BACKGROUND: Among the factors contributing to the occurrence of motion sickness (MS)
symptoms, the presence of smells in the environment is very often reported by individuals who are
susceptible to MS. The aim of the present work was to compare olfactory function in MS sensitive
(MS+) and insensitive (MS-) subjects.
METHODS: Olfactory testing included determination of odor detection thresholds, subjective
evaluation of the quality (intensity, hedonicity and familiarity) of three different odorants
(limonene, isovaleric acid and petrol) as well as measures of skin conductance responses to these
three odorants.
RESULTS: Results showed no difference in olfactory sensitivity between MS+ and MS- subjects.
However, findings of both subjective (odor quality self-rating) and objective (psychophysiological
responses) measures did reveal that the affective response to petrol odor was significantly different
in MS+ and in MS- subjects. Indeed, on a scale from 0 (unpleasant) to 10 (pleasant) MS+ subjects
rated petrol odor as more unpleasant (mean = 2.52) than MS- subjects (mean = 4.15) and rise-time
of skin conductance responses to petrol odor was significantly longer in MS+ (mean = 5.98 sec)
compared to MS- subjects (mean =3.22 sec).
DISCUSSION: Our study delves further into the knowledge of the relationship between motion
sickness and olfaction by underlying a modified olfactory perception in motion sickness sensitive
subjects at both psychophysical and psychophysiological levels.
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Introduction
Travel in modern vehicles (cars, boats, planes, helicopters, spacelab, etc.) can cause a large
panel of symptoms such as nausea, headache, and postural discomfort, which are defined as motion
sickness (see 14 for review). Anyone with a healthy vestibular system can become motion sick with
a sufficiently provocative and long motion stimulus. For this reason, a variety of research has been
conducted to get a better understanding of this problem. In addition of being unpleasant, it has been
highlighted that motion sickness can negatively affect performance of complex tasks requiring
sustained performance 20. In particular, motion sickness can even slow air and in-simulators training
for pilots and aircrews 5.
Three main theories offer a clear explanation about motion sickness mechanisms. The ‘toxin
detector’ hypothesis 32 suggests that the brain can identify any mismatch of expected patterns of
vestibular, visual, and kinaesthetic cues as a sign of central nervous system breakdown and a
possible ingested neurotoxin, and thus will initiate vomiting as a defence mechanism. The
vestibular–cardiovascular reflex hypothesis 4 defines motion sickness as a consequence of visceral
discomfort after activation of vestibular autonomic reflexes due to the convergence of vestibular
and autonomic afferent information in the brainstem and cerebellum. The most widely accepted
theory is the sensory conflict or sensory mismatch theory 29, which postulates that motion sickness
originates from a sensory mismatch between actual versus expected invariant patterns of vestibular,
visual and somatosensory inputs. However, whereas motion sickness mechanisms are now well
understood by the scientific community, there is still no actual behavioral or pharmaceutical
technique that could cure motion sickness without side effects.
Among factors that may contribute to motion sickness, such as no view of the road ahead 34, caloric
food 10,22 or nicotine 16, the presence of strong smells is frequently reported by individuals suffering
from motion sickness as a factor that contributes to the occurrence of motion sickness symptoms.
More precisely, some authors suggested that unpleasant odors could contribute to motion sickness
10,13, 34. A recent study investigating motion sickness in rally car co-drivers showed that on-board
4
smells were one of the three main risk factors for motion sickness 27. Recently, our team 25
evaluated more precisely the relationship between motion sickness and olfaction. In this study,
subjects were submitted to three sessions of nauseogenic stimulations, Off Vertical Axis Rotation
(OVAR), performed under conditions of olfactory stimulation with limonene (pleasant odor), petrol
(unpleasant odor) or distilled water (as a control). Motion sickness was assessed before, during and
after each OVAR session. This study showed that OVAR consistently increased the induced-motion
sickness. However, the addition of an odor that is pleasant or unpleasant during the rotation did not
affect the occurrence of motion sickness symptoms compared to the control condition. This study
also showed that intensity of odors was significantly increased after OVAR and the intensity was
significantly higher for an unpleasant odor than for a pleasant one. For the hedonicity, OVAR made
unpleasant odor more unpleasant while it made limonene odor slightly more pleasant. Paillard et al.
25 highlighted the lack of influence of odors in motion-induced sickness but an impact of a
nauseogenic test on olfactory perception. Following Paillard et al. 25, one could question whether
sensitivity to odors is higher in motion sickness sensitive subjects.
