MadSci Network: Neuroscience
Query:

Re: What causes motion sickness?

Date: Thu May 13 20:12:19 1999
Posted By: Jeffrey Utz, M.D., Neuroscience, pediatrics, Allegheny University
Area of science: Neuroscience
ID: 925956005.Ns
Message:

There are two main things that your brain uses to tell where you are. 

One of them, as you learned, is the semicircular canal. The cochlea is 
involved only in hearing. The semicircular canals are able to tell your 
brain when your head is moving, and often your body (because, they are 
attached to one another). 

The other is your eyes. Your eyes are also really important for giving you 
brain information about where you are. This is because vision is a really 
important sense for us and we get a lot of information from what we see. 

For example, if you are riding in a very smooth airplane, it feels like 
you are not moving. But if you look outside, you can see cities and towns 
going by. It is mostly your sense of vision that tells you you are moving,
not what your semicircular canals sense. 

If you watch a movie, like Star Wars, you can get a sense of motion when 
you are looking out from a fighter or other ship, and you see things 
wizzing past you, even though you are sitting perfectly still in 
your chair. The same thing can happen while you are playing video games.

If you are in a car and are looking outside at another car that starts 
moving forward (while your car is still), you feel like you are moving 
backwards. (The other car has to take up most of the area you are looking 
at for this to work; this also works great with trains and subways.)

These are all examples of when your sense of vision tells your brain you 
are moving, even though you are sitting (or standing) perfectly still.

Your brain is programmed to tell you something is wrong when one set of 
information coming into your brain tells you you're moving one way, and 
the other set of information tells you you are moving another way (or 
staying still). Your brain gives you motion sickness. If you are riding 
in a car, you whole body goes up and down and turns too, as the car goes 
over bumps and makes turns. But what you see is different. You see the 
seat in front of you or a book if you are reading. You do not see these 
things moving as fast you feel yourself moving. 

Your brain then says: "Something is wrong. I feel like I am moving, but I 
do not see things move (as fast). Better make you feel sick."

You are probably wondering why this is so. The reason why is that when you 
eat a poison, you sometimes feel like you are moving when you are not. If 
it makes you feel really bad, it is better for you to get rid of the stuff 
than to keep it in your body. 

So you get rid of it by vomiting (throwing-up). It is better for you to 
lose your lunch (or other meal) than to get sick for a long time. 
Unfortunately, as we evolved, we did not have cars, planes and video 
games (we did not even have video games when I was growing up). So our 
bodies overreact to the motion of moving vehicles by making us feel sick, 
and, if we feel really sick, vomit.

If you think this is bad, astronauts have it worse. We have gravity to 
help us orient our bodies and tell us which way is down. Astronauts don't, 
because they do not feel gravity in space. Their eyes tell them one thing, 
their semicircular canals tell them something different. Some of them 
really get sick. But they get used to it, and in a day or two, they feel 
much better. In fact, there is an airplane that comes down in such a way 
that it makes the people inside feel like there is no gravity. When this 
airplane flies, it's flight path is parabolic (so it feels like there is 
no gravity at the top of its parabolic path). It does this many times in 
a day. Many people get sick on it, so they call it the "vomit comet."

There is one other reason why you might feel sick in a car. As you know 
most cars are powered by gasoline and have gasoline in the fuel tanks. 
Either fumes from the fuel tanks or fumes from the engine can sometimes 
make you feel sick. It may help if you open a window or move to a different 
part of the car or bus. 

There are other things you can do. It may help you to either read or play 
games while you are in a car (to take your mind off of being sick) or look 
outside, so it feels more like you are moving. Finally, sitting in the 
front seat sometimes helps, because you can see better. When you see the 
things moving in front of you better, what you see agrees better with what 
you feel, and you do not get sick. (Hint: Use this to get to sit in the 
front seat instead of your big brother! But only if you do not have 
airbags up there and you wear your seat belt.)

There is one added bonus: Feeling in control also helps. So when you learn 
to drive, you know when to expect to turn and stop, so this helps your 
brain see and feel the same thing, and you do not get sick.




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