Elsevier

Brain Research Bulletin

Volume 47, Issue 5, 15 November 1998, Pages 465-469
Brain Research Bulletin

Original Articles
Evaluating sensory conflict and postural instability. theories of motion sickness

https://doi.org/10.1016/S0361-9230(98)00090-2Get rights and content

Abstract

Two experiments were carried out to evaluate the sensory conflict and the postural instability theories of motion sickness. The central hypothesis of sensory conflict theory is that motion sickness is caused by conflict between the current pattern of sensory inputs about self-movement and the pattern that is expected on the basis of previous experience. A subsidiary hypothesis is that the degree of motion sickness is proportional to the magnitude of sensory conflict. The central hypothesis of postural instability theory is that motion sickness is caused by loss of postural control. A subsidiary hypothesis is that the degree of motion sickness is proportional to amount of postural instability, which can be manipulated by physical restraint. In both experiments there were two levels of sensory conflict and two levels of postural restraint. Dependent variables were latency of onset and severity of motion sickness. The widespread occurrence of motion sickness in both experiments clearly confirmed the main hypothesis of sensory conflict theory. The results from Experiment 1, that there was significantly more motion sickness in the restrained condition, and from Experiment 2, that there was no significant difference in symptoms between the two restraint conditions, provide no support for the subsidiary hypothesis of postural instability theory. Evidence relating to the subsidiary proposition of sensory conflict theory was inconsistent.

Introduction

Motion sickness may be defined in terms of a variable set of responses to the stimulus of real or illusory self-movement. Usually, the four cardinal symptoms (responses) of cold sweating, pallor, nausea and vomiting are preceded by some combination of prodromal features (responses) such as lethargy, salivation, enhanced visceral awareness, difficulty in focussing, dizziness, panting and feelings of warmth.

The almost universally accepted explanation of motion sickness is that it is due to a single necessary and sufficient cause referred to as sensory conflict or sensory mismatch. The theory has been developed in detail by Reason and Brand [2], who summarised it as follows: “situations which provoke motion sickness are all characterised by a condition in which the motion signals transmitted by the eyes, the vestibular system and the nonvestibular proprioceptors are at variance with one another, and hence—and this is the crucial factor—with what is expected on the basis of previous transactions with the environment” (p. 264). Although this is the central proposition of the theory, there are corollary propositions. One of these is that “the extent and severity of symptoms are assumed to be a reflection of the mismatch signal, which in turn is directly related to extent of the mismatch” (pp. 169–170).

Although sensory conflict theory has been widely accepted, one detailed critique of the theory has been published by Stoffregen and Riccio [4] who have also proposed an alternative analysis [3]. Their theory invokes the ideas of postural control, which they define as “the coordinated stabilisation of all body segments” (p. 199), and of postural stability, which they define as “the state in which uncontrolled movements of the perception and action systems are minimised” (p. 202). Their central hypothesis is “that prolonged postural instability is the cause of motion sickness” (p. 205). Among several corollary hypotheses is the suggestion that “Symptoms may … scale directly to the magnitude of instability” (p. 206). They propose that “reductions in demands on postural control should reduce the incidence or severity of motion sickness” (p. 206). Resting the head and lying down are given as examples of ways to reduce the demands on postural control.

There seems to have been almost no published work dedicated to evaluating the postural instability theory of motion sickness, except for one report by Warwick-Evans and Beaumont [5]. This was intended to test the hypothesis from sensory conflict theory that motion sickness is proportional to the magnitude of the sensory conflict and the hypothesis from postural instability theory that reducing the demands on postural control will reduce motion sickness. To reduce demands on postural control, all subjects sat on a hard chair, resting their heads against a restraining wooden block each side and to the rear of their heads at 45° to the sagittal plane and leaning their foreheads on hard plastic goggles set into a heavy rigid framework. While seated, all subjects watched a film previously taken from the eye-level perspective of someone walking inside and outside buildings on the campus. To manipulate the magnitude of sensory conflict, half the subjects were shown the film played at normal speed and half were shown the film speeded up by 20%. Vestibular and somatosensory information did not differ between the two groups. But the faster film speed generated the faster perceptual flow associated with walking at a greater velocity and, in particular, with greater linear and angular accelerations. It therefore produced greater sensory conflict. The results were that 1) all subjects reported symptoms of motion sickness, and 2) there was significantly more motion sickness associated with the normal speed of viewing. The authors interpreted the first result as not supporting the hypothesis that reducing the demands of postural control reduces the incidence or severity of motion sickness; the second result was seen as disconfirming the prediction that motion sickness symptoms are proportional to the magnitude of the sensory conflict.

One criticism of the above study is that there was no condition in which a different level of restraint and hence of postural stability could be compared with the sitting condition for its effect on motion sickness. The following experiment was designed in view of this criticism to compare two levels of postural instability so as to evaluate the postural control hypothesis that motion sickness is a function of postural instability and to investigate further the sensory conflict hypothesis that motion sickness is proportional to the magnitude of the conflict.

Section snippets

Subjects

Forty female students at the University of Southampton participated as subjects. Ages ranged from 18 to 25 years, with a mean of 20 years, 6 months. They were matched in groups of four to within five points on the basis of their scores on Reason’s Motion Sickness Questionnaire, with one member of each group being randomally allocated to each of the four conditions (see next section). They were instructed to report as soon as they experienced any symptoms of motion sickness that had previously

Analysis based on equal visually specified distance travelled

In the following three analyses the data are based on the full running time of each film, 330 s for the fast speed of viewing and 660 s for the normal speed of viewing. Thus, the total “distance travelled” is equal across all conditions.

Hypothesis 1 (motion sickness is proportional to the magnitude of postural instability)

In the equal-time analysis the mean symptom severity scores were 6.50 for the lying condition and 4.95 in the free-standing condition, a nonsignificant difference in the direction opposite to that predicted by the postural instability theory. In the equal distance analysis, the mean symptom severity scores were 7.25 for the lying condition and 7.60 in the free-standing condition, a nonsignificant difference in the direction predicted by the postural instability theory. So, with symptom severity

Subjects

Instructions were as described for Experiment 1 and subjects were selected as described for Experiment 1 but with the additional constraint that they had not participated in the previous experiment.

Design and procedure

As in Experiment 1 there were two independent variables (film speed and postural stability) with two levels of each. But to equate viewing speeds for novelty and realism, the film was shown 20% faster than normal in the high-conflict condition and 20% slower than normal in the low-conflict condition.

Analysis based on equal visually specified distance travelled

In the following four analyses the data are based on the first 600 s of the fast film and on all 900 s of the normal-speed film, so that comparisons are made between subjects who have visually “travelled” the same distance.

Hypothesis 1 (reduced postural instability will reduce motion sickness)

Although the data show that almost all subjects experienced symptoms of motion sickness, all six ANOVAs found no significant difference between the two levels of postural restraint. Hypothesis 1 is therefore not supported.

Hypothesis 2 (motion sickness is proportional to the magnitude of the conflict)

Because all six ANOVAs found no significant difference between the two levels of speed of presentation of the the film, this hypothesis was not confirmed.

General discussion

The intention behind both experiments was to produce motion sickness so that the sensory conflict and the postural instability theories could each be evaluated for their ability to account for the pattern of the results. The widespread occurrence of motion sickness could be accounted for by each theory. The sensory conflict explanation is that in every condition the relationship between visual and nonvisual information about self-movement was at variance (in conflict) with what was expected.

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