Neurons ‘Tag’ New Memories For Storage During Sleep

IRA FLATOW: Some memories stick around, and others quickly fade away. But how does your brain push those memories into long-term storage? And how does your brain recognize which memories should be kept and which should be discarded?

You know, this topic has been bashed around for decades, and now scientists are beginning to better understand this process. And you know what? Sleep. Sleep may be the key. Here’s the explanation in brief.

During the day, as we are forming memories, cells in the hippocampus in our brain fire in a formation called sharp wave ripples. These are markers that tell the brain to keep those memories for later. And then when we’re asleep, those same sharp wave ripples activate again and lock in those memories. Pretty cool.

Joining me now to explain more about the findings of his recent study published in the journal Science is my guest, Dr. Gyorgy Buzsaki, professor of neuroscience at NYU Health, based in New York City. Welcome to Science Friday.

DR. GYORGY BUZSAKI: Thank you. Good meeting you, Ira.

IRA FLATOW: You’re welcome. Let’s start at the beginning. What’s going on in our brain when we form memories throughout the day? What’s going on in there?

DR. GYORGY BUZSAKI: What I would say is that we interact with the world constantly and continuously during the day. But those are not memories yet. In order to deposit them and to make memories, you have to go through some kind of a selection process.

We have known it for a long time that sleep is a special state because it helps either rehearse our memories or experiences, and then the sleep itself is useful for consolidating the experiences that we had. And as you mentioned, the key thing is that, what fraction of the many, many things that we experience during the day will be deposited for long-term storage?

IRA FLATOW: And how does that work in the brain? What determines what gets deposited for long-term storage? What, physiologically, is going on there?

DR. GYORGY BUZSAKI: There are structures in the brain. The hippocampus is known to be a critical structure in the brain that forms our episodic memories. The mechanism by which it happens is a particularly interesting pattern called hippocampal sharp wave ripples. Now this hippocampal sharp ripple is a fast oscillation, and it lasts for about 100 millisecond, and it is the most synchronous pattern in the mammalian brain, at least. And it’s a great tool to reach the vast part of the neocortex and tell something to the neocortex.

Now what is it telling? These sharp wave ripples are representations, if you want, of what happened to us during the day. But the discussion that we are having with each other now will be chopped up into 100 millisecond segments mixed together and mixed with our previous experiences, and that’s called the sleep consolidation. This is what we have known.

So the question was, what process in the brain– what is the mechanism that allows to put tags to our experiences– to say one goes to trash can, the other one would be retained. And that mechanism– of course, it sounds like a decision-making process. And for very long, we thought there is just a– jokingly speaking, there is a little man, or a homunculus, that decides for us.

IRA FLATOW: You mean, there isn’t? There isn’t one?

DR. GYORGY BUZSAKI: Well, there is one, but it’s not a little man. It’s called the sharp wave ripple. There are three steps. The first one is experience, and it’s a much slower process. It goes in real time. And then when the brain switches from this exploratory attentive state, that allows this other algorithm to kick in, and this is when the sharp waves occur. And if they occur, and depending how many of them occur in the waking state, then those experiences in the vicinity of these sharp waves will have a priority of replaying that part of the daily experience what we have during sleep.

IRA FLATOW: Now, can I say to myself– you know, I’m having an experience. I want to remember this wonderful conversation I had. Or I’m here in a beautiful countryside, and I want to remember that. Can I activate these sharp wave ripples?

DR. GYORGY BUZSAKI: Not really. Sharp waves is the opposite of your will, if you want. Sharp wave ripple occurs exactly when you don’t think hard– when you are not consciously attending to things. These happen in the absence of our effort. But you can try to maintain your mind throughout this conversation. If your brain is not allowed to switch into the sharp wave ripple state, you don’t learn a lot.

For example, if you take amphetamine– which I did when I was a student. It’s known that all known subcortical neuromodulators, such as acetylcholine, norepinephrine, serotonin– they all suppress sharp waves. So when you maintain this attentive state, but you don’t switch to the other state, you will not remember anything later.

IRA FLATOW: So it’s true, then, that taking a break from learning something and then maybe going back to it can actually help you?

DR. GYORGY BUZSAKI: Exactly. You got it right. Just coming in and coming in and coming in is not enough because there is a phenomenon called interference. So if you go to a French class after an Italian class, then most of the knowledge of the French class will be erased. It’s much better to have sleep or idling brain state before that. Take a walk.

IRA FLATOW: Do you actually have to be asleep to do this, or can you just be relaxed?

DR. GYORGY BUZSAKI: The biggest change, of course, that we have is sleep-waking. But the other changes happen all the time. You cannot attend a one-hour lecture to be attentive all the time.

IRA FLATOW: But you said that the sharp waves will be reactivated during sleep. Is that only during sleep? I’m trying to understand if just the resting period also reactivates those waves in the brain, and that’s why you remember it.

DR. GYORGY BUZSAKI: So if the resting period is previously preceded by something important or some experience, then that particular experience will be marked also, and that will be replayed during sleep. So there are many of these events occur during the day, and those marked events are the only ones that will have the chance to enter in long-term storage because the unmarked ones will be not retained.

So if you would allow me to record from your brain from the depth of the brain, and I would see whether you have sharp waves after some of the sentences that I utter, and I could tell with some precision which parts of the conversation you would remember.

IRA FLATOW: Does that mean if I don’t get sleep at night, I’m not actually being able to consolidate some of those experiences?

DR. GYORGY BUZSAKI: Correct.

IRA FLATOW: That’s why that’s important.

DR. GYORGY BUZSAKI: Yes. Sleep has a lot of important processes, but one of them is making memories.

IRA FLATOW: And how can we use this now for people who are not good at making memories or have memory problems. Is there something useful here?

DR. GYORGY BUZSAKI: Well, every psychiatric disease is associated with some kind of memory problems. Every animal model that has been investigated– from autism to schizophrenia to Alzheimer disease– have altered sharp waves. So we have known that.

Then, how can we exploit this observation that we just made? We can perhaps manipulate sharp waves, and it can be done with drugs and so on. The easy thing at the moment is to erase them. There are several subcortical neuromodulators that have corresponding drugs, and we can erase memories.

Is it useful? Well, I can think of one disease, which is post-traumatic stress disorder, PTSD, where you don’t want to remember things, and maybe that would be an immediate avenue– potentially useful application.

IRA FLATOW: So where do you go now? What’s next for your research?

DR. GYORGY BUZSAKI: There are many, many ways. One of them would be going into these applications. The other one is manipulate the sharp waves– manipulate the dopaminergic system, for example, which is also known to be related to reward. How reward is facilitating the occurrence of the sharp waves. Why the switches are important. Which brain systems are involved, more and less, in this process. We can manipulate them separately. So these are directions that will entertain us for the next several years.

IRA FLATOW: Well, I hope you’ll get entertained to come back and talk to us more about it.

DR. GYORGY BUZSAKI: I would be happy to.

IRA FLATOW: Thank you for taking time to be with us today.

DR. GYORGY BUZSAKI: Thank you.

IRA FLATOW: Dr. Gyorgy Buzsaki, professor of neuroscience at NYU Health, based in New York City.

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