This is Poisonous (Maybe)

Right now, you’re reading. And hopefully, what you’re reading has made your day a little bit better.

But if you’re reading while in a moving vehicle — the passenger seat of a car, say — maybe that’s not the case. (If you’re in the driver’s seat, Now I Know can wait until you’re no longer driving.) Reading while going for a ride may end up making you feel nauseous. The phenomenon called motion sickness — often labeled “car sickness” or “seasickness” given the modes of transport — is to blame. But what causes motion sickness?

According to the prevailing theory, it’s an evolutionary mistake.

When people move around — historically, that is — we do so by foot. We walk, we run, maybe we skip a bit. Regardless, our bodies take in a lot of data as we do all this. It’s not just our legs moving. Among other things, our eyes are scanning the area, changing focus as things become closer and our vestibular systems (in our inner ear) help us maintain balance. For our subconscious selves, this all adds up: we’re moving, and everything’s fine.

When we’re driving around in a car or other vehicle, though, the equation fails. We’re sitting still, relative to the car itself at least, so our legs and muscular system in general act as if we’re not moving at all. The vestibular system, though, detects the motion. As author Dean Barnett of the book “The Idiot Brainexplained in the Guardian, “the fluid [in the inner ear] responds to the laws of physics, so [it] moves about in response to acceleration and gravity.” This mismatch is a problem, but there’s an easy remedy, at least if you’re in the car. Barnett explained in an NPR interview:

When you’re in a car, you can look out the window. You can see things going by. You can see the passage and movement itself, so that sort of balances the system. The brain’s going, oh, look, things moving – I must be moving – and then sort of calms down the sickness response.

And, on the flipside, if you start reading, your eyes fixate on the book or phone screen, somewhat ignoring a lot of the motion you’d otherwise pick up peripherally. Your brain thinks you’re not moving, except for some weird data from your ears and your peripheral vision, and again, there’s that mismatch.

But that still doesn’t explain why we get sick — why do our brains, sensing something is amiss, decide to make us feel like we’re going to vomit? For that, we have to go back to the time before we rode around in vehicles and instead moved around primarily (if not only) by foot. Per Barnett (again via the Guardian), there weren’t a lot of reasons why you’d have the mismatch. The most likely, if not only explanation: poisoning. And the fastest way to get the poisons out is by puking.

So, we get nauseous, if not worse. And it’s all because parts of our brains are stuck in the days when cars and other vehicles didn’t exist, but poisons did.

Bonus fact: Many hearing-impaired people are immune to seasickness, according to a 1977 study (available here). Because many types of hearing impairment also affect the vestibular system, there’s no signal to the brain that the body is in motion (or the signal isn’t as strong). As a result, there’s no mismatch — and therefore, no false detection of toxins.

From the Archives: Temporary Blindness: Why you can’t see your eyes move when you look at them in a mirror.

Related: “The Idiot Brain: What Your Head is Really Up To” by Dean Burnett. 4.5 stars on 17 reviews.