What causes motion sickness?

Posted February 19th, 2009 in Biology. Tags: , , .

I used to go scuba diving, but I routinely got seasick on the boat. Since I had nothing better to do while leaning over the water, I wondered why I had to go through this wretched experience. I understood the origins of physical pain– an animal that didn’t realize it had sprained an ankle would likely hurt itself even more rather than waiting for it to heal. But why should I feel nauseous when on a boat? I wasn’t being hurt by the waves, so this incapacitating condition wouldn’t have provided any advantage to my ancestors and therefore shouldn’t have been favored by natural selection.

I’ve heard some proposed explanations for this mystery. A woman once told me that she thought motion sickness was a relic of our tree-dwelling ancestors. Young monkeys are probably similar to young humans in the sense that they’re full of energy and devoid of caution. Motion sickness acted as negative reinforcement that discouraged them from swinging through the trees too wildly and losing their grip on the branches.

Another explanation comes from Alastair Reynolds’s novel Redemption Ark. He notes that evolution shaped our genes for many generations before boats and other moving vehicles were invented. Being inside a moving vehicle isn’t a situation evolution ever had to deal with before, so we haven’t been optimized for riding in them. Normally, our sense of balance is maintained by the vestibular system in our inner ears combined with our vision. You can test this by standing on one foot with your eyes open, then close your eyes and find out how much harder it is to keep your balance.

It’s usually impossible for your visual cues to contradict the signals from your inner ears– unless you’re in a moving vehicle. Then the deck of the boat appears stationary to your eyes, but your inner ear insists that it’s moving. It’s not obvious to me why this conflict results in the specific symptoms of motion sickness, though. For instance, why does seasickness manifest as nausea rather than a headache?

Reynolds suggests that motion sickness arose as a defense against hallucinogenic plants. An animal that accidentally eats peyote will experience hallucinations that cause the inner ear to conflict with visual cues just like being inside a moving vehicle. This state of intoxication is potentially deadly to a prey animal who wouldn’t be alert enough to notice a stalking predator. Because the psychoactive chemical is absorbed through the stomach, throwing up is the quickest way to end these hallucinations.

So whenever I get seasick, I’m experiencing the side effects of an evolutionary war between plants and animals. Plants evolved thorns and disorienting chemicals to deter animals from eating them. Animals, in turn, evolved a physiological response to these chemicals which is accidentally triggered by riding on a moving vehicle.

Interesting idea, but how can it be tested? The only falsifications I can think of are that hallucinogenic plants had to have evolved before motion sickness, and species whose ancestors never encountered these plants wouldn’t get seasick. Those seem nearly impossible to verify, but some people seem to be immune to motion sickness, so their genomes could be compared to people like me to identify the genes responsible. I’ve also briefly looked into the role of the area postrema, the region of the brain that controls vomiting. Reynolds suggests this part of the brain can be activated by a conflict between the inner ear and visual cues, but this page notes that it lies outside the blood-brain barrier which suggests that it induces vomiting based on detecting toxins in the blood rather than as the result of sensory conflicts.

Last modified February 6th, 2012

6 Responses to “What causes motion sickness?”

  1. Al Reynolds posted on 2009-02-20 at 02:53


    I’d love to take credit for that theory of motion sickness, but the truth is it was just one of the several theories I read about while researching that scene in Redemption Ark. I’m afraid I don’t recall the whereabouts of the original article.

    I suffer terribly from motion sickness myself, as it happens.

    kind regards,


    • I feel your pain. I used to want to ride one of those parabolic flights that simulate microgravity, until I realized that it wouldn’t be a pleasant experience for me or anyone else on board. If someone could literally turn off motion sickness as Skade did in your book, I’d be willing to pay an awful lot of money for the treatment.

      Also, I missed a couple of details in my analysis. This article notes that proprioception helps to establish our sense of balance as well, which might be why blind people can suffer from motion sickness even though they wouldn’t suffer from the same sensory conflict as sighted people.

      Furthermore, Professor Stoffregen questions the role of sensory conflicts altogether, noting that all his test subjects who feel sick also fidget much more than unaffected subjects. Even when strapped to the chair, people in the throes of motion sickness “wriggle” and “move in really weird ways.”