The aim of the present study was to compare olfactory function in motion sickness sensitive
subjects and in motion sickness insensitive subjects using both psychophysical and
psychophysiological measurement. Psychophysical measures included olfactory detection
thresholds tests and self-rating of intensity, familiarity and hedonicity of three odorants. In addition,
psychophysiological responses to these odorants were analyzed using skin conductance
measurements. Indeed, in the olfactory modality, it is well known that skin conductance can be
modulated by the perception of an odorant 30,35 and specifically that it could be modulated by odor
pleasantness 1,2. Thus, it appears relevant to determine whether psychophysiological responses to
odorants differ between motion sickness sensitive and insensitive subjects.
Methods
Subjects
5
The inclusion criteria of the present study were: i) to be non-smokers; ii) to report normal smell
sensitivity and no history of nasal/sinus disease or extensive exposure to chemicals with potential
toxicity; (iii) to be free of any vestibular and neurological disorders.
A group of 85 healthy volunteers (students from the University of Franche-Comté) who fit with
these inclusion criteria were asked to complete the Motion Sickness Susceptibility Questionnaire
(MSSQ12). The first stage of the study was a triage phase where we made sure that our initially
selected subjects has either a MSB score equal or higher than 15 (thus belong to the MS+ group) or
below 2 (thus belong to the MS-group). This criterion has been decided according to Golding15.
Out of these 85 students, 42 volunteers were asked and agreed to take part in the second stage of the
study, i.e. olfactory testing. The sample of subjects included 34 women and 8 men and their age
ranged from 20 to 30 (mean age 22 years 4 months).
The MS+ group, i.e subjects who are very sensitive to motion sickness included 21 subjects (19
women and 2 men, mean age 22 years 5 months; MSB score range between 15 and 40.50). The MSgroup,
i.e subjects who are not sensitive to motion sickness included 21 subjects (15 women and 6
men; mean age 22 years 3 months; MSB score range between 0 and 2).
The study was reviewed and approved by the local ethics committee and declared to the national
authority (N° UF: 1013; DGS 2006/0494) in accordance with the Declaration of Helsinki on
biomedical research involving human subjects. Participation required the completion of an informed
consent form.
For electrodermal recordings 10 subjects in the MS+ group and 12 subjects in the MS- group were
excluded due to low skin conductance responses (SCR <0.02 μS) as described below, or due to the
lack of distinct SCRs during the entire experiment. Thus, 11 subjects in the MS+ group and 9
subjects in the MS- group were available for skin conductance data analyses.
Materials
6
Olfactory detection thresholds were determined using n-butanol [C4H10O; molecular weight =
74,12] (Sigma-Aldrich, France). A dilution series (factor 2) was prepared in odorless mineral oil
(Sigma-Aldrich, France). After successive dilutions, the full series include steps 1 to 25 (step 25 is
the highest concentration). Four milliliters of each concentration was placed into glass tubes (7,5 cm
high, 1 cm in diameter at the opening). Another tube was filled with 4 mL of mineral oil only.
For subjective ratings and recording of skin conductance responses three specific odorants were
used: (R)-(+)-limonene [C10H16; molecular weight = 136,23] (Sigma-Aldrich, France), a pleasant
orange-like odor; isovaleric acid [C5H12O2; molecular weight = 102,3] (Sigma-Aldrich, France), an
unpleasant cheesy odor and petrol as a travel related smell. The dilutions used in our study have
been determined according to a pretest carried out on ten subjects. The dilutions that reach a
moderate intensity perception have been chosen for the tests. Specifically, limonene was used
without dilution (100% of the stock solution), while isovaleric acid and petrol were diluted at 50%
in mineral oil and at 25% in water respectively. Nasal stimuli were presented in glass bottles (6 cm
high, 2.5 cm in diameter at the opening) filled with ten milliliters of each solution.
Procedure
The experiment was carried out in two separate sessions. The first session was dedicated to the
psychophysiological evaluation of olfactory function. Olfactory detection thresholds to n-butanol
were determined using a classical ascending binary (stimulus vs. blank) forced-choice method. A
trial consisted in the presentation of two tubes, one being the blank (mineral oil) and the other
containing the dilution of the odorant (n-butanol). The subject indicated which one of the two
randomly presented tubes contained the odorant. Even no sensations were perceived or if no
difference was apparent between the tubes, the subject was required to choose one tube or the other.
No feedback was given regarding the correctness of the responses. Testing began at the weakest
concentration so as to ovoid olfactory receptor's saturation. For each concentration, the test was
performed three