For decades scientists have theorized that memories are formed in the hippocampus and then move to the neocortex for long-term storage. They believe this process takes place as we sleep. This concept has been researched recently by a study conducted by Brown University and previously thought theories have been dispelled.
Lead neuroscientist, Mayank Mehta working with Nobel Prize winning physiologist Bert Sakmann tried to prove the previous theory that the hippocampus creates the memories and the neocortext transforms in our dreams was correct, but were surprised at their findings.Â Their work, published in Nature Neuroscience, reported that instead of the hippocampus uploading information to the neocortex in a burst of brain cell communication, they found just the opposite; the neocortex seems to initiate the dialogue with the hippocampus.
â€œLong-term memory making may be a very different process than we previously thought,â€ said Mehta, an assistant professor in the Department of Neuroscience at Brown. â€œEither this reversed dialogue is, somehow, a part of memory storage or this transfer of information from the old to the new brain may not occur during sleep. Either way, the results call into question commonly held theories about the role of cortico-hippocampal dialogue in sleep.â€
Professor Edvard Moser, of the Norwegian University of Science and Technology, and director of the Centre for the Biology of Memory, is a leading expert on memory processes in the hippocampus. Moser says, â€œThis technically sophisticated study may significantly influence our view of hippocampal-neocortical interactions during sleep-related memory consolidation processes.â€
The researchers studied electrical activity in the brains of rats. The rats were anesthetized to imitate our deepest state of sleep and fitted with electrodes. One electrode measured the electrical activity of the neocortex, which involved thousands of cells. The other electrode measured electrical activity of a single inhibitory cell in the hippocampus. They found the electrodes in the neocortex were excitatory â€“ communicating between nerve cells.
With this single-cell recording technique, researchers made a significant finding â€“ during our deepest sleep both the hippocampus and neocortex show a great deal of activity, and cells in both the old and new parts of the brain are working in synch.
This discovery was surprising because previous studies had shown cells in the neocortex with much activity while the activity in the hippocampus was erratic.Â Mehta and his colleagues were stumped because if their new findings are correct these two parts of the brain talk to each other as we sleep they didnâ€™t appear to be speaking the same language. They realized that they actually were, but you have to listen to the inhibitory, not the excitatory cells in the hippocampus. The timing of their conversation was the same in both brain regions although there was a slight delay in the hippocampus.
This communication, known as â€˜phase-lockingâ€ has two key presumptions:
1.Â Â Â Â Â The neocortex, and not the hippocampus, drives the communication between the two parts of the brain during deep sleep.
2.Â Â Â Â Â The inhibitory neurons control the conversation
Mehta believes the findings may change the way neuroscientists look at past experimental data and the way they conduct future research. â€œWe now have a way, experimentally and theoretically, to see how the two parts of the brain talk to each other,â€ he said. â€œThis will help us better understand the mechanisms behind learning and memory. But what is really exciting is that this method â€“ simultaneously studying two different cell types in two different brain areas â€“ could be used to study other aspects of brain function, such as perception, emotion, movement. It could open important new avenues for basic and applied research.â€
Science Daily – Speak, Memory: Research Challenges Theory Of Memory Storage: http://www.sciencedaily.com/releases/2006/11/061114175231.htm