motion sickness

motion sickness At some point in their lives, most individuals experience motion sickness, in one form or another, while being transported in moving vehicles. There is a wide variability in susceptibility, with a greater prevalence in females than males, and in children between 3 and 12 years of age. It is characterized by initial feelings of dizziness, general discomfort, pallor, and cold sweating, followed by nausea, vomiting, and apathy. It has been given many specific names relating to the mode of transport involved (such as sea-, air-, space-, or even camel-sickness), but all forms are thought to arise from the same basic cause.

The first known report of sea-sickness was by Hippocrates, and the word ‘nausea’ actually derives from the Greek for ship (naus). The precise cause has been difficult to establish, although it has long been realized that it is associated with stimulation of the vestibular system of the inner ear. Thus, motion sickness is never experienced by individuals who have no vestibular function. Older texts often suggest that the cause is over-stimulation of the vestibular system, but current opinion favours an explanation in terms of what has been called the sensory conflict theory of motion sickness. This is based on the notion that the stimuli which cause motion sickness are those that generate sensations that do not conform with a repertoire of expected sensations that has been built up and stored in the brain on the basis of past experience of the sensory stimulation associated with motion.

Two major sources of sensory conflict are recognized: intra-vestibular and visual–vestibular. Intra-vestibular conflict arises from the fact that there are two types of sensory organ in the vestibular system, the semicircular canals, which respond to rotation, and the otolith organs, which respond to linear motion and to changes in orientation with respect to gravity. If a rotational movement of the head is made, such as pitching the head forward to look down at the ground, the otoliths and canals independently give signals about the magnitude of the movement during and after the motion. Because such head movements are made very frequently, an internal representation of the association between these two signals is built up, so that they are then accepted as compatible. However, if the same head movement is made in a different context, the signals may become incompatible. For example, if such a head movement is made during prolonged rotation on a fairground carousel the sensation of head rotation will be influenced by the rotation of the carousel, resulting in a conflict with signals arising from the otolith organs. Equally, head movements made in space, where gravitational acceleration is almost eliminated, will elicit sensations of turning from the canals, which are not matched by the normal otolithic signals.

Visual–vestibular conflict may arise from the fact that the vestibular stimulation experienced with head motion stimuli is normally associated with a compatible visual impression of movement. If the motion of the visual scene is modified, as it is for example when viewing the world through binoculars from a moving vehicle, the incompatibility between the vestibular sensation and the visual motion sensation may be sufficient to induce motion sickness. Viewing the horizon from the deck of a ship on a rough sea helps to reduce motion sickness because the vestibular sensation is compatible with a stable visual world, whereas viewing the wave motion of the sea itself is likely to generate conflict. Apparent motion of the visual world, as generated by large, projected moving images (e.g. cinerama), in the absence of the head movement that would normally occur when seeing such images, may also provide sufficient conflict to induce motion sickness.

Associated with the concept of the sensory conflict hypothesis is the notion that continued exposure to new combinations of visual and vestibular motion stimuli should lead to adaptation of the internal model and thereby to reduced susceptibility. This is supported by the observation that individuals may initially experience motion sickness on a sea voyage, but quickly adapt and gain their ‘sea-legs’ within a period of 3–6 days. For those susceptible individuals who are continually exposed to provocative motion stimuli it is possible to carry out a process of adaptation based on this principle. For more limited periods of exposure there are some anti-motion sickness drugs (scopolamine, dimenhydrinate) that are effective in preventing the onset of sickness, at least for the period of the journey.

Graham Barnes

See also vestibular system.