[DIGEST: NPR, Science Alert, New York Magazine]
If you get motion sickness, you’re not alone. About 33 percent of the population is susceptible to motion sickness. While you may assume that the queasiness comes from being bumped and jostled about, the actual reason may be a bit stranger.
According to Cardiff University neuroscientist Dean Burnett, author of the book Idiot Brain: What Your Head Is Really Up To, motion sickness is actually the brain’s response to thinking it has been poisoned.
Burnett explained that a part of the brain called the thalamus is responsible for interpreting sensory signals and feeding those signals to other parts of the brain. The thalamus interprets the motion of the muscular system, the balance sensors in the ears, the input from the eyes and so forth. All these sensory cues are compiled by the thalamus which gives us an impression of what’s happening in the world around us.
Until very recently, evolutionarily speaking, if we were moving, our muscular system was moving as well. However, when in a car or other vehicle, we are sitting still. This leaves the brain with mixed signals—the fluids in your ears tell you that you are moving, but other cues are telling you that you are not.
“There’s a sensory mismatch there,” said Burnett. “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 being poisoned. When it’s been poisoned, the first thing it does is get rid of the poison, aka throwing up.”
The mismatch becomes even more severe when the visual cues are impaired – for instance, if you are reading a book instead of looking out the window. When you are processing the scenery in a moving vehicle, “the brain’s going—oh look, things are moving—I must be moving—and then sort of calms down the sickness response.”
But when you’re reading, “you’re looking at sort of a small, static square and shutting out the external information that cues that you are moving.” This increases the sensory mismatch, making motion sickness more likely.
Being the driver can also help with car sickness because there is more visual evidence that you are genuinely moving, and you are the one in control of the movement.
Burnett said there was no clear explanation as to why some people get motion sickness and others do not, calling it a “quirk of development.” A 2013 study found that people with more “body sway”—those who naturally move more often even when stationary—are more susceptible to seasickness. This finding caused the researchers to conclude that people more prone to motion sickness may move differently in general.
There also does not seem to be an explanation as to why some people grow out of car sickness while others do not.
Despite not yet knowing all the “whys” around motion sickness, researchers are attempting to find a cure for that uncomfortable queasy feeling. In the meantime, give your thalamus a break on your next road trip and put away that book.