What causes seasickness? A glitch in the brain

By Dean Burnett 

A lot of people, when they travel by car, ship, plane or whatever, end up feeling sick. They’re fine before they get into the vehicle, they’re typically fine when they get out. But whilst in transit, they feel sick. Why?

One theory is that it’s due to a weird glitch that means your brain gets confused and thinks it’s being poisoned. This may seem surprising; not even the shoddiest low-budget airline would get away with pumping toxins into the passengers (airline food doesn’t count, and that joke is out of date). So where does the brain get this idea that it’s being poisoned?

Despite being a very “mobile” species, humans have evolved for certain types of movement. Specifically, walking, or running. Walking has a specific set of neurological processes tied into it, so we’ve had millions of years to adapt to it.

Think of all the things going on in your body when you’re walking, and how the brain would pick up on these. There’s the steady thud-thud-thud and pressure on your feet and lower legs. There’s all the signals from your muscles and the movement of your body, meaning the motor cortex (which controls conscious movement of muscles) and proprioception (the sense of the arrangement of your body in space, hence you can know, for example, where your arm is behind your back without looking at it directly) are all supplying particular signals.

There’s also the vestibular system, which includes the balance sensors; tiny fluid-filled tubes in our ears. The fluid responds to the laws of physics, so moves about in response to acceleration and gravity, so we can tell when we’re upside down, for example. And, of course, there’s our vision. When we walk, the world travels past on our retinas at a steady rate, and there’s the gentle side-to-side rocking caused by our hips and legs etc.

When we’re walking, all of this sensory information is fed into the fundamental, subconscious areas of the brain, like the thalamus, that integrate it into one coherent and rich perception of ourselves and the world around us.

However, ships and vehicles haven’t been around long enough for our brains, at such fundamental levels, to “recognise” when we’re travelling in one. Because when you’re travelling, all the usual signals of movement are absent. Your muscles are still. You’re sat down. This all results in sensory information that says to the fundamental brains regions “we are stationary”.

Not the vestibular system though; the fluid in your ears obeys physics, travelling at high speeds means it sloshes around even more than usual, so it’s telling the brain “we are really moving”. That means these fundamental regions are getting mixed signals; usually reliable senses are now disagreeing. What the hell can cause that? As far as the lower brain is concerned, only one thing; neurotoxin, aka poison. And what’s the quickest way to get rid of poison? Throw up. And so, we feel nauseous, and often vomit.

Dean Burnett‘s debut book The Idiot Brain is available now in the UK, USA and Canada and includes lots more useful information about motion sickness.

Article Courtesy of Guardian News & Media Ltd.