Exp Brain Res (2014) 232:827–836 DOI 10.1007/s00221-013-3793-9
RESEARCH ARTICLE
Combined effects of auditory and visual cues on the perception of vection Behrang Keshavarz · Lawrence J. Hettinger · Daniel Vena · Jennifer L. Campos
Received: 25 September 2013 / Accepted: 20 November 2013 / Published online: 4 December 2013 © Springer-Verlag Berlin Heidelberg 2013
Abstract Vection is the illusion of self-motion in the absence of real physical movement. The aim of the present study was to analyze how multisensory inputs (visual and auditory) contribute to the perception of vection. Participants were seated in a stationary position in front of a large, curved projection display and were exposed to a virtual scene that constantly rotated around the yaw-axis, simulating a 360° rotation. The virtual scene contained either only visual, only auditory, or a combination of visual and auditory cues. Additionally, simulated rotation speed (90°/s vs. 60°/s) and the number of sound sources (1 vs. 3) were varied for all three stimulus conditions. All participants were exposed to every condition in a randomized order. Data specific to vection latency, vection strength, the severity of motion sickness (MS), and postural steadiness were collected. Results revealed reduced vection onset latencies and increased vection strength when auditory cues were added to the visual stimuli, whereas MS and postural steadiness were not affected by the presence of auditory cues. Half of the participants reported experiencing auditorily induced vection, although the sensation was rather weak and less robust than visually induced vection. Results demonstrate
B. Keshavarz (*) · D. Vena · J. L. Campos Department of Research, Toronto Rehabilitation Institute, Technology Team/iDAPT, 550 University Avenue, Toronto, ON M5G 2A2, Canada e-mail: [email protected] L. J. Hettinger Center for Behavioral Sciences, Liberty Mutual Research Institute for Safety, Hopkinton, MA, USA J. L. Campos Department of Psychology, University of Toronto, Toronto, ON, Canada
that the combination of visual and auditory cues can enhance the sensation of vection. Keywords Illusory self-motion · Vection · Multisensory integration · Motion sickness · Postural sway · Virtual reality
Introduction Vection is a well-known phenomenon and describes the illusion of self-motion in the absence of real, physical movement (Dichgans and Brandt 1978; Fischer and Kornmüller 1930). Vection can be frequently experienced in real-life situations (e.g., while sitting in a stationary car or a stationary train when the vehicles around you begin to move) or in virtual environments (e.g., in fixed-base driving or flight simulators) (for an overview, see Hettinger 2002). In these cases, observers are under the impression that they are moving, even though they remain physically stationary. Physical behaviors evidenced during real motion, such as postural adjustments, are also frequently observed with vection (Fushiki et al. 2005). The first scientific reports of vection can be dated back to more than a century ago (e.g., Mach 1875), and vection induced by visual stimulation has been extensively discussed ever since. In contrast, the influence of nonvisual information on the experience of vection has been largely neglected. In fact, vection can also be induced solely by auditory cues (see Väljamäe 2009 for an overview), a finding reported by Dodge (1923) as early as in the beginning of the twentieth century. However, only a few investigators had followed up on Dodge’s findings (e.g., Lackner 1977) until recently. As of late, research studying auditory vection has been revived and has
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piqued the interest of several research groups (Riecke et al. 2005; Sakamoto et al. 2004; Kapralos et al. 2004). Results of these studies have shown auditory vection to be less compelling and less frequently experienced than visual vection. Riecke et al. (2009) estimated that auditory vection is experienced by 25–60 % of all participants across different studies and, when auditory vection was perceived, the sensation was relatively weak. Research regarding auditory vection is still in the early stages and the precise nature of auditory vection is yet to be clearly defined. This is unlike the stimulus parameters underlying visually induced vection, which have been widely investigated for some time. Additionally, research regarding the effects of crossmodal integration of visual and nonvisual information on the perception of vection is almost nonexistent. To our knowledge, Riecke et al. (2009) were the only ones to analyze the influence of auditory cues when combined with visual stimulation. In their study, the authors used a virtual reality (VR) setup including a realistic visual stimulus (i.e., a high-resolution panoramic photograph of a city’s marketplace) that constantly rotated around the participants’ yaw-axis to create circular vection and added spatial 3D sound that was synchronized with the corresponding visual scene. Results demonstrated that the addition of auditory cues indeed enhanced vection (i.e., reduced vection onset time), highlighting the facilitative effects of multisensory cue combination. Additional vection-increasing effects of cross-modal integration have also been recently reported through demonstrations of increased auditory vection in the presence of vibro-tactile and haptic cues (e.g., vibration of the participants’ seat) (Riecke et al. 