Dreams: the stuff memories are made of?
Animal Cognition and Human Sentience
Dreams feel meaningful — drawn from a mishmash of content from our waking lives. But it's a hot debate among scientists, who are yet to confirm why we sleep, let alone dream. Neuroscientist Matthew Wilson's extraordinary experiments involve eavesdropping on the sleeping minds of rats. He proposes dreaming is central to how we remember and learn.
Matthew Wilson: Memory is really much more than simply a record of experience, it is what really defines who we are and who we are going to be. We really have to rethink the term itself — memory.
Edwin Robertson: There has been an intuitive connection for literally thousands of years between sleep and memory. I mean even such simple phrases as, 'Oh, I'll need to sleep on it,' suggests that we have an intuitive notion that sleep is important for memory processing.
Natasha Mitchell: I'm Natasha Mitchell… Memory — is it the stuff that dreams are made of, literally? Sit back, start counting those sheep. Believe it or not, we're dreaming one neuron at a time… On the question of whether a psychoanalyst's take on dreams can ever be reconciled with a neuroscientist's — here's what he says:
Robert Bosnak: Oh yes, I think they are being reconciled as we speak. For instance we're finding in neuroscience that cognition and the neocortex is involved in dreaming and that therefore meaning can come from dreaming itself. I think that more of these connections are going to be found. You have to see that neuroscience is very young: MRIs started in 1993 so we've been doing MRIs for 16 years and the resolution on MRIs is about as good as photography in the 1820s. So it is just beginning, and I think that neuroscience as it matures will find more and more connections between their field and ours.
Natasha Mitchell: So today we're considering a vexed question in neuroscience — do we dream in order to remember? One of the reasons this is tricky for science is that, surprisingly, we don't really know why it is we sleep, let alone dream.
Edwin Robertson: Yes, it is surprising that we spend so much of our time asleep but yet we still don't really have a clear understanding of why it is that we do sleep.
Natasha Mitchell: Edwin Robertson is assistant professor of neurology at Harvard Medical School.
Edwin Robertson: I think that part of the problem very often is that we imagine that all animals use sleep for the same purpose. The function of sleep in humans might be quite different from the function that a koala uses sleep for. So the koala most likely uses sleep for the regulation and conservation of energy and that's why it ends up sleeping for 20 hours a day, whereas our human need for sleep may be driven by other alternative factors.
Natasha Mitchell: Is sleep there to help our body recover metabolically, or is it actually fundamentally about allowing us to consolidate the stuff that goes through our brain during the day? What do you think?
Matthew Wilson: Well I think it's a combination of all of these things. Clearly there is a metabolic component to it. So this is something cells should do, that bodies should do. The question is whether that's all it does.
Natasha Mitchell: Matthew Wilson is professor of neuroscience at MIT's Picower Institute for Learning and Memory in Boston.
Matthew Wilson: The resting brain is not really resting at all, it's extremely active and that activity in many respects is indistinguishable from the activity that's engaged in during wakefulness. Can all of that be going on to no useful end? And I think that the answer to that is probably no. But to be fair to those critics that argue that nothing goes on during sleep, there is no processing of memory, there is no learning, it is something that has not yet been proven unequivocally but this is simply a matter of time.
Natasha Mitchell: As you'll hear, Matthew Wilson is leading the charge with some extraordinary experiments testing a theory that sleep is key to remembering and learning. And without the capacity to learn we wouldn't really be human would we? We'll come to where dreams fit in in a moment but let's start with why the machinations of our memory might need sleep. Each time we remember something a vast network of brain cells fire together corresponding to that memory, they chemically communicate with each other across the synapses or junctions between them. The more a memory is recalled, the stronger those synaptic connections are. Edwin Robertson.
Edwin Robertson: Sleep is often viewed as a special environment for memory processing. Some people might argue that this special environment is merely that it performs protective function, it's a protective cocoon, because when you're asleep you can't get any further experiences during that time and it prevents memories from being interfered with. Other theories would say no, that's incorrect, that there's actually profoundly important electrophysiological events that occur in the sleeping brain that are important for memory processing per se.
Natasha Mitchell: What's the other prevailing theory for why sleep is so crucial for memory?
