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On TV, or in novels, patients in comas are usually described the same way: like they're sleeping. On the outside, that makes sense.

After all, they'll have the same closed eyes and maybe even slower breathing or a peaceful expression. But while a sleeping person and a coma patient might look the same on the outside, on the inside — especially inside their heads — they're very different. For the brain, being in a coma is almost nothing like being asleep.

It's actually more like being under general anesthesia. But what's happening in there is pretty fascinating. Comas are what clinicians call disorders of consciousness.

There are a few of them, including comas and vegetative states, and they're caused by some sort of damage to the brain. This could come from something like a stroke, oxygen deprivation, or a major hit to the head, like in a car accident. The specific mechanism that causes these disorders is still up for debate, but regardless, some damage triggers a deep unconsciousness that can last for days or months.

On a philosophical level, what it means to be conscious is debatable. But medically speaking, a conscious person is generally two things: awake and aware. Being awake is pretty simple: It's having the reflexes to do things like open your eyes.

Being aware means that you can respond to something happening around you — like by squeezing someone's hand if they ask, or holding up a certain number of fingers. When someone has a disorder of consciousness, one or both of these things is disrupted. And with comas, it's both.

A coma patient will have some basic, automatic responses that show their brain is somewhat functional — like pupil dilation — but they won't open their eyes if someone pokes them, even painfully. And they can't respond to what's going on around them. Now, even though that might sound like you after your last all-night study session, comas are nothing like being in a heavy sleep.

For one, the brain is using a lot less energy during a coma — about 10 to 20% less than even the deepest part of sleep, and 50 to 60% less than when you're awake. This is a sign of significantly lowered activity all across the brain, and it plays out in a few ways. One is that the comatose brain doesn't go through the regular cycles of sleep.

Normally, during sleep, brain goes through periods of rapid eye movement, or REM sleep. During these times, your brain is actually generating electrical activity at a similar level that it does when it's awake. But the comatose brain doesn't produce those high levels of activity, so, as far as we can tell from brain scans, it doesn't go through REM cycles.

Since REM is the phase of sleep associated with vivid dreams, this also means that coma patients probably don't dream, either — although many have reported them on their way out of a coma. Probably the biggest difference between comas and other conditions, though, is the activity in the brain's cerebral cortex -- or what we tend to think of as the main part of the brain. During a coma, there's activation in some basic, sensory areas, but they're not being processed normally.

So the brain can't actually make sense of what those sensory signals mean. For example, a study done in the journal Brain in 2000 showed that five patients in vegetative states and comas had some activity in their auditory cortex in response to sound. But those signals weren't sent on to a place in the brain where they could be understood.

So, you might've heard that someone in a coma can understand you, even if they can't respond — but that's not totally true. They can technically hear you, but according to most research, they won't be able to process anything you say. It's different than what happens when you, say, fall asleep during lecture.

According to studies, the sleeping brain can process what it hears, even if you don't realize or remember it. This lack of processing happens because, during a coma, two major networks in the cortex are disrupted: one that covers internal awareness, and one that does external awareness. Internal awareness deals with things like talking inside your head and wandering thoughts.

External awareness is more about how you process external stimuli, like what my face looks like. These networks can get interrupted in other disorders, too — like locked-in syndrome, where someone is conscious but can't move normally. But in that case, only one network is disrupted.

With comas, it's both, so there's not a ton of higher-level processing happening. In general, the comatose brain is a lot more disconnected from the things happening around it than in some other conditions or when you're asleep. If anything, it's actually most similar to being under general anesthesia.

Some physicians even call anesthesia a “reversible coma”, and it can be used to study real comas without putting someone in serious danger. Ultimately, depending on the severity of the damage, a coma patient might heal and slowly come back to regular consciousness, becoming both awake and aware. Once they do, they probably won't have any memory of the experience — even if they do remember the process of waking up.

Which probably isn't such a bad thing. Although doctors work to get patients out of comas as safely and quickly as possible, studying them and how they work are really important. After all, comas are key ways for us to understand the root of consciousness in our brains.

By untangling the things that are happening inside our heads when we lose connection with the world, we're able to get a better idea of the biology that links us to reality in the first place. And that's definitely worth understanding. Thanks for watching this episode of SciShow Psych!

If you'd like to keep exploring the human mind with us, you can go to youtube.com/scishowpsych and subscribe. [♪ OUTRO ].