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Motion sickness in public road transport: The relative importance of motion, vision and individual differences.

Mark Turner [*]

Department of Psychology, University of Portsmouth, UK

Human Factors Research Unit, Institute of Sound and Vibration Research, University of Southampton, UK

The relative importance of vehicle motion, a view of the road ahead and passenger characteristics in the causation of motion sickness in road transport has been investigated using survey data from 3256 coach passengers and measurements of coach motion. Overall, 28% of passengers said they felt unwell during coach travel. Prior experience of sickness, travel regularity and age were the factors most highly correlated with illness. Increased vehicle motion and poorer forward vision also correlated with illness. Little difference in illness was apparent with a good view of the road ahead, regardless of motion exposure, although vision alone was not sufficient to eliminate passenger sickness entirely. The results suggest that travel sickness could be significantly reduced by improved forward external vision and that improved forward vision may be particularly beneficial for individuals new to coach travel and for those who travel less often.

Motion sickness occurrence is related to motion exposure (e.g. Golding & Markey, 1996; Lawther & Griffin, 1988; McCauley, Royal & Wylie, 1976). However, motion of the head or body is not a necessary requirement for the provocation of motion sickness. Movement of all or part of the visual scene can be a sufficient stimulus to cause sickness in the absence of bodily motion (Griffin, 1990). Visual input has been shown to increase (e.g. Lackner & Graybiel, 1979) and decrease (e.g. Rolnick & Bles, 1989) the sickness occurrence during exposure to nauseogenic motion. However, vision is not an essential, but rather a secondary aetiological factor in sickness occurrence. The visually impaired are known to be susceptible (Graybiel, 1970) whilst a functioning vestibular system is necessary for motion sickness to occur: Cheung, Howard & Money (1991) demonstrated that a rotating visual field capable of causing sickness in normal participants did not lead to sickness in labyrinthine-defective participants.

The most popular explanation of motion sickness, the sensory conflict theory (Reason & Brand, 1975; Oman 1990) suggests that motion sickness results from the misinterpretation of concurrent sensory input from the visual and vestibular systems. In particular, stimulation of the otolithic membranes of the vestibular system may lead to the incorrect identification of low frequency (below 0.5 Hz) translational forces occurring during road travel as a change in body orientation or tilting of the head with respect to gravity (Griffin, 1990). This interpretation of the translational motion experienced will be inconsistent with cues obtained from the visual system, which is assumed to indicate the true orientation of the body, or from the semicircular canals of the vestibular system, which are assumed to be insensitive to translational motion. Increases in sensory conflict are thought to produce proportional increases in motion sickness (Oman, 1990). If the magnitudes of low frequency motion experienced by road pass engers are regarded as a function of the sensory conflict produced, the theory would predict that the greater the amount of low frequency motion experienced during road travel, the greater the incidence of motion sickness.

According to the sensory conflict theory, motion sickness may be explained by more than one type of conflict, with the dominant cause of sickness depending on the specific conditions of travel and the assumed significance of visual or vestibular stimulation. Where passengers are seated to look from the side or rear window of a moving vehicle, visual information obtained from viewing the passing landscape or other moving vehicles will provide cues to motion which conflict with the cues sensed (or expected) by the vestibular system. Under these circumstances, the vestibular apparatus is assumed to indicate the true nature of vehicle motions whilst the relative motion between the observer and the external visual scene provides misleading visual cues (Reason & Brand, 1975). When the passenger has no view of the external environment, for example when reading, travelling in an enclosed vehicle or during night travel, a stationary visual field is perceived because the passenger and the visual reference frame share the same relative motion. Vestibular cues to the motion of the vehicle will then conflict with the absence of expected visual cues to motion. Forward-facing seats that provide good visibility of an external, stable visual field are predicted to yield a better correlation between visual and vestibular cues to motion and may lead to a lower incidence of sickness. Under these circumstances, movement of the vehicle will be compared to a fixed, visual reference frame that is expected to facilitate the interpretation of visual information as motion of the observer with respect to a stable environment.

Most studies examining motion sickness occurrence with different visual conditions have been constrained to laboratory settings using highly provocative tests which are not typical of the motion and environmental conditions experienced during everyday travel. Such experimental studies tend to adopt a passive view of the individual (Yardley, 1992) and ignore the complex combined effects of voluntary motion and the characteristics or experience of participants. Some evidence suggests that individuals differ in their utilization of visual cues during travel. Novice and experienced drivers have been shown to attend to different aspects of the visual scene when driving (Crundall & Underwood, 1998) whilst differences in visual strategies have been related to motion sickness occurrence in the laboratory. Yardley, Lerwill, Hall & Gresty (1992) report that susceptible individuals are less able to ignore misleading visual information during exposure to nauseogenic motion.

