In the past decade-or-so, we’ve learned a lot about the sleep-memory link. We know, for instance, that people recall facts and experiences more easily and more accurately when they get consistent, sufficient and high-quality rest. We also know that slow-wave sleep and REM sleep facilitate recollection in different, but synergistic ways. Oh, and we know the primary brain regions and networks involved in memory-centric neural processes. What we’re missing, however, is a lot of the stitching holding the fabric of memory together. A new study on reactivated hippocampal neural firing patterns, published in the journal Cell Reports, is another piece of the patchwork quilt.
Before we get to that, here’s a refresher on sleeping to help us remember (and not forget):
Research suggests that sleep fuels our powers of recall in several ways. It locks in new knowledge (see below). It runs “retroactive interference,” meaning it acts as an antidote to forgetting and helps us unearth memories that already started to slip away. The process also helps us prune — prioritize and sharpen memories we need and dispose of the junk-thoughts bogging down our brains.
During the day, we learn new information and soak up the world around us, a process called memory encoding. At night, we convert these freshly collected factoids and experiences into longer-term memories, a process called memory consolidation. For the most part, memory consolidation appears to take place during slow-wave sleep. It’s thought that consolidation involves the transfer of information from the hippocampus, which acts like a temporary storage area, to the neocortex, a more permanent storage unit from which we can retrieve memories when we need them.
When we talk about consolidating memories, we’re mostly talking about the declarative kind, meaning those we can consciously recall. This category further breaks down into semantic memory, which pertains to facts we can learn and recite, and episodic memory, which deals with personal experiences.
But, recent work has illuminated the role of REM sleep, and the vivid dreaming that occurs during it, in preserving emotional memories.
So. This new study — the new piece in the sleep-memory connection — concerns consolidation and fills in a research gap that’s narrow but needs filling.
During slow-wave sleep memory consolidation, the hippocampus reactivates neural firing patterns that were active during memory encoding. This means that, to help a memory stick, the brain “replays” the brain activity that occurred when the memory was initially processed. (The neuro-physical changes that take place in the brain to form memories are called “engrams.”)
It’s not news that these reactivated firing patterns support memory. As Jack Mellor, co-author of the new study, explained, previous research has shown that preventing these reactivated firing patterns in rats and humans causes memory deficits. But, while we'd figured out what neural firing reactivation does, we didn’t know why it was so important to memory. In other words, what crucial memory process does reactivation support?
Now we have an answer: Reactivation helps strengthen the synapses (connections between nerve cells), which enable the passage of electrical messages. And this new study, Mellor explained, also tells us that strengthening these nerve connections requires surprisingly few reactivations — “as few as 10 reactivations occurring in a 5 minute sleep time are sufficient,” he said.
Memories, which are (most likely) stored in synapses, harden when synapses strengthen. The brain is a fairly small mass of fat, nerve fibers and blood vessels that’s home to a lot of interconnected neurons and the electric signals whirring among them. So, as in Manhattan and San Francisco, space is a premium. Neurons must be be bound together by synapses in a way that lets one neural activity pattern (representing a memory) stand out from surrounding neural firing patterns. And this strengthening process occurs before memories can leave the hippocampus (again: the temporary closet where reactivated firing happens) and join stored memories in the neocortex, a layer of gray matter covering the hippocampus.
So, it’s important to figure out that reactivated firing (a mechanism) supports synaptic strengthening (a process). Knowing this lets us further understand how sleep directly supports a critical part of human consciousness: our memories.