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IT STARTS behind your eyes, a niggling ache that heads down towards your stomach where it tumbles and turns before building towards a climax of vomit. Bleurgh! Motion sickness.

This has been a human affliction pretty much since we began travelling on anything but two legs. Most of us have experienced it, and it is likely to become even more prevalent when we all become passengers as driverless cars roll out, space tourism takes off and virtual reality headsets take over, both in the gaming industry and, increasingly, for virtual meetings. Even before covid-19, environmentally conscious businesses had started adopting VR technology to bring international clients together.

Motion sickness is clearly related to the movement of our body and head, but why this results in nausea has been a long-standing mystery. Now, however, evidence from brain imaging and genetics is helping scientists get to the bottom of it 鈥� as well as suggesting new ways to solve the problem. It turns out that there is far more to motion sickness than you might think. Your genes, gender and diet all have an influence. It might even come down to your foot size.

The word nausea derives from the Greek for 鈥渟hip鈥�. But motion sickness goes way beyond the odd queasy sailor. Seasickness has had a , influencing the outcome of several military conflicts, from the battle of the Red Cliffs, which marked the end of the Han dynasty in ancient China to the defeat of the Spanish Armada by the English in 1588. And, of course, motion sickness isn鈥檛 confined to the high seas. There are reports of ancient Greeks and Chinese feeling nauseous while being carried aloft in Sedan chairs or travelling by horse and buggy. Their solutions included fasting, drinking the urine of young boys and hiding earth from the kitchen hearth in their hair.

Today, there are more ways to induce motion sickness than ever. One in three of us easily succumb, another third will experience it in rough seas or on a roller-coaster. Nobody is completely immune. We aren鈥檛 the only species affected, either: . In fact, the only animals that don鈥檛 are those without a vestibular system.

This gives us our first clue as to what the cause might be. The vestibular system is a delicate assemblage of structures within the ear that detects motion (see diagram). It includes three semicircular canals filled with fluid that sit at right angles to one another. As the fluid sloshes around, the system sends signals about rotational movement to two places. The first is the brain鈥檚 cerebellum 鈥� the region responsible for balance and movement 鈥� and the other is the brainstem, which links the brain to the rest of the body and includes areas that trigger nausea and vomiting. The vestibular system also transmits signals to the eyes, which stop the world from blurring as we move our head.

Early ideas about motion sickness put it down to . However, if that were true, why would sailors experience nausea after returning to land? Why don鈥檛 we suffer motion sickness when jumping around on a dance floor? And why is the driver of a car less likely to feel sick than the passengers?

Attempting to address such anomalies, a second idea posits that motion sickness results from . When you read in a car, for example, your vision of the book and the dashboard tell your brain that you are stationary, but with all the bumps and turns, your vestibular system is convinced you are moving. These conflicting signals make it difficult for the brain to create a coherent sense of balance, which triggers nausea.

Something unexpected

This 鈥渟ensory-conflict鈥� idea has problems, too, however. If a novice and a seasoned sailor stand on the same deck, for example, they receive the same sensory signals but have different levels of nausea. There is a third possible explanation for why. Rather than resulting from conflicting signals between your eyes and ears, this idea frames motion sickness as a conflict between these signals and .

embraced by Charles Oman at the Massachusetts Institute of Technology. In 1990, he argued that when you make a movement, your brain subtracts the actual sensory input from the expected pattern of neural activity, and what鈥檚 left is a 鈥渟ensory-motor conflict signal鈥�. This is generally small, but it spikes when an unexpected obstacle or motion is encountered, resulting in a corrective response from the brain鈥檚 motor systems. You then rebalance yourself and the conflict is extinguished. However, if the conflict signal is stimulated over a longer period, it triggers motion sickness.

This would explain why we don鈥檛 feel nauseous when we jive around a dance floor, but do when we are swayed by high seas. It also explains why sailors get motion sickness on dry land 鈥� their sensory expectations don鈥檛 fit with the stable environment. It even explains the mysterious immunity bestowed on the driver of a vehicle: being in control gives you more accurate expectations of your movements. There was just one problem with Oman鈥檚 idea 鈥� nobody could find evidence for it in the brain. The required network of neurons was missing.

鈥淲hy don鈥檛 we experience motion sickness when jumping around on a dance floor?鈥�

Meanwhile, Thomas Stoffregen, now at the University of Minnesota, had another idea. His research, measuring the subtle movements that people make to maintain their balance at sea, led him to believe that an . His studies also indicated a : when sailors increased the width of their stance, making them more stable, they felt less nauseous.

The debate looked set to continue. Then, Kathleen Cullen, now at Johns Hopkins University in Baltimore, inadvertently . Her team had trained monkeys to move in a specific way to get a treat. Occasionally, the researchers would disrupt this movement 鈥� by placing a weight on top of the animal鈥檚 head, for instance 鈥� causing a mismatch between the animal鈥檚 expected and actual head motion. When this happened, there was a spike in the activity in some neurons in the cerebellum, cells that didn鈥檛 seem to be active when the animal generated its own movement. 鈥淚t suggests that the cerebellum can perform this amazing computation within a millisecond, in which it compares an internal model of sensory expectation based on prior experience with actual sensory information,鈥� says Cullen. Now her team has shown that when an animal subsequently adapts its movement, the and the neurons stop firing.

