Running head: THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 1. The Influence of Self-Motion Expectation on Motion Sickness Severity

Transcription

1 Running head: THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 1 The Influence of Self-Motion Expectation on Motion Sickness Severity Marius M. Keppel Utrecht University Author Note This paper is the master thesis of Marius M. Keppel. It serves as the end product of the master study Applied Cognitive Psychology at Utrecht University. The corresponding project was supervised by prof. dr. Jelte E. Bos, researcher at TNO Human Factors Soesterberg and VU University Amsterdam, and Stella F. Donker, researcher at Utrecht University. Assignment: Master s thesis ECTs: 27.5 Date: 7 July 2017 First reviewer: dr. Stella F. Donker Second reviewer: dr. Anouk Keizer Student: Marius M. Keppel Student number:

2 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 2 Abstract According to the spatial orientation motion sickness model, motion sickness occurs when the sensed vertical as determined by sensory information is at variance with the expected vertical based on previous experience. Since it is known that cognition can affect self-motion perception, the expected vertical was manipulated through imagination to discover whether motion sickness severity could be increased with incongruent imagination, and decreased with congruent imagination, both with respect to actual self-motion. Subjects were blindfolded, and exposed to lateral motion on a parallel swing. Motion sickness severity was found to increase over time, in the incongruent condition only. However, post hoc analyses showed no differences in motion sickness severity across individual time points. The findings are discussed in terms of statistical procedures and the experimental setup. Recommendations for future research are proposed. Keywords: cognition, imagination, motion sickness, passive self-motion

3 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 3 The Influence of Self-Motion Expectation on Motion Sickness Severity Motion sickness is a malady caused by certain kinds of motion (Money, 1970). For example, day-to-day motion as sensed in busses, ferries, and transport planes can be experienced as provocative (Lawther, & Griffin, 1988; Turner, 1999; Turner, Griffin, & Holland, 2000). Motion sickness symptoms include dizziness, sweating, tiredness, and nausea (Bos, MacKinnon, & Patterson, 2005), and may result in substantial impairment in task performance (Bos, 2015; Lackner, 2014; Stevens & Parsons, 2002). These consequences of motion sickness may affect many, since it is suggested that almost all healthy individuals, with varying thresholds of susceptibility, can experience motion sickness in specific conditions (Murdin, Golding, & Bronstein, 2011). Even though motion sickness medication is found to be effective, it may elicit a variety of negative side effects such as a dry mouth, and drowsiness (Brainard & Gresham, 2014), and an overall reduction in task performance (Cowings, Toscano, DeRoshia, & Miller, 2000). Therefore, broadening knowledge on motion sickness occurrence could be beneficial for optimizing both human behaviour and environment with respect to provocative situations. Several motion sickness models have been developed, based on a variety of theories, to describe the mechanisms underlying motion sickness occurrence (see Golding, 2006, for an overview). Nonetheless, still no consensus on the specific mechanisms behind motion sickness has been achieved (Golding, 2006). A widely accepted motion sickness model is based on sensory conflict or sensory mismatch (Oman, 1987; Reason & Brand, 1975). An example of a sensory mismatch is reading a newspaper while sitting in a bus. One s visual input is then a still image of the newspaper while, in contrast, the vestibular input is constructed by different kinds of motion as produced by a travelling bus. Sensory mismatch models hold that such a prolonged mismatch would result in motion sickness. One version of this theory, called the subjective vertical mismatch theory (Bles, Bos, de Graaf, Groen, & Wertheim, 1998), ascribes the occurrence and severity of motion sickness to a mismatch between vertical-sensory information, and verticality expectation. More specifically, it states that all situations which provoke motion sickness are characterised by a condition in which the sensed vertical as determined on the basis of integrated information from the eyes, the vestibular system and the nonvestibular proprioceptors is at variance with the subjective vertical as expected from previous experience (Bles et. al, 1998). Even though this model ascribes occurrence of motion sickness merely to the difference between the sensed and subjective

4 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 4 vertical, it is able to account for motion sickness in different kinds of situations (Bles et. al, 1998; Bles, Bos, & Kruit, 2000). The subjective vertical mismatch theory is further elaborated based on recent insights, and is currently known as the spatial orientation and motion sickness model (SO motion sickness model; Bos, Bles, & Groen, 2008). This model is shown in Figure 1. ud P C m ue B u = one s desired body state = preparatory phase = controller = efferent signals = external disturbance = body = resulting position som vis vest LP us c K = somatosensation = visual sensation = vestibular sensation = low-pass filter = position sensation = conflict = weighed amount H MS EM SV MP = transfer function for motion sickness = motion sickness = eye movements = subjective vertical = motion perception = summation of inputs, or complex (weighted) functions Figure 1. Overview of the spatial orientation and motion sickness model. Within this model, the grey areas represent the two spatial orientation submodels, which are essential for describing motion sickness. Within the internal model, all functions have been indicated with a prime. Adapted from A theory on visually induced motion sickness by J.E. Bos, W. Bles, and E.L. Groen (2008), Displays, 29(2). p. 49. Copyright 2007 by Elsevier Science Incorporation. The main input of the SO motion sickness model is a desired body state (ud). A body state has a position in space, and an orientation with respect to external forces (e.g., an assumption of verticality with respect to gravity). This state enters a prepatory phase (P), in

