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Effect of economically friendly acustimulation approach against cybersickness in video-watching tasks using consumer virtual reality devices
Applied Ergonomics 82 (2020) 102946
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Effect of economically friendly acustimulation approach against cybersickness in video-watching tasks using consumer virtual reality devices
T
Ruijun Liua,b, Chu Zhuangc, Rui Yanga,b, Liang Mad,∗ a
School of Computer Information and Engineering, Beijing Technology and Business University, 100048, Beijing, PR China Beijing Key Laboratory of Big Data Technology for Food Safety, 100048, Beijing, PR China c Social Science Division, University of Chicago, Chicago, IL, 60637, USA d Department of Industrial Engineering, Tsinghua University, 100084, Beijing, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: VR headset Cybersickness Acustimulation
Background: Consumer virtual reality (VR) devices are becoming more prevalent in the market, but cybersickness induced by VR devices limits their potential application and promotion. Acustimulation has been found effective in reducing cybersickness symptoms. However, in previous forms, the more effective way of acustimulation is either intrusive or electrical which is hard to be applied to daily VR use. Purpose: In this study, we aimed to find a both simple and more effective acustimulation approach, acupressure plus acupaste (AcP+) to reducing the adverse effects caused by cybersickness from VR applications. Method: In this study, we set three conditions: acupressure plus acupaste (AcP+) (main condition of interest), acupressure with fake acupaste (AcP), and a no acustimulation condition (NoAcP). In AcP and AcP + conditions, we applied acupressure or acupressure with true acupaste on P6 point before conducting video-watching tasks using VR headsets, while in NoAcP condition, participants received no special treatment before video-watching tasks. We used questionnaires to measure symptoms of cybersickness and compared the results between these 3 conditions, especially between acupressure plus acupaste (AcP+) and acupressure (AcP) to examine the effect of AcP+, and compared AcP and AcP+ with NoAcP to confirm the effect of acustimulation. Result: Participants reported significant fewer symptoms of cybersickness nausea feelings in both acustimulation methods, compared with NoAcP; and AcP+ was more effective than AcP against cybersickness on visual oculomotor aspect, and facilitated cybersickness recovery. Implication: It would be promising to develop acupressure equipment and apply stimulation before VR application to reduce cybersickness.
1. Introduction The year 2016 is known as the year of virtual reality (VR), and since then, consumer immersive visual display products are becoming more and more prevalent (Rutkin, 2016). Head-mounted displays (HMDs), or VR headsets, are one of the major types of device on the market, and the type with the highest demand, which is continually increasing. For example, Google has announced the intent to produce a consumer heads-up display (Bilton, 2012), and Facebook has acquired Oculus VR for over $2 billion (Parkin, 2014). An HMD is a closed display that places two separately rendered images in different screens in front of the eyes to create stereoscopic views for the user (Rebenitsch and Owen, 2016). However, cybersickness induced by these VR devices (Rutkin, 2016; Rebenitsch and Owen, 2016; So and Ujike, 2010) are frequently reported, which limits their potential application and ∗
promotion. Cybersickness is defined as the onset of nausea, oculomotor, and/or disorientation while experiencing virtual environments in headmounted displays, large screens, and curved screen systems (Rebenitsch and Owen, 2016). Cybersickness is also considered the digital version of motion sickness, or visually-induced motion sickness. Symptoms of cybersickness include not only symptoms similar to motion sickness, such as nausea, pale skin, cold sweats, vomiting, dizziness, headache, increased salivation, and fatigue, but also eyestrain and focusing difficulty, because the virtual environment puts additional strains on the eyes (Ehrlich, 2012). The uncomfortable feelings caused by using VR devices becomes a great challenge to promoting VR applications, and tremendous effort has been made in both industrial practice and academic research to overcome this obstacle. Most current efforts against cybersickness are
Corresponding author. S611, Shunde Building, Dept. of Industrial Engineering, Tsinghua University, 100084, Beijing, PR China. E-mail addresses: [email protected] (R. Liu), [email protected] (C. Zhuang), [email protected] (R. Yang), [email protected] (L. Ma).
https://doi.org/10.1016/j.apergo.2019.102946 Received 12 May 2018; Received in revised form 21 August 2019; Accepted 27 August 2019 0003-6870/ © 2019 Elsevier Ltd. All rights reserved.
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explanation for the effectiveness of acupuncture against cybersickness (Chu et al., 2012). Because of the intrusiveness of acupuncture and apprehension to the piercing needles, other forms of nonintrusive acupuncture, more broadly named acustimulation, such as acupressure and electro-acupuncture, were developed to substitute the effect of acupuncture. Both acupressure and electro-acupuncture have been tested to be more or less effective in reducing motion sickness as well (Chu et al., 2012; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001). However, the effects of these substituted forms of acustimulation were limited and unstable; till the intensity of acustimulation was high enough, they could approximately reach the full effect of acupuncture (Chu et al., 2012; Hu et al., 1992). For electro-acupuncture, it is easy to fulfill the high intensity requirement and reach comparable effect of acupuncture, while this approach is still relatively intrusive that electricity exerted on human skin which might arouse mental discomforts. Among acustimulation approaches, except for increasing the intensity of acupressure, acupaste, which contains small stones that could facilitate the effect of acupressure, is developed as a common nonintrusive substitute for acupuncture needles, to achieve the full function of acupuncture. However, the effect of acupressure plus acupaste has not been tested in reducing motion sickness and cybersickness yet. Considering the easiness of conducting acupressure, the little mental discomforts it could induce and the low cost of this approach, we predict that acupressure plus acupaste could be a promising countermeasure, both simple and more effective, for cybersickness in VR applications, especially for low-cost VR devices. Therefore, in this study, we aim to examine the effectiveness of acupressure plus acupaste in reducing the adverse effects of cybersickness in VR applications. In this study, to examine the effectiveness of acupressure plus acupaste, we designed three conditions, acupressure plus acupaste (AcP+), acupressure (AcP, sham-condition) and a no acustimulation condition (NoAcP, control condition). In AcP+ and AcP conditions, acupressure in conjunction with acupaste or only acupressure was exerted on participants before the VR experience, and in NoAcP condition, no special treatment, either acupressure or acupaste was applied to participants. For AcP+ and AcP conditions, they were controlled to be as same as possible except that AcP + used true acupaste, while AcP used a ‘fake’ invalid one but has the same appearance with the true one (detailed descriptions of these two forms of paste were provided in Section 2Methods). Therefore, in this experiment, AcP conditions was served as a sham condition and NoAcP as control condition. The differences between the acupressure plus acupaste (AcP+) and acupressure (AcP), AcP+ and control condition (NoAcP), are of main interests to examine the effect of our newly proposed approach; and the difference between AcP and control condition is also of interests to confirm the effect of acustimulation which has been proved. Pre- and post-VR cybersickness were measured during the experiment. The detailed procedure is introduced in Section 2. Results and discussions regarding the effectiveness of acustimulation are shown in Sections 3 and 4.
explored from the technology perspective (Chen et al., 2016; Kolasinski, 1995; Stanney and Salvendy, 1998; LaViola, 2000; Rebenitsch and Owen, 2016), such as providing scenes with better quality (Jennifer et al., 2004; So et al., 2001), blurring the screen during rotational movements (Budhiraja et al., 2017), reducing the time-delay of scene update (Draper et al., 2001), changing the field of view (Bos et al., 2010; van Emmerik et al., 2011), or providing additional reference frames or spectral filters in the view (Chang et al., 2013; Prothero et al., 1999; Wikins and Evans, 2010). Technology improvement against cybersickness partially solves the cybersickness problem. However, while improvements in technology solve challenging technical problems, especially difficulties in reducing the unavoidable delay of update and the inherent mismatch of visual and vestibular inputs (Rebenitsch and Owen, 2016), they also greatly increase the cost. Therefore, owing to the hardware limitations of low-end consumer VR devices, such as small view fields, low refresh frequency, heavy weight, poor air permeability, and the poor quality of display content (e.g., resolution, frame rate, scene change), improvements against cybersickness in terms of technology may be too costly. In addition to technology improvement, behavioral countermeasures against cybersickness have also been explored through intervention by VR users. These measures cost much less and are relatively easy to implement, such as adaptation (Hill and Howarth, 2000; Regan, 1995; Sugita et al., 2007), re-allocating attention (Wei et al., 2018), relaxing music (Keshavarz and Hecht, 2014), and diaphragmatic breathing (Russell et al., 2014). However, certain limits exist with respect to these measures for normal daily VR use. For example, adaptation requires that users view the same VR material at least two times in order to develop resistance against cybersickness, and cybersickness is not effectively reduced when using VR for the first time. Music constitutes the auditory cue and restricts audio input and breathing occupies considerable cognitive capacity while experiencing VR. Originating from traditional Chinese medicine, acupuncture is a popular and mature method with a long medical history for treating nausea and motion sickness (Dundee et al., 1986). Acupuncture is an intrusive medical approach that involves piercing the skin with acupuncture needles and applying stimulation at specific points on the human body. For example, point P6 (see Fig. 3, Nei-Kuan Point) is one of the most frequently-used points for treating nausea-related symptoms (Dundee et al., 1986; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001). Many studies have also shown that acupuncture at P6 could effectively suppress the nausea symptoms of motion sickness (Chu et al., 2012; Stern et al., 2001) and also of vection-induced motion sickness (VIMS) in simulators (Hu et al., 1992; Hu et al., 1995). The underlying mechanism of why acupuncture was effective in reducing nausea symptoms, motion sickness and cybersickness has been discussed (Dundee et al., 1986; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001; Streitberger et al., 2006). According to the most prevalent cybersickness theory-sensory conflict theory (Davis et al., 2014; Rebenitsch and Owen, 2016), VIMS and cybersickness is evoked by the conflict among somatosensory, visual, and vestibular inputs. The underlying mechanism of why acupuncture was effective in reducing nausea symptoms, motion sickness and cybersickness has been discussed (Dundee et al., 1986; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001; Streitberger et al., 2006). According to the most prevalent cybersickness theory-sensory conflict theory (Davis et al., 2014; Rebenitsch and Owen, 2016), VIMS and cybersickness is evoked by the conflict among somatosensory, visual, and vestibular inputs. Autonomic nervous system (ANS) has physiological connections with vestibular and somatosensory systems (Schmidt and Thews, 1989). When conflicts among somatosensory, visual and vestibular inputs happened, ANS will be activated, which leads to sickness symptoms (Money, 1970; Ohyama et al., 2007; Previc, 1993). It has been examined that acupuncture could successfully influence ANS responses, through which mediates nausea symptoms, and even enhance visuospatial abilities, postural control, and cognitive function, which provided the currently most accepted
2. Methods 2.1. Subjects A total of 29 graduate students in college (12 males and 17 females, with an average age of 24.0 years, SD = 1.0 year) were recruited in this study. All subjects received health-related tests, including vestibular and visual tests, before the experiment to ensure that they were healthy and free of medication and illness, especially any illness which could cause motion sickness. Each subject provided written informed consent in which the possible risks of the experiment were stated. Subjects were instructed to refrain from the use of medication, alcoholic substances, and caffeinated drinks for at least 24 h before each experiment. The subjects who completed the experiment were entitled to a payment of 150 RMB. The study protocol was approved in advance by the 2
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channel, the video material could effectively stimulate related symptoms of cybersickness. Fig. 2 shows the scene that subjects saw in the VR device. The VR device could display the video at a frame rate of 50 fps with a refresh rate of 60 Hz, which are common technical specifications of VR headsets on the market. We used questionnaires to measure cybersickness in this study, including the Motion Sickness Susceptibility Questionnaire (MSSQ), Nausea Rating Test (NRT), and Simulator Sickness Questionnaire (SSQ). The MSSQ (Golding, 1998) is often used to test subjects’ past history of vertigo, and analyzes the frequency with which an individual has become ill from motion in the past, as cybersickness studies may be confounded by the susceptibility of the subjects. The NRT (Lo and So, 2001) is often used as a rapid assessment of cybersickness during VR application. From no symptom to moderate nausea, the NRT measures nausea on a seven-point scale (see Table 1). The NRT needs subjects to verbally report their current discomfort level every 5 min. We used the NRT during the VR video watching. We used SSQ to measure the physiological status and comfort degree of subjects (Kennedy et al., 1993). The SSQ was developed by Kennedy, and it is a standard questionnaire and most frequently used in assessing cybersickness (Kennedy et al., 1993). In comparison with the NRT, SSQ is able to distinguish cybersickness in three different dimensions—nausea (stomach awareness, increased salivation etc.), oculomotor (eyestrain, difficulty focusing, blurred vision, etc.), and disorientation (dizzy-eyes opened, vertigo) on a scale of 0–3, and thus can provide richer information for further analysis. Due to its multiple dimensions, we used the SSQ only before and after the VR application. In this study, except for the control condition that no acustimulation was given to the participants, both acupressure condition (AcP) and acupressure plus acupaste condition (AcP+) adopted nonintrusive acustimulation before the VR experience. In the AcP + condition, we exerted acupressure on the Nei-Kuan Point (P6) in conjunction with acupaste (Fig. 3); while in the AcP condition, we exerted acupressure in conjunction with a fake paste, which is of the same size, shape and apperance as the true acupaste, which served as the sham/baseline condition compared with AcP+. The acupaste (acupuncture paste) is a new therapy of acupuncture, which consists of a stone needle and lowsensitivity medical pressure-sensitive adhesive. It follows the same principle as traditional acupuncture, namely by stimulating the meridian acupuncture points to treat disease (Zhang, 2015). In the AcP condition, we exerted acupressure in conjunction with a fake paste, carefully made by medical proof fabric (with no special function but physical protection and stabilization), and as the exact same size, circle shape and color as the true acupaste. Hence, AcP condition served as the sham/baseline condition compared with AcP+. During the pressing intervention, we asked subjects to close their eyes and keep their body relaxed. The intervention process lasted about 2 min, and the acupaste remained stuck to the subject's P6 point until the end of the experiment.
