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Scientists show how memories are linked in the brain


Memories form an important part of our lives. Some memories seem etched in our mind forever. In fact, if we think about it we will realize that some memories are quite connected. For example, if we think about an important experience in our life, we might also closely remember another experience that happened around the same, like your graduation ceremony and the party you had after that, or the memory of exchanging vows at your wedding, and then how your friends danced to the music in the party later that same night. Somehow there seems to be a connection between the two memories in your mind.

In a new study led by the Hospital for Sick Children (SickKids) published recently in an online edition of Science, the connection between memories and how they become linked in the brain has been explored.

This study is supported by Canadian Institutes of Health Research, Natural Sciences and Engineering Research Council of Canada, Brain Canada, Brain and Behavior Research Foundation grants, and SickKids Foundation

The principal investigator of this study, Dr. Sheena Josselyn and her lab have been working for many years now, on how the brain forms, stores and organizes memories. Previous research from her lab had revealed about collections of neurons (engram) in the amygdala that store specific memories in mice. The amygdala is a brain region that is important in encoding important memories. The amygdala consists of thousands of neurons, but the question is how is it decided by the brain which collection of neurons encodes a particular memory?

Josselyn, who is also a Senior Scientist in Neurosciences & Mental Health at SickKids revealed that in their study they found that the activity, or excitability, of neurons in the amygdala fluctuates, and the neurons that are most excitable when an event occurs are most likely to ‘grab’ the memory. During the study, it was also noted that once a memory is encoded, the engram cells stay active for s few hours before their activity level decreases. The researchers found that if another event takes place within the activity window which is less than six hours, the memory is then encoded in the same set of neurons. Since these two memories are encoded in the same population of cells they become linked. Likewise, if a second event occurs outside the activity window of the activated neurons, they get encoded in a different population of cells. Hence, those two memories are not linked and are stored in the brain as entirely separate events.

This study have found that a key factor in determining whether two memories are linked or not is – the neuron’s excitability. The researchers also showed that by artificially manipulating the neuron’s excitability, two different memories could be encoded in the same set of amygdala neurons. Similarly, it was also possible to separate two memories that would normally be encoded in the same population of neurons.

Josselyn who is also Associate Professor in the Departments of Physiology and Psychology at the University of Toronto remarked that their study uncovers the principles of how we organize our memories”how remembering one event conjures up memories of closely related episodes.

The findings of this study can be applied to various psychiatric conditions opined Paul Frankland, co“principal investigator on the study. By understanding how memories are linked may provide hints as to how they become inappropriately connected in conditions such as schizophrenia and in the long run it might help in finding viable solutions to such problems.

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