5 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 5 which a controller in the brain (C) generates efferent signals (m). Those efferent signals are sent to the required muscles in the body (B) to achieve the desired body state. The body, however, is subject to external disturbances from the environment (u e ), such as when being pushed by someone. With those disturbances taken into account, this process results into an actual body state (u). The actual body state is then detected by means of somatic, visual, and vestibular sensory information (som, vis, vest). Visual and vestibular sensory information is then processed through a low-pass filter (LP). In this way, gravito-inertial acceleration (the sum of gravity, and accelerations due to motion) is divided into a motion and a gravity component. This division is needed to control both body and eye movements (Bos & Bles, 2002). The brain is provided with the resulting sensory information (us) in the form of afferent signals. In this way, one is provided with sensed verticality in the current body state. When one is about to initiate a movement, the central nervous system calculates a prediction of the expected resulting self-motion. When efferent signals are sent to the muscles to achieve a desired body state, therefore, a copy of those signals, called efference copies, is generated. These efference copies (m ) are then sent to an assumed internal model of the actual body and sensory dynamics. This internal model can be thought of as a neural store that is created and updated based on experience, and it uses its experience to predict what the afferent signals of the desired body state will be like. The internal model includes a copy of the primary path, consisting of B, som, vis, vest, and LP, to construct this expectation. Using the efference copies, the internal model provides an estimate of the body state (u ), which is then compared to the desired body state (ud). This results in an error signal (e) that feeds back into the prepatory phase to correct an initiated movement so that the desired body state can be achieved more precisely. Furthermore, the estimation (u ) results in the subjective vertical (SV). The subjective vertical is verticality as someone would expect it to be in the near future. Besides forming an expectation about the afferent signals of a future body state, the internal model has another main goal. If one senses the current body state by means of afferent signals (us), it may be in conflict with the expectation about that body state by the internal model (us ). This may happen, for example, by a prolonged external disturbance from the environment. This conflict (quantified by c = us us ) is found to be directly correlated with motion sickness (MS) through a transformation matrix (H). To minimise this sickness, the conflict is weighted by a weighting coefficient, or Kalman gain (K). However, whether this weighting coefficient should

6 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 6 be small or large in a given situation may be unknown to the central nervous system in a specific situation. Therefore, it is not guaranteed that the conflict is reduced to a desired level, and motion sickness may thus still occur. A lot of research has been conducted to study the physical-sensory aspects of motion sickness (e.g., Hettinger & Riccio, 1992; Hu, Grant, Stern, & Koch, 1991; Lawther & Griffin, 1987; O'Hanlon & McCauley, 1973). However, verticality expectation is thought to incorporate cognitive inputs as well (Bos & Bles, 1998; Bos, Bles, & Groen, 2008). Multiple studies have shown that self-motion perception can be affected by cognitive processes. Wertheim, Mesland, and Bles (2001) found in their study that subjects knowledge about the experimental setup of their study has a direct influence on subjects otolith responsiveness to motion. Furthermore, research shows that self-motion imagination that is incongruent with actual self-motion leads an altered self-motion perception compared to self-motion imagination that is congruent with actual self-motion (Mertz, Belkhenchir, & Lepecq, 2000; Nigmatullina et al., 2015). Therefore, in terms of the SO motion sickness model, one could assume that verticality expectation could be manipulated through cognitive input. In that way, cognitive input could influence verticality expectation in such a way that it differs from the sensed vertical, which, in turn, should lead to (more) motion sickness. Also, cognitive input could influence the expected vertical in such a way that it equals the sensed vertical, which, in turn, should lead to no (or less) motion sickness. If this assumption could be validated in an experiment, it would add to the plausibility of the use of an internal model by our central nervous system and the validity of the SO motion sickness model as shown in Figure 1. To test this hypothesis in the current study, blindfolded subjects were exposed to passive lateral motion on a parallel swing. They were tasked to imagine themselves moving passively in cycles towards two objects in their environment that were communicated to them in two conditions. In the congruent condition, the communicated object was the object that subjects were actually moving towards. In the incongruent condition, the communicated object was the object they were actually moving away from. The main hypothesis of this study is that cognitive information that is incongruent with respect to actual self-motion leads to more motion sickness, while cognitive information that is congruent with respect to actual self-motion leads to less motion sickness.