Fig. 1. (a) Seating posture and experimental environment during videowatching VR application; (b) Samsung Gear VR SM-R323 and Samsung S7 Edge.
Institutional Review Board.
3. Materials We set up the experiment in a closed room (area = 40 m2). The air quality and temperature were controlled at 23 °C by air conditioning. Subjects could sit and act freely on a swivel chair (i.e., looking up or rotating) but could not move from the chair while wearing the VR device (Fig. 1). In this study, we used the VR headset Samsung Gear VR (SM-R323) with the Samsung S7 Edge smartphone, because this Samsung VR headset is one of the best-selling consumer VR devices on the market. In the first quarter of 2017, the Samsung Gear VR had the largest share of the VR market with shipments of approximately 782,000 headsets (SuperData Research, 2017). The SM-R323 weighs about 312 g alone and weighs 469 g in total with the Samsung S7 Edge. The viewing angle is up to 101°, and the motion-to-photons latency is less than 20 m s. We chose a video-watching task as the VR application in this study, because video watching is a typical VR application and it requires both visual and auditory cues. The video watching task normally does not involve large body movement, so disturbances from body movement could be greatly limited, and thus we could reduce other possible causes of cybersickness. We used a 20-min high resolution spliced video as 3D VR content for this experiment. The video had a resolution of 4096 pixels × 2160 pixels, and it was made up of five separate video clips. These clips were commonly used virtual reality scene videos that included slow and fast motion, with different levels of exposure to 3D scenes. After a preliminary test, we found that through the visual
Fig. 2. 3D virtual environment consisting of five separate video clips. 3
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Fig. 3. (a) Illustration of Nei-Kuan pressure point and application of acupressure to subjects; (b) Sample and instructions of Acupaste. Table 1 Nausea rating test scale (Lo and So, 2001). Scale
Symptom
0 1 2 3 4 5 6
No symptom Any unpleasant symptom, however slight Mild unpleasant symptom Mild nausea Mild to moderate nausea Moderate nausea but can continue Moderate nausea, want to stop
We carried out all the pressing operations in the experiment under the guidance and instruction of a professional medical practitioner.
Fig. 4. Illustration of the experiment procedure for one experimental session.
3.1. Experimental protocol
In Phase 1 of each session, subjects were asked to wait in the lounge for 5–8 min until they fully calmed down and could start the experiment in a normal state, checked by both subjective reporting and experimenter's standardized observation (E.g. no sweating, breathing calmly, normal consciousness etc). After entering the laboratory, subject was firstly asked to fill out the pre-SSQ. While helping the subject put on the VR device, we explained the NRT to the subject. After putting on the VR device, the subject had to freeze the screen at the main interface before watching the video, and we applied the pre-assigned acustimulation to the subject. The acustimulation for AcP and AcP + lasted for about 2 min. Of NoAcP condition, this 2 min s period would be occupied by VR equipment adjusting; no other special manipulation was conducted. After acupressure application, we asked subjects to watch the VR video for 20 min, and then took their nausea ratings using NRT every 5 min. After watching the video for 20 min, we asked subjects to fill out the post-SSQ and move to the rest area for about 10 min. Within those 10 min, we conducted a recovery test on the subject in compliance with scale of the NRT every 2 min. In addition, we asked the subject several experiment-related questions, such as about the fidelity of the simulation, whether the video segment induced uncomfortable feelings and the degree of the symptoms, and how they were feeling compared with the previous experiment. Ten minutes later, we checked whether the subject was able to conduct normal activities without symptoms of cybersickness (E.g. able to walk straight normally, with no blurred vision, eyestrain and nausea feelings at all). If yes, the session would be over; otherwise, the subject had to rest until self-reported full recovery from cybersickness.
We used a within-subject experimental design in this study. Each subject received all of the three conditions (NoAcP, AcP, AcP+) in three separate experiment sessions conducted at the same time on 3 different days. Regarding the influence of adaptation, there was a twoday interval between the two successive sessions (Hill and Howarth, 2000; Regan, 1995; Sugita et al., 2007). In order to limit the order effect, all the subjects were randomly assigned to three groups (groups A, B, and C), and each group followed an experiment sequence different from the other two groups. A Latin square experimental design was used to determine the sequence of treatments received by each group. The detailed experimental arrangement is shown in Table 2. Before the first treatment, each subject was asked to fill in the MSSQ to measure their past history of vertigo. Each subject needed to finish the three experimental sessions under different acustimulation conditions, and each session was composed of four phases: Phase 1 (8 min), experiment set-up and preparation; Phase 2 (2 min), pre-VR acustimulation; Phase 3 (20 min), VR video watching and cybersickness measurement; Phase 4 (10 min), post-VR nausea assessment (see Fig. 4). Table 2 Treatment sequence for each group.
Group A Group B Group C
Day 1
Day 2
Day 3
AcP NoAcP AcP+
AcP+ AcP NoAcP
NoAcP AcP+ AcP
4
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Table 3 Mean (SD) of MSSQ scores.
MSSQ scores
Group A
Group B
Group C
8.04 (4.78)
8.84 (12.37)
8.61 (9.06)
3.2. Data analysis We used SPSS Version-19 for data analysis and R Version-3.4.1 for drawing figures. The priori significance level was set to α = 0.05. Two subjects from experimental group B were excluded from the data analysis, because they reported a very low level of cybersickness during the experiments (score of 0/6 on the NRT during experiments), and the effect of acustimulation could not be verified using the data. Therefore, data analysis was conducted based on the data from the remaining 27 subjects (10, 7, 9 of each group). 4. Result
Fig. 5. Post-SSQ scores of 3 experimental conditions (* denotes significance level of p < 0.05, ** denotes significance level of p < 0.01).
4.1. MSSQ and SSQ pre-scores
compared with the NoAcP and AcP + conditions either (t(26) = 1.278, p = .212, t(26) = 0.880, p = .387, t(26) = 1.710, p = .099, t (26) = 0.286, p = .777; the oculomotor subscale and the total-score for the control and AcP+, respectively). Moreover, to further valid the results, a two-way repeated measures ANOVA was also calculated, with the condition (NoAcP, AcP+, AcP) and report timing (pre-SSQ, post-SSQ) as within-subjects factors, on SSQ total-score, and its three subscales nausea, oculomotor, and disorientation. Results showed significant effect of report timing on totalscore and all three subscales (total-score, F(1,26) = 48.491, p < 0.0001, ηp2 = 0.651; nausea, F(1,26) = 40.877, p < 0.0001, ηp2 = 0.611; oculomotor, F(1,26) = 29.023, p < 0.0001, ηp2 = 0.527; disorientation, F(1,26) = 37.217, p < 0.0001, ηp2 = 0.589). As for the interaction effect of condition and report timing, results demonstrated that on total-score and nausea subscale, the interaction effect was significant (total-score, F(2,52) = 3.648, p = .033, ηp2 = 0.123; nausea, F (2,52) = 4.301, p = .019, ηp2 = 0.142), while on oculomotor subscale it was marginally significant (F(2,52) = 3.047, p = .056, ηp2 = 0.105), and on disorientation subscale it was not significant. Main effect of condition on all subscales were not significant. The results were corresponding with previous analysis.