Few studies have examined motion sickness occurrence during land based travel. Those which have, tend to be small in scale and based on extreme rather than typical driving manoeuvres. Probst, Krafczyk, Buchele & Brandt (1982) reported that motion sickness was least in car passengers exposed to repeated acceleration and braking when a clear external view was provided, compared to eyes closed or when reading from an artificial, stationary visual field. This led the authors to conclude that providing ample peripheral vision of the moving surroundings was the best strategy for alleviating car sickness. However, Stern, Hu, Anderson, Leibowitz & Koch (1990) suggest that sickness can be reduced by fixating a central target in a moving visual field or by restricting the visual field to a 15 degree angle, compared with viewing the whole visual field. Visual field movement tends to be more effective in inducing illusory self-motion (vection) and also motion sickness when presented to peripheral vision (Kano, 1991; Yar dley, 1992), as may often be the case in moving vehicles. No studies have investigated the effect of visual reference and its relationship with motion exposure on sickness occurrence in everyday travel conditions. Consequently, the influence of these factors on travel sickness remains unclear.

Using data obtained from a large-scale survey of coach passengers, the aim of the current paper was to examine the relative importance of motion and visual stimuli in the causation of motion sickness in road transport. The paper seeks to address whether visual cues are sufficient to alleviate the occurrence of sickness during coach travel when a passenger's visual experience is not experimentally enforced. The relationships between different vehicle motions and the occurrence of sickness found in this study have been reported elsewhere (Turner & Griffin, in press).

The importance of other non-motion, secondary aetiological factors known to influence sickness occurrence such as age, gender and experience (e.g. Money, 1970; Griffin, 1990) are also considered. The results obtained are used to determine a composite model for the prediction of motion sickness amongst passengers in public road transport.

Method

Passenger survey

A questionnaire survey of 3256 passengers was undertaken on 56 coach journeys. Information on passenger travel sickness occurrence was gathered by means of a single page questionnaire. An 8-point checklist was created from the more detailed diagnostic criteria for grading motion sickness suggested by Graybiel, Wood, Miller & Cramer (1968). Passengers were asked to report on whether they had experienced nausea, vomiting, dizziness, drowsiness, mouth watering, pallor, headaches or feeling hot at any stage during the journey. To assess subjective feelings of illness independently of specific symptoms, a 4-point Illness Rating (IR) was used. Passengers were asked to describe which of the following four statements most accurately described the worst they had felt during the journal: 'I felt all right' (rated as 0); 'I felt slightly unwell' (rated as 1); 'I felt quite ill' (rated as 2); 'I felt absolutely dreadful' (rated as 3). The validity of this scale has been examined in a study by Lawther & Griffin (1986).

Visual reference was assessed by asking passengers to rate subjectively how well they could see the road ahead during their journey as 'not at all', 'not very well', 'quite well' or 'extremely well'. For the purposes of subsequent analyses, passenger responses to this item were categorized as 'no view of the road ahead' (not at all), 'poor view of the road ahead' (not very well), or 'good view of the road ahead' (quite well and extremely well). Information on gender, age, seating position, travel regularity and susceptibility to motion sickness in other travel environments was also gathered. Passengers were asked to indicate whether they had previously experienced motion sickness in any of five transport environments including cars, coaches, ships, aircraft and trains. Of those surveyed, 1361 (41.8%) had never previously experienced sickness and 1895 (58.2%) had previously experienced sickness in one or more of the five modes of transport considered.

Questionnaires were administered approximately 5 min before the end of each journey. Of those surveyed, 1594(49%) were male and 1662(51 %) were female. Passenger ages ranged from 8 to 80 years (M = 28.3). All journeys were by private hire arrangement between the coach company and an organizing body, and would have taken place without the conduct of the present study. All passengers participated voluntarily and were free to withdraw from the study without prejudice at any stage. This was made explicit to passengers prior to the administration of the questionnaire. The study met with the requirements of the Human Experimentation Safety and Ethics Committee of our organization.

Physical measures

The occurrence of low frequency accelerations (below 0.5 Hz) are thought to be an important factor in the occurrence of motion sickness (Griffin, 1990). Such motions occur in road coaches mainly in the fore-and-aft and lateral directions (Turner, 1994). Continuous measurements of vehicle acceleration in the frequency range 0 to 2 Hz were made throughout all 56 journeys using translational accelerometers aligned with the fore-and-aft and lateral vehicle axes. Accelerometers were attached rigidly to the vehicle floor. The resulting acceleration-time histories were then used to calculate fore-and-aft and lateral Motion Sickness Dose Values [1] which represent the cumulative magnitude of nauseogenic motion experienced during each journey. Air temperature was recorded on the lower and upper vehicle decks (where relevant) at the beginning, mid-point and end of each journey and the mean temperature was calculated. To investigate time-of-day effects, the departure time and duration of each journey were also recorded.