7 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 7 Method Subjects A total of sixteen subjects participated in this study, three males, thirteen females, with an average age of 23.6 ± 5.7 years. They were recruited via an online recruitment platform and posters that were spread throughout different universities. Subjects were selected by means of a screening procedure. Firstly, candidate subjects completed the Motion Sickness Susceptibility Questionnaire (MSSQ; Golding, 1998) in Dutch (Appendix A). The MSSQ predicts motion sickness due to a variety of provocative environments and consists of 24 questions. Candidate subjects answered how often they felt sick in several situations on a 5-point Likert scale, such as Over the last 10 years, how often you felt sick or nauseated [in] Buses or Coaches. If candidate subjects stated that they never experienced discomfort caused by motion (i.e., MSSQ = 0), they were excluded from participation. Furthermore, candidate subjects completed Bett s Questionnaire upon Mental Imagery (QMI; Sheehan, 1967) in Dutch (Appendix B), which measures the ability to imagine with respect to a variety of sensory modalities. Candidate subjects answered how clear and vivid their imagination was in 35 specific examples on a 7-point Likert scale, such as How clear and vivid can you imagine what you do when you Climb the stairs? If candidate subjects stated that they could not imagine clearly and vividly (i.e., QMI = 35), they were excluded from participation. Other exclusion criteria were being aware of having a dysfunctional vestibular system, using medication affecting alertness or balance, and being aware of having trouble differentiating left from right. Candidate subjects were questioned on these matters directly during the screening procedure. If candidate subjects passed the screening procedure, they were asked to participate in the experiment in two consecutive sessions. Between both sessions, at least 24 hours passed to prevent motion sickness aftereffects from interfering with motion sickness symptoms in the second session. As compensation, subjects received 10 euros per session, irrespective of whether the session was completed or not, and a bonus of 10 euros for participating in both sessions, leading to 30 euros in total.

8 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 8 Materials and Procedures Parallel swing. A custom-made parallel swing was used to expose subjects to lateral motion. The experimental setup was specifically designed to limit subjects sensory input to vestibular sensory information only, since subjects were tasked to imagine incongruent selfmotion in the incongruent condition. Such imagination was assumed to be achieved more easily when subjects were deprived from sensory input that would otherwise directly indicate their actual self-motion direction. The m swing consisted of a support frame made out of six interlocked aluminium beams with two wooden boards of cm attached on top to create a smooth surface. The frame was attached to the ceiling with four ropes with a length of 6.41 m each. A memory foam mattress was placed on top of the wooden board for the subjects sitting comfort, and to prevent them from sensing too much shear stress on their skin, caused by mass inertia during the motion direction changes at the peaks of the swing motion. Using this type of swing is beneficial in this experiment due to the smoothness of the swing motion which does not allow for sudden acceleration changes to occur. Especially at the position turning position, such a direct change could be a cue for subjects to determine the direction of their actual self-motion. Moreover, this swing results in mere linear acceleration, thus resulting in a physically unambiguous simple stimulus without a complicating angular component. Although a parallel swing also results in a vertical acceleration, this component will be negligible for angles of the suspending ropes less than 15 degrees. The swing was manually driven using elastic luggage straps to minimise detectability of the pulling to one side. This procedure, however, caused therefore a slight variability in amplitude (1.375 ± m). Nevertheless, the period for the swing motion is independent of its amplitude in accordance to the formula for a simple gravity pendulum. This formula is used to calculate the period T as a function of the arm (rope) length L and the gravitational acceleration g as T 2π L g Given the rope length of 6.41 m, this results in a period of approximately 5.1 seconds. With this period, a frequency of 0.20 Hz was achieved for the swing motion, which is known to

9 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 9 be sickening (Golding, Finsch & Scott, 1997; Golding & Markey, 1996; Golding, Phil, Mueller, & Gresty, 2001; McCauley et al., 1976). Figure 2. Picture of a subject during the swing motion (permission was granted). The subject wears latex gloves, an overall, headphones, and a blindfold, and is positioned on a memory foam mattress. Subjects were instructed to sit in the middle of the swing with their legs and back straightened, and position their head as if they were looking straight forward. Cognitive input. To test the hypothesis that cognition can influence motion sickness severity, the following operationalization was used. Two objects in the experimental room served as cognitive anchor points for the subjects during the swing motion: 1) the entrance door through which the subjects entered the experimental room, which was positioned on the left from the subjects perspective, and 2) a computer desk, which was positioned on the right, from the subjects perspective. As is described in the Procedures section, subjects were given time before the swing motion to observe the objects, memorize them, and then describe the objects to the experimenter as they memorized them. In this way, the subjects were forced to make an appeal to the mental visualization of the objects when they needed to describe them based on memory. During the swing motion, subjects were tasked to imagine themselves moving towards the objects by means of provided verbal information. For example, in the congruent condition,