Edwin Robertson: There's an idea that essentially sleep allows a homeostatic readjustment of synapses during awake you're bombarded by experiences and that leads to multiple synapses being engaged and their weights being changed. And then what sleep allows—and specifically actually slow wave sleep—is a pruning back of those unimportant synapses. So it's kind of like I imagine in some senses the judicial pruning of your roses to get a nicer flower. So the idea is that the slow wave sleep is able to prune back synapses that are energetically wasteful but are also wasteful in an information processing account because they are adding noise. And that clearer crisper signal then translates into improved performance the next day.
Natasha Mitchell: So from the possible role of sleep to that of dreams. Is it simply a coincidence that we often dream about things we happen to remember, things that have happened to us or that we've learned from our waking lives? Some scientists think of dreams as epi-phenomena that is meaningless, random by-products of the real business of the thinking brain. Neuroscientist Matthew Wilson—controversially for some—disagrees.
Matthew Wilson: I think they are not meaningless. It's easier to see what the meaning, that potential meaning, might be when we study animals like rats, whose life experience is much simpler than ours. So when we study the dreams of rats we're studying animals that have only had months of experience and we've controlled all of that experience, and what we see reflects very closely their actual experience. Now a human, when we think about our own human dreams, we're thinking about dreams that now have access to decades of experience. They may seem complex and obscure because they are bringing together, combining and evaluating decades of memories and experience.
But if we think about dreams not as a process of simply retrieving, of replaying memories, but of re-evaluating, reorganising something—akin to taking piles of paper that have accumulated and now one needs to organise it. As you go through this process picking up one piece of paper and another they may not seem related but as you organise them the end product is something that is actually more useful.
So if we think about dreams and the seemingly chaotic structure and nature of dreams as reflecting this process of reorganisation, I think we get a better idea of what might be going on during sleep. Again, not simply taking memories, replaying them and transferring them to other parts of the brain, but really re-evaluating, reorganising.
Natasha Mitchell: So in a sense it's a filing process and some people think that sleep and dreams allow for memories to be organised into a sort of more efficient storage system.
Matthew Wilson: Correct. The most efficient again being the discovery of rules and relationships and condensing it into something that now captures all of the relationships that were present—the rule—and so discovering the rule may be precisely the kind of complex, difficult to understand processing that goes on during sleep and dreams.
Natasha Mitchell; It's so interesting, because our dreaming brain seems to want to try and make sense of all the elements that it's you know messing about with. I mean my dreams I've got to say are grand epic narratives every night on a scale of Gone with the Wind, I've got to tell you, it's exhausting. It certainly seems much more anarchic than the way we normally remember in our waking lives.
Matthew Wilson: I think if we think about this kind of process not simply about retrieving memories and storing them, but taking them and trying to imagine a future that could have come from them, synthesising rather than simply storing. Perhaps your epic narratives are in a way projecting where you would like to go, where your memories and your experience feel that you could go. You know you may never get there but the brain is trying to understand in a sense, pushed to its limits, where experience tells it it might go.
Natasha Mitchell: Memory is imagining the future, not just storing the past — nice idea. Matthew Wilson comes to dreams from his first research love which is memory. And if you thought sleep was scientifically elusive, try memory. It's that complexity that AS Byatt reflects on in her recent collection, Memory and Anthology a compilation of literary and scientific musings on the many faces of memory.
[Reading]: I have a memory I think of as 'The Memory'. It is seen from the point of view of a small person seeing over the wall of a playground in East Hardwick Elementary School. The stone is hot, and is that kind that flakes into gold slivers. The sun is very bright. There's a tree overhead and leaves catch the light and are golden and in the shade they are blue/green.
Over the wall and across the road is a field full of daisies and buttercups and speedwell and shepherd's purse. On the horizon are trees with thick trunks and solid branches. The child thinks I am always going to remember this, then she thinks, what is remembering? This is the point where myself then and myself now confuse themselves into one.
I know I've added to this memory every time I have thought about it, or brought it out to look at it. It has acquired notes of Paradise Lost which I don't think it had when I was 5 or 6. It has got further away and brighter, more or less real.
I always associate it with one of my very few good memories of my maternal grandmother, a perpetually cross person who never smiled. The year she died she began to forget, and forgot to be irritated. She said to me sitting by the fire at Christmas, do you remember all the beautiful young men in the fields? And she smiled at me like a sensuous young girl. She may have been talking about the airmen who were billeted on her in the war, or she may have been remembering something from long before my mother was born. I shall never know. But I can see the young men in the fields.