Results

Overall, 22% of passengers said they felt 'slightly unwell', 4% felt 'quite ill' and 2% felt 'absolutely dreadful'. Table 1 shows the distribution of motion sickness symptoms experienced by passengers. Not all passengers who experienced symptoms of motion sickness also reported subjective feelings of illness. Passengers who reported vomiting, nausea, dizziness, headaches or pallor were most likely to report concurrent feelings of illness. Reports of drowsiness or feeling hot were equally frequent among passengers who did and did not report illness.

Although IR is a convenient method of describing motion sickness severity, it cannot be used as the dependent variable in a parametric regression model due to the strong positive skew of the response distribution and the restricted range of possible scores. Illness rating data were re-coded as a dichotomous dependent variable whereby an IR of 0 was associated with the value 0, and an IR of greater than 0 was associated with the value 1. Overall, 2330 (71.6%) of coach passengers reported not feeling ill (IR = 0) and 926 (28.4%) of passengers reported some level of illness (IR [greater than] 0). With illness data coded in this manner, a stepwise logistic regression analysis was performed which allowed the probability of illness occurring during coach travel to be predicted from categorical measures of coach motion, vision and other individual passenger and environmental characteristics. The larger the probability value, the greater the likelihood of illness occurring (Table 2). The Hosmer-Lemeshow goodness-of- fit [X.sup.2] test was calculated for the logistic model ([X.sup.2](8, N = 3256) = 2.7, p = .95, n.s.). This suggests passenger illness predicted by the model did not differ significantly from observed illness responses.

The significance of the relationship between illness and each independent variable may be ascertained by examining changes in the deviance of the logistic model (Hosmer & Lemeshow, 1989). When a variable is entered or removed from the model, a change in the deviance of the model occurs which is represented by the likelihood ratio [X.sup.2] value (-2 Log LR). A statistically significant likelihood ratio [X.sup.2] value indicates a significant improvement in illness prediction compared to a regression model based on a constant value alone (i.e. a model which suggests no relationship between dependent and independent variables). No significant relationships were found between the probability of illness and fore-and-aft coach motion, time of departure or air temperature during the journey. All other variables were found to have a significant impact on illness probability (Table 2).

The contribution of each variable to the overall regression, and consequently its relative importance to the occurrence of motion sickness, may be ascertained by examining the partial correlation, R between the dependent variable and each independent variable. In logistic regression, a positive partial correlation indicates that as the variable (e.g. lateral motion) increases in value so does the likelihood of sickness occurring. For negative partial correlations, the opposite is true.

Individual passenger characteristics (previous susceptibility, regularity of travel and age) contributed most to the prediction of illness. The probability of illness was greater for passengers who had previously felt ill during travel, for passengers who made few coach journeys per year, and for younger passengers.

The logistic model suggests lateral coach motion to be more influential than self-ratings of forward visibility in determining illness probability. The probability of illness increased substantially for lateral MSDVs above 16 [ms.sup.-1.5] (see Table 2). Illness probability reduced for passengers with a good view of the road ahead. Differences in seating also affected illness. The probability of illness increased towards the rear of the vehicle and was greater for passengers sitting in aisle seats as opposed to window seats. The probability of illness was greater for female than for male passengers.

The effect of vision was further examined by comparing mean illness ratings as a function of forward visibility and coach motion magnitude (Fig. 1). No differences in illness ratings were found as a function of forward visibility for coach motion below 12 [ms.sup.-1.5] MSDV ([X.sup.2](6, N = 815) = 6.59, p = .36, n.s.). Illness ratings were significantly lower for passengers with good forward visibility compared to passengers with poor or no forward view at motion magnitudes of 12-14 [ms.sup.-1.5] MSDV ([X.sup.2](6, N = 819) = 30.8, p [less than].001). Differences in illness with forward visibility became more pronounced for higher magnitudes of lateral coach motion (14-16 [ms.sup.-1.5] MSDV, [X.sup.2](6, N = 813) = 53.5, p [less than].001; above 16 [ms.sup.-1.5] MSDV, [X.sup.2](6, N = 809) = 53.5, p [less than].001).

Mean illness ratings also varied as a function of forward vision and travel regularity. Differences in mean illness ratings with forward visibility were greatest when 0-2 coach journeys were completed per year ([X.sup.2](6. N = 1282) = 67.2, p [less than].001), but the effect of vision was reduced when a greater number of journeys were undertaken (Fig. 2).

