellular learning processes and learning processes that occur in the context of the whole brain, such as the long-term information storage in the cerebral cortex.

In further experiments, the scientists could show that the induction of such ultra fast network oscillations depends on the activation of a specific glutamate receptor, the N-methyl-D-aspartate (NMDA) receptor. The NMDA receptor is an important building block at the contact points between nerve cells and plays a significant role in cellular learning processes. This is a further proof for cellular learning processes being a prerequisite for long-term information storage in the cortex. The possibility to now also generate high frequent oscillations in brain sections offers new perspectives for research, since in this preparation, the cells are much easier accessible than in the living brain.


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Prof. Dr. Uwe Heinemann

Induction of sharp wave-ripple complexes in vitro and reorganization of hippocampal networks

Institute for Neurophysiology, Charité Berlin
Tucholskystrasse 2
10117 Berlin
Phone: ++ 49 (0)30 450-52 81

Article in Nature Neuroscience, 2005 Nov; 8(11):1560-7.
(URL: http://dx.doi.org/10.1038/nn1571)

 

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