The MSSQ scores of the three experimental groups, as shown in Table 3, were first examined to rule out a possible between-groups effect. A one-way ANOVA including the between subjects factor of the experimental groups revealed no significant differences in the MSSQ total scores (F(2,26) = 0.019, p = 0.981). To preclude the influence of participants' sickness level before the experiment, a one-way repeated measures ANOVA, with the conditions (NoAcP, AcP+, AcP) as the within-subjects factor, was conducted on the pre-SSQ scores (Table 4). Results revealed no significant differences for the SSQ total-score, F(2,52) = 0.068, p = .935, subscale nausea, F (2,52) = 0.271, p = .764, oculomotor, F(2,52) = 0.049, p = .953, disorientation, F(2,52) = 0.357, p = .702. 4.2. SSQ-post scores To examine the effects of different acustimulation intervention methods, a one-way repeated measures ANOVA was calculated, with the condition (NoAcP, AcP+, AcP) as the within-subjects factor, on the post SSQ total-score, and its subscales nausea, oculomotor, and disorientation (the SSQ scores were shown in Fig. 5). Results demonstrated a significant effect on the nausea subscale, F(2,52) = 5.970, p = .005, ηp2 = 0.187, with a marginal significant trend on the total-score, F (2,52) = 2.938, p = .062, ηp2 = 0.102, and oculomotor subscale, F (2,52) = 2.603, p = .084, ηp2 = 0.091. A non-significant effect was observed on the disorientation subscale, F(2,52) = 0.527, p = .594. Then, post-hoc analysis (a paired-sample t-test) was conducted on the nausea and oculomotor subscales and the total score. Results showed that, on the nausea subscale, both the AcP + condition (t (26) = 2.820, p = .009, Cohen's d = 0.46) and the AcP condition (t (26) = 2.820, p = .009, Cohen's d = 0.64) were significant compared with the NoAcP condition, while on the oculomotor subscale and the total score of SSQ, only the AcP + condition (the oculomotor subscale, t (26) = 2.320, p = .028, Cohen's d = 0.43; total-score, t(26) = 2.129, p = .043, Cohen's d = 0.39) showed a significant effect compared with the NoAcP condition. The AcP condition showed no significant trend
4.3. NRT during and after experiments A two-way repeated measures ANOVA, 3 [conditions (NoAcP, AcP +, AcP), within-subject] * 5 [time (0 min, 5 min, 10 min, 15 min, 20 min), within-subject], was conducted on the NRT scores recorded during the experiments (as depicted in Fig. 6) to examine the intervention effects. A significant effect of time was shown, F (4,108) = 61.379, p < 0.001, ηp2 = 0.70, suggesting that the subjective feeling of cybersickness increased with time. However, the main effect of condition, and the interaction effect were not significant, (F (2,54) = 2.276, p = .112, F(8,216) = 0.804, p = .599). Similarly, a two-way repeated measures ANOVA, 2 [condition (NoAcP, AcP+, AcP), within-subject] * 6 [time (0 min, 2 min, 4 min, 6 min, 8 min, 10 min), within-subject], was conducted on the NRT scores recorded after the experiments to examine whether acustimulation interventions were effective in the recovery period after the experiments (shown in Fig. 7). Results showed a significant trend involving time as well, F(5,130) = 104.651, p < 0.001, ηp2 = 0.80, which indicates a decreasing level of cybersickness with time after experiments. Moreover, a marginally significant main effect of the intervention condition, and a marginally significant interaction effect were revealed (F(2,52) = 3.015, p = .058, ηp2 = 0.10, F(10,260) = 1.747,
Table 4 Mean (SD) of pre-SSQ scores.
Total-score Nausea Oculomotor Disorientation
AcP+
AcP
NoAcP
11.77 (14.80) 10.60 (15.28) 10.11 (12.08) 9.80 (18.03)
10.80 (10.85) 8.13 (9.79) 10.95 (10.99) 8.25 (14.58)
10.80 (10.39) 9.19 (11.06) 10.67 (10.35) 7.22 (11.82)
5
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condition (NoAcP), both acustimulation methods, acupressure (AcP) and acupressure plus acupaste (AcP+) had some effects on the reported level of cybersickness. For AcP condition, results showed that it had a significant effect on the nausea subscale of the self reported level of cybersickness (SSQ), which was consistent with previous studies of acupressure in treating motion sickness, of which nausea was the core symptom (Chu et al., 2012; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001). For the main focus of this study, our newly proposed acustimulation approach–AcP+ (acupressure plus acupaste), results revealed that compared with NoAcP, AcP + had a significant effect not only on nausea subscale, but also on oculomotor subscale and total score of SSQ, while the differences were not significant compared with AcP condition. Moreover, during the recovery period, in AcP + condition, compared with NoAcP, participants recovered more quickly indicated by NRT scores, while this trend was not significant compared with AcP condition. Detailed discussion was provided in Section 4.3. 5.2. Confounding factors 5.2.1. Placebo effect In this study, the effectiveness of acupressure against VIMS, specifically cybersickness, on nausea-related symptoms, was proven and replicated by comparison between AcP and NoAcP conditions. Based on this finding, we further found that acupressure along with acupaste, compared with control condition, significantly reduced cybersickness symptoms, shown on nausea and oculomotor subscales, total score of SSQ, and facilitated the recovery process indicated by NRT scores. The effects could be more closely examined through two steps in ruling out possible placebo effects: 1) the validation and replication of the effectiveness of acupressure (AcP), the basic form of acustimulation. Comparison between AcP and NoAcP conditions showed that, AcP significantly reduced nausea symptoms on nausea subscale of post-SSQ (while non-significant on other subscales of post-SSQ, nor NRT scores during and after experiments), which was in line with previous findings–acupressure was effective in reducing VIMS, especially on the nausea-related symptoms (Chu et al., 2012; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001, detailed explanations and underlying mechanisms were discussed in the following Section 4.3.1); 2) Based on the validated effectiveness of acupressure, we further examined the effect of AcP+, acupuncture with acupaste, the condition of main interest. During experiments, AcP+ and AcP conditions appeared totally the same to subjects. In both conditions, acupressure was adopted in conjunction with an acupaste of same appearance, while one was a true acupaste, another was an invalid one. The difference between the true and fake acupaste could not be distinguished by participants, either visually or tactually. Considering that all the perceptible factors were kept aligned between these two conditions, we could possibly rule out the influence of subjective bias, and attribute the additional effects of AcP + on oculomotor aspect and recovery period to our manipulation-the upgraded version of acustimulation, acupressure in conjunction with acupaste. However, we had to admit that in this study, the differences between AcP+ and AcP conditions were not significant. Further studies were needed to better clarify the effectiveness of acupaste, whether it just simply magnified the effect of acupressure or had additional ameliorating effects in general.
Fig. 6. Subjective ratings of cybersickness during experiments.
Fig. 7. Subjective ratings of cybersickness after experiments.
p = .071, ηp2 = 0.06). Post-hoc analysis of the condition main effect showed that AcP + had a significant effect compared with the NoAcP condition, (t(26) = 2.395, p = .024, Cohen's d = 0.48), while AcP was not significantly different from the AcP+ (t(26) = 1.693, p = .102), and the NoAcP (t(26) = 0.630, p = .534). Taken together, the acupressure plus acupaste intervention approach was more effective during the recovery process after the experiments. 5. Discussion 5.1. Main finding The purpose of this study was to examine the effect of acupressure plus acupaste, the simpler approach of acustimulation, against cybersickness. In the VR video-watching application, cybersickness measured by NRT showed a trend of increasing under each condition, and gradually faded during the recovery phase (see Figs. 6 and 7). This suggested that for the majority of people, they would suffer a mild level of cybersickness after a period of VR video watching. The longer time immersed in the VR application, the severer the cybersickness will be. Experiments’ results showed that compared with the control
5.2.2. Adaptation effect Another confounding factor of the present study was the probable adaptation effect (symptoms could be lessened through repeated exposure to the same stimulus), as each participant viewed the same VR stimulus three times, with two days between viewings, in three different intervention conditions. To exclude the influence of adaptation, all cybersickness measurements were examined under two variables–intervention conditions, and times (the 1st, 2nd, or 3rd time 6
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condition and previous studies about motion sickness. The explanation of this different finding might be two-folds: 1) compared with motion sickness, visual stress and oculomotor sickness were more distinctive of cybersickness (Guo et al., 2017; Jennifer et al., 2004; Rebenitsch and Owen, 2016), thus acupressure showing a more prominent improvement in VR applications on the oculomotor subscale after intervention; 2) the differences between these two acustimulation methods and the non-negligible effect of acupaste, which commonly served as a substitute non-intrusive alternative of acupuncture. Consisting of a stone “needle” and pressure-sensitive adhesives, Acupaste might greatly magnify the effect of acupressure. Therefore, with the help of acupaste, AcP + turned out to be a more intensive form of acustimulation compared with the pure form of acupressure. Another intensive form of acustimulation–electro-acupressure (Chu et al., 2012) was also demonstrated to be both effective in reducing nausea symptoms, and promoting visuospatial abilities as well (the effet size of it could not be compared with the current study, because only figure, but no numeric data was provided), and even cognitive function by influencing cardiovascular responses through ANS reflex. Intensity of acustimulation has been discussed and paid much attention in this field (Chu et al., 2012; Hu et al., 1992). Moreover, in previous works (Bruce et al., 1990; Hu et al., 1992; Warwick-Evans et al., 1991), not all studies of acupressure against motion sickness found effective. Intensity of acupressure was regarded as an important factor-acupressure, only owning certain level of intensity, could effectively reduce motion sickness symptoms and possibly realize the full functions of acupuncture (e.g. suppress nausea symptoms, enhance motor function, visuospatial abilities, postural control, and cognitive function, Chu et al., 2012; Hu et al., 1992). Therefore, in this study, we might infer that the extra effects of acupressure plus acupaste condition (AcP+) resulted from the magnification effect of acupaste which possibly more effectively activated the ANS reflex and rendered participants more capable to recover quickly. Another interesting result needed further discussion was that, although AcP + significantly facilitated the recovery process, indicated by subjective NRT scores, similar effect was not found during VR experience under AcP + condition, while acupaste was kept stuck with participants during both periods. One possible guess was that the acupressure plus acupaste approach might take some time to take effect. To prove this, we could manipulate the length of VR experience and then examine the cybersickness reported level in future studies. Another feasible explanation of why significant results found after VR reviewing while not during VR viewing, was that considering the nature of NRT, which required participants to report their general subjective cybersickness level at the same time viewing VR video, participants might not own enough cognitive capacity to be well aware of their cybersickness level and report it accurately during VR viewing. The VR viewing conduct itself might influence the subjective report of NRT scores. Especially under the mild cybersickness, any tiny influence could tune the results. Overall, although based on current experiment design, we could not reach a definite answer about the underlying reasons behind this phenomenon, it was of great interest and worthy more efforts to carefully examine its causes.