Discussion

The likelihood of motion sickness occurrence during road travel increases with increasing exposure to low frequency lateral acceleration and decreases when passengers are provided with a good forward external view. The results suggest that the influences of motion and vision are interactive: vision may moderate rather than provoke sickness induced by motion during travel. No difference in illness was found on journeys involving little lateral motion, regardless of the external view provided. Illness levels on journeys involving higher levels of lateral motion, were exacerbated when no forward view, or only a poor forward view, was provided. Little difference in illness was apparent when forward visibility was good, regardless of motion exposure. However, forward visibility does not offer immunity from illness: 229 passengers became ill even with what they categorized to be a good view of the road ahead.

Ostensibly, these findings are consistent with the predictions made by the sensory conflict theory of motion sickness. In a moving vehicle, when no external stable visual reference is afforded, a visuo-vestibular conflict will arise between the internal, static visual field and the dynamic motion sensed by the vestibular system. Such conditions in the present study were found to result in an increased likelihood of sickness occurrence, although this does not necessarily preclude other theoretical explanations.

Visual reference may explain some of the variations in illness as a function of seat location. Seating at the front of a vehicle provides the passenger with easy access to a stable, external visual reference from which appropriate cues to motion and orientation may be gained. Seating towards the rear of modern coaches affords little external forward visibility to the passenger. At the rear of a coach, external visual reference will be restricted to transient, fast moving scenes viewed from side windows that provide few visual cues to orientation. Such visual field movement may be mainly presented to the peripheral visual field of forward-facing passengers. In the present study the probability of illness among passengers sitting at the rear of the coaches was more than twice that of passengers sitting at the front. However, changes in vision are not the only possible explanation of greater sickness at the rear of coaches: during cornering, passengers at the rear of the vehicle are also exposed to greater late ral acceleration than passengers located further forward.

An interaction was observed between the quality of the external visual field and travel regularity. Differences in illness levels as a function of forward visibility were pronounced for inexperienced travellers but were relatively small for experienced travellers. This could be explained by a secondary role for vision in the development of coach sickness: experienced travellers are better able to cope with the absence of visual information due to prior experience of the motion stimulus (i.e. habituation). Alternatively, the use of visual information may change with increased familiarity with the motion environment. The latter interpretation is consistent with previous research relating motion sickness to the postural behaviour of passengers. Fukuda (1976) observed that bus drivers and experienced passengers, who rarely experience sickness, tend to adopt a 'centripetal posture', rotating their head and upper torso to face the direction of rotation as a vehicle corners. In contrast, in experienced passengers w ere found to adopt a 'centrifugal posture', looking away from the direction of travel. Further data reported by the same author demonstrated that fixing a participant's head to face the direction of motion during laboratory exposure to rotation led to a significant reduction in visual disturbances arising from postrotatory nystagmus. Whilst the occurrence of sickness during road travel is clearly related to vision, the utility of visual information is not simply determined by the availability, or restriction, of visual cues but also by the individual selection of appropriate cues for judging motion and orientation. Due to the scale of the present study, the collection of information on vision was limited to subjective ratings of visual reference rather than quantitative analysis of eye movements, visual attention or visual field angles. Further research into physical measures of visual reference is needed to provide information on the visual moderation of sickness occurrence in travel environments.

Information from provocative motion tests and motion sickness history questionnaires has been used previously to predict susceptibility to motion sickness (e.g. Lin & Reschke 1987; Reschke 1990). In the present study, composite information on individual passenger characteristics, motion exposure and nonmotion aspects of the travel environment was used to determine the relative importance of different travel factors in the occurrence of motion sickness. Whilst the role of motion and vision in motion sickness occurrence are unequivocal, the influence of both factors when predicting illness probability was relatively small compared to individual passenger characteristics, such as previous experience of motion sickness, travel regularity and age. Prior experience of motion sickness was the most influential factor in predicting illness probability, in agreement with Homick, Reschke & Vanderploeg (1987) who found the best predictor of space motion sickness to be sickness occurrence during previous spaceflights.

Conclusions

Motion and vision influence motion sickness in public road transport; the influences of motion and vision have been found to be interactive. During coach travel, passenger illness was significantly reduced with a good view of the road ahead. Illness experienced by infrequent coach travellers was dependent on the extent of the view of the road ahead, suggesting that improved forward vision will be particularly beneficial for individuals new to coach travel. However, a view of the road ahead was not sufficient to eliminate passenger sickness altogether, for any level of coach motion. Predictions of motion sickness based on only single factors, such as the motion exposure or the visual field, will exclude individual variables and other environmental variables which influence the occurrence of sickness in travel environments.

