How the Brain Builds Memory Chains

Sara Chodosh in Scientific American:

BrainThink about the first time you met your college roommate. You were probably nervous, talking a little too loudly and laughing a little too heartily. 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. Now neuroscientists are starting to figure out why. When two events happen in short succession, they feel somehow linked to each other. 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 this week’s Science. “Intuitively we know that there’s a structure to our memory,” says neuroscientist Paul Frankland, affiliated with both the University of Toronto and SickKids. “These experiments are starting to scratch the surface of how memories are linked in the brain.”

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 clusters of connected cells are known as engrams, or memory traces. When a mouse receives a light shock to the foot in a particular cage, an engram forms to encode the memory of that event. Once that memory forms the set of neurons that make up the engram are more likely to fire. 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. 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. 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.

More here.