To understand what causes motion sickness, we first have to understand how the visual system and the balancing system of the inner ear (vestibular system) normally work together:
As you may already be aware, balance relies on the fluid (endolymph) within the semi-circular canals and other organs of the inner ears. Movement of the head displaces the fluid relative to the motion, triggering signals that inform the brain about linear acceleration (movement in a straight line), the direction and the speed of rotation of the head. The brain compares signals from the ears to incoming signals from the eyes and both systems work together to keep objects in focus during motion (vestibule-ocular reflex).
The combined input of both senses enables the brain to build an accurate picture of the environment and how your position relates to it, allowing you to interact and respond to the environment appropriately i.e. walk around without falling over or walking into things!
However, when input from these two systems is in conflict, the symptoms of motion sickness -dizziness, nausea and vomiting- can arise. To illustrate how this conflict can occur, imagine a typical car journey:When sitting in a moving car you are either looking out of the window with the scenery whizzing by informing your brain (via the eyes) that you are moving at speed, while the ears are sensing relatively little movement; or your visual focus is on the (stationary) interior of the car, perhaps reading a map or selecting driving anthems on the iPod, yet the acceleration, bumps and hills of the road are registering as movement via small displacements of the fluid in the inner ears.
Sensory Conflict: when things go wrong!
Either scenario reduces synchronicity due to two very different analyses of the environment, one from the ears and a conflicting signal from the eyes, which the brain has to try and then resolve. So why does this make some people sick? One way the brain might interpret this sensory conflict is as a brain malfunction, and one possibility of brain malfunction is the ingestion of a neurotoxin (poison). The human body has evolved many protective mechanisms and responses against poisons, of which the emetic response (vomiting) is probably the best known. It allows the body to immediately rid itself of any harmful toxic substances before serious damage or illness occurs. Although no neurotoxins have been ingested in this scenario, the symptoms of motion sickness are essentially so similar to what would be experienced if you’d been poisoned that the body responds appropriately: vomit first ask questions later!
Why do children seem to suffer the most?
Susceptibility to motion sickness decreases with age and experience (the brain can learn to override this evolutionary response), although studies indicate that two thirds of people will suffer with motion sickness in severe travelling conditions. Children are less experienced travellers and are often stuck in the back where their field of vision consists mainly of the seat in front, meaning they are less able to reconcile the sensation of movement with a changing horizon