10 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 10 subjects were told to imagine themselves moving towards the computer desk while they were actually moving towards the computer desk (congruent cognitive input). In the incongruent condition, subjects were told to imagine themselves moving towards the computer desk while they were actually moving towards the entrance (incongruent cognitive input). The tasked selfmotion imagination direction was thus either right or wrong with respect to subjects actual selfmotion direction. Figure 3. The two objects that the subjects were instructed to use for their passive self-motion imagination: the entrance, which was left from the subjects perspective (left image), and the computer desk, which was right from the subjects perspective (right image). MISC. During the experiment, a Dutch misery scale (MISC; Bos, MacKinnon, & Patterson, 2005) was used to assess subjects feeling of discomfort on an 11-point scale. Answers were filled in by the experimenter, based on the subjects verbal rating of their discomfort (see Table 1). A score of 0 indicates no problems; a score of 10 indicates vomiting. At any point during the experiment, when a MISC score of 6 or higher was reported (i.e., indicating nausea), the experiment was ceased immediately.

11 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 11 Table 1 The 11-point MIsery SCale (MISC), which was used to assess subjects discomfort Symptoms MISC No problems 0 Some discomfort, but no specific symptoms 1 Dizziness, cold/warm, headache, stomach/throat awareness, sweating, blurred vision, yawning, burping, tiredness, salivation, but no nausea Nausea Vague Little Rather Severe Little Rather Severe Retching Vomiting 10 Procedure Ethical approval by The Scientific and Ethical Review Board (Vaste Commissie Wetenschap en Ethiek) of the Faculty of Behaviour & Movement Sciences at VU University Amsterdam was obtained for this experiment in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki. Informed consent was obtained from all subjects (Appendix C). Prior to participation, subjects received only general information concerning the experimental procedure (Appendix D). They were informed that the study concerned visual imagination during motion, and that they would be exposed to motion while sitting on a platform. The true goal of the study was withheld since prior knowledge concerning the (in)congruency of the given passive self-motion information could interfere with possible effects of the manipulation. In the experimental room, subjects put on an overall with an attached hood (Tyvek Classic plus) and latex gloves to minimise haptic sensation of wind that could indicate subjects self-motion direction. Next, subjects were instructed to sit in the middle of the swing with their legs and back straightened, and to observe two objects (the entrance door, and the computer

12 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 12 desk, left and right to them respectively) in great detail. The experimenter asked questions so that the description involved several specific details such as colours and shapes within the objects. Subjects were asked to describe the objects with their eyes closed while the experimenter checked if that description was in line with the actual object. If this was not the case, or if the description was not sufficient, subjects were allowed to take another look until the description was correct and sufficiently detailed. This was assumed to aid the subject in forming a detailed mental visualization of both objects, possibly resulting in a more accessible passive self-motion imagination later in the experiment. Then, the MISC was explained to the subjects, and they were asked to report their MISC score at that moment. This served as a base measurement of their discomfort prior to being exposed to the swing motion. Next, subjects were blindfolded to obscure their vision during the actual experiment (Mindfold Relaxation Mask). Then, they were asked to keep both visualized objects in mind and actively imagine themselves moving towards the object when that object was named through the headphones. They were also explicitly instructed not to focus on the sensed motion itself but on their own imagination. Then, subjects put on the headphones (Sennheiser HD 201) which were furthermore used to present constant pink noise at 80 db, thereby masking environmental sounds that could indicate subjects actual self-motion direction. The experiment was divided into two consecutive phases (see Table 2 for a detailed overview). Pre-experimental phase. The goal of the pre-experimental phase was solely to provoke motion sickness in the subjects. They were therefore instructed to rotate their head, combining a yaw and pitch motion, while being exposed to the swing motion. With respect to the SO motion sickness model, this was assumed to result in noisy and uncertain sensory afferent signals about their current body state. During this phase, subjects were asked to report a MISC score, and to rotate their head in the opposed direction every 90 seconds. After 10 minutes, a total of 6 MISC scores were acquired per subject. However, when a MISC score of 3 or higher was reported, the experiment was proceeded to the experimental phase. This cut-off score of 3 ensured enough space to detect both a decrease (to a minimum of 0, i.e., no problems) and an increase (to a maximum of 6, i.e., nausea, the stop criterion) in MISC score during the experimental phase.

