What’s the Weird Face We Make When We Touch Our Eyes?

[ ♪OUTRO ] Have you ever noticed that when you use eye
drops or put on mascara, your mouth hangs open? Although there aren’t any hard numbers on
how many people experience this “mascara mouth” phenomenon,
it’s definitely pretty common. Just watch next time you’re around
someone putting in their contacts. So why is it that messing with our eyes often
leaves our mouth agape? No one knows for sure, but there are some
best guesses as to what might be causing it. We can start by looking at the nerves that
control facial movements — of which there are quite a few. The trigeminal nerve is a large, complex nerve
that controls the lateral pterygoid muscle, which along
with other muscles, opens and closes your mouth. And the oculomotor nerve is responsible for
controlling a wide range of muscles in and around the eye — including the muscle
that raises and lowers your upper eyelid. Although these nerves do pretty different things, they originate close to one another in the
brain stem. So one hypothesis to explain mascara mouth is that the two nerves are so close together,
they get their signals crossed. When you activate your oculomotor nerve and open your eyes wide, this might also accidentally trigger your
trigeminal nerve, causing your mouth to open. Some researchers have made a connection to
this response with a very rare genetic disorder called Marcus
Gunn syndrome. This syndrome causes one eyelid to move rapidly
every time the jaw moves. It’s thought to be caused by an abnormal
connection between the trigeminal and oculomotor nerves. And researchers aren’t totally sure why yet. But some think that it’s a more extreme
version of this “mascara mouth” response that most of us seem to have. Which could help explain why individuals who
don’t have this syndrome will still open their mouths while poking
around their eyes. Another interesting connection between our eyes and our mouths shows itself in a reaction called
the corneomandibular reflex. This is an automatic, completely involuntary
movement of the jaw which happens when touching the cornea, the
transparent front layer of the eye. Touching the cornea makes you blink, and the
jaw movement seems to happen in response to that. It almost always shows up in patients with
brain related damage or diseases, like those with Parkinson’s or ALS. It’s well known in comatose patients, suggesting
it can happen when you’re totally unconscious. It’s thought that this reflex is present
when there is either a brain stem dysfunction, or structural damage to nerve pathways
— possibly another mixed message between the oculomotor and trigeminal nerves. So while there are definitely a number of
connections between our eye and mouth movements,
we’re still really not sure what exactly is happening to cause your mouth to hang open
when you’re trying to convince your stubborn contact lens to stick to your eyeball. What we do know is that it’s hard to avoid
looking silly. Thanks to our patron Katy for sending in this question, and thanks to everyone who voted in our Patreon
question poll. If you want a shot at getting your questions answered or just want to vote in the polls, head on
over to patreon.com/scishow. [ ♪OUTRO ]


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