Acknowledgements

This research was supported by the Engineering and Physical Sciences Research Council and was made possible by the kind cooperation of Red Ensign Coaches of Portswood, Southampton and Eassons Coaches of Itchen, Southampton.

(*.) Requests for reprints should be addressed to Mark Turner, Department of Psychology, University of Portsmouth, King Henry Building, King Henry I Street, Portsmouth P01 2DY, UK (e-mail: Mark.Turner@port.ac.uk).

(1.) British Standard 6841 (British Standards Institution, 1987) defines an acceleration frequency weighting, [W.sub.t] for assessing low frequency motion with respect to motion sickness. The weighting is defined over the range 0.1 to 0.5 Hz and assumes greatest sensitivity to acceleration in the frequency range 0.125 Hz to 0.25 Hz. The [W.sub.f] weighting and its associated formulation of motion dose, the Motion Sickness Dose Value (MSDV), are specifically intended for the evaluation of vertical motion. MSDV is used more generally here to report exposures to non-vertical motion.

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Number of passengers reporting each symptom as a function of concurrent
reports of illness
                       Symptom reported
                       Feeling
                         hot            Headache Nausea  Drowsiness Dizziness
Passengers               432              430      411      253        171
 reporting illness     (13.3%)          (13.2%)  (12.6%)   (7.8%)    (5.3%)
 (IR [greater than] 0)
Passengers not           496              137       6       255        31
 reporting illness     (15.2%)           (4.2%)   (0.2%)   (7.8%)    (1.0%)
 (IR = 0)
                       Increased
                       salivation Pallor Vomiting
Passengers                123       79      55
 reporting illness       (3.8%)   (2.4%)  (1.7%)
 (IR [greater than] 0)
Passengers not            111       16      0
 reporting illness       (3.4%)   (0.5%)  (0.0%)
 (IR = 0)
Note: Percentage of total survey population are shown in
parentheses (N = 3256).
Forward step, likelihood-ratio logistic regression model for the prediction
       of illness from individual and environmental characteristics
                 Step    -2 Log LR
Variable        entered ([X.sup.2]) d.f.    Significance        R
Previous           1      231.33     1   p [less than] .001   .22
 susceptibility
Regularity of      2       93.94     2   p [less than] .001 --.16
 travel
Lateral            3       77.25     3   p [less than] .001   .09
 motion
Age group          4       69.59     1   p [less than] .001 --.13
Seat position      5       20.86     2   p [less than] .001   .06
Forward            6       20.58     2   p [less than] .001 --.06
 visibility
Seat type          7       16.59     1   p [less than] .001 --.06
Gender             8        4.36     1        P = .04         .03
Fore-and-aft      --        6.48     3        p = .09          --
 motion
Time of           --        1.84     1        p = .17          --
 departure
Air               --        0.43     1        p = .51          --
 temperature
                                                       Probability
Variable                      Category             N   of illness
Previous        Never felt ill during travel      1361     .16
 susceptibility Previously felt ill during travel 1895     .53
Regularity of   Two or fewer journeys per year    1282     .32
 travel         three to six journeys per year    1061     .29
                More than six journeys per year    913     .03
Lateral         Below 12 [ms.sup.-1.5]             815     .15
 motion         12-14 [ms.sup.-1.5]                819     .16
                14-16 [ms.sup.-1.5]                813     .20
                Above 16 [ms.sup.-1.5]             809     .77
Age group       Under 15                          1473     .48
                15 or over                        1783     .11
Seat position   Front of vehicle                   928     .16
                Middle of vehicle                 1329     .22
                Rear of vehicle                    999     .38
Forward         No view of road ahead              944     .25
 visibility     Poor view of road ahead           1102     .24
                Good view of road ahead           1210     .10
Seat type       Aisle seat                        1505     .24
                Window seat                       1751     .16
Gender          Male                              1594     .19
                Female                            1662     .24
Fore-and-aft    Below 9 [ms.sup.-1.5]              825     --
 motion         9-11 [ms.sup.-1.5]                 809     --
                11-13 [ms.sup.-1.5]                782     --
                Above 13 [ms.sup.-1.5]             840     --
Time of         a.m.                              2273     --
 departure      p.m.                               983     --
Air             21 [degrees]C or below            1447     --
 temperature    Above 21 [degrees]C               1908     --
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Author:Turner, Mark; Griffin, Michael J.
Publication:British Journal of Psychology
Article Type:Statistical Data Included
Geographic Code:4EUUK
Date:Nov 1, 1999
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