Hank Green: When I say hallucination, you probably think of one of two things: psychotropic experimentation, like a dude in a hemp shirt trippin' at a Phish show, watching rainbows melt into puddles; or mental illness, like someone doing things because the voices in their head told them to and ending up in the hospital, or in jail.

A few centuries ago, most people would've ascribed hallucinations to the influences of ghosts, gods, or witches. Today, we understand them as the result of neurological disturbances and the truth is, hallucinations aren't necessarily a sign of mental illness, drug use, or spirit vengeance - rather, they can be caused by lots of different things.

So while not everyone has experienced hallucinations, it's important to understand that everyone is capable of hallucinating under the right circumstances. In fact, a good number of us will experience some type of hallucination in our lifetime - I know I have. Like if you've lost the use of one of your senses, like your sight or smell, but still smell things from time to time, then you've hallucinated. If you've been under a ton of stress and heard someone tell you what to do, only to find that there's no-one there, then you've hallucinated. If you've driven by a bus stop and for a split second saw your deceased father sitting on the bench reading a newspaper, then you've hallucinated.

It's weird, I know, but the one thing I can assure you is that the science behind these phenomena is very real.

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Hank: Whether they're fun, enlightening, confusing, or terrifying, all hallucinations involve sensing things that appear to be real, but actually are a product of the mind. They're not dreams - you have to be awake to truly hallucinate - instead, they're sensory disturbances, coming in a variety of flavors. You can see light patterns and objects that aren't there, smell imaginary odors, hear voices and other sounds, or feel physical sensations like crawling skin or your own organs moving. Even out-of-body, near-death, and sleep paralysis experiences are types of hallucinations.

But to understand what's not real, you first have to understand how your brain judges something to be real in the first place. This involves the sister processes of sensation and perception. Two different things. Your sensory organs receive visual, auditory, olfactory, and tactile clues pulled from the outside world in a process called sensation. That raw data is then sent to the brain where it's processed, giving it meaning and putting it into context in the process of perception. So basically, the brain turns sensory input into meaningful information to create your own model understanding of the world.

The more sensory data you collect, the more they inform that model of how you perceive the world to work, and most people's models more or less agree on the big stuff - the sky is blue, the dogs bark, kittens cannot fly, and snow is cold. But if that normal, copacetic brain activity gets disturbed by, say, a dose of LSD, a seizure, a disease, or a mega blow to the head, this model can get shaky, eventually diverging from its normal representation of the world. Then suddenly a flock of kittens fly by in fiery, rose-scented rain while calling your name. Depending on the type of brain tweak, these hallucinations may take different forms - interesting, random, or scary - the good news is they all give us potential to learn about what the heck is going on up here.

Obviously, outside substances can spur some wacky activity in your mind, and most ancient cultures have some tradition of vision questing, aided by some psychotropic substance. I'm not gonna get too into drugs today, but you should understand that different compounds, whether found in plants, animals, fungus, or synthetically made concoctions, affect the brain very differently, meaning that even if they all cause hallucinations, they may be using very different pathways. For example, most researchers agree that LSD garbles the senses in a psycho-biochemical way by affecting specific feel-good serotonin receptors found in the brains cerebral cortex and thalamus regions, responsible for sensory perception.

Caffeine, on the other hand, works in an entirely different way. Australian researchers have linked consuming excessive amounts of coffee with increased risk of auditory hallucinations. Caffeine sorta gooses the physiological effects of stress, including the increased production of the body's naturally occurring stress hormone cortisol, which can lead you to hear things that are not actually there. But there are many reports of healthy, non-caffeinated people experiencing vivid auditory hallucinations, often when under extreme stress. There have been recorded cases of, for example, someone breaking their leg while alone in the woods and just wanting to curl up and die, but a strong voice, not just a thought, but something that seemed actually audible, telling them they needed to get up and move to survive.

Grief is its own kind of stress, of course, and hallucinations have been known to occur in otherwise healthy people during periods of extreme grief. In fact, people seeing a departed loved one or hearing their voice can be a common part of the grieving process. But hearing voices is the symptom we typically associate with mental illness, and while it's true that seventy percent of schizophrenics report hearing voices regularly, it's not necessarily a scary thing. In fact, one Dutch study found that a good number of mentally stable people enjoyed hearing pleasant, supportive voices in their heads.

Exactly how a person experiences auditory hallucinations is still fuzzy, but some neural imaging studies suggest that it may have to do with the failure of the brain's frontal speech area. This is the part that usually lets your sensory areas know that the sound they're hearing is self-generated, like something you're just thinking to yourself. But if that signal doesn't get sent, then the brain gets confused as to where that voice it's hearing is coming from. Brain scans have also shown that people with schizophrenia have abnormalities in the areas associated with memory. That might exacerbate this failure to identify the words that show up in their mind as being part of their own inner monologue.

Here's something interesting, though: in the total absence of sensation, the brain still leaps to perception, wanting to maintain that connection. For example, some people who suddenly lose their ability to smell often experience phantosmia, a type of olfactory hallucination. And yeah, if you smelled wildflowers and baking cookies, it might not be so bad, but unfortunately, the most common odors are the bad ones, like puke, poop, and rotting stuff. In these cases, your brain is overcompensating for its lack of olfactory input by pulling from the memories of old scents and making it seem like you're smelling them for real. And it's possible that the brain is offering up these disagreeable odors because those very smells had just been suppressed by certain neurons that have since turned off.

And it doesn't just stop with smells - in fact, one of the most fascinating ways to see visual hallucinations is to go blind. Charles Bonnet syndrome, or CBS, is a condition that can cause otherwise healthy people with macular degeneration or loss of vision to experience complex visual hallucinations. The condition was first described by Swiss naturalist Charles Bonnet in 1760, after watching his newly blind grandfather describe seeing visions of birds, buildings, and ladies, among other things. Patients with CBS report seeing images ranging from simple geometric shapes and colors to vivid detailed scenes, often involving faces and - for some reason - lots of little people. These images are not interactive; the experience is more like watching a movie with the sound off, and what patients see is unrelated to anything they're thinking, feeling, or doing at the time. And none of the faces are familiar. Some are pleasing, but others are deformed, and they often have big, misshapen teeth.

These types of hallucinations are very different from psychotic hallucinations, which tend to be interactive and actively address the patient, whether to be support, berate, threaten, or jeer at them. Instead, CBS hallucinations are more like those people with olfactory damage who start to hallucinate smells - when a person's vision no longer functions, the visual part of their brain can become hyperactive in the absence of stimuli that it was used to. So it starts firing off images, trying to sorta keep the story going, guessing what it might be seeing if it could still see.

Interestingly, the same phenomenon can be achieved under visual sensory deprivation. One study blindfolded volunteers for four days, and in the end, ten of the thirteen subjects reported seeing various levels of hallucinations. Different localized parts of the brain are often involved in specific hallucinations, too, like there's an area for seeing buildings and landscapes, and one just for cartoons. There's also a region in the temporal lobe called the fusiform gyrus, which is where we process the images of people's faces, so damage to this area can cause prosopagnosia, or face-blindness: the inability to recognize faces, sometimes even your own. But over activity in this sliver of the brain can cause a person to hallucinate and start seeing - you guessed it - faces. Not only that, but the front part of this region specializes in registering teeth and eyes specifically, so it may be involved in the phenomenon of hallucinators seeing faces with crazy big teeth.

So even though you and your brain may sometimes see things a little differently, and sometimes a lot differently, you can think of hallucinations as particularly vivid reminders of just how delicate, complicated, and freaking amazing your brain is. Take care of it, and it will likely do its best to take care of you.

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