![]() “We were blown away by that,” Josselyn says. Decreasing the excitability of the neurons in the first memory during the second event seemed to prevent the second memory from forming. When they tried to disentangle memories that happened six hours apart, however, they ran into trouble. Mice should normally form separate memories when events happen 24 hours apart, but when the researchers re-excited the neurons in the first memory engram while the second memory was forming, they could artificially link those experiences. Josselyn, a neuroscientist at SickKids and the University of Toronto, and Frankland’s group was also able to tinker with the link between two memories by adjusting the excitability of neurons during different time points. The rodents who formed the same memories 24 hours apart had separate sets of neurons related to each memory. Mice who formed a fear memory-one where they were given a foot shock in a particular environment-and then formed a second memory six hours later had formed those two memories in overlapping engrams. It follows then that a memory formed soon after the first might be encoded in an overlapping population of neurons, which is exactly what Frankland and study co-lead author Sheena Josselyn, found. The question was, did that principle apply to two memories that happen close together in time? Neurons in a newly formed memory trace are subsequently more excitable than neighboring brain cells for a transient period of time. Furthermore, more excitable neurons-that is, brain cells that activate easily-are more likely to be recruited into an engram, so if you increase the excitability of particular neurons, you can preferentially include them in a new engram. Once that memory forms the set of neurons that make up the engram are more likely to fire. When a mouse receives a light shock to the foot in a particular cage, an engram forms to encode the memory of that event. ![]() These clusters of connected cells are known as engrams, or memory traces. In your brain, and in the brains of lab mice, recollections are physically represented as collections of neurons with strengthened connections to one another. “These experiments are starting to scratch the surface of how memories are linked in the brain.” “Intuitively we know that there’s a structure to our memory,” says neuroscientist Paul Frankland, affiliated with both the University of Toronto and SickKids. It turns out that apparent link has a physical manifestation in our brains, as researchers from the Hospital for Sick Children in Toronto (SickKids), the University of Toronto and Stanford University describe in Science. When two events happen in short succession, they feel somehow linked to each other. ![]() Now neuroscientists are starting to figure out why. What else does that memory bring to mind? The lunch you shared later? The dorm mates you met that night? Memories beget memories, and as soon as you think of one, you think of more. You were probably nervous, talking a little too loudly and laughing a little too heartily. ![]() Think about the first time you met your college roommate. Recollections of successive events physically entangle each other when brain cells store them Neuromorphogenesis: How the Brain Builds Memory Chains
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