Use of virtual reality (VR) technology is often accompanied by a series of unwanted symptoms, including nausea and headache, which are characterised as ‘simulator sickness’. Sensory mismatch has been thought to lie at the heart of the problem and recent studies have shown that reducing cue mismatch in VR can have a therapeutic effect. Specifically, electrical stimulation of vestibular afferent nerves (galvanic vestibular stimulation; GVS) can reduce simulator sickness in VR. However, GVS poses a risk to certain populations and can also result in negative symptoms in normal, healthy individuals. Here, we tested whether noisy vestibular stimulation through bone-vibration can also reduce symptoms of simulator sickness. We carried out two experiments in which participants performed a spatial navigation task in VR and completed the Simulator Sickness Questionnaire over a series of trials. Experiment 1 was conducted using a high-end projection-based VR display, whereas Experiment 2 involved the use of a consumer head mounted display. During each trial, vestibular stimulation was either: 1) absent; 2) coupled with large angular accelerations of the projection camera; or 3) applied randomly throughout each trial. In half of the trials, participants actively navigated using a motion controller, and in the other half they were moved passively through the environment along pre-recorded motion trajectories. In both experiments we obtained lower simulator sickness scores when vestibular stimulation was coupled with angular accelerations of the camera. This effect was obtained for both active and passive movement control conditions, which did not differ. The results suggest that noisy vestibular stimulation can reduce simulator sickness, and that this effect appears to generalize across VR conditions. We propose further examination of this stimulation technique.

Introduction

Recently, technological advances have supported a proliferation of inexpensive and powerful consumer-oriented virtual reality (VR) hardware devices. This advancement creates an urgent need to solve some of the key problems of VR exposure. Perhaps the principle problem is a phenomenon known as ‘simulator sickness’ (also known as ‘cybersickness’ [1–2]). Around 80% of VR users typically experience some symptoms of sickness, with as many as 50% experiencing symptoms with such severity that they are compelled to terminate a session of VR early [3]. The most common adverse effects of virtual environment immersion include nausea, headache, sweating, and vomiting. These symptoms can persist for several hours following exposure to the environment [4–5]. The symptoms are often sufficient to compel users to avoid further use of VR entirely [6–7]. Given that VR technology offers a valuable method for use in skills training, education, and clinical rehabilitation, there has been a substantial amount of research into the causes of simulator sickness in VR [3].

Causes of simulator sickness A number of contributing factors have been implicated in the etiology of simulator sickness, including visual flicker, low refresh-rate, and high motion-to-photon latency [6,8–10]. As tracking and display technology continues to develop, user comfort is expected to increase—although some display improvements may in fact exacerbate symptoms, such as increases in the field-of-view [11]. Frequently, VR experiences simulate self-motion through an environment using optic flow, and this manner of simulation appears to be a particular trigger for sickness [10–13]. It is well known that optic flow is sufficient to specify motion of an observer through their environment [14–15]. However, if the vestibular sense does not receive stimulation at the moment of motion onset and offset to indicate body accelerations, sensory information is incongruent. Symptoms are thought to occur as a result of the nervous system attempting to respond appropriately to sensory mismatch, which is a situation that might have been caused internally (e.g., as the results of accidental ingestion of a neurotoxic substance [13,16–17]). In that case, nausea and the emptying of the stomach could be considered an adaptive function, although the response becomes severely maladaptive when the sensory conflicts result from curve navigation in a driving simulator. Another explanation for simulator sickness has focused on the postural instability produced by exposure to VR technology [18–19]. It is possible that decreased postural stability in VR increases the number and magnitude of cue-conflicts that may underlie symptoms of discomfort, although empirical evidence is as of yet unclear [20–21].