2008, 2009). Thus, the perception of vection seems to be moderated by multiple sensory modalities. Note that, in the past, vection has also been linked to the phenomenon of visually induced motion sickness (MS; Flanagan et al. 2004; Hettinger et al. 1990). Visually induced MS is a sensation very similar to traditional MS, and typical symptoms include (but are not limited to) pallor, cold sweat, nausea, dizziness, fatigue, and (rarely) vomiting (Hettinger and Riccio 1992; Kennedy et al. 1992). Vection is often considered to be involved in the genesis of visually induced MS, as most participants who suffer from visually induced MS also experience vection at the same time (e.g., Hettinger and Riccio 1992; Smart et al. 2002). However, others have demonstrated that vection does not necessarily or always lead to visually induced MS (Webb and Griffin 2002; Palmisano et al. 2008; Bonato et al. 2008). Hence, the precise relationship between vection and visually induced MS has yet to be defined. As with vection, the role of multisensory inputs and the role of auditory cues in particular in the genesis of MS have only been vaguely discussed so far (except for Keshavarz and Hecht 2012a;
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Exp Brain Res (2014) 232:827–836
Nichols et al. 2000). In the present study, we collected data germane to MS to control for possible MS symptomatology during and after stimulus exposure. In sum, the main goals of the study were (1) to analyze the role of combined auditory and visual cues in the perception of vection, and (2) to determine the strength of auditory vection compared to its visual counterpart. Additionally, we also collected data specific to the occurrence of MS to analyze the potential impact of auditory and visual vection on MS. The present study is the first to combine visual and auditory cues with respect to their impact both on vection and MS at the same time, with the same participants. In the present study, stationary participants were exposed to a VR scene that rotated about their perceived yaw-axis at a constant velocity for 360°. The virtual environment provided either only visual cues, only auditory cues, or a combination of visual and auditory cues. Additionally, simulated rotation speed (60°/s vs. 90°/s) and the number of sound sources (1 vs. 3) were varied. We collected subjective ratings on vection onset, vection strength, MS severity, and also recorded participant’s postural steadiness after each trial using center of pressure measured via a force plate. The relationship between auditory vection and postural steadiness has never before been examined. We hypothesized that auditory vection would be less pronounced than visual vection, whereas we expected that the combination of visual and auditory cues would generate the strongest vection. Based on the previous findings (Larsson et al. 2004), we assumed that adding more sound sources (3 vs. 1) and increasing the stimulus speed (90°/s vs. 60°/s) should enhance the strength of vection.
Methods Participants Thirteen male (Mage = 26.54, SDage = 5.77) and 7 female (Mage = 30.14, SDage = 7.99) adults participated in this experiment. All participants gave written consent prior to the experiment and stated that they were in a normal state of health. Participants had normal or corrected-to-normal vision and were naïve with respect to the purpose of the study. The stimuli were administered in accordance with the Declaration of Helsinki to ensure research ethics in human experimentation. The study protocol was reviewed and approved by the research ethics board of the Toronto Rehabilitation Institute. Participants were paid $10 and were informed that they were free to abort the experiment at any time without negative consequences. However, no participant chose to abort the study prior to regular termination.
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Exp Brain Res (2014) 232:827–836
Fig. 1 The left panel shows a schematic illustration of the laboratory (top-down view). Participants were seated in a rotatable chair, facing the curved projection screen (thick inner circle). A force plate (FP) was used to measure postural steadiness post-vection stimulus. Seven speakers (small white ovals) were used to present the rotating sound sources. The black-filled circles represent the 3 sound source
locations used in the experiment and their spatial arrangement with respect to the participant (S1 = church bells, S2 = streetcar, and S3 = car engine). The sound sources were synchronously rotated counterclockwise for 360° along the participants’ yaw-axis (dotted line). The right panel shows the experimental settings and a screenshot of the visual stimulus
Design, apparatus, and stimuli
and the VR scene was created using customized OpenScene Graph application. No components within the virtual scene were animated (e.g., no motion of cars or pedestrians). For the visual + auditory condition, three different sound sources were added to the visual scene: the sound of church bells, the engine sound of a nonmoving car, and the ringing sound of a stationary streetcar/tram. The loudness of each sound was set to 75 dB, and the distance between each sound source was held constant. For the trials containing only a single sound source, the sound of the church bells was chosen. In the auditory condition, visual stimulation was excluded by blackening the projector screen and by blindfolding the participants with a sleeping mask.
