Reading Makes You Carsick Because Your Brain Thinks It’s Being Poisoned

Photo: Toni Barth/EyeEm

There are those who can happily while away a long road trip with the companionship of a good book, their minds flying away with the plot even as their bodies remain motionless in the passenger seat of a car plowing along a freeway. And then there are those — well, there are those for whom this all sounds very nice in theory but who know that in reality it would probably result in nausea at best, actual vomit at worst. Carsickness is an annoying quirk of human physiology, but it’s one that has more to do with your “idiot brain” than you’d probably think, as neuroscientist and author Dean Burnett explained yesterday in an interview with NPR’s Fresh Air.

That’s the title of Burnett’s book — Idiot Brain: What Your Head Is Really Up To — which Science of Us excerpted earlier this week. Burnett is a refreshing sort of neuroscientist in that he does not seem to consider the brain with any particular reverence; if anything, he seems mildly impatient about its many ridiculous failings. Motion sickness is one of them. It’s the job of the thalamus, he explained, to interpret all the sensory signals the body sends its way. Typically, when you’re moving about, your muscles are in motion, your eyes are observing the distance you’ve covered, and, whether you know it or not, you’re also relying on the “balance sensors” in your inner ears. These are “little tiny little tubes full of fluid,” Burnett explained, “and the motion of that fluid tells us where we’re going. So, if we’re upside down, we can tell. And if we’re going fast, we can tell, because this fluid just obeys the laws of physics.” The thalamus pieces all of this information into a kind of “explain it to me like I’m 4” message to your mind about where you are in space.

But when you’re in a car, your unsuspecting thalamus is picking up all sorts of mixed signals. Your muscles are motionless, and yet your eyes can see that you are, in fact, moving along, and quite quickly. And then there’s the problem of the aforementioned fluid in your inner ears, which are “rocking around and sloshing because you actually are moving,” Burnett explained. More on that:

So what’s happening there is the brain’s getting mixed messages. It’s getting signals from the muscles and the eyes saying we are still and signals from the balance sensors saying we’re in motion. Both of these cannot be correct. There’s a sensory mismatch there. And in evolutionary terms, the only thing that can cause a sensory mismatch like that is a neurotoxin or poison. So the brain thinks, essentially, it’s been being poisoned. When it’s been poisoned, the first thing it does is get rid of the poison, a.k.a. throwing up. And as a result — so, like, as soon as the brain gets confused by anything like that, it says, oh, I don’t know what to do, so just be sick, just in case. And as a result, we get motion sickness because the brain’s constantly worried about being poisoned.

Your poor dumb brain is only trying to help, in the best way it knows how, in other words. For some, reading a book only compounds the confusion. “When you’re in a car, you can look out the window. You can see things going by. You can see the passage and the movement itself,” which means that, for some people, the sight of the world outside the car passing by “sort of balances the system,” Burnett said. “The brain’s going — oh, look, things moving — I must be moving — and then sort of calms down the sickness response.” But when you read, you’re focused on the page, and as such, “you’re shutting out a lot of external visual information,” he continued. “So it sort of increases the sensory mismatch, which is causing the sickness in the first place … You’ve got no visual information to try and help allay the brain’s concerns.” That’s also, by the way, the reigning theory for seasickness. When you’re on a ship, it doesn’t really look like you’re moving — again, you’re not getting that visual feedback to help prove to your brain that your body is, in fact, in motion, much like what happens when your eyes are trained on a book.

Burnett doesn’t explain why this happens to some people and not to others, because, he says, there is no clear reason why it should happen to some and not others; it’s simply a “quirk of development.” There also doesn’t seem to be an explanation as to why some people grow out of carsickness, beyond, well, some people just grow out of these things; some brains get used to the mixed signals and others do not. Regardless: Now you are armed with a fun fact to know and tell on your next road trip, an at least momentary distraction for yourself or your fellow passengers from the misery of motion sickness.

A Very Weird Explanation for Carsickness