The Neuroscience of Christopher Nolan's Inception

A Google search for "dream interpretation" brings up over eleven million results. Pop psychology certainly loves this bit of Sigmund Freud's psychoanalytic theory. For Freud and his intellectual successors, dreams were a psychological gold mine. They offered a unique glimpse into the unconscious mind while the conscious mind slept. And Freud was hardly alone in his fascination with our nightly visions: almost every human culture has ascribed some kind of importance to dreams. Joan of Arc supposedly predicted her own death in dreams. The "inexplicable cultural phenomenon" (MST3K guys' words, not mine) that is Twilight occurred to author Stephanie Meyer in a dream. And now director Christopher Nolan takes us into a world of dreams with drugs and a strange machine that sends Leonardo DiCaprio into an individual's dreaming mind.

His purpose? To steal ideas. Inception is based around the premise that individuals can share dream states, and that bizarre machine we've seen in snatches of trailers seems to be the key. So how realistic is all this?

First, a quick crash course on the neuroscience of sleep and dreams. Sleep, typically considered a passive state where nothing's happening, is actually a time of great changes in brain activity. When we sleep, we go through cycles of approximately ninety minutes a piece, with multiple stages of sleep per cycle (conveniently numbered in chronological order). Stage 1 and 2 sleep are relatively shallow--in fact, if participants in sleep experiments are awakened during this period, they might not even remember being asleep. Stages 3 and 4 are what scientists call "slow-wave sleep," after the large slow delta waves that show up on EEGs measuring brain activity. If awakened from this very deep sleep, people are typically confused and groggy.

Yet these four stages have nothing to do with dreaming. In fact, it's only after going through stages 1-4 that we reach the dreaming state of sleep--REM sleep. REM, which stands for rapid eye movement, refers to the state of sleep in which the brain is insanely active. Brain activity of a person in REM sleep bears a lot of resemblance to that of a person who's awake. If startled from REM, people will be alert, responsive, and almost always report that they've been dreaming. However, REM itself doesn't account for much of the time we're asleep. With about four to five sleep cycles per night and about a half hour of REM sleep per cycle, we only enter REM for about two to two and a half hours per night.

That's two hours, give or take a half hour, that we're dreaming and conscious. That's right--conscious. Strike one against Inception, which assumes, as common sense would have it, that dreamers are unconscious. Startlingly, it turns out that we're actually not down and out for the count while we're dreaming, and our brains are aware of what's going on. For example, say you're having a dream about running along the beach. The parts of your brain responsible for motor control become active, as if you were actually running. The visual association cortex of your brain--the part responsible for making sense of information from your eyes--is extremely active, literally "seeing" the beach. Even your eye motions are not random--they resemble the eye movements of a conscious person scanning a scene for information. Your heart rate and blood flow to the brain increases, as does your brain's oxygen consumption.

So why, if you're actually conscious, do dreams make so little sense when you wake? How do you get the crazy dream-scapes seen in the trailers of Inception? One possible explanation is the lack of activity in certain parts of the brain during REM. The prefrontal cortex, located behind your forehead and eyeballs, shows low activity during REM. The prefrontal cortex's job is to make sense of things, plan, execute higher order functions like logic and reasoning, among other functions. The lack of activity in this part of the brain while dreaming may explain why dreams are very vivid, but don't make a whole lot of sense upon waking, as Leo informs Ellen Page in the movie trailer.

Here's where the premise of Inception begins to get a bit implausible. There are indeed common patterns among mental states. Generally speaking, individuals show consistent patterns of brain activation depending on what their mental state is. Someone looking at faces has brain activation in the fusiform face area, on the underside of the brain. Someone listening to music shows activation in the temporal lobe, responsible for hearing and sound. In theory, it would be plausible for two people's brains to share the exact same pattern of brain activation at a given moment in time. We obviously have no technology nowadays that would achieve such a feat, whatever movies like Inception or Avatar might imply, but it is theoretically possible.

However, Leo would be in trouble if he had the exact same brain activation pattern as the dreaming person. Remember that prefrontal cortex? The bit of your brain that makes logical sense of things? His prefrontal cortex would have to remain active in order for him to pull off a heist inside someone else's mind. Otherwise, he'd just be sucked into the dream with no particularly coherent sense of time or planning, just like the rest of us when we dream. So whatever technology he's using would have to replicate the mental patterns of the dreaming person in the sensory areas of his brain, but leave Leo's own prefrontal cortex unaltered.

But let's assume for the moment that Leo can share a dream state with another person, and still keep his own sense of planning, logic, and reason active. What will he find in another person's dreams? Deep dark corporate secrets? Not likely. The premise of Inception is that it's easier to steal secrets from someone's dreaming mind, when the mind is supposedly more vulnerable. However, there's no evidence that REM has anything to do with unconscious desires or deep dark secrets. There is evidence that in animals, REM sleep helps with learning new information, though the evidence is somewhat thinner with humans. Learning is correlated with the amount of REM sleep in humans, but there's no telling if one causes the other. Bottom line is this--Leo might have spent all that time and effort getting into someone's dreaming mind to find nothing useful there.

Even if there is something useful there, there is still the fundamental problem that dreamers are conscious. In neuroscience terms, consciousness roughly refers to awareness of thoughts, perceptions, emotions, and memories. When experimenters wake sleeping participants from REM sleep, they are alert and perceptive. When asked to describe their dreams, they show signs of consciousness, giving details of how their dreams looked, what they felt, what they did within the dream, etc. Even if their prefrontal cortices were inactive and their dreams are strange and disjointed, participants were still conscious in their dreams, even if it was different from wakeful consciousness.

Bottom line is this--Leo would most likely go through all that trouble with futuristic technology to enter someone else's dream state, only to find a dream that has none of the information he wants, and a very much conscious dreamer capable of reacting to events in the dream, including his intrusion. But that's just half of the equation. What about inception, his ultimate challenge of planting an idea in a dreamer's mind rather than taking it?

The film synopsis and marketing so far have made a huge deal of the difficulty of inception, but neuroscience and psychology tell us that planting an idea inside someone's head without them noticing is actually surprisingly easy, even when that person is awake. Hysteria about subliminal messages and brainwashing aside, it is possible to "prime" people subliminally in visual experiments. Priming involves rapidly flashing a word, too quickly for it to be consciously processed. If primed with one word, participants will tend to pick related words, like picking the word bedroom over the word elephant if primed with the word "house." This is just one example of the subtle but real ways in which ideas can be influenced without the person's conscious awareness.

In conclusion, Leo would really have an easier time of getting what he wanted if he stuck to good, old-fashioned conversation with a person while conscious. The premise of Inception is not entirely unrealistic, but it certainly is extremely impractical, even if the strange dream-sharing technology in the film did exist.