A 3 × 2 × 2 within-subjects design including stimulus condition (visual, auditory, and visual + auditory), simulated rotation speed (60°/s and 90°/s), and number of sound sources (1 and 3) was chosen. As the visual-only condition did not include sound at all, simulated rotation speed was the only factor that was varied within it. Thus, the total number of trials resulted in 10 (instead of 12 trials). Every participant was exposed to all 10 trials in a randomized order, and each trial was presented for 60 s. Participants were seated in a dimly lit, dome-shaped laboratory in a rotatable chair, 100 cm in front of a curved projection screen (see Fig. 1). Six projectors (Eyevis ESPLED series with LED technology) were used to create a visual image with a field of view of 240° horizontally and 120° vertically. Three-dimensional sound was delivered by 7 speakers located behind the projection screen (Meyersound MP-4XP) and a subwoofer (Meyersound MP10XP). The height of the chair was fixed at 70 cm above the laboratory’s floor. A headrest was mounted to the back of the chair to minimize head movements and to exclude head movements as a potential confounding factor accounting for changes in vection and MS perception. Participants were asked to have their feet hanging loosely in the air without touching the ground or any other surface. The visual stimulus consisted of a VR scene of downtown Toronto that was continuously rotated for 360° about the observer’s yaw-axis. Simulated rotation speed was set to 60°/s or 90°/s, depending on the experimental trial. No acceleration and deceleration were used at video onset and offset, respectively. Picture resolution was 6.5 arcmin/OLP
Response measures Two different vection parameters were collected, including vection onset time and vection strength. To measure the onset time of vection, participants verbally indicated whenever they started to feel vection. Data regarding the strength of vection were collected after each trial using an 11-point Likert scale (0 = not at all, 10 = very strong). Motion sickness data were collected using the Fast Motion Sickness Scale (FMS; Keshavarz and Hecht 2011). The FMS is a verbal rating scale ranging from 0 (no sickness at all) to 20 (severe sickness) that was designed to monitor the time course and the severity of MS (i.e., nausea and general discomfort in particular) and has been validated using the well-established Simulator Sickness Questionnaire (Kennedy et al. 1993). In the past, high correlations between the FMS and the SSQ (e.g., r = .83) suggested that the FMS scale is a valid and reliable tool to capture the severity of
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MS (Keshavarz and Hecht 2011). The FMS is a fast and easy-to-assess tool and allows continuous capturing of MS without disrupting task performance. Hence, participants were asked to verbally report their level of sickness by choosing a single score using the FMS after each trial. To make sure that no MS-related carryover effects occurred, participants were provided with a rest period after each trial until any MS symptoms had subsided to normal (i.e., FMS score of zero) before continuing with the experiment. Additionally, we measured participants’ postural steadiness, as several studies have shown that postural steadiness is linked to MS (e.g., Flanagan et al. 2004; Stoffregen et al. 2000; Smart et al. 2002). We captured steadiness data once before the experiment began and once after each trial. Participants were instructed to stand on a force plate (AMTI BP12001200), align their feet with predefined markings in a parallel position (distance between ankle bones approximately 7 cm), and close their eyes for 30 s. Postural steadiness was examined using the center of pressure (COP) calculated from the force plate. Data from the first 25 s of the 30-s test period of quiet standing were used across all participants. Force data were filtered using a sixth-order dualpass butterworth filter with a 6-Hz cutoff frequency. Matlab R2012 was used to compute COP measures of postural steadiness. COP-based measures used for the analysis were variability (standard deviation) of COP excursion in the medial–lateral (ML) and anterior–posterior (AP) directions, total length of the COP path (COP length), and the 95 % confidence area surrounding the COP (COP ellipse). More specifically, COP length was calculated as the sum of the distance between consecutive points for the 25-s dataset. The 95 % confidence ellipse is defined as the area containing the center of the COP points with a 95 % probability. Procedure Before the first trial was presented, a practice trial was used to familiarize participants with the sensation of vection and with the data collection procedure. For this purpose, the stimulus that was predicted to produce the strongest sense of vection (i.e., visual + auditory stimulus containing a simulated rotation speed of 90°/s and 3 sound sources) was presented to the participants until they reported fully saturated vection (i.e., vection strength of 10). After the practice trial, participants were exposed to all 10 trials in randomized order.1 The first measure of postural steadiness was carried out after the practice trial and acted as a baseline with respect to the subsequent measurements. Between each of the 10
