Anyone who survived the ’80s and ’90s has gone through the aggravating ritual of rewinding a video or cassette tape. Wanting to catch what someone said in a movie scene or listen to that rad new Def Leppard track one more time meant pressing the rewind button and waiting until the film was inevitably wound too far back or not far enough. Our brains are capable of something similar when it comes to replaying memories.
Memory replay is like the brain’s version of rewinding a VCR. The brain backtracks and replays an experience that had been “encoded,” a mechanism that is necessary for recall and spatial working memory, reinforcement learning, navigation, and consolidating memories. Sometimes it can also be like a Back to the Future-esque clip of time travel. Replaying memories involves many brain regions, especially the hippocampus, whose neurons become active while a task is being carried out, but keep firing when the task is completed and even after sleep sets in. It’s thought these “replayed” events may help with remembering locations in space—and they can even be edited to reveal a new understanding. Neuroscientist Zhe Sage Chen, PhD, of New York University has been studying this phenomenon for decades.
“Some people see the hippocampus like a GPS system,” says Chen. “This is one of the areas we bring up the most when it comes to episodic memory, which is reliving what happened in the past. It basically allows you to memorize some of the things that you experience during the day, but it’s also like a GPS system that guides you with spatial navigation and so on. It also goes beyond the spatial [realm]… Humans can obviously navigate in abstract space.”
Replaying memories is possible because of what are known as place cells and grid cells in the hippocampus. In 1971, Nobel Prize winner John O’Keefe, PhD, was working with rats navigating mazes in his lab when he discovered place cells. These are a type of neuron that is part of our internal GPS. It essentially maps the space around us. O’Keefe had been trying to figure out how the brain controls behavior when he recorded nerve cells and realized those in the hippocampus were activated when the rats stayed in a particular place. In 2005, May-Britt, PhD and her former husband Edvard Moser, PhD, saw that another group of neurons activated as rats passed several locations—these became known as grid cells. They create invisible coordinates that allow us to position ourselves and find our way.
We experience our environment through the perception and positioning that having a sense of place allows for. O’Keefe found that place cells were accumulating memories of an environment, and the Mosers realized cells in the entorhinal cortex were activated when rats made their way through a grid. Replaying these memories tells us where we are. The three scientists jointly won a Nobel Prize for their work in 2014. Fascinated by their discoveries, neurologist Matt Wilson, PhD, of Massachusetts Institute of Technology (MIT) decided to delve further into the role of place and grid cells in memory recall when he was a postdoc (Chen was also formerly an MIT postdoc). His own experiments with rats gave him more insight into how the human brain rewinds and replays.
“The idea was to look at how memories of space are formed as an animal first explores an environment,” Wilson says. “So that was really what I was interested in. I was interested in the formation of memory as animals experience the world. It just happened that as I was doing recordings after these experiments, I would look at the activity of these cells to ensure the quality of the recordings and make sure the electrodes on the animal hadn’t moved, but I also had signals coming off these electrodes routed through an audio amplifier. That was how I could hear brain activity.”
What surprised Wilson was that he kept hearing these signals as his furry test subject fell asleep. Not only were place cells still firing, but they were firing the same way in REM sleep as they had been when the rat had been awake and exploring its surroundings.
It turned out the rat had been replaying memories while asleep—and human brains do the same. Because place cells fire when you are in certain locations, they work as a sort of code, with individual cells firing in different combinations depending on where you might be. Every location has its own code in the brain. That code is then expressed again by both visual and spatial cells during sleep, like a movie scene being rewound and replayed ad nauseam.
“It’s like you’re in like a video editor,” Wilson says. “You have these memory sequences like you had the video recorder on. That’s the online mode. You recorded all this stuff, and then, after you’ve recorded it, you’re sitting in your studio in this offline state. You’re not taking in new information. Now you can go back. You can revisit. You can review, go forward, you can go backward. Then you can possibly edit these things.”
Wilson found out he could create something like a memory movie out of these sequences. If he had a scene from one memory sequence, he could put it together with a piece of another memory sequence that might not have been formed at the same time, but is still related. This made him realize that offline mode—as opposed to online mode—does more than just replay memory. It uses prior experiences for both generating and understanding new memories. Using replay in this way not only gives more insight into memory, but can even shape plans for the future. What is learned from the past can create a future based on memory.
Chen sees memory consolidation during sleep as one of the most important functions related to remembering these past experiences. While short-term memory is temporarily stored in the hippocampus, some of these memories are ranked higher by the brain than others. This is where episodic memory comes in. Episodes of memory, like episodes of a TV series, can be replayed. Some of them can be consolidated when the information in a shorter memory is transferred from the hippocampus to the more stable cerebral cortex. The cortex communicates with the hippocampus to make its availability known, so the hippocampus will transfer information to the cortex, something like going through old videos and deciding which episodes to keep and transfer to a digital format years after they aired.
Sleep can help consolidate memories more effectively than the waking brain. Seeing college students napping right before an exam is no accident. Even if they haven’t studied enough psychology to know what exactly is going on in their gray matter, what they have realized is that they somehow tend to do better on exams when they study and then sleep. There is a much higher chance of memories being consolidated if the brain reaches the phase of REM sleep. Wilson found that it is not only areas associated with memory, like the hippocampus, which are involved in this process, but also sensory areas such as the visual cortex, so memories are being replayed with visual imagery of an event. Memories are then retrieved upon waking. This explains why taking a short nap means potentially remembering more of what is going to be on the exam while still making it to Psych 101 on time.
Memories can also change with each replay because memory is dynamic. Think of recording the same episode on a videotape every time, but there are different commercials, and even things that may have previously gone unnoticed about the episode itself become more obvious. The brain keeps overwriting one version of a memory with another version. After the original recording is overwritten, additional information is filled in, not just to compensate for memory decay, but because communication between neurons responsible for memory is influenced by outside factors. Rewatching that video in a week may reveal what was forgotten.
“I think this happens because the memory itself is not storing something permanently,” Chen says. “You read out the information, but then you put it back. It is read and retrieved. Later on, you rewrite it back into the brain, so the information is dynamic. You selectively believe these are positive memories, and you keep adding information to that. Everything is subjective.”
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