Techniques for reducing simulator sickness Despite the understanding acquired about the causes of simulator sickness, its prevention and treatment have received less attention. One preventative approach has been to avoid situations that generate sensory mismatch: For example, Dorado and Figueroa [22] implemented camera movement in VR that avoids accelerations as much as possible. They showed that using ramps instead of staircases for changing elevation in the environment can reduce the degree of simulator sickness experience by the user. Recently a ‘point and teleport’ method for moving in a virtual world has gained popularity, where a user specifies a position to which they will relocate upon a button press [23]–this technique also minimizes the accelerations of the visual scene. Another method has focused on preventing sensory mismatch by ‘recoupling’ the visual and vestibular systems during navigation of a VR environment. Some approaches involve using motion platforms to move the body along with visually-simulated motion [24–25], and several consumer-oriented motion simulators are beginning to emerge. The efficacy of motion base simulators in reducing simulator sickness is not clear, however. While studies show improved comfort for moving base compared to fixed base simulators [26–27], others indicate no effect of motion cueing [28], or even an exacerbating effect on symptom severity [29]. Compared to their significant expense and technical complexity, the current balance of research shows little evidence that motion platforms effectively reduce symptom severity compared to stationary conditions [9,30–31]. Another technique, galvanic vestibular stimulation (GVS), has been effective in preventing symptoms of simulator sickness in virtual environments. This technique involves applying an electrical current to electrodes near the mastoid processes in order to stimulate vestibular afferent nerves. Applying GVS to recouple visual and vestibular cues was shown to reduce the incidence of simulator sickness in a flight simulator task [32]. The technique has found additional support in a study by Reed-Jones and colleagues [33], where simulator sickness was reduced by using GVS in a driving simulator. This is also supported by additional evidence, showing a preventive effect of galvanic stimulation on simulator sickness in a driving task, regardless of whether stimulation is applied during curve maneuvers or intermittently throughout the task [34]. The visual-vestibular recoupling approach to simulator sickness has led to the development of preliminary consumer-oriented GVS devices [35]. Nonetheless, a series of practical issues remain in terms of the use of GVS in VR experiences. Previous research indicates that GVS use is associated with symptoms of discomfort in some healthy users [36]. For certain individuals, such as pacemaker users, there are serious risks involved in applying direct current stimulation to the surface of the body, as is the case with GVS [37]. An additional obstacle to the widespread adoption of GVS is the precise match between vision and vestibular stimulation required in order to accurately replace the expected vestibular signals. Small errors between directional cues derived from vision and those that are applied using GVS could engender sensory mismatches that impact performance and comfort significantly [37]. Recent research from our group has employed a vestibular stimulation method that presents a possible solution to both the problem of invasiveness and the problem of precision described above. The method we have used involves applying noisy stimulation to the vestibular system using bone-conducted vibration (BCV) that is applied at the mastoid processes. This technique has been shown to evoke the oculomotor and myogenic responses similar to those produced by linear accelerations of the otolith organs [38–42]. Unlike with GVS, there are no known populations for whom BCV produces adverse effects, according to evidence obtained with well over 3000 participants [43]. At the same time, we contend that the use of BCV to reduce sensory mismatch does not require a precise mapping between the expected vestibular signal and the applied vestibular signal, given that the intention of the approach is to add sensory noise to the vestibular system. This reduced constraint therefore renders BCV easier to implement than GVS, where the aim is typically to ‘recouple’ vision and vestibular cues [32–34]. In addition, we recently proposed that BCV reduces the sensory reliability of the vestibular system, which has the consequence of upweighting visual self-motion information that is obtained during stimulation. This theory was presented on the basis of evidence that visually evoked illusions of self-motion (vection) are facilitated by noisy stimulation of the vestibular system with both BCV and noisy GVS [44]. The idea builds on a Bayesian cue integration framework where sensory cues are inversely weighted by their reliability [45–47]. The results of previous work conducted by our group [44] provided strong evidence that BCV–an otolith stimulation [38, 43]–facilitates quicker vection when it is applied in conditions in which no otolith stimulation would be expected (e.g., yaw rotation about the vertical axis). This finding points towards a general effect of BCV on vestibular processing, which we attributed to a reduction in vestibular reliability. The same study also disputes the possibility that noisy vestibular stimulation simply masks the input to vestibular organs, since we observed similar effects between BCV (otolith) and noisy GVS (non-specific vestibular afferent stimulation [48]). In the context of the relationship between sensory conflict and simulator sickness proposed by Reason and Brand [13], we expected that reducing vestibular reliability in this manner would give rise to reduced conflict and improved comfort in VR. We designed the current study to test this possibility.