1 Statistical analyses revealed no significant effect of trial order and demonstrated successful randomization.
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Exp Brain Res (2014) 232:827–836
trials, postural steadiness data were collected immediately after stimulus offset using the same procedure. This resulted in a total of 11 steadiness measurements. A short rest break between each trial was provided. During each stimulus presentation, participants were asked to avoid head and body movements by positioning their heads into the headrest and focusing on the stimuli. Additionally, during the stimulus presentation, participants were simply asked to verbally indicate whenever they started to feel vection. Vection strength and sickness ratings were collected verbally after each trial. Before leaving the laboratory, all subjects were debriefed and the experimenter ensured that all MS-related symptoms had subsided to normal.
Results For all statistical analyses, SPSS (Statistical Package for Social Sciences, IBM, Version 21) was used. A priori significance level was set to α = .05. Mean ratings for vection onset time and vection strength are shown in Fig. 2, and MS ratings are given in Fig. 3. To analyze the influence of simulated rotation speed and the number of sound sources in the trials including visual cues, a 3 × 2 repeated-measures (rm) ANOVA including the within-subjects factors sound (no sound vs. 1 vs. 3) and speed (60°/s vs. 90°/s) was calculated for each dependent variable, that is, vection onset time, vection strength, FMS scores (motion sickness), and COP data. Note that in the case of absent vection (i.e., participants did not report feeling vection at all), the maximum onset time of 60 s was chosen and transferred to the data. Results showed a significant effect of sound with respect to vection strength, F(2,38) = 9.38, p
RESEARCH ARTICLE
Combined effects of auditory and visual cues on the perception of vection Behrang Keshavarz · Lawrence J. Hettinger · Daniel Vena · Jennifer L. Campos
Received: 25 September 2013 / Accepted: 20 November 2013 / Published online: 4 December 2013 © Springer-Verlag Berlin Heidelberg 2013
Abstract Vection is the illusion of self-motion in the absence of real physical movement. The aim of the present study was to analyze how multisensory inputs (visual and auditory) contribute to the perception of vection. Participants were seated in a stationary position in front of a large, curved projection display and were exposed to a virtual scene that constantly rotated around the yaw-axis, simulating a 360° rotation. The virtual scene contained either only visual, only auditory, or a combination of visual and auditory cues. Additionally, simulated rotation speed (90°/s vs. 60°/s) and the number of sound sources (1 vs. 3) were varied for all three stimulus conditions. All participants were exposed to every condition in a randomized order. Data specific to vection latency, vection strength, the severity of motion sickness (MS), and postural steadiness were collected. Results revealed reduced vection onset latencies and increased vection strength when auditory cues were added to the visual stimuli, whereas MS and postural steadiness were not affected by the presence of auditory cues. Half of the participants reported experiencing auditorily induced vection, although the sensation was rather weak and less robust than visually induced vection. Results demonstrate
B. Keshavarz (*) · D. Vena · J. L. Campos Department of Research, Toronto Rehabilitation Institute, Technology Team/iDAPT, 550 University Avenue, Toronto, ON M5G 2A2, Canada e-mail: [email protected] L. J. Hettinger Center for Behavioral Sciences, Liberty Mutual Research Institute for Safety, Hopkinton, MA, USA J. L. Campos Department of Psychology, University of Toronto, Toronto, ON, Canada
that the combination of visual and auditory cues can enhance the sensation of vection. Keywords Illusory self-motion · Vection · Multisensory integration · Motion sickness · Postural sway · Virtual reality
Introduction Vection is a well-known phenomenon and describes the illusion of self-motion in the absence of real, physical movement (Dichgans and Brandt 1978; Fischer and Kornmüller 1930). Vection can be frequently experienced in real-life situations (e.g., while sitting in a stationary car or a stationary train when the vehicles around you begin to move) or in virtual environments (e.g., in fixed-base driving or flight simulators) (for an overview, see Hettinger 2002). In these cases, observers are under the impression that they are moving, even though they remain physically stationary. Physical behaviors evidenced during real motion, such as postural adjustments, are also frequently observed with vection (Fushiki et al. 2005). The first scientific reports of vection can be dated back to more than a century ago (e.g., Mach 1875), and vection induced by visual stimulation has been extensively discussed ever since. In contrast, the influence of nonvisual information on the experience of vection has been largely neglected. In fact, vection can also be induced solely by auditory cues (see Väljamäe 2009 for an overview), a finding reported by Dodge (1923) as early as in the beginning of the twentieth century. However, only a few investigators had followed up on Dodge’s findings (e.g., Lackner 1977) until recently. As of late, research studying auditory vection has been revived and has
