Imagine walking down a crowded street and on every person you pass, their nose and mouth and eyes are stretching and drooping to the bottom of their face.

Like, It almost looks like  their faces are melting. And this isn’t the plot of a horror  movie or an Edvard Munch painting.

This is a reality for some patients affected by an extremely rare facial processing disorder. And as horrifying as this experience can be, it’s helping scientists learn  more about how we process faces when everything in the brain is going right. [♪ INTRO] Today we’re talking about Prosopometamorphopsia, which we’ll call PMO so I  don’t have to say that again. And PMO is a disorder that causes  facial features to appear distorted, with mouths that stretch wide,  or noses that are twisted, or eyes that droop down the face.

And we’re able to envision  what that might look like, because one patient saw  distortions when he looked at faces in person, but not when looking  at them on a computer screen. After teaming up with researchers,  he looked at a person’s face and made changes to a picture of  them until the computerized image matched what he was seeing in person. The patient himself described  these faces as “demonic”.

Way back in the 60s, there was  even a patient who was also an artist and put his experiences to canvas. His paintings show not just distorted shapes, but florid colors and a sense  of overwhelming strangeness. So we can put a face on what these patients see.

Which is huge for understanding this condition, because you can’t usually look  through someone else’s eyes. Now, PMO is a super rare disorder, with just over 80 cases documented  in the research literature, though it could be under-reported. But even among those few cases,  there’s a lot of variability.

Most of the time, the  distortions only affect faces, not other objects, and not other body parts. In some cases, other people’s faces  are the ones that are distorted. In other cases it's the  patient’s own face in the mirror, and for some people, it’s both.

For some people, these distortions  only affect the right or left half of people’s faces, while for  others it’s the whole face. And these symptoms can last  anywhere from hours to years. Understandably, the condition  can be terrifying for patients.

And not because they believe these distorted faces are real or that they’re actually demonic. It’s kind of the opposite. They tend to think they’re developing some kind of severe mental illness.

They often end up isolating  themselves, either physically, because the facial distortions  freak them out so much that they want to avoid looking at  people, or emotionally because they don’t want to confess to their  friends or family what they’re seeing. When patients do seek out medical help, they’re often misdiagnosed as  having psychiatric disorders like schizophrenia, when what they actually have is a visual system disorder. So two totally different things.

In fact, studying patients with  PMO is helping scientists better understand the visual system and  facial processing in general. In about 75% of cases, the disorder is caused by some type of brain damage. In about half of cases, there was  damage in the back of the brain, where a lot of very basic  visual processing happens.

There are also cases of damage  along the side of the brain as well as in the corpus  callosum, the bundle of neurons that connects the right and left sides. When brain areas go offline,  doctors can learn something about how those areas are involved in processing specific types of information,  based on what functions are lost. In PMO, even though people see individual facial features as distorted,  they can usually still recognize the people those faces belong to.

So it seems likely that our abilities  to process the spatial layout of faces, and to recognize specific  faces, are separate things. Also, damage to the left side of  the brain has only been associated with distortions on the right side of faces, while damage on the right side  of the brain has been associated with distortions on the left side  of faces, the right side of faces, and both sides simultaneously. This leads to the hypothesis  that each side of the brain processes the side of faces opposite to it, but then it’s up to the right side of the brain to put those two images together.

Damage in so many different areas  resulting in similar symptoms may also suggest that more  parts of the brain contribute to processing facial features  than we previously thought. Hey! Sorry for butting into  a really cool science story, but I’ve got another science learning opportunity to share from the people who  support this SciShow video.

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Okay, you can keep learning about  the topic of this video now. PMO patients are also helping us better understand what happens at each stage of visual processing and which parts of the brain are involved. In the earliest stages of processing, our brains think about what we see in terms of where something falls on our retinas.

In other words, where it  is is relative to our eyes. In the middle stages, we group features together and begin processing them as a whole, so we think about where individual features are relative to the thing itself. So we can tell that the small rectangle  is on the right side of the big rectangle regardless of whether the  big rectangle is on our left or right.

And we assign meaning to those things  relatively late in the process, deciding that the small rectangle is a book and the big rectangle is a bookshelf, and it’s still a book on the  right side of a bookshelf regardless of whether you look  at it up, down, or sideways. At least, that’s how it works with objects. We’ve long thought that faces are special.

The idea was that we only see a face as a face under the right frame of reference, and that our brains don’t see  upside down faces as faces, the way you’d see an upside  down bookshelf as a bookshelf. But one patient with corpus callosum damage may have changed that. They always saw distortions  on the right side of faces.

And I mean always. They didn’t see distortions on faces  of the right side of their vision, and they didn’t see distortions  on the right side of face shapes. When they saw a full face,  the right side was distorted, if they were only shown the right half of a face, the whole thing was distorted,  and if they were only shown the left half, it was perfectly normal.

That suggests that the problem  is relatively late in processing, when the frame of reference for left  and right is the meaningful thing. In this case, the face. But importantly, they also saw distortions on the same side whether the face  was upside down or right side up.

So this particular patient suggests  that at some point, in some way, upside-down faces actually do  get processed as faces, possibly because the brain starts using  the face as a frame of reference. And this is what we learned  from one single patient. There are dozens of other patients  with symptoms just as instructive.

So if you ever do find yourself  surrounded by melting faces, don’t be afraid to report it to your  doctor, and you can also contact the Social Perception Lab at Dartmouth University. They’re interested in working with PMO patients not just to learn more about face processing, but also to identify potential  treatments for the disorder. We’ll put their info in  the description down below.

So while seeing melting,  demonic faces are terrifying, sharing your experience can  help make sure no one else has to go through the same thing, and can teach us more about brains in general. [♪ OUTRO]