13 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 13 When the pre-experimental phase ended, subjects were instructed to stop rotating their head, and to position their head as if they were looking straight forward during the rest of the experiment. Table 2 Schematic overview of the content of one session, with the accompanied duration. Whether the presented verbal information was congruent, or incongruent, with respect to actual self-motion, depended on the condition the subject was assigned to in that session Session element Content Duration Pre-experimental phase Experimental phase (1) Experimental phase (2) Experimental phase (3) Induce motion sickness; ask for MISC score every 90 seconds Block 1: Verbal (in)congruent information Block 2: No verbal information Block 3: Subjects were asked for imagination vividness score, self-motion direction, and MISC score Block 1: Verbal (in)congruent information Block 2: No verbal information Block 3: Subjects were asked for imagination vividness score, self-motion direction, and MISC score Block 1: Verbal (in)congruent information Block 2: No verbal information Block 3: Subjects were asked for imagination vividness score, self-motion direction, and MISC score 10 minutes (resulting in 6 MISC scores in total), or until MISC 3 60 seconds 30 seconds Until questions were answered 60 seconds 30 seconds Until questions were answered 60 seconds 30 seconds Until questions were answered Experimental phase. The goal of the experimental phase was to stimulate passive selfmotion imagination, to check for the subject s evaluation of their imagination vividness, and to ask subjects for their experienced self-motion direction. This phase consisted of three blocks. In the first block, with a duration of 60 seconds, subjects continually received verbal information about their self-motion direction. They were tasked to use this information for their self-motion imagination. The verbal information consisted of alternately the object subjects were moving towards, or the direction of their self-motion. In the congruent condition, this verbal information was congruent with respect to the subjects actual self-motion (i.e., toward the

14 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 14 entrance when they were moving towards the entrance, and towards the computer desk when they were moving towards the computer desk; to the left when they were moving leftwards, to the right when they were moving rightwards). In this way, subjects verticality expectation was in congruence with their verticality sensation. This was assumed to lead to less severe motion sickness, and thus a lower MISC score, relative to subjects MISC score at the end of the preexperimental phase. In the incongruent condition, the verbal information was incongruent with respect to subjects actual self-motion (i.e., towards the entrance when they were moving towards the computer desk, and towards the computer desk when they were moving towards the entrance; to the left when they were moving rightwards, to the right when they were moving leftwards). In this way, it was attempted to make subjects verticality expectation differ from their sensed verticality. This was assumed to lead to more severe motion sickness, and thus a higher MISC score relative to subjects MISC score at the end of the pre-experimental phase. In the second block, with a duration of 30 seconds, no verbal information was provided. Subjects were tasked to continue their self-motion imagination according to the verbal information that was given in the first block. The goal of the third block was to obtain responses from the subjects. Firstly, subjects were asked if their imagination regarding the entrance and computer desk was still vivid. This could be rated in the following terms: very vague, a little vague, a little sharp, or very sharp. Next, subjects were asked to report their experienced self-motion direction. This could be rated as either left or right. This was asked directly after the swing switched its motion direction, right after it had reached a far end. Questioning subjects on this matter enabled to check if they detected their self-motion direction correctly, or if they were successfully mistaken by their own incongruent self-motion imagination in the incongruent condition. Finally, subjects were asked to report their MISC score. This block ended when subjects answered all questions. The experimental phase (consisting of block 1, 2, and 3) was repeated for 3 times as long as MISC scores lower than 6 were still reported. If subjects did experience discomfort (MISC 3) after the experiment was finished, they were advised to stay in the experimental room until the discomfort was lowered to a more acceptable level (MISC 2). To counterbalance any learning effects, the order of the conditions was randomized between subjects.

15 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 15 Data analysis Of the sixteen subjects who participated, data from nine participants was analysed. Four subjects did not experience any discomfort during both sessions (i.e., MISC = 0), two subjects did not want to participate in the second session due to nausea in the first session, and one subject did not understand the meaning of imagination in the second session and showed deviant behaviour during the experiment. This study used a within-subjects design. The independent variable was cognitive input (congruent/incongruent, with respect to the subjects actual self-motion; nominal). The dependent variables were MISC score (0-10; ordinal), imagination vividness scores (very vague, a little vague, a little sharp, or very sharp; ordinal), direction of self-motion (left, or right; nominal), and QMI scores (35-245; ordinal). To represent the rise or fall of the MISC score in the experimental phase relative to the last MISC score in the pre-experimental phase, corrected MISC scores were calculated (denoted by cmisc for the rest of this paper). This was done by subtracting each raw MISC score in the experimental phase from the last raw MISC score in the pre-experimental phase. For the pre-experimental phase, a Friedman s analysis of variance (ANOVA) was conducted to analyse raw MISC score development over time. This enabled to check if subjects did experience motion sickness over time due to the procedure in the pre-experimental phase. For the experimental phases, a 2 3 (Condition [congruent, incongruent] (Time [experimental phase 1, experimental phase 2, experimental phase 3]) repeated measures ANOVA was conducted to analyse cmisc score development over time per condition. This enabled to check if subjects did get more motion sick over time in one condition compared to the other condition. However, since the MISC is formally an ordinal scale, and the parametric ANOVA assumes a dependent variable on a continuous level, separate nonparametric Friedman ANOVAs were conducted to analyse cmisc score development over time per condition as well. Secondary analyses were conducted to test for remaining relationships. The same analyses that were conducted for cmisc scores were conducted for imagination vividness scores as well (i.e., 2 3 (Condition [congruent, incongruent] (Time [experimental phase 1, experimental phase 2, experimental phase 3]) repeated measures ANOVA, and separate Friedman ANOVAs). This enabled to check if the imagination vividness was stable over time per condition.

