Inception and the Neuroscience of Sleep

By Malcolm MacIver | August 10, 2010 11:28 pm

sleepingChristopher Nolan’s Inception is a film about a time when we have the power to enter into each other’s dreams, and actively steer the dream’s course to implant an idea in the dreamer.

The film raises the issue of how much we understand about the neuroscience of dreams. Due to its need for invasive experiments, neuroscience typically works with non-human animals, which raises a significant difficulty: how do you know that a rat is dreaming? You can’t wake it up from REM sleep and ask. (Well, you can, but don’t expect a cogent response.) There’s no accepted objective indicator that a person or animal is having a dream, as opposed to sleeping. But, we can still learn something useful by looking at the neuroscience of sleep.

The neuroscience of sleep has told us a few important things over the years. For example, we know that our pattern of sleep and wakefulness (the “circadian rhythm”) has much of its basis in the activity of the suprachiasmatic nucleus, a rice-grain-sized group of cells just above where the optic nerves from our eyes crossover. We know that our free running rhythm—what we go to if we are completely in the dark, with no indicator of solar activity—is slightly over 24 hours, and that the length of the rhythm can be affected by things like cannabinoids found in pot. We know that the brain activity of a person dreaming is very similar to that of an awake person—were it not for the fact that our body is paralyzed during dreaming, we’d probably do a lot of things we’d regret.

While we’ve made a lot of progress in understanding sleep, we’ve a long way to go to understand dreaming. What makes it a challenge, perhaps as big a challenge as understanding consciousness itself, is the subjective aspect of dreaming. For example, we know that vivid dreaming occurs during REM sleep in humans. We also know that other animals have REM sleep. Do they also dream? How can we know, since, as I mentioned above, we can’t wake them during REM sleep and ask (the way we determined this fact with humans)? How we can go from objective facts like the presence of REM sleep to subjective ones, like a dream of a pink elephant bouncing down along a high tension power line (from one of my own dreams) is as unclear as how we get from neurons firing to awareness. Nonetheless, significant work has occurred on some of the neuronal correlates of REM sleeping in rodents and songbirds.

The most intriguing result from recent work is that during sleeping, the brain appears to “play back” patterns of activity that occurred during the day. For example, Matt Wilson and colleagues have found that patterns of “place cell” activity — brain cells that light up, like crumbs left on Hansel and Gretel’s path in the woods, corresponding to a specific path that the animal (in these experiments, a rodent) took during the day — and this playback seems to be integral to the animal learning the path it took. In birdsong, from work by Dan Margoliash and others, we’ve learned that birds playback patterns of activity almost identical to singing while they sleep, and again, it seems to be integral to the bird learning its songs from its tutor. Why does the brain play back patterns of daytime activity at night? It isn’t completely understood, but some backstory on memory research helps motivate one hypothesis.

It’s been known for some time that a structure called the hippocampus is responsible for acquisition of new memories. Without it, we still have our memories, but anything new that happens is completely lost (think of the movie Memento, one of Christpher Nolan’s previous films) — we are stuck in the continual present. Real-life patient HM taught us this many years ago, after he had this structure removed as part of an experimental operation to cure his epilepsy. He, and many similar cases, lose all memory but for those events that happened some time before the loss of their hippocampus, typically a few months. Over time, the idea has emerged that perhaps the hippocampus “trains” the neural networks in other regions of the brain to store memories through repeated playback during sleep. Like crickets trying to attract females in the night, in the world of memory nothing succeeds like persistent repetition.

So if in REM sleep the brain is repeating patterns of activity from periods of wakefulness, perhaps that process helps the brain to remember, over the long term, the items that are temporarily stored in the hippocampus.

What is not understood from these studies, which were done in rodents and song birds after all, is the basis of all the strange subjective aspects of dreaming — such as how or why in our dreaming we seem to borrow from real experience while adding a good dollop of stuff from elsewhere. This aspect of dreaming seems like it would be crucial in order to have any hope of building a dream experience a la Inception. There is not a whole lot of creative potential in simply regurgitating the day’s brain patterns.