viewing the stimulus)–at the same time. A two-way repeated measures ANOVA was calculated on the SSQ total-score and its three sub-scales. A three-way repeated measures ANOVA was calculated on the NRT scores recorded during and after experiments. Results showed no significant trend of repetition in any form of measurement (the SSQ total-score, F (2,18) = 1.182, p = .329; nausea, F(2,18) = 1.760, p = .200; oculomotor, F(2,18) = 1.316, p = .293; disorientation, F(2,18) = 0.342, p = .715; subjective ratings during experiments, F(2,20) = 0.694, p = .511; after experiments, F(2,20) = 0.048, p = .953), ruling out the possible influence of an adaptation effect. Previous studies (Gavgani et al., 2017; Hill and Howarth, 2000; Regan, 1995; Sugita et al., 2007) also indicated that adaptation might not always have an effect on cybersickness and could largely depend on individual traits. Besides the placebo and adaptation effects mentioned above, there were also other factors which might influence the results interpretation, such as the head motion, attention location differences between conditions. In this experiment, we tried to keep these factors identical among conditions as much as possible, viewing the same video in each session, conducting the experiment at the same time slot of each day for every participant, keeping the height and location of the swivel chair still all through the experiment etc. Based on the strict experiment control among conditions, the influence from these covariates factors could be neglected. 5.3. Interpretation 5.3.1. Acupressure Acupressure had been proven to be effective in reducing motion sickness and cybersickness in previous studies and the underlying mechanisms had been hotly discussed (Dundee et al., 1986; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001; Streitberger et al., 2006). Sensory conflict theory is one of the most prominent explanations for both motion sickness and cybersickness (Oman, 1990; Rebenitsch and Owen, 2016; Reason, 1978). According to sensory conflict theory, the mismatch among somatosensory, vestibular, and visual inputs give rise to the feelings of nausea and related sickness symptoms. Since vestibular and somatosensory systems have physiological pathways connected with autonomic nervous system (ANS), the conflicting sensory inputs would simultaneously activate autonomic responses (Money, 1970; Ohyama et al., 2007; Previc, 1993). Moreover, ANS further has multiple interactions with “nausea neurosis” and plays a vital role in the development of nausea. Therefore, intervening through ANS is a feasible way to mediate nausea symptoms (Himi et al., 2004; Muth et al., 1999). Many studies have shown that mediation through the sympathetic and parasympathetic nervous system, two components of the ANS, was effective in reducing nausea symptoms and autonomic reactions to motion stimuli (David, Parker, and William, 1978; Farmer et al., 2014; Himi et al., 2004). Functional magnetic resonance imaging studies also directly proved that P6-stimulation had effects on gastric myoelectrical activity, vagal modulation and cerebellar vestibular activities (Streitberger et al., 2006). In addition, using physiological methods, Chu et al. (2012) found that electro-acupuncture effectively mediated motion sickness symptoms mainly by counteraction in the ANS pathways that were stimulated by motion sickness. Arguably, the effect induced by acupressure in this study against cybersickness–nausea feelings might influence the same pathways of ANS, although further physiological validation was still needed to fully explain the underlying mechanism.
5.4. Implications In the current study, it was found that acupressure with acupaste (AcP+) was not only effective in reducing cybersickness nausea feelings, but also helpful in reducing oculomotor discomforts and facilitated cybersickness recovery process, although the underlying physiological mechanism still needs further examination. Current findings of the acupaste effect is still promising, since reducing oculomotor discomforts is extremely important in cybersickness (also called visually-induced motion sickness, VIMS). Visual stress has been found to be one of the leading factors influencing the experience of cybersickness in VR (Rebenitsch and Owen, 2016; So and Ujike, 2010). Therefore, the
5.3.2. Acupressure plus acupaste Except for against nausea feelings, in this study the main condition of interest-acupressure plus acupaste (AcP+) was found to be more effective in reducing cybersickness that it also had significant effects on oculomotor subscale and total score of post-SSQ, and facilitated the recovery process indicated by significant NRT scores, compared with the NoAcP control condition, which were not found on the AcP sham7
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Conflicts of interest
intervention AcP+, acupressure with acupaste, may hold great value for future VR use. Treatments of cybersickness nowadays mainly involved interventions from the technological and hardware aspects, aiming to improve the quality of the scenes presented by virtual reality, for example, the field of view (Bos et al., 2010; van Emmerik et al., 2011) and the timedelay of update (Draper et al., 2001). Other than addressing the technological aspects, behavioral intervention methods tried to solve the problem as well. For example, music therapy (Keshavarz and Hecht, 2014) and diaphragmatic breathing (Russell et al., 2014) were both proven to be effective in reducing cybersickness in VR. However, these methods had some shortcomings, such as occupying too much extracognitive capacity, which could greatly influence users’ VR experience and limit its application and generalization. In summary, compared with these methods, acupressure stimulation methods, especially with acupaste, offered several advantages: 1) lowcost intervention through the user end, which could avoid the difficulties and high expense of promoting a VR hardware system; 2) an easy-to-conduct, non-intrusive technique which took only a few minutes of physical pressure exerted on a particular point; 4) effective, not only for nausea symptoms, but also for the visual and oculomotor afflictions, which were central to cybersickness.
None. Acknowledgements The authors would like to address their special thanks to the financial support of the National Science Foundation of China under grant number 71471095, and the Innovation-Method Fund of China's Ministry of Science & Technology (Project No. 2016IM010200). References Bos, J.E., de Vries, S.C., van Emmerik, M.L., Groen, E.L., 2010. The effect of internal and external fields of view on visually induced motion sickness. Appl. Ergon. 41 (4), 516–521. Bruce, D.G., Golding, J.F., Hockenhull, N., Pethybridge, R.J., 1990. Acupressure and motion sickness. Aviat. Space Environ. Med. 61 (4), 361. Budhiraja, P., Miller, M.R., Modi, A.K., Forsyth, D., 2017. Rotation Blurring: Use of Artificial Blurring to Reduce Cybersickness in Virtual Reality First Person Shooters. Chang, E., Hwang, I., Jeon, H., Chun, Y., Kim, H.T., Park, C., 2013. Effects of rest frames on cybersickness and oscillatory brain activity. In: Brain-Computer Interface (BCI), 2013 International Winter Workshop on. IEEE, pp. 62–64. Chen, D., Bao, B., Zhao, Y., So, R.H.Y., 2016. Visually induced motion sickness when viewing visual oscillations of different frequencies along the fore-and-aft axis: keeping velocity constant versus amplitude constant. Ergonomics 59 (4), 582–590. Chu, H., Li, M.H., Juan, S.H., Chiou, W.Y., 2012. Effects of transcutaneous electrical nerve stimulation on motion sickness induced by rotary chair: a crossover study. J. Altern. Complement. Med. 18 (5), 494–500. Davis, S., Nesbitt, K., Nalivaiko, E., 2014. A Systematic Review of Cybersickness. pp. 1–9. Draper, M.H., Viire, E.S., Furness, T.A., Gawron, V.J., 2001. Effects of image scale and system time delay on simulator sickness within head-coupled virtual environments. Hum. Factors 43 (1), 129–146. Dundee, J.W., Chestnutt, W.N., Ghaly, R.G., Lynas, A.G., 1986. Traditional Chinese acupuncture: a potentially useful antiemetic? Br. Med. J. 293 (6547), 583–584. Farmer, A.D., Al, O.Y., Aziz, Q., Andrews, P.L., 2014. The role of the parasympathetic nervous system in visually induced motion sickness: systematic review and metaanalysis. Exp. Brain Res. 232 (8), 2665–2673. Gavgani, A.M., Nesbitt, K.V., Blackmore, K.L., Nalivaiko, E., 2017. Profiling subjective symptoms and autonomic changes associated with cybersickness. Auton. Neurosci. 203, 41–50. Golding, J.F., 1998. Motion sickness susceptibility questionnaire revised and its relationship to other forms of sickness. Brain Res. Bull. 47 (5), 507. Guo, C.C., Chen, D.J., Wei, I.Y., So, R.H., Cheung, R.T., 2017. Correlations between individual susceptibility to visually induced motion sickness and decaying time constant of after-nystagmus. Appl. Ergon. 63, 1–8. Hill, K.J., Howarth, P.A., 2000. Habituation to the side effects of immersion in a virtual environment. Displays 21 (1), 25–30. Himi, N., Koga, T., Nakamura, E., Kobashi, M., Yamane, M., Tsujioka, K., 2004. Differences in autonomic responses between subjects with and without nausea while watching an irregularly oscillating video. Auton. Neurosci. Basic Clin. 116 (1–2), 46. Hu, S., Stern, R.M., Koch, K.L., 1992. Electrical acustimulation relieves vection-induced motion sickness. Gastroenterology 102 (6), 1854–1858. Hu, S., Stritzel, R., Chandler, A., Stern, R.M., 1995. P6 acupressure reduces symptoms of vection-induced motion sickness. Aviat. Space Environ. Med. 66 (7), 631–634. Jennifer, T.J., Felix, W.L., Richard, H.S., 2004. Integrating a computational model of optical flow into the cybersickness dose value prediction model. In: Proceedings of the Human Factors and Ergonomics Society Annual Meeting, vol. 48. SAGE Publications, Sage CA: Los Angeles, CA, pp. 2667–2671 No. 23. Kennedy, R.S., Lane, N.E., Berbaum, K.S., Lilienthal, M.G., 1993. Simulator sickness questionnaire: an enhanced method for quantifying simulator sickness. Int. J. Aviat. Psychol. 3 (3), 203–220. Keshavarz, B., Hecht, H., 2014. Pleasant music as a countermeasure against visually induced motion sickness. Appl. Ergon. 45 (3), 521–527. Kolasinski, E.M., 1995. Simulator Sickness in Virtual Environments. Army Research Institute for the Behavioral and Social Sciences, Alexandria VA No. ARI-TR-1027. LaViola Jr., J.J., 2000. A discussion of cybersickness in virtual environments. ACM SIGCHI Bull. 32 (1), 47–56. Lo, W.T., So, R.H., 2001. Cybersickness in the presence of scene rotational movements along different axes. Appl. Ergon. 32 (1), 1–14. Money, K.E., 1970. Motion sickness. Physiol. Rev. 50 (1), 1–39. Muth, E.R., Koch, K.L., Stern, R.M., Thayer, J.F., 1999. Effect of autonomic nervous system manipulations on gastric myoelectrical activity and emotional responses in healthy human subjects. Psychosom. Med. 61 (3), 297. Ohyama, S., Nishiike, S., Watanabe, H., Matsuoka, K., Akizuki, H., Takeda, N., et al., 2007. Autonomic responses during motion sickness induced by virtual reality. Auris Nasus Larynx 34 (3), 303. Oman, C.M., 1990. Motion sickness: a synthesis and evaluation of the sensory conflict theory. Can. J. Physiol. Pharmacol. 68 (2), 294–303. Parker, David M., Wilsoncroft, William E., 1978. Intensity of motion sickness symptoms as a function of apparent autonomic balance. J. Gen. Psychol. 98 (2d Half), 253. Previc, F.H., 1993. Do the organs of the labyrinth differentially influence the sympathetic
5.5. Limitations Although we came to a promising finding, our study owned many limitations and left several significant questions which needed to be answered in the future. Firstly, in this study, it lacked of important objective measurements to confirm the subjective rating results. Especially under different experiment conditions, participants' subjective evaluation might be unconsciously influenced by experiment manipulations. Also, the stimulus continuously coming from videowatching tasks could influence participants’ ability to accurately report their sickness level during experiments greatly as well. Objective methods were especially needed and should be compensated in future studies to validate current findings. Secondly, while we found promising effect of an easy-to-conduct acustimulation approach-acupressure plus acupaste, its effectiveness was preliminary and limited. As subjective report NRT shown, the effect of acupressure plus acupaste was manifested only during recovery period. Both its underlying reasons and whether acupaste was effective for sickness prevention were required further exploration. Moreover, the effect of AcP+ was only significant from the NoAcP conditions, but not significant from AcP condition. Based on the current study, we could not figure out whether it was resulted from the experiment design-the level of cybersickness was too small to manifest its full effect and separate from AcP-or not. The full function and effectiveness of AcP + should be better investigated and clarified in future various experiment settings. Thirdly, the intervention effect of acustimulation on disorientation was still unclear. In the current study, subjects were required to sit on a chair and thus they could not stand and walk, having a limited range of movements. Therefore, possible intervention effects on the disorientation aspect of cybersickness, which was indispensable as well, needed further efforts of exploration. 6. Conclusion In the current study, it was found that acupressure plus acupaste, a simple and easy-to-conduct acustimulation approach, was significantly effective in reducing cybersickness nausea feelings, oculomotor discomforts and could facilitate cybersickness recovery, compared with control condition. Although the underlying mechanism needed further investigation and the full effect of acupaste still needed exploration, this finding was promising and of great value for low-cost VR application. 8
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Stern, R.M., Jokerst, M.D., Muth, E.R., Hollis, C., 2001. Acupressure relieves the symptoms of motion sickness and reduces abnormal gatric activity. Altern. Ther. Health Med. 7 (4), 91. Streitberger, K., Ezzo, J., Schneider, A., 2006. Acupuncture for nausea and vomiting: an update of clinical and experimental studies. Auton. Neurosci. Basic Clin. 129 (1), 107–117. Sugita, N., Yoshizawa, M., Abe, M., Tanaka, A., Watanabe, T., Chiba, S., Nitta, S.I., 2007. Evaluation of adaptation to visually induced motion sickness based on the maximum cross-correlation between pulse transmission time and heart rate. J. NeuroEng. Rehabil. 4 (1), 35. Super Data Research, 2017. Virtual Reality Market and Consumers. Retrieved Septempber 21, 2017 from Super Data Research website: https://www. superdataresearch.com/market-data/virtual-reality-industry-report. van Emmerik, M.L., de Vries, S.C., Bos, J.E., 2011. Internal and external fields of view affect cybersickness. Displays 32 (4), 169–174. Warwick-Evans, L.A., Masters, I.J., Redstone, S.B., 1991. A double-blind placebo controlled evaluation of acupressure in the treatment of motion sickness. Aviat. Space Environ. Med. 62 (8), 776. Wei, Y., Zheng, J., So, R.H.Y., 2018. Allocating less attention to central vision during vection is correlated with less motion sickness. Ergonomics 61 (7), 933–946. Wilkins, A.J., Evans, B.J., 2010. Visual stress, its treatment with spectral filters, and its relationship to visually induced motion sickness. Appl. Ergon. 41 (4), 509–515. Zhang, Hude, 2015. The model of inheritance and innovation: talk about acupuncture's contribution to the motherland medicine (in Chinese). Clin. J. Chin. Med. (18), 115–116.
