What Is Motion Sickness?

You are on a cruise ship in your room and your stomach starts to rumble. Or maybe you are in a car reading a book in the back seat and get nauseated. Why does this happen? We are not 100% sure but it may be due to what is called the “sensory conflict theory” or “neural mismatch”. Here is how it works:

The balance system works by combining input from your vision, your inner ear function, and your proprioception. This input is sent to the brain where it is processed and keeps you oriented in space. The sensory conflict theory is the idea that what your eyes are telling you is different than what your proprioception or inner ear function is telling you, causing a mismatch and triggering your nervous system causing nausea or GI distress.

So what are these three systems? The visual system operates with a reflex called the VOR (vestibulo-ocular reflex). It is a complicated reflex but to keep it simple, it keeps your gaze stable when you are moving. If we didn’t have this reflex, the world would look like the Blair witch project when riding in a car.

Your inner ear function is not your hearing (that is outer ear). It’s a series of semi-circular circular canals that contain two layers of fluids, and hair cells that are anchored by crystals (otoliths). It operates in a push-pull mechanism. When you turn your head one way, the fluid places a drag on the hair cells and trigger the vestibular nerve like a switch, sending input to the VOR (that’s the vestibular part of the vestibulo-ocular reflex).

Proprioception is what gives you kinesthetic awareness. It allows you to know what your joints are doing during movement. If you close your eyes and bend your elbow, you can ‘feel’ it bend. That is proprioception.

Normally, the input from all three systems corresponds and we are able to stay balanced. Now, let’s go back to the cruise ship. You are in your cabin. Your feet (proprioception) feel movement, but the room is moving exactly the same as you are, so visually you do not see movement and this is what causes the discrepancy. The same thing happens in the car. With a book in your lap it moves with your body with every bump in the road and you don’t see movement, you only feel it. This is why looking at the horizon or looking out the window helps, you resolve the discrepancy.