Until these and many other mysteries of dreaming are solved, what the research is showing is that the best way to architect a dream is to architect the experience you have during wakefulness, since dreaming appears to be a lot about learning patterns you were exposed to while awake. Our understanding of the coupling is not clear enough to think about designing dreams by structuring our awake behavior, but perhaps with further research we will come to that point where we can do inception of ideas into our own heads.

Image: Midlands Rat Club
Corrections: Aug 11, 2010: “…neuroscience typically works with animals, rather than humans…” adjusted to “…neuroscience typically works with non-human animals.” Reference to length of circadian rhythm also adjusted. Aug 12, 2010: “integral to the bird learning its large repertoire of over a million syllables” changed to “integral to the bird learning its songs from its tutor.”

Comments (28)

  1. Staring at rats while they sleep is one of those jobs that is sort of hard to play off when you’re trying to pick up the hostess at TGI Fridays. If she asks what you do, say “Biologist” and move the conversation along.

  2. megan

    I suppose eons of reported observance of pets moving in their sleep as if running or playing or hunting isn’t official scientific observation.

  3. Eina

    Megan – you beat me to it – I used to watch my dog dreaming as he chased cats or whatever in his sleep. I am sure all dogs do that.

    And paralysed while dreaming? Can’t they do a study on dogs for that one too? My dog used to get quite agitated!

  4. This looks so much similar to caching in computer science. Caching is a process that puts a fast, small storage in front of a slow, larger storage, like for example a Hard Disk. Data are written and read from the cache for fast access, but periodically the cache “fills up”, or a synchronization against the slower storage must be performed. A technique to achieve this is “write-back”: areas of the cache that are modified are marked as “dirty” and synchronized against the slow storage when the cache is emptied.

    It seems like brain is the slow storage. Hippocampus is the temporary fast cache for daily information. Sleep is the period of time where hippocampus is emptied and its new contents are written to the brain through a “write-back” mechanism.

  5. Malcolm MacIver

    Stefano: precisely right. It looks like the hippocampus has evolved to be a fast interconnect between different sensory modalities (it’s needed to learn things that come through different senses, and to generalized what you learn across senses), and uses a high amount of metabolic energy so that it can do “one shot” learning while other parts of the brain seem to require large amounts of repetition. In diseases and under oxygen deprivation, cells in the hippocampus are often the first to die, for example. The rest of the brain can’t do this stuff, and it may be because it would take too much energy to make the rest of the system so plastic.

    Eina and Megan: I think comparing muscle twitches during REM sleep in animals to muscle activation patterns while awake could make for a very interesting and useful study (the paralysis, as you point out, is not 100%). It could give us insight into whether what looks like real “running after the squirrel” in a dream really is that!

  6. Here is another correction:

    The birds studied by Dan Margoliash (zebra finches) do not have a “large repertoire of over a million syllables.” In fact, they have a very small repertoire (one song of 6-7 syllables) that they sing repeatedly. Hence, the analysis in the article you referenced analyzed “over a million [total] syllables” that were sung during the birds development, NOT over a million *unique* syllables.

    Some species (i.e., European starlings, humans) do have larger repertoires, but nowhere near 1 million unique syllables.

  7. Malcolm MacIver

    Justin: you are correct. Repertoire was a bad word choice. Derégnaucourt’s study looked at learning over the course of vocalizations encompassing around one million syllables per bird. I’ve fixed that. Thanks.

  8. It’s very important the research of dreams because we will understand the construction of personality across the dreams of a dreamer. Hope that this kind of articles can motivate the researching on different areas.
    Thank you for this article.

  9. Sam

    Quantum interactions take place on a scale far, far, far smaller than the scales of processes that are known to affect biological activity. It’s likely but not proven that quantum effects don’t play any meaningful role in the brain, especially one that would be as complex as telepathy.