and parasympathetic systems? Neurosci. Biobehav. Rev. 17 (4), 397–404. Prothero, J.D., Draper, M.H., Furness 3rd, T.A., Parker, D.E., Wells, M.J., 1999. The use of an independent visual background to reduce simulator side-effects. Aviat. Space Environ. Med. 70 (3 Pt 1), 277–283. Reason, J.T., 1978. Motion sickness adaptation: a neural mismatch model. J. R. Soc. Med. 71 (11), 819–829. Rebenitsch, L., Owen, C., 2016. Review on cybersickness in applications and visual displays. Virtual Real. 20 (2), 101–125. Regan, E.C., 1995. Some evidence of adaptation to immersion in virtual reality. Displays 16 (3), 135–139. Russell, M.E.B., Hoffman, B., Stromberg, S., Carlson, C.R., 2014. Use of controlled diaphragmatic breathing for the management of motion sickness in a virtual reality environment. Appl. Psychophysiol. Biofeedback 39 (3–4), 269–277. Rutkin, A., 2016. Vr to make you nauseous. New Sci. 232 (3100), 26. Schmidt, A., Thews, G., 1989. Autonomic nervous system. In: Janig, W. (Ed.), Human Physiology, 2 ed. Springer-Verlag, New York, NY, pp. 333–370. So, R.H.Y., Ujike, H., 2010. Visually induced motion sickness, visual stress and photosensitive epileptic seizures: what do they have in common? - preface to the special issue. Appl. Ergon. 41 (4) 491-393. So, R.H., Ho, A., Lo, W.T., 2001. A metric to quantify virtual scene movement for the study of cybersickness: definition, implementation, and verification. Presence Teleoperators Virtual Environ. 10 (2), 193–215. Stanney, K., Salvendy, G., 1998. Aftereffects and sense of presence in virtual environments: formulation of a research and development agenda. Int. J. Hum. Comput. Interact. 10 (2), 135–187.
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Contents lists available at ScienceDirect
Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo
Effect of economically friendly acustimulation approach against cybersickness in video-watching tasks using consumer virtual reality devices
T
Ruijun Liua,b, Chu Zhuangc, Rui Yanga,b, Liang Mad,∗ a
School of Computer Information and Engineering, Beijing Technology and Business University, 100048, Beijing, PR China Beijing Key Laboratory of Big Data Technology for Food Safety, 100048, Beijing, PR China c Social Science Division, University of Chicago, Chicago, IL, 60637, USA d Department of Industrial Engineering, Tsinghua University, 100084, Beijing, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: VR headset Cybersickness Acustimulation
Background: Consumer virtual reality (VR) devices are becoming more prevalent in the market, but cybersickness induced by VR devices limits their potential application and promotion. Acustimulation has been found effective in reducing cybersickness symptoms. However, in previous forms, the more effective way of acustimulation is either intrusive or electrical which is hard to be applied to daily VR use. Purpose: In this study, we aimed to find a both simple and more effective acustimulation approach, acupressure plus acupaste (AcP+) to reducing the adverse effects caused by cybersickness from VR applications. Method: In this study, we set three conditions: acupressure plus acupaste (AcP+) (main condition of interest), acupressure with fake acupaste (AcP), and a no acustimulation condition (NoAcP). In AcP and AcP + conditions, we applied acupressure or acupressure with true acupaste on P6 point before conducting video-watching tasks using VR headsets, while in NoAcP condition, participants received no special treatment before video-watching tasks. We used questionnaires to measure symptoms of cybersickness and compared the results between these 3 conditions, especially between acupressure plus acupaste (AcP+) and acupressure (AcP) to examine the effect of AcP+, and compared AcP and AcP+ with NoAcP to confirm the effect of acustimulation. Result: Participants reported significant fewer symptoms of cybersickness nausea feelings in both acustimulation methods, compared with NoAcP; and AcP+ was more effective than AcP against cybersickness on visual oculomotor aspect, and facilitated cybersickness recovery. Implication: It would be promising to develop acupressure equipment and apply stimulation before VR application to reduce cybersickness.
1. Introduction The year 2016 is known as the year of virtual reality (VR), and since then, consumer immersive visual display products are becoming more and more prevalent (Rutkin, 2016). Head-mounted displays (HMDs), or VR headsets, are one of the major types of device on the market, and the type with the highest demand, which is continually increasing. For example, Google has announced the intent to produce a consumer heads-up display (Bilton, 2012), and Facebook has acquired Oculus VR for over $2 billion (Parkin, 2014). An HMD is a closed display that places two separately rendered images in different screens in front of the eyes to create stereoscopic views for the user (Rebenitsch and Owen, 2016). However, cybersickness induced by these VR devices (Rutkin, 2016; Rebenitsch and Owen, 2016; So and Ujike, 2010) are frequently reported, which limits their potential application and ∗
promotion. Cybersickness is defined as the onset of nausea, oculomotor, and/or disorientation while experiencing virtual environments in headmounted displays, large screens, and curved screen systems (Rebenitsch and Owen, 2016). Cybersickness is also considered the digital version of motion sickness, or visually-induced motion sickness. Symptoms of cybersickness include not only symptoms similar to motion sickness, such as nausea, pale skin, cold sweats, vomiting, dizziness, headache, increased salivation, and fatigue, but also eyestrain and focusing difficulty, because the virtual environment puts additional strains on the eyes (Ehrlich, 2012). The uncomfortable feelings caused by using VR devices becomes a great challenge to promoting VR applications, and tremendous effort has been made in both industrial practice and academic research to overcome this obstacle. Most current efforts against cybersickness are
Corresponding author. S611, Shunde Building, Dept. of Industrial Engineering, Tsinghua University, 100084, Beijing, PR China. E-mail addresses: [email protected] (R. Liu), [email protected] (C. Zhuang), [email protected] (R. Yang), [email protected] (L. Ma).
https://doi.org/10.1016/j.apergo.2019.102946 Received 12 May 2018; Received in revised form 21 August 2019; Accepted 27 August 2019 0003-6870/ © 2019 Elsevier Ltd. All rights reserved.
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explanation for the effectiveness of acupuncture against cybersickness (Chu et al., 2012). Because of the intrusiveness of acupuncture and apprehension to the piercing needles, other forms of nonintrusive acupuncture, more broadly named acustimulation, such as acupressure and electro-acupuncture, were developed to substitute the effect of acupuncture. Both acupressure and electro-acupuncture have been tested to be more or less effective in reducing motion sickness as well (Chu et al., 2012; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001). However, the effects of these substituted forms of acustimulation were limited and unstable; till the intensity of acustimulation was high enough, they could approximately reach the full effect of acupuncture (Chu et al., 2012; Hu et al., 1992). For electro-acupuncture, it is easy to fulfill the high intensity requirement and reach comparable effect of acupuncture, while this approach is still relatively intrusive that electricity exerted on human skin which might arouse mental discomforts. Among acustimulation approaches, except for increasing the intensity of acupressure, acupaste, which contains small stones that could facilitate the effect of acupressure, is developed as a common nonintrusive substitute for acupuncture needles, to achieve the full function of acupuncture. However, the effect of acupressure plus acupaste has not been tested in reducing motion sickness and cybersickness yet. Considering the easiness of conducting acupressure, the little mental discomforts it could induce and the low cost of this approach, we predict that acupressure plus acupaste could be a promising countermeasure, both simple and more effective, for cybersickness in VR applications, especially for low-cost VR devices. Therefore, in this study, we aim to examine the effectiveness of acupressure plus acupaste in reducing the adverse effects of cybersickness in VR applications. In this study, to examine the effectiveness of acupressure plus acupaste, we designed three conditions, acupressure plus acupaste (AcP+), acupressure (AcP, sham-condition) and a no acustimulation condition (NoAcP, control condition). In AcP+ and AcP conditions, acupressure in conjunction with acupaste or only acupressure was exerted on participants before the VR experience, and in NoAcP condition, no special treatment, either acupressure or acupaste was applied to participants. For AcP+ and AcP conditions, they were controlled to be as same as possible except that AcP + used true acupaste, while AcP used a ‘fake’ invalid one but has the same appearance with the true one (detailed descriptions of these two forms of paste were provided in Section 2Methods). Therefore, in this experiment, AcP conditions was served as a sham condition and NoAcP as control condition. The differences between the acupressure plus acupaste (AcP+) and acupressure (AcP), AcP+ and control condition (NoAcP), are of main interests to examine the effect of our newly proposed approach; and the difference between AcP and control condition is also of interests to confirm the effect of acustimulation which has been proved. Pre- and post-VR cybersickness were measured during the experiment. The detailed procedure is introduced in Section 2. Results and discussions regarding the effectiveness of acustimulation are shown in Sections 3 and 4.
