Some memories seem to naturally go together. For instance, if you think of an important experience in your life (exchanging your vows at the church, for example), you may also be likely to think about another experience that happened around that time (later on your friend drank too much wine at the reception). These two memories seem to be somehow linked in your mind. In a new study, our labs have figured out how memories how this might happen in the brain.
Our memories are thought to be represented in the brain by small collections of cells (neurons). In recent years, our labs have identified collections of neurons (or engrams) in the brain’s amygdala that store specific memories. But there are many thousands of neurons in the amygdala. How does the brain decide which collections of neurons encode a particular memory? Sheena’s lab found that the activity (or excitability) of neurons in the amygdala fluctuates, and only the neurons that are most excitable when an event occurs are likely to ‘grab’ the memory.
In the new paper, we take this one step further and ask how are memories for separate events linked in the brain. We find that after encoding a memory, these ‘engram’ cells remain active for a few hours before their excitability levels fall back in line with their neighbors. We show that if a second event occurs within this window (i.e., <6 hours), then the memory for that event is encoded in exactly the same collection of neurons as the first memory. These two memories have been encoded in the same population of cells and have become linked.
In contrast, if the second event occurs outside of this window (e.g., a day later) then these cells are no longer excitable, and the memory for that event is encoded in a different population of cells. These two memories are not linked.
We went onto to prove that a neuron’s excitability was the key factor determining whether two memories were linked or not. By artificially manipulating neuron excitability we could link two memories that would normally be encoded in separate populations of amygdala neurons. Conversely, they could separate two memories that would normally be encoded in the same population of neurons.
Asim Rashid (in Sheena’s lab) is the lead author on this work. His contributions to the lab selfie project are above, and his paper is available here.