Neurologists Deconstruct Motion Sickness

Motion sickness has generated many theories on the mechanisms and stimili involved. Researchers review the literature.

Motion sickness can be a miserable malady, researchers agree. But there are competing theories on why some people get it and others do not.

Swiss researchers reviewed the literature on motion sickness, focusing on vestibular-only motion sickness, in order to list, clarify, and frame the stimuli and sensory signals involved within the context of current theories.

The review was conducted by Giovanni Bertolini of the Department of Neurology at the University Hospital Zurich, in Switzerland, and colleagues, and was published in Frontiers in Neurology on February 15.

The reviewers begin by considering the sensory systems that contribute to motion sickness, including vestibular, visual and somatosensory.

They then turn to the the theories on motion sickness and name the sensory mismatch theory as the most widely accepted. They describe the theory as stating that “conflict of motion stimuli alone is not sufficient to cause motion sickness.” They go on to explain, “This conflict is only perceived as nauseogenic, if the present pattern of the rearranged sensory motion signals is at variance with what is expected from previous experience.”

Next, the reviewers looked at classifications of motion sickness, and name two main categories: “(1) conflict between angular (semicircular canals) and linear (otolith organs) vestibular input and (2) conflict between visual and vestibular input.”

Vestibular motion sickness is rooted in conflicting signals. The authors offer a classic example, saying, “this kind of conflict occurs when the semicircular canals perceive head rotations about an axis that is not aligned with the estimated gravity vector, but the latter does not change its orientation relative to the head accordingly.” This is called cross-coupling and is the most well-known and understood form of vestibular only conflict.

In addition to cross-coupling, linear oscillation has been studied as a cause of motion sickness.

According to the reviewers, “The motion sickness induced by linear oscillations may apparently contrast with the sensory conflict theory, since they cause variations of a single sensory signal.” They go on to say that it is not at all clear that oscillatory movements can cause motion sickness.

The researchers conclude by considering examples of vestibular motions sickness in everyday life. One example is that of a tilting train. The researchers say,tilting the car bodies of trains compensates for the centripetal acceleration during turns by bringing the vertical axes of the cars closer to the gravitoinertial force vector.” The problem is that passengers often develop motion sickness as a result.

Another common example is motion sickness while riding in cars and public road transport. One practical finding is that drivers themselves rarely if ever get motion sickness.

The researchers believe that is because drivers unconsciously reposition themselves according to changes in the road or view that would otherwise trigger motion sickness.

If self-driving cars become a reality, studying the mechanisms that trigger this type of nausea will be necessary--such as providing ways for passengers to reposition themselves so that horizontal oscillations in their view do not make them ill.