explored from the technology perspective (Chen et al., 2016; Kolasinski, 1995; Stanney and Salvendy, 1998; LaViola, 2000; Rebenitsch and Owen, 2016), such as providing scenes with better quality (Jennifer et al., 2004; So et al., 2001), blurring the screen during rotational movements (Budhiraja et al., 2017), reducing the time-delay of scene update (Draper et al., 2001), changing the field of view (Bos et al., 2010; van Emmerik et al., 2011), or providing additional reference frames or spectral filters in the view (Chang et al., 2013; Prothero et al., 1999; Wikins and Evans, 2010). Technology improvement against cybersickness partially solves the cybersickness problem. However, while improvements in technology solve challenging technical problems, especially difficulties in reducing the unavoidable delay of update and the inherent mismatch of visual and vestibular inputs (Rebenitsch and Owen, 2016), they also greatly increase the cost. Therefore, owing to the hardware limitations of low-end consumer VR devices, such as small view fields, low refresh frequency, heavy weight, poor air permeability, and the poor quality of display content (e.g., resolution, frame rate, scene change), improvements against cybersickness in terms of technology may be too costly. In addition to technology improvement, behavioral countermeasures against cybersickness have also been explored through intervention by VR users. These measures cost much less and are relatively easy to implement, such as adaptation (Hill and Howarth, 2000; Regan, 1995; Sugita et al., 2007), re-allocating attention (Wei et al., 2018), relaxing music (Keshavarz and Hecht, 2014), and diaphragmatic breathing (Russell et al., 2014). However, certain limits exist with respect to these measures for normal daily VR use. For example, adaptation requires that users view the same VR material at least two times in order to develop resistance against cybersickness, and cybersickness is not effectively reduced when using VR for the first time. Music constitutes the auditory cue and restricts audio input and breathing occupies considerable cognitive capacity while experiencing VR. Originating from traditional Chinese medicine, acupuncture is a popular and mature method with a long medical history for treating nausea and motion sickness (Dundee et al., 1986). Acupuncture is an intrusive medical approach that involves piercing the skin with acupuncture needles and applying stimulation at specific points on the human body. For example, point P6 (see Fig. 3, Nei-Kuan Point) is one of the most frequently-used points for treating nausea-related symptoms (Dundee et al., 1986; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001). Many studies have also shown that acupuncture at P6 could effectively suppress the nausea symptoms of motion sickness (Chu et al., 2012; Stern et al., 2001) and also of vection-induced motion sickness (VIMS) in simulators (Hu et al., 1992; Hu et al., 1995). The underlying mechanism of why acupuncture was effective in reducing nausea symptoms, motion sickness and cybersickness has been discussed (Dundee et al., 1986; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001; Streitberger et al., 2006). According to the most prevalent cybersickness theory-sensory conflict theory (Davis et al., 2014; Rebenitsch and Owen, 2016), VIMS and cybersickness is evoked by the conflict among somatosensory, visual, and vestibular inputs. The underlying mechanism of why acupuncture was effective in reducing nausea symptoms, motion sickness and cybersickness has been discussed (Dundee et al., 1986; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001; Streitberger et al., 2006). According to the most prevalent cybersickness theory-sensory conflict theory (Davis et al., 2014; Rebenitsch and Owen, 2016), VIMS and cybersickness is evoked by the conflict among somatosensory, visual, and vestibular inputs. Autonomic nervous system (ANS) has physiological connections with vestibular and somatosensory systems (Schmidt and Thews, 1989). When conflicts among somatosensory, visual and vestibular inputs happened, ANS will be activated, which leads to sickness symptoms (Money, 1970; Ohyama et al., 2007; Previc, 1993). It has been examined that acupuncture could successfully influence ANS responses, through which mediates nausea symptoms, and even enhance visuospatial abilities, postural control, and cognitive function, which provided the currently most accepted
2. Methods 2.1. Subjects A total of 29 graduate students in college (12 males and 17 females, with an average age of 24.0 years, SD = 1.0 year) were recruited in this study. All subjects received health-related tests, including vestibular and visual tests, before the experiment to ensure that they were healthy and free of medication and illness, especially any illness which could cause motion sickness. Each subject provided written informed consent in which the possible risks of the experiment were stated. Subjects were instructed to refrain from the use of medication, alcoholic substances, and caffeinated drinks for at least 24 h before each experiment. The subjects who completed the experiment were entitled to a payment of 150 RMB. The study protocol was approved in advance by the 2
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channel, the video material could effectively stimulate related symptoms of cybersickness. Fig. 2 shows the scene that subjects saw in the VR device. The VR device could display the video at a frame rate of 50 fps with a refresh rate of 60 Hz, which are common technical specifications of VR headsets on the market. We used questionnaires to measure cybersickness in this study, including the Motion Sickness Susceptibility Questionnaire (MSSQ), Nausea Rating Test (NRT), and Simulator Sickness Questionnaire (SSQ). The MSSQ (Golding, 1998) is often used to test subjects’ past history of vertigo, and analyzes the frequency with which an individual has become ill from motion in the past, as cybersickness studies may be confounded by the susceptibility of the subjects. The NRT (Lo and So, 2001) is often used as a rapid assessment of cybersickness during VR application. From no symptom to moderate nausea, the NRT measures nausea on a seven-point scale (see Table 1). The NRT needs subjects to verbally report their current discomfort level every 5 min. We used the NRT during the VR video watching. We used SSQ to measure the physiological status and comfort degree of subjects (Kennedy et al., 1993). The SSQ was developed by Kennedy, and it is a standard questionnaire and most frequently used in assessing cybersickness (Kennedy et al., 1993). In comparison with the NRT, SSQ is able to distinguish cybersickness in three different dimensions—nausea (stomach awareness, increased salivation etc.), oculomotor (eyestrain, difficulty focusing, blurred vision, etc.), and disorientation (dizzy-eyes opened, vertigo) on a scale of 0–3, and thus can provide richer information for further analysis. Due to its multiple dimensions, we used the SSQ only before and after the VR application. In this study, except for the control condition that no acustimulation was given to the participants, both acupressure condition (AcP) and acupressure plus acupaste condition (AcP+) adopted nonintrusive acustimulation before the VR experience. In the AcP + condition, we exerted acupressure on the Nei-Kuan Point (P6) in conjunction with acupaste (Fig. 3); while in the AcP condition, we exerted acupressure in conjunction with a fake paste, which is of the same size, shape and apperance as the true acupaste, which served as the sham/baseline condition compared with AcP+. The acupaste (acupuncture paste) is a new therapy of acupuncture, which consists of a stone needle and lowsensitivity medical pressure-sensitive adhesive. It follows the same principle as traditional acupuncture, namely by stimulating the meridian acupuncture points to treat disease (Zhang, 2015). In the AcP condition, we exerted acupressure in conjunction with a fake paste, carefully made by medical proof fabric (with no special function but physical protection and stabilization), and as the exact same size, circle shape and color as the true acupaste. Hence, AcP condition served as the sham/baseline condition compared with AcP+. During the pressing intervention, we asked subjects to close their eyes and keep their body relaxed. The intervention process lasted about 2 min, and the acupaste remained stuck to the subject's P6 point until the end of the experiment.
Fig. 1. (a) Seating posture and experimental environment during videowatching VR application; (b) Samsung Gear VR SM-R323 and Samsung S7 Edge.
Institutional Review Board.
3. Materials We set up the experiment in a closed room (area = 40 m2). The air quality and temperature were controlled at 23 °C by air conditioning. Subjects could sit and act freely on a swivel chair (i.e., looking up or rotating) but could not move from the chair while wearing the VR device (Fig. 1). In this study, we used the VR headset Samsung Gear VR (SM-R323) with the Samsung S7 Edge smartphone, because this Samsung VR headset is one of the best-selling consumer VR devices on the market. In the first quarter of 2017, the Samsung Gear VR had the largest share of the VR market with shipments of approximately 782,000 headsets (SuperData Research, 2017). The SM-R323 weighs about 312 g alone and weighs 469 g in total with the Samsung S7 Edge. The viewing angle is up to 101°, and the motion-to-photons latency is less than 20 m s. We chose a video-watching task as the VR application in this study, because video watching is a typical VR application and it requires both visual and auditory cues. The video watching task normally does not involve large body movement, so disturbances from body movement could be greatly limited, and thus we could reduce other possible causes of cybersickness. We used a 20-min high resolution spliced video as 3D VR content for this experiment. The video had a resolution of 4096 pixels × 2160 pixels, and it was made up of five separate video clips. These clips were commonly used virtual reality scene videos that included slow and fast motion, with different levels of exposure to 3D scenes. After a preliminary test, we found that through the visual
Fig. 2. 3D virtual environment consisting of five separate video clips. 3
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Fig. 3. (a) Illustration of Nei-Kuan pressure point and application of acupressure to subjects; (b) Sample and instructions of Acupaste. Table 1 Nausea rating test scale (Lo and So, 2001). Scale
Symptom
0 1 2 3 4 5 6
No symptom Any unpleasant symptom, however slight Mild unpleasant symptom Mild nausea Mild to moderate nausea Moderate nausea but can continue Moderate nausea, want to stop
We carried out all the pressing operations in the experiment under the guidance and instruction of a professional medical practitioner.
Fig. 4. Illustration of the experiment procedure for one experimental session.
3.1. Experimental protocol
In Phase 1 of each session, subjects were asked to wait in the lounge for 5–8 min until they fully calmed down and could start the experiment in a normal state, checked by both subjective reporting and experimenter's standardized observation (E.g. no sweating, breathing calmly, normal consciousness etc). After entering the laboratory, subject was firstly asked to fill out the pre-SSQ. While helping the subject put on the VR device, we explained the NRT to the subject. After putting on the VR device, the subject had to freeze the screen at the main interface before watching the video, and we applied the pre-assigned acustimulation to the subject. The acustimulation for AcP and AcP + lasted for about 2 min. Of NoAcP condition, this 2 min s period would be occupied by VR equipment adjusting; no other special manipulation was conducted. After acupressure application, we asked subjects to watch the VR video for 20 min, and then took their nausea ratings using NRT every 5 min. After watching the video for 20 min, we asked subjects to fill out the post-SSQ and move to the rest area for about 10 min. Within those 10 min, we conducted a recovery test on the subject in compliance with scale of the NRT every 2 min. In addition, we asked the subject several experiment-related questions, such as about the fidelity of the simulation, whether the video segment induced uncomfortable feelings and the degree of the symptoms, and how they were feeling compared with the previous experiment. Ten minutes later, we checked whether the subject was able to conduct normal activities without symptoms of cybersickness (E.g. able to walk straight normally, with no blurred vision, eyestrain and nausea feelings at all). If yes, the session would be over; otherwise, the subject had to rest until self-reported full recovery from cybersickness.