16 MISC score THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 16 Furthermore, a Chi square test was conducted to test whether the conditions were independent of each other with respect to correct self-motion direction determination. A Spearman rank-order correlation was conducted to test the relationship between QMI scores and the mode of subjects imagination vividness scores, and between cmisc scores and imagination vividness scores. Results To test whether the raw MISC scores increased over time during the pre-experimental phase, which did not differ between conditions, a Friedman s ANOVA was conducted. It was found that raw MISC scores significantly increased over time during the pre-experimental phase, χ 2 (6) = , p <.001 (see Figure 4) Time (minutes) Figure 4. A line graph representing average MISC scores in the pre-experimental phase. In the pre-experimental phase, every 90 seconds, subjects were asked to report a MISC score. The average MISC score at 0 minutes represents subjects base MISC score. Error bars represent the standard error of the mean (SEM). To test whether the cmisc scores in the experimental phases changed over time differently in the congruent condition than in the incongruent condition (Figure 6), a 2 3 (Condition [congruent, incongruent] (Time [experimental phase 1, experimental phase 2,

17 cmisc score MISC score THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS Congruent condition Incongruent condition Experimental phase (1) Experimental phase (2) Experimental phase (3) Figure 5. A bar graph representing the average MISC score at the third block of each experimental phase per condition. Error bars represent the standard error of the mean (SEM). 2 Congruent condition Incongruent condition Experimental phase (1) Experimental phase (2) Experimental phase (3) Figure 6. A bar graph representing the average cmisc scores (MISC score in the third block of each experimental phase, minus the last MISC score in the pre-experimental phase). Error bars represent the standard error of the mean (SEM).

18 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 18 experimental phase 3]) repeated measures ANOVA was conducted. A significant increase in 2 cmisc score over time was found, F(2, 16) = , p <.001, η p =.672. However, no main effect of condition, F(1, 8) = 1.280, p =.291, and no interaction effect of time and condition, F(2, 16) = 0.200, p =.821, was found. Since for the ANOVA the assumption of normality for the residuals was violated, and the cmisc scores are formally ordinal and thus violate the assumption of a dependent variable on a continuous level, separate Friedman s ANOVAs were conducted to analyse the development of cmisc scores over time per condition. Friedman s ANOVA confirmed a significant increase in cmisc scores over time across the experimental phases, χ 2 (2) = 8.000, p =.018. For the congruent condition, however, no significant difference in cmisc scores over time was found, χ 2 (2) = 1.625, p =.444. In contrast, there was a significant increase in cmisc scores over time in these phases for the incongruent condition, χ 2 (2) = 7.185, p =.028. Post hoc analysis with Wilcoxon signed-rank tests was conducted with an applied Bonferroni correction, resulting in a significance level of p <.017. Despite an overall increase in cmisc scores over time in the incongruent condition, post hoc analyses showed there were no significant differences between the first and second cmisc scores (z = , p =.257), between the second and third cmisc scores (z = , p =.059), and between the first and third cmisc scores (z = , p =.099). As part of the secondary analyses, a 2 3 (Condition [congruent, incongruent] (Time [experimental phase 1, experimental phase 2, experimental phase 3]) repeated measures ANOVA was conducted for imagination vividness scores. No significant time effect was found, F(2, 16) = 2.141, p =.150. Furthermore, no main effect of condition, F(1, 8) = 0.308, p =.594, and no interaction effect of time and condition, F(2, 16) = 1.931, p =.177, was found. The Friedman ANOVAs confirmed these results, since no overall time effect was found χ 2 (2) = 3.455, p =.178. Furthermore, no time effect was found for the congruent condition, χ 2 (2) = 0.500, p =.779. However, the analysed time effect for the incongruent condition was close to significance, χ 2 (2) = 5.429, p =.066. Additionally, a Chi square test was conducted to test whether the conditions were independent of each other with respect to correct self-motion direction determination. It was found that conditions were not independent of each other with respect to this aspect, χ 2 (1) = 0.353, p =.552. In the congruent condition, subjects reported experienced self-motion direction

