eli5: what makes people throw up when driving up a hill when they wouldn’t throw up during a normal drive?

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eli5: what makes people throw up when driving up a hill when they wouldn’t throw up during a normal drive?

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If you’re driving up a hill everything “looks” flat because you’re tilted along with the hill. So your eyes say “we are driving on a flat plane” but gravity tells your body “you are going up a hill.” This discrepancy between sensory inputs confuses your brain and is similar to symptoms of certain kinds of toxins that we have evolved responses to. So your brain thinks, “oh shit, we’ve ingested some sort of poison, better puke it out.”

You inner ears contain some liquid that moves around as your body or your head moves. The walls of the tubes the liquid is in are full of tiny hairs that sense the moving liquid and they send this information to your brain as “position”.

You brain also sees what your eyes are reporting about motion.

It combines these two inputs and that’s were you get your balance from.

Normally your ear liquid and your eyes are telling the same story. “We’re moving”, “we’re falling”, “we’re leaning over”.

In our evolution the only significant time those two signals *disagreed* with each other was when you ingested some sort of poison. Using alcohol as an example, alcohol will make the liquid in your ears “thinner” (like, less syrupy) which screws up the signals the ear hairs send your brain. This gives you those drunk staggers you might be familiar with, you can’t get your balance.

So the brain takes these weird signals and says “Woh! I’m poisoned! Gotta get that poison out!” and then you start barfing your brains out.

Unfortunately, traveling in cars or boats or planes can cause the same effect in terms of earhairs reporting “motion!” but your eyes say “Uhm, nope, we’re staying still”. That deep-monkey part of your brain goes back to “We’re poisoned!” Hence. Baaarrrfff.