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piqued the interest of several research groups (Riecke et al. 2005; Sakamoto et al. 2004; Kapralos et al. 2004). Results of these studies have shown auditory vection to be less compelling and less frequently experienced than visual vection. Riecke et al. (2009) estimated that auditory vection is experienced by 25–60 % of all participants across different studies and, when auditory vection was perceived, the sensation was relatively weak. Research regarding auditory vection is still in the early stages and the precise nature of auditory vection is yet to be clearly defined. This is unlike the stimulus parameters underlying visually induced vection, which have been widely investigated for some time. Additionally, research regarding the effects of crossmodal integration of visual and nonvisual information on the perception of vection is almost nonexistent. To our knowledge, Riecke et al. (2009) were the only ones to analyze the influence of auditory cues when combined with visual stimulation. In their study, the authors used a virtual reality (VR) setup including a realistic visual stimulus (i.e., a high-resolution panoramic photograph of a city’s marketplace) that constantly rotated around the participants’ yaw-axis to create circular vection and added spatial 3D sound that was synchronized with the corresponding visual scene. Results demonstrated that the addition of auditory cues indeed enhanced vection (i.e., reduced vection onset time), highlighting the facilitative effects of multisensory cue combination. Additional vection-increasing effects of cross-modal integration have also been recently reported through demonstrations of increased auditory vection in the presence of vibro-tactile and haptic cues (e.g., vibration of the participants’ seat) (Riecke et al. 2008, 2009). Thus, the perception of vection seems to be moderated by multiple sensory modalities. Note that, in the past, vection has also been linked to the phenomenon of visually induced motion sickness (MS; Flanagan et al. 2004; Hettinger et al. 1990). Visually induced MS is a sensation very similar to traditional MS, and typical symptoms include (but are not limited to) pallor, cold sweat, nausea, dizziness, fatigue, and (rarely) vomiting (Hettinger and Riccio 1992; Kennedy et al. 1992). Vection is often considered to be involved in the genesis of visually induced MS, as most participants who suffer from visually induced MS also experience vection at the same time (e.g., Hettinger and Riccio 1992; Smart et al. 2002). However, others have demonstrated that vection does not necessarily or always lead to visually induced MS (Webb and Griffin 2002; Palmisano et al. 2008; Bonato et al. 2008). Hence, the precise relationship between vection and visually induced MS has yet to be defined. As with vection, the role of multisensory inputs and the role of auditory cues in particular in the genesis of MS have only been vaguely discussed so far (except for Keshavarz and Hecht 2012a;
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Exp Brain Res (2014) 232:827–836
Nichols et al. 2000). In the present study, we collected data germane to MS to control for possible MS symptomatology during and after stimulus exposure. In sum, the main goals of the study were (1) to analyze the role of combined auditory and visual cues in the perception of vection, and (2) to determine the strength of auditory vection compared to its visual counterpart. Additionally, we also collected data specific to the occurrence of MS to analyze the potential impact of auditory and visual vection on MS. The present study is the first to combine visual and auditory cues with respect to their impact both on vection and MS at the same time, with the same participants. In the present study, stationary participants were exposed to a VR scene that rotated about their perceived yaw-axis at a constant velocity for 360°. The virtual environment provided either only visual cues, only auditory cues, or a combination of visual and auditory cues. Additionally, simulated rotation speed (60°/s vs. 90°/s) and the number of sound sources (1 vs. 3) were varied. We collected subjective ratings on vection onset, vection strength, MS severity, and also recorded participant’s postural steadiness after each trial using center of pressure measured via a force plate. The relationship between auditory vection and postural steadiness has never before been examined. We hypothesized that auditory vection would be less pronounced than visual vection, whereas we expected that the combination of visual and auditory cues would generate the strongest vection. Based on the previous findings (Larsson et al. 2004), we assumed that adding more sound sources (3 vs. 1) and increasing the stimulus speed (90°/s vs. 60°/s) should enhance the strength of vection.