19 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 19 correctly in 96% of cases; in the incongruent condition, this was in 93% of the cases (both with respect to the subjects actual self-motion direction). Next, a Spearman's rank-order correlation was run to test whether there was a relationship between subjects QMI score, and the mode of their imagination vividness scores. A strong and significant positive correlation was found, rs(7) =.745, p =.021. Furthermore, a Spearman's rank-order correlation was conducted to analyse the relationship between subjects imagination vividness scores and subjects cmisc scores at that same time. There was no significant correlation, rs(52) = -.116, p =.402. Even though for the congruent condition this effect was still not significant, rs(25) =.266, p =.179, a significant negative correlation was found for the incongruent condition, rs(25) = -.507, p =.007. Discussion This study was conducted to test the hypothesis that cognitive information that is incongruent with respect to actual self-motion leads to more severe motion sickness, while cognitive information that is congruent with respect to actual self-motion leads to less severe motion sickness. Before drawing any conclusions, some critical notes concerning the conducted analyses should be taken into account. The assumption of both normally distributed residuals and a dependent variable on a continuous level were violated for the two-way repeated measures ANOVA, since the MISC scores that were used are formally ordinal. Even though two Friedman s ANOVAs were conducted per condition to account for this, these analyses do only take the magnitude of the differences between the cmisc scores into account to a certain extent. For example, it might occur that a subject in the congruent condition reports a relatively weak increase in MISC score, while reporting a relatively strong increase in MISC score in the incongruent condition. With Friedman s ANOVA, these increases would be treated in the exact same way, so no effect of the magnitude of the difference in both conditions would be found in that case, even though an effect might be present if this same example applies to multiple subjects. With the current data, no test was found that could test for the main hypothesis without violating assumptions, or accompanying the results with a critical note. Additionally, several subjects were excluded from analysis, which resulted in a reduced power to find significant effects. This might explain why in the conducted Wilcoxon-signed rank tests, which served as

20 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 20 post hoc analyses for Friedman s ANOVA, no significant differences were found between cmisc scores in different time points, even though there was an overall time effect for the incongruent condition. The results that were found, however, do point in the direction of the hypothesis (Figure 6). Even though motion sickness severity was not found to decrease with congruent cognitive information, subjects were indeed found to get more motion sick over time with incongruent cognitive information. It seems therefore, at first sight, that a conflict between self-motion sensation and cognitive self-motion expectation does indeed lead to (more) motion sickness, as would be predicted by the SO motion sickness model. However, as mentioned above, no differences between motion sickness severity over specific time points for the incongruent condition were found. In addition, a global analysis of the individual data per subject shows no convincing trend of a difference between motion sickness severity development over time between the congruent and the incongruent condition. These additional observations do question whether the main effect that was found is convincing enough to validate the use of an internal model as described in the SO motion sickness model. One could therefore question whether the difference between verticality expectation and verticality sensation in the incongruent condition was apparent enough to the central nervous system to be interpreted as a serious conflict, and thus would, or would not, have resulted in a noticeable difference in motion sickness severity. Even though self-motion sensation was attempted to be limited to vestibular sensation only, some subjects reported they could still feel wind caused by the overall pressing against their skin. Additionally, several subjects reported variable pressure on their wrists since they used their hands as support to keep sitting upright during the swing motion (as is apparent in Figure 2). It is conceivable that subjects used this wrist pressure variability to aid themselves in determining their self-motion direction. In the incongruent condition, subjects reported their experienced self-motion direction, with respect to their actual motion direction, correctly in 93% of the cases. It could be assumed that such an apparent sense of actual self-motion could be disadvantageous to the vividness of the incongruent self-motion imagination, an effect described by Nigmatullina et. al (2015). In their study, subjects were exposed to passive rotation on a chair in a dark room. When they were tasked to imagine themselves rotating towards the other side than they actually did, they reported a relatively lower imagination vividness than when they

21 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 21 imagined themselves rotating in the actual rotation direction. In the current experiment, however, it is unclear whether that same effect has been present for incongruent imagination vividness as well. Even though imagination vividness was not found to decrease over time in the incongruent condition, this effect was found to be close to significance. Furthermore, it was found that there is a negative relationship between imagination vividness and motion sickness severity for the incongruent condition only. Although no causation can be implied from this finding, it could be assumed that incongruent self-motion imagination requires relatively more cognitive effort than congruent self-motion imagination. Since it is known that motion sickness may result in difficulties with concentration (Bos, MacKinnon, & Patterson, 2005), it could be that when one is motion sick, he or she may experience difficulty in fully focusing on this (relatively difficult) incongruent imagination, which therefore reduces its vividness. The other way around, it could be that when one is focused on their imagination, they are mentally distracted from their motion sickness symptoms, which therefore leads to a decrease in motion sickness severity, an effect described by Bos (2015). However, from the SO motion sickness model, it would be expected that a high imagination vividness (and thus a convincing expectation about one s future body state) should result in a powerful conflict between body state sensation and body state expectation. To the central nervous system, such an apparent conflict is assumed to lead to an increase in motion sickness severity, which is in contrast with the negative relationship that was found between imagination vividness and motion sickness severity. One last assumption could therefore be that the central nervous system resolves this conflict by reducing the incongruent imagination vividness, in an attempt to achieve a correct perception of the body state. However, no literature was found to support this assumption. Therefore, it would be interesting to further study these relationships in more detail in future research. Several recommendations are proposed for future research. Firstly, since it may be that imagination vividness declines over time in the incongruent condition, subjects may benefit from more representative visual object exposure. In the current study, when subjects were given time to observe both objects prior to the swing motion, they observed the objects as being still. However, during the swing motion, they were asked to imagine themselves moving towards one of the objects. There is, therefore, a discrepancy between the initial exposure to the objects and the eventually tasked imagination (i.e., the objects appear bigger on the retina as a function of the relative distance to them that is covered on the swing over time, and, when being tilted, the

