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A vaguely familiar image in the crowd – who is it: a former classmate who has changed a lot over the years, or a complete stranger whom we have never seen before?
Previously, neurophysiologists assumed that two areas of the hippocampus, the dentate gyrus and CA3 (Cornu Ammonis 3), are responsible for distinguishing a completely new image from an already known but changed one. At the same time, it was believed that the dentate gyrus automatically “read” any image as a completely new one, and CA3 “tries” to find a correspondence between the new image and the old one stored in memory. So, because of a new hairstyle or glasses, the dentate gyrus determines the person they meet as a stranger, and CA3 compares this image with what is already stored in memory.
New data from a team led by Johns Hopkins University professor of neurophysiology James J. Knierim shows that the role of CA3 is more complex than previously thought – different parts of CA3 can “make” different decisions and communicate them to other areas brain.
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“In the end, it is up to CA3 to decide – is it the same image or a different one? If the person you meet is familiar to you, just slightly changed over time, this is one story. But if you’re wrong and he really is a stranger, your brain needs to create a new memory, completely separate from the memories of the person they are similar to,” says James Knierim.
His group was experimenting with rodents. The rats were implanted with electrodes in the hippocampus and forced to run along a specially built path, along which chocolate chips were scattered. The surface of the track was covered with four different materials – sandpaper, carpet, strong adhesive tape and rubber. The path was surrounded by a black curtain, to which various objects were attached. For ten days, the rats remembered the entire territory well.
Then the experimenters changed the setting. They turned the walkway counterclockwise and the curtain clockwise, disorienting the rats. “To understand how the rats felt, imagine that you came to your house and saw that the furniture had been moved and the paintings were hanging differently. What would you think: that this is your house or that you have the wrong door? This kind of experience is very disorienting and very uncomfortable.”
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Even with small changes in the interior, the activity in one of the CA3 subdivisions in the brain of rats completely changed – they memorized the new location of objects. However, in another subdivision, activity remained practically the same: memories of the past location of objects were maintained there, even if the situation changed dramatically. Obviously, this part “understood” that the environment, despite the changes, remained the same.
Why do scientists study the process of the hippocampus in such detail? Among other things, such studies are important for understanding the mechanisms of memory loss in Alzheimer’s disease and developing new methods of its treatment.
Подробнее см. J. Knierim et al. «Neural population evidence of functional heterogeneity along the CA3 transverse axis: Pattern completion versus pattern separation», Neuron, August 2015.