Methods Participants Thirteen male (Mage = 26.54, SDage = 5.77) and 7 female (Mage = 30.14, SDage = 7.99) adults participated in this experiment. All participants gave written consent prior to the experiment and stated that they were in a normal state of health. Participants had normal or corrected-to-normal vision and were naïve with respect to the purpose of the study. The stimuli were administered in accordance with the Declaration of Helsinki to ensure research ethics in human experimentation. The study protocol was reviewed and approved by the research ethics board of the Toronto Rehabilitation Institute. Participants were paid $10 and were informed that they were free to abort the experiment at any time without negative consequences. However, no participant chose to abort the study prior to regular termination.
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Exp Brain Res (2014) 232:827–836
Fig. 1 The left panel shows a schematic illustration of the laboratory (top-down view). Participants were seated in a rotatable chair, facing the curved projection screen (thick inner circle). A force plate (FP) was used to measure postural steadiness post-vection stimulus. Seven speakers (small white ovals) were used to present the rotating sound sources. The black-filled circles represent the 3 sound source
locations used in the experiment and their spatial arrangement with respect to the participant (S1 = church bells, S2 = streetcar, and S3 = car engine). The sound sources were synchronously rotated counterclockwise for 360° along the participants’ yaw-axis (dotted line). The right panel shows the experimental settings and a screenshot of the visual stimulus
Design, apparatus, and stimuli
and the VR scene was created using customized OpenScene Graph application. No components within the virtual scene were animated (e.g., no motion of cars or pedestrians). For the visual + auditory condition, three different sound sources were added to the visual scene: the sound of church bells, the engine sound of a nonmoving car, and the ringing sound of a stationary streetcar/tram. The loudness of each sound was set to 75 dB, and the distance between each sound source was held constant. For the trials containing only a single sound source, the sound of the church bells was chosen. In the auditory condition, visual stimulation was excluded by blackening the projector screen and by blindfolding the participants with a sleeping mask.
A 3 × 2 × 2 within-subjects design including stimulus condition (visual, auditory, and visual + auditory), simulated rotation speed (60°/s and 90°/s), and number of sound sources (1 and 3) was chosen. As the visual-only condition did not include sound at all, simulated rotation speed was the only factor that was varied within it. Thus, the total number of trials resulted in 10 (instead of 12 trials). Every participant was exposed to all 10 trials in a randomized order, and each trial was presented for 60 s. Participants were seated in a dimly lit, dome-shaped laboratory in a rotatable chair, 100 cm in front of a curved projection screen (see Fig. 1). Six projectors (Eyevis ESPLED series with LED technology) were used to create a visual image with a field of view of 240° horizontally and 120° vertically. Three-dimensional sound was delivered by 7 speakers located behind the projection screen (Meyersound MP-4XP) and a subwoofer (Meyersound MP10XP). The height of the chair was fixed at 70 cm above the laboratory’s floor. A headrest was mounted to the back of the chair to minimize head movements and to exclude head movements as a potential confounding factor accounting for changes in vection and MS perception. Participants were asked to have their feet hanging loosely in the air without touching the ground or any other surface. The visual stimulus consisted of a VR scene of downtown Toronto that was continuously rotated for 360° about the observer’s yaw-axis. Simulated rotation speed was set to 60°/s or 90°/s, depending on the experimental trial. No acceleration and deceleration were used at video onset and offset, respectively. Picture resolution was 6.5 arcmin/OLP
Response measures Two different vection parameters were collected, including vection onset time and vection strength. To measure the onset time of vection, participants verbally indicated whenever they started to feel vection. Data regarding the strength of vection were collected after each trial using an 11-point Likert scale (0 = not at all, 10 = very strong). Motion sickness data were collected using the Fast Motion Sickness Scale (FMS; Keshavarz and Hecht 2011). The FMS is a verbal rating scale ranging from 0 (no sickness at all) to 20 (severe sickness) that was designed to monitor the time course and the severity of MS (i.e., nausea and general discomfort in particular) and has been validated using the well-established Simulator Sickness Questionnaire (Kennedy et al. 1993). In the past, high correlations between the FMS and the SSQ (e.g., r = .83) suggested that the FMS scale is a valid and reliable tool to capture the severity of