Matthew Wilson: Memory is really more about learning, learning from the past rather than simply storing it. The challenge that the brain has is trying to form a model of the world; we are constantly trying to understand how the world works so that we can make predictions. Of course our biggest challenge is making good decisions in novel contexts or circumstances. Not simply repeating the past, often repeating the mistakes of the past, but rather trying to learn from that so that we can make decisions in unpredictable circumstances. And that's what really separates us, sentient organisms that are able to move forward in an unpredictable world, separates us from simple computing devices. And so my deeper interest is in this re-evaluation, revisitation—how memory is retrieved, restored and re-examined because this is memory put to use.
Natasha Mitchell: But take us inside your lab - you've developed some quite extraordinary experimental techniques to effectively eavesdrop on memories as they're formed in the brain cells of rats. What approach have you taken and what have you been able to measure?
Matthew Wilson: Well what we are measuring are the discharges of individual brain cells. Brain cells communicate through electrical signals, changes in voltages much like the pops and clicks that you might hear on the radio if you tune it to some place between stations on the dial. These little electrical discharges are driven by input from the outside world so there's a code that is created.
Now what we would like to be able to do is to listen in on these signals, and what that requires us to do is take very fine electrodes—electrodes are little wires about the size of a human hair, smaller than a human hair actually—we take these wires and we send them down into the brain where they can listen in to these signals. They are placed next to brain cells. We leave them there permanently so that we have little microphones distributed across the brain and as animals engage in normal behaviour: sitting, resting, sleeping, running, we can follow the activity of these brain cells. Not just individual brain cells but many of them because we implant many of these very fine electrodes.
So what we have are rodents, rats and mice, that have little badminton shaped hats, now it's these hats when we plug our electronics into them we're able to take the signals, the brain signals out, amplify them and record them on our computers, then follow the patterns as animals engage in normal daily experience.
Natasha Mitchell: Well as normal as the life of the rat in a lab can be, I guess.
The making of memory, ours and rats', involves a couple of key brain structures, our brain's ancient core which we share with reptiles called the limbic system and especially a structure in it called the hippocampus, and this communicates with the outer layer of our brains, the neocortex; newer in evolutionary terms, and busy with the processing of sensations, perceptions, thinking, planning, evaluating social behaviours etc. So back to that extraordinary rat rig-up we just heard about.
Matthew Wilson: One of the reasons we were interested in sleep is that during sleep the brain in a sense is cut off from the outside world. So that if we see patterns, or we see traces of past experience popping up once again, we know that it is in fact memory, because it's not being driven by anything the animal is currently experiencing. So we are using sleep as a way of looking for and examining the content and structure of memory. And what we found was that when animals would engage in very simple behaviours, running around in little mazes, and as the animals would run along these tracks in this part of the brain, the hippocampus, very unique patterns that allowed us to tell from moment to moment precisely where the animal was and what it was doing.
So very much like a video record of the animal's actual behaviour. Now we would look for these patterns as they changed over time, much like we would be watching a movie and then going back to see whether or not that movie was being replayed. And what we discovered was that during sleep, in fact small segments of these animals' experience running through the maze was replayed. But it was replayed in a form that was compressed: seconds or even minutes of experience would be re-expressed in just a fraction of a second. So little flashes of activity in the brain which would replay small segments of the animal's past experience. And this would go on over and over as the animal slept.
Natasha Mitchell: I wonder if that's a conscious process or an unconscious process?
Matthew Wilson: Well that's a great question, of course now we can't ask the rats whether or what it is that they're thinking about. We can only measure what it is that they are thinking about. So it is a bit of a stretch, I'm always a bit reluctant to refer to what we are looking at as a process of thought, but I have to say that if it looks like a duck, if it walks like a duck, and quacks like a duck it probably is a duck. And in this case I believe that these rats are thinking but we don't know how much of it is actually making its way to the level of consciousness that we might introspectively think we engage in.
Natasha Mitchell: How do you know that the patterns of neurons that you're measuring, firing in a sleeping rat who happens to be remembering an activity when they were awake, actually corresponds to those precise patterns that they also played out in their brain when they were doing the activity. I mean is the correlation that precise?