鈥淚 was startled when I saw their work,鈥� says Oman. 鈥淭hese neurons arguably play the critical role in motion sickness.鈥� But one piece of the puzzle is still missing, he says. Neurons are known to project from this region of the cerebellum to areas of the brainstem that trigger nausea. 鈥淚t would be a clincher if we could establish that they are the same neurons Cullen鈥檚 lab has been studying,鈥� he says.

Despite this progress in understanding motion sickness, questions remain about why some people are more prone to it than others. Why, for example, do than male ones? And why do women, especially pregnant women, consistently report more symptoms of motion sickness during VR experiments than men?

One possible explanation for this sex divide rests on a decades-old idea about why motion sickness exists at all. Vomiting is a handy reaction to ingesting bad food, but what is the purpose of being sick in reaction to movement? 鈥淢otion sickness is so physically disabling that if there were no positive reason for its presence, natural selection should have acted strongly to eliminate it,鈥� says Michel Treisman at the University of Oxford. Such thinking led him to suggest that it is a : the network involved in keeping us balanced is an ideal early warning system for detecting the unbalancing effects of toxins. Unfortunately, anything else that makes us unbalanced has the same result. If so, it would be beneficial for women to have a lower threshold for motion sickness so that, when they are pregnant, the fetus has increased protection from toxins.

It鈥檚 in the balance

Stoffregen鈥檚 research suggests another explanation for why women are more prone to motion sickness than men. 鈥淭hey have different distributions of body mass,鈥� he says. 鈥淲omen carry weight lower in their hips and have smaller feet even when compared with men of similar height.鈥� As a result, women may be less physically stable, he says, so find it harder to balance when faced with unexpected movements.

Genes also seem to play a role. In 2015, consumer genetic analysis company 23andMe conducted the first genome-wide study on motion sickness in some 80,000 people and found . The genes involved are related to the development of the eye and ear, and to glucose regulation 鈥� women who have particular versions of them are up to three times more prone to motion sickness than men who have them. How the genes influence motion sickness isn鈥檛 known, but people who experience more motion-induced nausea tend to have lower levels of insulin than those who don鈥檛 get sick. Because insulin helps us stabilise glucose levels in the body, the researchers suggest that stabilising glucose through your diet 鈥� by eating plenty of vegetables, fruits and whole grains, for example 鈥� might help to keep motion sickness at bay.

鈥淲hy do women report more symptoms of motion sickness in VR experiments than men?鈥�

Closing the case on what causes motion sickness and why some people are more susceptible might lead to new pharmacological targets. Currently, for it is called scopolamine, which works by blocking chemicals that transmit information from the vestibular system to nausea centres in the brain. However, it can cause dry eyes, drowsiness, headaches, palpitations and urinary retention. Antihistamines work too, and probably in a similar way, but they also have side effects.

听

Regardless of what might be making you susceptible, there are other things you can do to reduce sensory conflict that leads to nausea. Oman recommends to help synchronise visual information with motion, and keeping your head and body as still as possible to limit unexpected external forces. 鈥淪oldiers flying in transport planes found that strapping their head against the seat reduced their sickness,鈥� he says. . Just as sailors eventually get their sea legs, so prior exposure to the sorts of motion you are likely to experience while travelling can reduce your chances of becoming seasick.

Few other traditional remedies have been tested. 鈥淪ome people swear by things like pressure bands [worn on the wrist],鈥� says Oman. 鈥淭here鈥檚 no good evidence for them, but any clinician will tell you that placebos can be useful.鈥� There is evidence . 鈥淲hen I meet sailors who swear by these things, I say 鈥榠f you believe in it, do it鈥�,鈥� says Oman.

Some ideas do have a little evidence behind them. Studies suggest that ginger can reduce nausea during pregnancy and chemotherapy, and although the for motion-induced sickness, ginger activates the vagus nerve 鈥� which runs between the brain and gut and controls aspects of nausea 鈥� and there is evidence that can reduce motion sickness. at Queen Mary University of London found this when he fitted volunteers with an epilepsy device called NEMOS 鈥� an earphone containing two electrodes that stimulate the vagus nerve as it passes behind the ear. Another way to stimulate the vagus nerve is by breathing slowly and deeply. This simple measure has been found to .

The latest idea is a wacky-looking pair of glasses, which are in 94 per cent of users. 鈥淪eetro毛n鈥� spectacles, produced by carmaker Citro毛n, feature four rings 鈥� two framing the eyes and one at each side of the head 鈥� half-filled with a coloured liquid. 鈥淭he liquid moves in the same way as the liquid in the inner ear,鈥� says Antoine Jeannin, CEO of Boarding Ring, the company that developed the glasses. 鈥淚t鈥檚 like a visual inner ear, which helps reduce sensory conflict.鈥� Jelte Bos at Vrije University in the Netherlands thinks it is a nice seasickness idea. But the glasses only give information about two of the three planes of motion, he adds. 鈥淪o I鈥檓 a little bit sceptical,鈥� he says.

In future, the best solution for motion sickness might not be a pill or ginger or a pair of special glasses. There is a growing realisation that we need to improve the technologies that make us nauseous in the first place (see 鈥淪exist technology鈥�). 鈥淲e don鈥檛 design the oceans, but we do design VR and driverless cars,鈥� says Stoffregen. 鈥淲e can鈥檛 be held responsible for seasickness, but we are damn well responsible for the technology we create.鈥