Keshavarz, B. et al. Demonstrating the Potential for Dynamic Auditory Stimulation to Contribute to Motion Sickness

Keshavarz B, Hettinger LJ, Kennedy RS, Campos JL (2014) Demonstrating the Potential for Dynamic Auditory Stimulation to Contribute to Motion Sickness. PLoS ONE 9(7): e101016. doi:10.1371/journal.pone.0101016

Published: July 1, 2014

The present study is the first to demonstrate that motion sickness may be caused by pure auditory stimulation, which we refer to as “auditorily induced motion sickness”

Abstract

Auditory cues can create the illusion of self-motion (vection) in the absence of visual or physical stimulation. The present study aimed to determine whether auditory cues alone can also elicit motion sickness and how auditory cues contribute to motion sickness when added to visual motion stimuli. Twenty participants were seated in front of a curved projection display and were exposed to a virtual scene that constantly rotated around the participant’s vertical axis. The virtual scene contained either visual-only, auditory-only, or a combination of corresponding visual and auditory cues. All participants performed all three conditions in a counterbalanced order. Participants tilted their heads alternately towards the right or left shoulder in all conditions during stimulus exposure in order to create pseudo-Coriolis effects and to maximize the likelihood for motion sickness. Measurements of motion sickness (onset, severity), vection (latency, strength, duration), and postural steadiness (center of pressure) were recorded. Results showed that adding auditory cues to the visual stimuli did not, on average, affect motion sickness and postural steadiness, but it did reduce vection onset times and increased vection strength compared to pure visual or pure auditory stimulation. Eighteen of the 20 participants reported at least slight motion sickness in the two conditions including visual stimuli. More interestingly, six participants also reported slight motion sickness during pure auditory stimulation and two of the six participants stopped the pure auditory test session due to motion sickness. The present study is the first to demonstrate that motion sickness may be caused by pure auditory stimulation, which we refer to as “auditorily induced motion sickness”.

Introduction

Motion sickness (MS) is a well-known and widely reported malady. MS is not only a major issue among travelers (e.g., on cars, buses, trains, airplanes, or ships), but also for users of virtual environments (e.g., driving simulators or gaming systems). As real physical motion is typically absent in the latter scenarios (except for motion-based simulators), MS is often referred to as being visually induced in these cases (see [1] for an overview). An acute phase of MS is characterized by symptoms ranging from sensations of sudden warmth, pallor, sweating, drowsiness, and fatigue, to more severe stomach problems, increased salivation, retching, nausea, and/or vomiting [2], [3].

The precise nature of MS and its etiology is not fully understood and several theories exist (see [4] for an overview). For example, the “sensory conflict theory” [5] proposes that a mismatch between (or within) the visual, the vestibular, and/or the somatosensory senses causes MS. Based on this theory, a fixed-base simulator can cause visually induced MS due to the incongruent information delivered to the eyes (indicating self-motion, see [6] for an overview) and the vestibular and somatosensory senses (indicating a veridical, stable, and non-moving position). If the nature of the conflict is novel to the organism (i.e., no previous experience of this particular scenario), MS is possible [7]. In contrast, others postulate as a part of the “postural stability theory” that changes in the amount of postural steadiness (either reduced or increased) precede the occurrence of MS [8], [9]. A comprehensive overview of the most prominent theories explaining MS and a critical comparison of these theories is provided by [10].

None of the current theories explicitly address the role of auditory information in the genesis of MS, yet auditory cues might in fact contribute to MS in at least two ways. First, spatial sound can create illusory self-motion (vection) in the absence of visual, vestibular, or somatosensory information (see [11] for an overview). In this case, the information perceived by the auditory system contradicts the information given by other sensory modalities, thereby introducing a sensory conflict. Second, spatial sound is also known to create physical responses such as adjustments to posture [12], [13], which might influence self-motion perception and MS. Currently, it is not clear whether the strength of the response to spatially moving sound is enough to by-pass other cues to self-motion perception (i.e., visual, vestibular, somatosensory) in a way that leads to perceptual, behavioral, and physical responses such as MS or vection. Anecdotal reports indeed suggest that auditory cues can create MS symptoms (J. Lackner, personal communication, October 03, 2013), but scientific findings are non-existent (see [14], p. 33) and thus, the aim of the present study was to fill this gap.

In a recent study [15], we analyzed the effect of auditory information on vection [15]. Participants were exposed to a constantly rotating stimulus that contained either only visual, only auditory, or a combination of corresponding visual and auditory cues. These results demonstrated that auditory cues significantly increased vection strength and reduced the onset time of vection when they were added to visual cues (compared to pure visual or pure auditory stimulation). Motion sickness data was also collected as a control factor and results showed that auditory cues did not affect the level of MS. Note, however, that the primary focus in [15] was on vection and contributing factors, thus, the experimental settings were not optimized for producing and assessing MS; hence, MS-reports were generally very weak, likely resulting in a floor effect. The present study was intended to follow up on the findings by [15], but with a primary focus on introducing factors that were likely to maximize the chances of observing MS. We aimed to answer two questions: First, can auditory stimulation in the absence of visual cues elicit MS? Second, does the inclusion of dynamic auditory stimulation enhance the experience of MS when added to visual motion displays? We used the same apparatus as in [15] (e.g., laboratory, stimuli etc.), but modified the experimental parameters to be able to optimally measure MS. For instance, we prolonged the stimulus duration and we asked participants to tilt their heads to the right or left shoulder alternately while being exposed to the visually or auditorily rotating stimulus. Such head movements have previously been shown to cause pseudo-Coriolis sensations that can increase MS [16], [17], [18], [19]. Traditional Coriolis sensations (e.g., [20]) are experienced when the tilting of the head during full-body axis rotation causes an intra-vestibular canal-otolith mismatch that leads to severe MS. Pseudo-Coriolis sensations, on the other hand, are not related to the interactions between head rotations/vestibular feedback and physical rotations, but are induced via visual rotations. We assessed measurements of MS (severity, onset time), vection (vection strength, vection onset time, vection duration), and posture (center of pressure) using well-validated tools.

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