Matthew Wilson: It is that precise. Literally individual brain cells in this part of the brain, the hippocampus, they fire in a unique pattern, a unique sequence when individual animals engage in unique behaviour in a unique context. If we take an animal and we put it in one room and have it run back and forth for chocolate, we will see one pattern. If we have the animal do exactly the same thing but simply move it to another room, we see another unique pattern.
If we bring the animal back to the original room, that original pattern returns. So it is as though an animal's experience in a given place at a given time is captured. So it's the fact that we can see the unique fingerprints during awake experience that then allows us to go back and interpret these unique fingerprints of brain activity during sleep. So it is the remarkable property of this part of the brain, the hippocampus.
Natasha Mitchell: Eavesdropping on the neuronal fireworks going on in the hippocampus of a sleeping rat—it's incredible, but at what point does dreaming come into this story?
Matthew Wilson: We follow that up by looking elsewhere in the brain and discovered that when this part of the brain, the old part of the brain, the hippocampus, was replaying these memories that the new part, the neocortex, the part that deals with perception and action, was also replaying the same information. So as the hippocampus replayed memory of what the animal was doing, the visual cortex would replay what the animal was seeing.
Now you might think of that as being perhaps a description of what we would call a dream, events that seem to occur over time that carry with them the imagery of experience. And these rats seemed to be engaging in precisely that kind of behaviour during sleep—just as we introspectively feel that we do.
So it suggested that this part of sleep, slow wave sleep, is the kind of sleep that you enter into, it's a very primitive form of sleep, not what you typically associate with dream sleep but the slow wave sleep where you can see this memory replay going on. Now we realised that this kind of sleep like activity didn't simply go on during slow wave sleep, it also went on whenever animals were simply sitting quietly, resting. There was even more than simple replay going on. We found these events being replayed but in reverse time order, it was as though the animals were playing their memories backward, backward in time. Now that seemed curious, why would you want to play memories backward in time?
Natasha Mitchell: Why indeed?
Matthew Wilson: Well it turns out there had been a whole field of study in the area known as machine learning, trying to get computers to learn the way animals and humans do. And this strategy of playing things backward turned out to be important in learning for machines. It simply wasn't known, or perhaps even believed, that it could be done in biological systems, in animals or humans. And so here in a sense these rats were demonstrating that memory was being processed precisely in the way that it should if it were being used to drive learning. So it wasn't simply replaying it for the purpose of creating imagery, creating dreams, it was doing it in order to learn from it. So that when animals were sitting quietly they were pondering what they had done, perhaps planning what they were going to do for the purpose of learning, trying to figure things out, building models in order to try to anticipate future choices and decisions.
Natasha Mitchell: As convincing as the data from Matthew Wilson's lab rats sounds, linking learning and memory, there is a robust debate going on here and not everyone's convinced that memory and dreams are linked. Edwin Robertson.
Edwin Robertson: They've shown very nice work, showing that in the hippocampus during slow wave sleep or in the parietal cortex, in the motor cortex, in the prefrontal cortex, that during periods of sleep the patterns of neural activity that you see as an animal is moving around and navigating around a maze are re-instantiated, are replayed during periods of sleep.
The challenges that face the community in fully fleshing that idea out I think are several-fold. Firstly the replay that people see neurophysiologically is at a completely different time base than when the rat is finding its way around the maze neurons go a lot more slowly than when you see that replay occurring during sleep. So why is it time-compressed and can we really think that time compression as being really truly replayed?
I think the second point, which is a far stronger and more problematic element, is that no one has yet demonstrated that that replay is then linked to the benefits or the behavioural manifestation of memory consolidation. So it's never been shown for example if you disrupt that replay that you prevent memory consolidation. Nor that the amount of replay is related to memory consolidation.
So certainly does replay occur during sleep? Yes. Does it have anything to do with memory processing? Unfortunately we still don't know, it certainly as a neuro-physiological phenomenon it occurs, how that relates to my behaviour the next morning we still don't know unfortunately.
Natasha Mitchell: That said there is an effort to probe what happens to our capacity to learn and remember when we miss sleep—and don't we all—and it seems to be bad news for humans. Matthew Wilson.