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MS (Keshavarz and Hecht 2011). The FMS is a fast and easy-to-assess tool and allows continuous capturing of MS without disrupting task performance. Hence, participants were asked to verbally report their level of sickness by choosing a single score using the FMS after each trial. To make sure that no MS-related carryover effects occurred, participants were provided with a rest period after each trial until any MS symptoms had subsided to normal (i.e., FMS score of zero) before continuing with the experiment. Additionally, we measured participants’ postural steadiness, as several studies have shown that postural steadiness is linked to MS (e.g., Flanagan et al. 2004; Stoffregen et al. 2000; Smart et al. 2002). We captured steadiness data once before the experiment began and once after each trial. Participants were instructed to stand on a force plate (AMTI BP12001200), align their feet with predefined markings in a parallel position (distance between ankle bones approximately 7 cm), and close their eyes for 30 s. Postural steadiness was examined using the center of pressure (COP) calculated from the force plate. Data from the first 25 s of the 30-s test period of quiet standing were used across all participants. Force data were filtered using a sixth-order dualpass butterworth filter with a 6-Hz cutoff frequency. Matlab R2012 was used to compute COP measures of postural steadiness. COP-based measures used for the analysis were variability (standard deviation) of COP excursion in the medial–lateral (ML) and anterior–posterior (AP) directions, total length of the COP path (COP length), and the 95 % confidence area surrounding the COP (COP ellipse). More specifically, COP length was calculated as the sum of the distance between consecutive points for the 25-s dataset. The 95 % confidence ellipse is defined as the area containing the center of the COP points with a 95 % probability. Procedure Before the first trial was presented, a practice trial was used to familiarize participants with the sensation of vection and with the data collection procedure. For this purpose, the stimulus that was predicted to produce the strongest sense of vection (i.e., visual + auditory stimulus containing a simulated rotation speed of 90°/s and 3 sound sources) was presented to the participants until they reported fully saturated vection (i.e., vection strength of 10). After the practice trial, participants were exposed to all 10 trials in randomized order.1 The first measure of postural steadiness was carried out after the practice trial and acted as a baseline with respect to the subsequent measurements. Between each of the 10
1 Statistical analyses revealed no significant effect of trial order and demonstrated successful randomization.
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Exp Brain Res (2014) 232:827–836
trials, postural steadiness data were collected immediately after stimulus offset using the same procedure. This resulted in a total of 11 steadiness measurements. A short rest break between each trial was provided. During each stimulus presentation, participants were asked to avoid head and body movements by positioning their heads into the headrest and focusing on the stimuli. Additionally, during the stimulus presentation, participants were simply asked to verbally indicate whenever they started to feel vection. Vection strength and sickness ratings were collected verbally after each trial. Before leaving the laboratory, all subjects were debriefed and the experimenter ensured that all MS-related symptoms had subsided to normal.
Results For all statistical analyses, SPSS (Statistical Package for Social Sciences, IBM, Version 21) was used. A priori significance level was set to α = .05. Mean ratings for vection onset time and vection strength are shown in Fig. 2, and MS ratings are given in Fig. 3. To analyze the influence of simulated rotation speed and the number of sound sources in the trials including visual cues, a 3 × 2 repeated-measures (rm) ANOVA including the within-subjects factors sound (no sound vs. 1 vs. 3) and speed (60°/s vs. 90°/s) was calculated for each dependent variable, that is, vection onset time, vection strength, FMS scores (motion sickness), and COP data. Note that in the case of absent vection (i.e., participants did not report feeling vection at all), the maximum onset time of 60 s was chosen and transferred to the data. Results showed a significant effect of sound with respect to vection strength, F(2,38) = 9.38, p