  10. I saw the data at a presentation given by Matt Wilson and one thing I found interesting is that the playback of place cell activity during sleep happens at an accelerated rate. I forget by how much, 10x speedup rings a bell. This might show why time seems to dilate during dreaming. When I dream during a short nap, the dream seems much longer than the time actually elapsed…maybe my perception of time is accelerated due to a similar speedup of neuron firing.

  11. It’s a dumb interpretation because to interpret a movie you must take what happens in the movie as evidence for your conclusion. To say that the entire movie was a dream is possible if he was in limbo the entire time but to get to limbo he had to at least use the same ideas of sharing dreams.
    I wouldn’t say it’s sci fi for idiots, I will say it wasn’t anywhere near as complex as people made it out to be (I got it the first time, got my mind blown once reading digg later but figured out without much difficulty) but I thought the movie was very well done in all ways. Excellent writing, directing, special, effects, and acting in my opinion. There is one flaw to the movie that my friend pointed out to me though (or I think it’s an error, like Nolan didn’t realize it but maybe it was intended), if the entire point of Cobb (Leo) doing the last job was to get to his kids in America, and their grandma was in America with them, and their grandpa was in France, why not just have the kids move out of the country instead of risking life and limb? Wouldn’t have made a good story but if that isn’t a mistake by Nolan he had to be in Limbo the whole time.

  12. Ben


    Cobb was trying to avoid extradition to the USA to stand trial for his “crimes”, so having his children move to France would have helped him no more than having them in the US, as he would still be facing extradition and trial. The point of his final job was to clear his name so he could live a normal life (be it France or the US) with his children – regardless of whether he was in limbo or not.

  13. Maybe I’m just not thinking outside the box, but is this something we really want people having the technology to know how to do?

  14. Lydia

    What if dreaming is the true language of the evolved specie. Ever notice how we don’t actually “talk” in dreams, although yes there is language in them, but more symbol than actual language. Maybe some day we will dream to each other, no language required. If an animal were suddenly removed to another cultural environment, could it then communicate with its own species anyway? Yes. Could we? No. My idea of the reason for dreaming is to predict what we need to do next.

  15. Arthur

    I remember a class years ago at UCLA in psychology with a teacher with a physiological bent who put hamsters in refrigerators to interfere with what was called “reverberation” at the time; a kind of rehearsal. That slowed the metabolic rate and interfered with learning. This supports the metabolic cost idea. When we are active and awake we do not have enough energy to work the hippocampus. Sleeping gives us the spare energy to do that.

  16. boy

    I saw the data at a presentation given by Matt Wilson and one thing I found interesting is that the playback of place cell activity during sleep happens at an accelerated rate. I forget by how much, 10x speedup rings a bell. This might show why time seems to dilate during dreaming. When I dream during a short nap, the dream seems much longer than the time actually elapsed…maybe my perception of time is accelerated due to a similar speedup of neuron firing.

  17. Great post, indeed Useful blog actuallly. good timing for me actually as I was researching all this stuff when stumbling across you :-) I will join, keep up the useful efforts. Take care

  18. Soren

    The stuff about patterns is something i can recognize very well. the last week ive been watching a serie nonstop, like 20 eps a day, and atm there is a 100% chance i will dream of that, cause its really all i did that day. but sometimes it can be very annoying. i remember when i was 14 playing diablo 2 day in and day out, sometimes when i would dream i would be stuck with this thought of d2, even as i was lying in bed awake, i would have to get up and turn the lights on and do something else before there was anyway of making my mind let go, now thats annoying, cause dreams about games are usually just the same thing over and over and over

  19. Really, even i have come across many facts on sleep. Sleep deprivation can cause high blood pressure and mental problems. Sleep problems relates to behavioral problems as well.

  20. I liked what we call as Limbo in Inception. Limbo was the state of lock up and something in which ages would have passed.


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