Matthew Wilson: Now one can also see that there are effects of sleep when you train an animal to perform a difficult task. One sees that the structure of their sleep changes, that the amount of this kind of processing that we see changes, that they use sleep to try to go back and study, replay, re-evaluate things that were important in solving tasks.
Natasha Mitchell: Well that's interesting, isn't it, because science is certainly starting to tangibly prove that if we miss sleep our ability to learn and remember is fundamentally affected.
Matthew Wilson: That's correct, there's a lot of evidence that points to that. Now again to be fair to the critics of the sleep, memory and learning hypothesis, that does not indicate that the memory processing during sleep is important; only that the sleep state itself is important—that when deprived of sleep it affects general things like attention, it affects stress; it's generally disruptive. Not specifically disruptive to memory; the sleep, memory and learning hypothesis really says that it is the information that the brain is actually processing, the things that you dream about, that lead to specific enhancements, specific learning when you wake up. And that's again something that requires more study. It is something that I firmly believe that we will answer.
Natasha Mitchell: We've heard about the rats replaying and reorganising memories, at least during slow wave sleep. We do dream in this phase but the dreams aren't as lively or as frequent as is the case for REM sleep. But some folk don't experience REM at all, and if memory and dreaming are linked according to Matthew Wilson's theory, does that mean they can't remember anything either? Matthew Wilson thinks REM sleep might in fact pay more of a value-adding role after slow wave sleep has done its work, a sort of mental testing ground for creativity and imagination, new ideas and possibilities. Higher order thinking—but is that something rats can do though?
Traditionally I guess we've possibly thought that animals can't possibly have as rich a dream life as us.
Matthew Wilson: Or a dream life at all.
Natasha Mitchell: Or a dream life at all.
Matthew Wilson: Absolutely, I think that the idea that animals live in the present, this is a very common, persistent and overwhelmingly dominant view: memory allows them to modify the brain, to change the way they act in the present, but they simply don't live in the past, they don't think into the future. I think that that is changing; what we're seeing is that they do both, and that the way in which they do it may not be entirely dissimilar from the way in which humans do that. So that all of cognition, not just human cognition, may possess this kind of rich tapestry of experience. And I think that that is something that, to me, is very reassuring, that we are not alone, we are not unique in the domain of animals and organisms, that the world is a much richer place for all of us.
Natasha Mitchell: Matthew Wilson you are a neuroscientist and an engineer, you're a long way away from the realm of Dr Sigmund Freud, but he embraced dreams and memory with equal passion to yourself, and I wonder if there's an interesting potential for a convergence between the thinking of psychoanalysists like Jung and Freud and your own investigations of dreams and memory.
Matthew Wilson: Well I think to the extent that we start on common ground and that is the belief that there is meaning to dreams, that these are windows into a level of brain function that's not normally accessible during our awake life. Now trying to interpret the imagery, you know the content, that's where things become difficult, that that's probably where basic neuroscience diverges from the Freudian psychoanalytical perspective: we're not simply trying to interpret the patterns that we see, we're trying to understand how the patterns contribute to the process, the construction of models of the world that we use to guide decisions.
Natasha Mitchell: Matthew Wilson from MIT, thanks for joining us on ABC Radio National, and sweet dreams.
Matthew Wilson: You're welcome, you too, Natasha.
Natasha Mitchell: Professor Matthew Wilson from the Picower Institute for Learning and Memory at MIT and before him Edwin Robertson from Harvard Medical School. And we'd love your comments on the last two shows on dreams on the blog, simple to do and it always sparks conversation among other listeners, we love that.
Thanks this week to reader Anne McInerney, producer Anita Barraud, studio engineer Carey Dell and I'm Natasha Mitchell. May your dreams be sweet.
- At Radio National (Australia) transcript from All In the Mind
Matthew A Wilson
Scholar, The Picower Institute for Learning and Memory
Departments of Brain and Cognitive Sciences and Biology
Investigator, RIKEN-MIT Neuroscience Research Center
Massachusetts Institute of Learning and Memory
Edwin M Robertson
Beth Israel Deaconess Medical Center
Harvard Medical School
Beth Israel Deaconess Medical Center
Harvard Medical School
from - http://www.abc.net.au/rn/allinthemind/stories/2010/2745024.htm
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