      In Stoffregen’s experiments, the subjects were stationary while their surroundings appeared to move. It almost seems like the first two senses (vision and the vestibular system) are inducing involuntary movements to force proprioception to “break the tie” by supporting the subject’s visual sense of motion. I wonder if the fidgeting he describes would take place if, instead, subjects were moving but their surroundings appeared stationary. That’s how I usually experience motion sickness; I can play the video games he describes with absolutely no symptoms.

      By the way, I’m a big fan of your novels. Anyone interested in the scene we’re discussing should know that it’s a good idea to read Revelation Space before reading Redemption Ark.

      • Al Reynolds posted on 2009-02-21 at 03:44

        FWIW, I’ve spoken to a few people who’ve done parabolic flights (and at least one who’s been up to the ISS) and I’m told that having a propensity for motion sickness on Earth/in boats/cars etc won’t necessarily mean you feel unwell on the plane/in space; it seems to be a somewhat different physiological response. However, like you I’d need serious persuasion before giving it a go.

        Thanks for the kind words on the books.

  2. Dae posted on 2009-02-23 at 01:10

    While I can’t offer an explanation to the underlying cause I can offer some data points with regard to inadvertent testing of some other people’s nausea response.

    I do something called First Person View (FPV) Video Piloting, which means I fly an R/C aircraft through a set of VR goggles receiving a live wireless video feed transmitted down from the plane. I have a head tracker on the goggles tied to the camera in the plane, so that when I pan or tilt my head, it pans or tilts the camera on the plane.

    Sometimes I give other people rides (they wear the VR goggles while I fly visually) and I find that about 30-40% of them experience some level of nausea, even when I fly very smoothly. I record the video from all my flights and I can see what happens to these people. What I find is that often the people who get nauseous the quickest, fail to anticipate the motion of the plane. They don’t look where the plane is going, but fixate on something that moves out of view (it’d be like fixating on every sign that you drive by and trying to watch it out the side window of the car as long as possible). In this case my passenger looks at a ground feature and as I start a gentle turn they track that same feature until the camera gimbals can’t turn any further and then suddenly their view is just blurry/rotating ground, or sky or whatever. As they get more disoriented they look less and less where I’m pointing the plane so the problem is self re-enforcing. What’s interesting is that some of my passengers are themselves real full scale pilots (airline, paraglider, hang glider) but they still fail to anticipate the motion.

    I’m not immune to motion sickness myself so I was a bit leary when I had a chance to ride passenger on my own FPV plane while someone else flew it. I found that given my experience as the pilot, I automatically anticipated the motion and looked where the plane was going even in relatively rough conditions. I’ve gotten motion sickness riding in cars, and planes and boats, but put me behind the controls it almost always ceases to be a problem. The motion is the same, the visual cues are the same, but my mental model and anticipation of future motion is different.

    I think for a lot of people the lack of a mental model for the motion is a huge factor that leads to nausea. What I started doing with my FPV plane is setting up a “buddy box” (second set of controls) and sometimes I just turn over control of the plane to my passenger so they can steer where they want and look where they’re going. I found it helps a lot. Even if they’re not very good pilots, they still create a better model of the motion of the plane and the view.

    For an example of the type of piloting I’m talking about check out http://www.vimeo.com/3047509
    It’s a pure glider (no motor). Notice I spend a lot of time looking sideways and down, but I keep my mental model up to date all the time.

    • I’ve soloed a Cessna before, and my results weren’t as encouraging. After more than 10 hours behind the stick, I never really became comfortable while flying. Towards the end I was feeling better, but only because my grandfather and I flew during calm, clear weather. I hated the lesson about stall recovery. Perhaps I needed more time to acclimate…

      I suspect there are several different kinds of motion sickness. One affects people even when they’re sitting still but their surroundings appear to move (like your example and Dr. Stoffregen’s research), and another affects people when they’re actually moving but their surroundings appear stationary (like seasickness). Perhaps a mental model helps with the first type but not the second?

      Also, FPV flying looks very fun!

      • sgroclkc posted on 2012-01-29 at 06:25

        There are three main types of motion sickness that are caused by infrasound ,axis rotation and visually induced .They are all different in certain ways. All three types of motion sickness can lead to nausea. Axis rotation and visually induced can lead to dizziness and infrasound do not. For example´╝îmotionsickness produces a whole range of symptoms, of which nausea and vomiting are the most severe. http://www.aeromedix.com/aeromedix_arti… index.html Axis rotation motion sickness can lead to a spinning sensation and others do not. Infrasound or carsickness, seasickness and airsickness can lead to drowsiness and others do not.

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