Why Do We Get Motion Sickness? Senses That Disagree About Movement

Three systems report on movement and position. The vestibular system in the inner ear contains semicircular canals detecting rotation in three planes and otolith organs detecting linear acceleration and the direction of gravity. Vision reports motion relative to the surroundings. Proprioception reports the position and loading of the body from muscles and joints. In ordinary movement all three agree. Motion sickness arises when they do not, and the mismatch takes two opposite forms. Reading in a car gives vestibular motion with no visual motion. A flight simulator or a film shot from a moving vehicle gives visual motion with no vestibular motion, which is why simulator sickness and cinema nausea exist. Seasickness combines low-frequency vertical motion, to which humans are most sensitive at around a fifth of a hertz, with a cabin interior that moves with the ship so the eyes see nothing moving.

The obvious question is why a sensory disagreement should produce vomiting rather than, say, a headache or confusion. The leading explanation, proposed by Michel Treisman in 1977, is that neurotoxins commonly disrupt coordination and balance, so an animal experiencing a sudden mismatch between what its senses report about its own movement has some reason to suspect it has eaten something poisonous, and the adaptive response to suspected poisoning is to empty the stomach. On that account motion sickness is a defence mechanism firing on a false alarm, since vehicles did not exist when the system evolved and passive transport reproduces the signature of intoxication. Supporting evidence includes the accompanying symptoms, which are those of the nausea response generally: salivation, pallor, cold sweating, a drop in stomach motility and eventually vomiting, along with the strong food aversion that follows, which is exactly what a poisoning defence should produce. The theory is not proven and remains the most coherent account available.

Susceptibility varies enormously and follows recognisable patterns:

• •Children between about two and twelve are most susceptible, with sensitivity typically declining through adolescence
• •Infants under two are almost immune, which is thought to reflect the immaturity of the integration involved
• •Women report motion sickness more than men in most studies, with hormonal and reporting explanations both proposed
• •Migraine sufferers are markedly more susceptible, which suggests shared underlying sensory processing
• •People with complete vestibular loss do not experience motion sickness at all, which is the strongest evidence that the vestibular system is essential to producing it
• •Control matters, since drivers are far less affected than passengers, apparently because anticipating the motion allows the brain to predict what the senses will report
• •Habituation occurs with repeated exposure, which is why sailors adapt and why the adaptation is lost after time ashore

Interventions that work mostly restore agreement between the senses. Looking at the horizon or far ahead gives the eyes motion information matching the vestibular signal, which is why sitting in the front of a vehicle helps and reading does not. Driving, or in a boat taking the helm, provides both a visual reference and anticipatory control. Choosing the position of least motion, over the wing in an aircraft or amidships and low in a ship, reduces the amplitude of the stimulus. Fresh air and avoiding strong smells help with the nausea rather than the cause. Medications fall into two groups, antihistamines and anticholinergics, both of which act on the vestibular pathways and both of which cause drowsiness, which is a genuine cost. Ginger has modest supporting evidence. Habituation through graded exposure is used for pilots and astronauts. For simulator and headset sickness, the technical fixes are reducing latency between head movement and display update, maintaining a high frame rate and providing a stable visual reference, since the mismatch there is one the designer can remove.

Motion sickness comes from disagreement between the vestibular system, vision and proprioception, either vestibular motion without visual motion as when reading in a car, or visual motion without vestibular motion as in a simulator. The leading explanation holds that the response evolved as a defence against neurotoxins, which disrupt balance, so vomiting is a false alarm. People with no vestibular function never experience it, drivers suffer less than passengers, and fixing the mismatch works better than treating the nausea.