22 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 22 perspective of the object changes). Even though it is known that objects can be mentally rotated (Shepard & Metzler, 1971), mental rotation capability may be decreased during vestibular sensation (Mast, Merfeld, & Kosslyn, 2006), which is experienced during swing motion. Furthermore, the ability to predict an object s appearance from different viewpoints is suggested to be based on experience (Asakura & Inui, 2011). If the swing would be moving while the subjects are observing the objects and forming a mental image of them, this, therefore, might aid them in forming a more accurate imagination. Secondly, to resolve the problem with respect to the subjects need to stabilize themselves using their body, one solution could be to let subjects lie down on top of the swing. However, motion sickness was not found to occur in this position during pilot studies, which may be attributed to being stable in the lying position. This effect is described by Riccio and Stoffregen (1991) who state that instability is, indeed, a requisite for motion sickness to occur. An alternative solution would be to place subjects on a regular swing. In contrast to a parallel swing, regular swing motion results in a variable net radial acceleration only (i.e., parallel to the cord of the swing), without any net tangential acceleration (i.e., in the direction of the motion). In this way, subjects are only pressed onto the swing seat, and thus do not need to stabilise their body using their hands. Such a setup might therefore lead to a more ambiguous self-motion sensation, while still including at least some postural instability. However, the radial component of the swing motion varies with twice the frequency of the parallel swing motion. In addition to the fact that the variance itself is smaller, the swing motion is then less provocative due to the frequency effect. Therefore, longer swing ropes would be needed, and the deviation of the swing should be greater than in the current experiment. Thirdly, replicating this study with more subjects may result in more convincing evidence for the existence of the hypothesized effect. Finding specific differences across time points within one condition would confirm the overall increase in motion sickness severity over time that was found in the current study. The hypothesis would therefore be validated more convincingly to add to the plausibility of the use of an internal model with respect to motion sickness occurrence. What is left to conclude from the current study are two main points. Firstly, even though this study did not add significant value to the plausibility of the usage of an internal model by the central nervous system, it also does not deny such existence. An effect of cognitively

23 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 23 manipulated self-motion expectation on motion sickness severity was indeed found. It may thus be interesting to study this effect in more detail in a follow-up study by exposing more subjects to more perceptually ambiguous lateral motion, for example on a regular swing. Secondly, what may not have been gained from a theoretical perspective, may have been gained from a methodological perspective. In the current experimental setup, subjects were successfully found to get motion sick, while still reporting vivid self-motion imagination. Research on the relationship between cognitive expectation with respect to self-motion and its consequences to motion sickness is, to the experimenters knowledge, entirely new. The current study has created a first stepping stone towards an understanding of this relationship.

24 THE INFLUENCE OF EXPECTATION ON MOTION SICKNESS 24 References Asakura, N., & Inui, T. (2011). Disambiguation of mental rotation by spatial frames of reference. Perception, 2(5), Bles, W., Bos, J. E., de Graaf, B., Groen, E., & Wertheim, A. H. (1998). Motion sickness: only one provocative conflict?. Brain research bulletin, 47(5), Bles, W., Bos, J. E., & Kruit, H. (2000). Motion sickness. Current opinion in neurology, 13(1), Bos, J. E., & Bles, W. (1998). Modelling motion sickness and subjective vertical mismatch detailed for vertical motions. Brain research bulletin, 47(5), Bos, J. E., & Bles, W. (2002). Theoretical considerations on canal otolith interaction and an observer model. Biological cybernetics, 86(3), Bos, J. E., MacKinnon, S. N., & Patterson, A. (2005). Motion sickness symptoms in a ship motion simulator: effects of inside, outside, and no view. Aviation, space, and environmental medicine, 76(12), Bos, J. E., Bles, W., & Groen, E. L. (2008). A theory on visually induced motion sickness. Displays, 29(2), Bos, J. E. (2015). Less sickness with more motion and/or mental distraction. Journal of Vestibular Research, 25(1), Brainard, A., & Gresham, C. (2014). Prevention and treatment of motion sickness. American family physician, 90(1). Cowings, P. S., Toscano, W. B., DeRoshia, C., & Miller, N. E. (2000). Promethazine as a motion sickness treatment: impact on human performance and mood states. Aviation, space, and environmental medicine, 71(10), Golding, J. F., & Markey, H. M. (1996). Effect of frequency of horizontal linear oscillation on motion sickness and somatogravic illusion. Aviation, space, and environmental medicine, 67(2), Golding, J. F., Finch, M. I., & Stott, J. R. (1997). Frequency effect of Hz horizontal translational oscillation on motion sickness and the somatogravic illusion. Aviation,