We used a within-subject experimental design in this study. Each subject received all of the three conditions (NoAcP, AcP, AcP+) in three separate experiment sessions conducted at the same time on 3 different days. Regarding the influence of adaptation, there was a twoday interval between the two successive sessions (Hill and Howarth, 2000; Regan, 1995; Sugita et al., 2007). In order to limit the order effect, all the subjects were randomly assigned to three groups (groups A, B, and C), and each group followed an experiment sequence different from the other two groups. A Latin square experimental design was used to determine the sequence of treatments received by each group. The detailed experimental arrangement is shown in Table 2. Before the first treatment, each subject was asked to fill in the MSSQ to measure their past history of vertigo. Each subject needed to finish the three experimental sessions under different acustimulation conditions, and each session was composed of four phases: Phase 1 (8 min), experiment set-up and preparation; Phase 2 (2 min), pre-VR acustimulation; Phase 3 (20 min), VR video watching and cybersickness measurement; Phase 4 (10 min), post-VR nausea assessment (see Fig. 4). Table 2 Treatment sequence for each group.
Group A Group B Group C
Day 1
Day 2
Day 3
AcP NoAcP AcP+
AcP+ AcP NoAcP
NoAcP AcP+ AcP
4
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Table 3 Mean (SD) of MSSQ scores.
MSSQ scores
Group A
Group B
Group C
8.04 (4.78)
8.84 (12.37)
8.61 (9.06)
3.2. Data analysis We used SPSS Version-19 for data analysis and R Version-3.4.1 for drawing figures. The priori significance level was set to α = 0.05. Two subjects from experimental group B were excluded from the data analysis, because they reported a very low level of cybersickness during the experiments (score of 0/6 on the NRT during experiments), and the effect of acustimulation could not be verified using the data. Therefore, data analysis was conducted based on the data from the remaining 27 subjects (10, 7, 9 of each group). 4. Result
Fig. 5. Post-SSQ scores of 3 experimental conditions (* denotes significance level of p < 0.05, ** denotes significance level of p < 0.01).
4.1. MSSQ and SSQ pre-scores
compared with the NoAcP and AcP + conditions either (t(26) = 1.278, p = .212, t(26) = 0.880, p = .387, t(26) = 1.710, p = .099, t (26) = 0.286, p = .777; the oculomotor subscale and the total-score for the control and AcP+, respectively). Moreover, to further valid the results, a two-way repeated measures ANOVA was also calculated, with the condition (NoAcP, AcP+, AcP) and report timing (pre-SSQ, post-SSQ) as within-subjects factors, on SSQ total-score, and its three subscales nausea, oculomotor, and disorientation. Results showed significant effect of report timing on totalscore and all three subscales (total-score, F(1,26) = 48.491, p < 0.0001, ηp2 = 0.651; nausea, F(1,26) = 40.877, p < 0.0001, ηp2 = 0.611; oculomotor, F(1,26) = 29.023, p < 0.0001, ηp2 = 0.527; disorientation, F(1,26) = 37.217, p < 0.0001, ηp2 = 0.589). As for the interaction effect of condition and report timing, results demonstrated that on total-score and nausea subscale, the interaction effect was significant (total-score, F(2,52) = 3.648, p = .033, ηp2 = 0.123; nausea, F (2,52) = 4.301, p = .019, ηp2 = 0.142), while on oculomotor subscale it was marginally significant (F(2,52) = 3.047, p = .056, ηp2 = 0.105), and on disorientation subscale it was not significant. Main effect of condition on all subscales were not significant. The results were corresponding with previous analysis.
The MSSQ scores of the three experimental groups, as shown in Table 3, were first examined to rule out a possible between-groups effect. A one-way ANOVA including the between subjects factor of the experimental groups revealed no significant differences in the MSSQ total scores (F(2,26) = 0.019, p = 0.981). To preclude the influence of participants' sickness level before the experiment, a one-way repeated measures ANOVA, with the conditions (NoAcP, AcP+, AcP) as the within-subjects factor, was conducted on the pre-SSQ scores (Table 4). Results revealed no significant differences for the SSQ total-score, F(2,52) = 0.068, p = .935, subscale nausea, F (2,52) = 0.271, p = .764, oculomotor, F(2,52) = 0.049, p = .953, disorientation, F(2,52) = 0.357, p = .702. 4.2. SSQ-post scores To examine the effects of different acustimulation intervention methods, a one-way repeated measures ANOVA was calculated, with the condition (NoAcP, AcP+, AcP) as the within-subjects factor, on the post SSQ total-score, and its subscales nausea, oculomotor, and disorientation (the SSQ scores were shown in Fig. 5). Results demonstrated a significant effect on the nausea subscale, F(2,52) = 5.970, p = .005, ηp2 = 0.187, with a marginal significant trend on the total-score, F (2,52) = 2.938, p = .062, ηp2 = 0.102, and oculomotor subscale, F (2,52) = 2.603, p = .084, ηp2 = 0.091. A non-significant effect was observed on the disorientation subscale, F(2,52) = 0.527, p = .594. Then, post-hoc analysis (a paired-sample t-test) was conducted on the nausea and oculomotor subscales and the total score. Results showed that, on the nausea subscale, both the AcP + condition (t (26) = 2.820, p = .009, Cohen's d = 0.46) and the AcP condition (t (26) = 2.820, p = .009, Cohen's d = 0.64) were significant compared with the NoAcP condition, while on the oculomotor subscale and the total score of SSQ, only the AcP + condition (the oculomotor subscale, t (26) = 2.320, p = .028, Cohen's d = 0.43; total-score, t(26) = 2.129, p = .043, Cohen's d = 0.39) showed a significant effect compared with the NoAcP condition. The AcP condition showed no significant trend
4.3. NRT during and after experiments A two-way repeated measures ANOVA, 3 [conditions (NoAcP, AcP +, AcP), within-subject] * 5 [time (0 min, 5 min, 10 min, 15 min, 20 min), within-subject], was conducted on the NRT scores recorded during the experiments (as depicted in Fig. 6) to examine the intervention effects. A significant effect of time was shown, F (4,108) = 61.379, p < 0.001, ηp2 = 0.70, suggesting that the subjective feeling of cybersickness increased with time. However, the main effect of condition, and the interaction effect were not significant, (F (2,54) = 2.276, p = .112, F(8,216) = 0.804, p = .599). Similarly, a two-way repeated measures ANOVA, 2 [condition (NoAcP, AcP+, AcP), within-subject] * 6 [time (0 min, 2 min, 4 min, 6 min, 8 min, 10 min), within-subject], was conducted on the NRT scores recorded after the experiments to examine whether acustimulation interventions were effective in the recovery period after the experiments (shown in Fig. 7). Results showed a significant trend involving time as well, F(5,130) = 104.651, p < 0.001, ηp2 = 0.80, which indicates a decreasing level of cybersickness with time after experiments. Moreover, a marginally significant main effect of the intervention condition, and a marginally significant interaction effect were revealed (F(2,52) = 3.015, p = .058, ηp2 = 0.10, F(10,260) = 1.747,
Table 4 Mean (SD) of pre-SSQ scores.
Total-score Nausea Oculomotor Disorientation
AcP+
AcP
NoAcP
11.77 (14.80) 10.60 (15.28) 10.11 (12.08) 9.80 (18.03)
10.80 (10.85) 8.13 (9.79) 10.95 (10.99) 8.25 (14.58)
10.80 (10.39) 9.19 (11.06) 10.67 (10.35) 7.22 (11.82)
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condition (NoAcP), both acustimulation methods, acupressure (AcP) and acupressure plus acupaste (AcP+) had some effects on the reported level of cybersickness. For AcP condition, results showed that it had a significant effect on the nausea subscale of the self reported level of cybersickness (SSQ), which was consistent with previous studies of acupressure in treating motion sickness, of which nausea was the core symptom (Chu et al., 2012; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001). For the main focus of this study, our newly proposed acustimulation approach–AcP+ (acupressure plus acupaste), results revealed that compared with NoAcP, AcP + had a significant effect not only on nausea subscale, but also on oculomotor subscale and total score of SSQ, while the differences were not significant compared with AcP condition. Moreover, during the recovery period, in AcP + condition, compared with NoAcP, participants recovered more quickly indicated by NRT scores, while this trend was not significant compared with AcP condition. Detailed discussion was provided in Section 4.3. 5.2. Confounding factors 5.2.1. Placebo effect In this study, the effectiveness of acupressure against VIMS, specifically cybersickness, on nausea-related symptoms, was proven and replicated by comparison between AcP and NoAcP conditions. Based on this finding, we further found that acupressure along with acupaste, compared with control condition, significantly reduced cybersickness symptoms, shown on nausea and oculomotor subscales, total score of SSQ, and facilitated the recovery process indicated by NRT scores. The effects could be more closely examined through two steps in ruling out possible placebo effects: 1) the validation and replication of the effectiveness of acupressure (AcP), the basic form of acustimulation. Comparison between AcP and NoAcP conditions showed that, AcP significantly reduced nausea symptoms on nausea subscale of post-SSQ (while non-significant on other subscales of post-SSQ, nor NRT scores during and after experiments), which was in line with previous findings–acupressure was effective in reducing VIMS, especially on the nausea-related symptoms (Chu et al., 2012; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001, detailed explanations and underlying mechanisms were discussed in the following Section 4.3.1); 2) Based on the validated effectiveness of acupressure, we further examined the effect of AcP+, acupuncture with acupaste, the condition of main interest. During experiments, AcP+ and AcP conditions appeared totally the same to subjects. In both conditions, acupressure was adopted in conjunction with an acupaste of same appearance, while one was a true acupaste, another was an invalid one. The difference between the true and fake acupaste could not be distinguished by participants, either visually or tactually. Considering that all the perceptible factors were kept aligned between these two conditions, we could possibly rule out the influence of subjective bias, and attribute the additional effects of AcP + on oculomotor aspect and recovery period to our manipulation-the upgraded version of acustimulation, acupressure in conjunction with acupaste. However, we had to admit that in this study, the differences between AcP+ and AcP conditions were not significant. Further studies were needed to better clarify the effectiveness of acupaste, whether it just simply magnified the effect of acupressure or had additional ameliorating effects in general.
Fig. 6. Subjective ratings of cybersickness during experiments.
Fig. 7. Subjective ratings of cybersickness after experiments.
p = .071, ηp2 = 0.06). Post-hoc analysis of the condition main effect showed that AcP + had a significant effect compared with the NoAcP condition, (t(26) = 2.395, p = .024, Cohen's d = 0.48), while AcP was not significantly different from the AcP+ (t(26) = 1.693, p = .102), and the NoAcP (t(26) = 0.630, p = .534). Taken together, the acupressure plus acupaste intervention approach was more effective during the recovery process after the experiments. 5. Discussion 5.1. Main finding The purpose of this study was to examine the effect of acupressure plus acupaste, the simpler approach of acustimulation, against cybersickness. In the VR video-watching application, cybersickness measured by NRT showed a trend of increasing under each condition, and gradually faded during the recovery phase (see Figs. 6 and 7). This suggested that for the majority of people, they would suffer a mild level of cybersickness after a period of VR video watching. The longer time immersed in the VR application, the severer the cybersickness will be. Experiments’ results showed that compared with the control
5.2.2. Adaptation effect Another confounding factor of the present study was the probable adaptation effect (symptoms could be lessened through repeated exposure to the same stimulus), as each participant viewed the same VR stimulus three times, with two days between viewings, in three different intervention conditions. To exclude the influence of adaptation, all cybersickness measurements were examined under two variables–intervention conditions, and times (the 1st, 2nd, or 3rd time 6
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condition and previous studies about motion sickness. The explanation of this different finding might be two-folds: 1) compared with motion sickness, visual stress and oculomotor sickness were more distinctive of cybersickness (Guo et al., 2017; Jennifer et al., 2004; Rebenitsch and Owen, 2016), thus acupressure showing a more prominent improvement in VR applications on the oculomotor subscale after intervention; 2) the differences between these two acustimulation methods and the non-negligible effect of acupaste, which commonly served as a substitute non-intrusive alternative of acupuncture. Consisting of a stone “needle” and pressure-sensitive adhesives, Acupaste might greatly magnify the effect of acupressure. Therefore, with the help of acupaste, AcP + turned out to be a more intensive form of acustimulation compared with the pure form of acupressure. Another intensive form of acustimulation–electro-acupressure (Chu et al., 2012) was also demonstrated to be both effective in reducing nausea symptoms, and promoting visuospatial abilities as well (the effet size of it could not be compared with the current study, because only figure, but no numeric data was provided), and even cognitive function by influencing cardiovascular responses through ANS reflex. Intensity of acustimulation has been discussed and paid much attention in this field (Chu et al., 2012; Hu et al., 1992). Moreover, in previous works (Bruce et al., 1990; Hu et al., 1992; Warwick-Evans et al., 1991), not all studies of acupressure against motion sickness found effective. Intensity of acupressure was regarded as an important factor-acupressure, only owning certain level of intensity, could effectively reduce motion sickness symptoms and possibly realize the full functions of acupuncture (e.g. suppress nausea symptoms, enhance motor function, visuospatial abilities, postural control, and cognitive function, Chu et al., 2012; Hu et al., 1992). Therefore, in this study, we might infer that the extra effects of acupressure plus acupaste condition (AcP+) resulted from the magnification effect of acupaste which possibly more effectively activated the ANS reflex and rendered participants more capable to recover quickly. Another interesting result needed further discussion was that, although AcP + significantly facilitated the recovery process, indicated by subjective NRT scores, similar effect was not found during VR experience under AcP + condition, while acupaste was kept stuck with participants during both periods. One possible guess was that the acupressure plus acupaste approach might take some time to take effect. To prove this, we could manipulate the length of VR experience and then examine the cybersickness reported level in future studies. Another feasible explanation of why significant results found after VR reviewing while not during VR viewing, was that considering the nature of NRT, which required participants to report their general subjective cybersickness level at the same time viewing VR video, participants might not own enough cognitive capacity to be well aware of their cybersickness level and report it accurately during VR viewing. The VR viewing conduct itself might influence the subjective report of NRT scores. Especially under the mild cybersickness, any tiny influence could tune the results. Overall, although based on current experiment design, we could not reach a definite answer about the underlying reasons behind this phenomenon, it was of great interest and worthy more efforts to carefully examine its causes.
viewing the stimulus)–at the same time. A two-way repeated measures ANOVA was calculated on the SSQ total-score and its three sub-scales. A three-way repeated measures ANOVA was calculated on the NRT scores recorded during and after experiments. Results showed no significant trend of repetition in any form of measurement (the SSQ total-score, F (2,18) = 1.182, p = .329; nausea, F(2,18) = 1.760, p = .200; oculomotor, F(2,18) = 1.316, p = .293; disorientation, F(2,18) = 0.342, p = .715; subjective ratings during experiments, F(2,20) = 0.694, p = .511; after experiments, F(2,20) = 0.048, p = .953), ruling out the possible influence of an adaptation effect. Previous studies (Gavgani et al., 2017; Hill and Howarth, 2000; Regan, 1995; Sugita et al., 2007) also indicated that adaptation might not always have an effect on cybersickness and could largely depend on individual traits. Besides the placebo and adaptation effects mentioned above, there were also other factors which might influence the results interpretation, such as the head motion, attention location differences between conditions. In this experiment, we tried to keep these factors identical among conditions as much as possible, viewing the same video in each session, conducting the experiment at the same time slot of each day for every participant, keeping the height and location of the swivel chair still all through the experiment etc. Based on the strict experiment control among conditions, the influence from these covariates factors could be neglected. 5.3. Interpretation 5.3.1. Acupressure Acupressure had been proven to be effective in reducing motion sickness and cybersickness in previous studies and the underlying mechanisms had been hotly discussed (Dundee et al., 1986; Hu et al., 1992; Hu et al., 1995; Stern et al., 2001; Streitberger et al., 2006). Sensory conflict theory is one of the most prominent explanations for both motion sickness and cybersickness (Oman, 1990; Rebenitsch and Owen, 2016; Reason, 1978). According to sensory conflict theory, the mismatch among somatosensory, vestibular, and visual inputs give rise to the feelings of nausea and related sickness symptoms. Since vestibular and somatosensory systems have physiological pathways connected with autonomic nervous system (ANS), the conflicting sensory inputs would simultaneously activate autonomic responses (Money, 1970; Ohyama et al., 2007; Previc, 1993). Moreover, ANS further has multiple interactions with “nausea neurosis” and plays a vital role in the development of nausea. Therefore, intervening through ANS is a feasible way to mediate nausea symptoms (Himi et al., 2004; Muth et al., 1999). Many studies have shown that mediation through the sympathetic and parasympathetic nervous system, two components of the ANS, was effective in reducing nausea symptoms and autonomic reactions to motion stimuli (David, Parker, and William, 1978; Farmer et al., 2014; Himi et al., 2004). Functional magnetic resonance imaging studies also directly proved that P6-stimulation had effects on gastric myoelectrical activity, vagal modulation and cerebellar vestibular activities (Streitberger et al., 2006). In addition, using physiological methods, Chu et al. (2012) found that electro-acupuncture effectively mediated motion sickness symptoms mainly by counteraction in the ANS pathways that were stimulated by motion sickness. Arguably, the effect induced by acupressure in this study against cybersickness–nausea feelings might influence the same pathways of ANS, although further physiological validation was still needed to fully explain the underlying mechanism.
5.4. Implications In the current study, it was found that acupressure with acupaste (AcP+) was not only effective in reducing cybersickness nausea feelings, but also helpful in reducing oculomotor discomforts and facilitated cybersickness recovery process, although the underlying physiological mechanism still needs further examination. Current findings of the acupaste effect is still promising, since reducing oculomotor discomforts is extremely important in cybersickness (also called visually-induced motion sickness, VIMS). Visual stress has been found to be one of the leading factors influencing the experience of cybersickness in VR (Rebenitsch and Owen, 2016; So and Ujike, 2010). Therefore, the
5.3.2. Acupressure plus acupaste Except for against nausea feelings, in this study the main condition of interest-acupressure plus acupaste (AcP+) was found to be more effective in reducing cybersickness that it also had significant effects on oculomotor subscale and total score of post-SSQ, and facilitated the recovery process indicated by significant NRT scores, compared with the NoAcP control condition, which were not found on the AcP sham7
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Conflicts of interest
intervention AcP+, acupressure with acupaste, may hold great value for future VR use. Treatments of cybersickness nowadays mainly involved interventions from the technological and hardware aspects, aiming to improve the quality of the scenes presented by virtual reality, for example, the field of view (Bos et al., 2010; van Emmerik et al., 2011) and the timedelay of update (Draper et al., 2001). Other than addressing the technological aspects, behavioral intervention methods tried to solve the problem as well. For example, music therapy (Keshavarz and Hecht, 2014) and diaphragmatic breathing (Russell et al., 2014) were both proven to be effective in reducing cybersickness in VR. However, these methods had some shortcomings, such as occupying too much extracognitive capacity, which could greatly influence users’ VR experience and limit its application and generalization. In summary, compared with these methods, acupressure stimulation methods, especially with acupaste, offered several advantages: 1) lowcost intervention through the user end, which could avoid the difficulties and high expense of promoting a VR hardware system; 2) an easy-to-conduct, non-intrusive technique which took only a few minutes of physical pressure exerted on a particular point; 4) effective, not only for nausea symptoms, but also for the visual and oculomotor afflictions, which were central to cybersickness.
None. Acknowledgements The authors would like to address their special thanks to the financial support of the National Science Foundation of China under grant number 71471095, and the Innovation-Method Fund of China's Ministry of Science & Technology (Project No. 2016IM010200). References Bos, J.E., de Vries, S.C., van Emmerik, M.L., Groen, E.L., 2010. The effect of internal and external fields of view on visually induced motion sickness. Appl. Ergon. 41 (4), 516–521. Bruce, D.G., Golding, J.F., Hockenhull, N., Pethybridge, R.J., 1990. Acupressure and motion sickness. Aviat. Space Environ. Med. 61 (4), 361. Budhiraja, P., Miller, M.R., Modi, A.K., Forsyth, D., 2017. Rotation Blurring: Use of Artificial Blurring to Reduce Cybersickness in Virtual Reality First Person Shooters. Chang, E., Hwang, I., Jeon, H., Chun, Y., Kim, H.T., Park, C., 2013. Effects of rest frames on cybersickness and oscillatory brain activity. 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5.5. Limitations Although we came to a promising finding, our study owned many limitations and left several significant questions which needed to be answered in the future. Firstly, in this study, it lacked of important objective measurements to confirm the subjective rating results. Especially under different experiment conditions, participants' subjective evaluation might be unconsciously influenced by experiment manipulations. Also, the stimulus continuously coming from videowatching tasks could influence participants’ ability to accurately report their sickness level during experiments greatly as well. Objective methods were especially needed and should be compensated in future studies to validate current findings. Secondly, while we found promising effect of an easy-to-conduct acustimulation approach-acupressure plus acupaste, its effectiveness was preliminary and limited. As subjective report NRT shown, the effect of acupressure plus acupaste was manifested only during recovery period. Both its underlying reasons and whether acupaste was effective for sickness prevention were required further exploration. Moreover, the effect of AcP+ was only significant from the NoAcP conditions, but not significant from AcP condition. Based on the current study, we could not figure out whether it was resulted from the experiment design-the level of cybersickness was too small to manifest its full effect and separate from AcP-or not. The full function and effectiveness of AcP + should be better investigated and clarified in future various experiment settings. Thirdly, the intervention effect of acustimulation on disorientation was still unclear. In the current study, subjects were required to sit on a chair and thus they could not stand and walk, having a limited range of movements. Therefore, possible intervention effects on the disorientation aspect of cybersickness, which was indispensable as well, needed further efforts of exploration. 6. Conclusion In the current study, it was found that acupressure plus acupaste, a simple and easy-to-conduct acustimulation approach, was significantly effective in reducing cybersickness nausea feelings, oculomotor discomforts and could facilitate cybersickness recovery, compared with control condition. Although the underlying mechanism needed further investigation and the full effect of acupaste still needed exploration, this finding was promising and of great value for low-cost VR application. 8
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