Stereoscopic three-dimensional (S3D) displays heighten viewer's sense of immersion by adding binocular depth cues to monocular ones displayed on a screen.1,2 To achieve this, each image is a view of the scene from a slightly different angle, thereby simulating the different views of the eyes in a real scene. As 3D cinemas, TVs, and games are made available to the viewers, there is a growing concern about visual and physical discomforts reported by some 3D viewers.3–6
Of reported viewing symptoms, blurred vision and double vision have been associated the conflicting vergence and accommodative demands encountered in S3D viewing,7,8 which arises from a significant difference in the stimulus to vergence (decided by image disparity) relative to accommodation (decided by the focal depth of S3D display).9–12 Since vergence responses normally are tightly coupled with accommodation neurophysiologically,13–17 the conflict could result in physiological stress and subsequent visual symptoms.
Motion sickness symptoms, such as dizziness, nausea, headache, and disorientation, are often associated with viewing moving stimuli displayed on a screen, where the perceived visual motion conflicts with vestibular or proprioceptive signals.18–20 S3D viewing often entails tracking a visual object through space and depth, and binocular disparity provides critical visual cues for detecting motion in depth.21,22 As such, stereoscopic viewing likely affords greater motion perception than 2D viewing and, hence, induces greater motion sickness symptoms due to a heightened conflict between vision and other senses.
Our recent studies support the contribution of S3D viewing to the above symptoms. In a pilot study, 96 adults viewed either 2D or 3D version of the same movie in a typical commercial theater setting. Visual and motion sickness-related symptoms were measured before and after viewing using a questionnaire developed in our laboratory.23,24 Compared with 2D viewing, S3D viewing resulted in a greater increase of perceived double vision, blurred vision, dizziness, or nausea. Twenty-one percentages of participants reported an increase to at least one of the measured symptoms after S3D viewing. In another study, 21 subjects viewed both 2D and 3D movies with a similar genre in a home theater setting (100 lux room illuminance, 2 meter viewing distance). A greater increase in the abovementioned symptoms for 3D viewing was observed compared with 2D viewing; the reported frequency of increased symptoms (51% of 21 participants) was greater compared with the commercial theater study.6
The different prevalence of reported viewing symptoms in the above studies might have resulted from differences in research methodologies. The viewing angle and distance in the commercial theater study were greater than in a home theater setting and might have placed a smaller conflict to the accommodative and vergence systems. In addition, in the home theater study, the tested movies (converted from 2D movie and displayed on shutter-based 3D TV) were much poorer than those chosen in the commercial theater study (true 3D content and circularly polarized 3D display). The greater cross talk (i.e., “bleed-through” of the other eye's image) could have contributed to the greater frequency of perceived blurred and double images. Finally, in the commercial theater study, participants had a wider age range, whereas in the home theater study, most subjects were young college students. Age-related individual differences might have contributed to the difference in the frequency of reported viewing symptoms observed in these two studies.
To measure the frequency of viewing symptoms in a more representative condition and address the aforementioned issues, in the present study we recruited 203 teen and adult participants and measured their perceived viewing symptoms before, during, and after sustained viewing of state-of-the-art 3D TV and content. We compared the symptoms in viewing 2D and 3D movies at different visual angles and distances. Furthermore, the effect of S3D viewing on perceived immersion and viewing symptoms were analyzed relative to age, gender, and prior symptoms in screen viewing.25
Two hundred three teens and adults (44% female; age, 13–23 years: n = 45; age, 24–34 years: n = 65; age, 35–45 years: n = 43; age 46+ years: n = 50) were recruited to participate in the present study. They viewed a movie in either 2D or 3D using their habitual optical correction, if any. Those who had seen the movie before were excluded from participating in the study. There were no other limitations (e.g., visual acuity and stereoacuity) to participating in this study. Participants were randomly assigned to 2D or 3D viewing groups.
A movie titled “Cloudy with a chance of meatballs” was shown in either 2D or 3D format from a Blu-ray DVD. The duration of the movie was about 90 min. The amount of depth budget (spatial disparity between the two images) was limited to within 2.5% of screen width (48 of 1920 pixels).
A 17-item “Visual and physical discomfort questionnaire,” as shown in Fig. 1, was used to measure subjective viewing symptoms before, during, and after movie viewing based on a 5-point scale. This questionnaire has been used in the previous studies in our laboratory and has been shown to be a valid tool in measuring changes in viewing symptoms.23,24 Wording of the questionnaire was altered for the measurements made before and after movie viewing. For measurements of viewing symptoms during and after movie viewing (both sets of responses taken after movie viewing), participants were specifically instructed to report their symptoms perceived during and after movie viewing. Participants were only asked to respond to questions related to ocular and physical symptoms perceived before and after, not during, movie viewing. This is because these symptoms were best evaluated in the present state rather than being recalled from earlier perception during movie viewing.
In addition, subjects completed a 4-question questionnaire to assess their previous 2D viewing experiences (Fig. 2, Panel A). These questions were only asked once before movie viewing.
A 5-item immersion questionnaire was derived from the Simulator Sickness Questionnaire to measure the viewers' sense of immersion, as shown in Fig. 2. This questionnaire has been shown to be effective in measuring user immersion.26
Home Theater Setting
Multiple viewing sessions took place in a media room in the Ronler Acres campus of Intel Corporation in Hillsboro, Oregon. Up to six participants were admitted to each viewing session and seated at different positions. The first five seat positions formed a horseshoe shape, with each seat oriented toward the center of the screen. They had the same viewing distance (11.1 feet) but different viewing angles in relation to the center of TV screen: seat 3 was directly in front of the TV; seating positions 1 and 2 were to the left of the TV (45 and 22.5° of visual angle respectively), and positions 5 and 4 to the right (45 and 22.5° of visual angle). Seat 6 was aligned to have the same viewing angle as seat 4, but located 15.8 feet away from the TV. The height of all seats was 20 inches.
A Samsung 55” C7000 HD3D LCD TV was used to display the 2D and 3D movies. It has an actual diagonal size of 54.6 inches, 1920 × 1080 native resolution, and 240 Hz vertical refresh rate (120 Hz for each eye in a 3D mode). The 3D image was delivered with a pair of wireless LCD shutter glasses, synchronized to TV display with infra-red signals. The TV was placed on a platform, with the center of TV screen 6 inch above the ground.
Participants were recruited from Intel campuses in Hillsboro, Oregon. Before viewing session, they first read a consent form approved by the Institutional Review Board of Pacific University. They were given opportunity to address any questions or concerns about the study. After providing consent, participants were seated in one of six seating positions, and encouraged to refrain from taking breaks during movie viewing. They then answered the visual and physical discomfort questionnaire as well as the questionnaire about their past viewing symptoms by selecting one of the five symptom levels (Figs. 1 and 2) that best describes their subjective assessment. Immediately after completing the viewing session, they responded the visual and physical discomfort questionnaire. Questions 1 to 12 assessed their symptoms after movie viewing, and the rest of questions (questions 13 to 17) asked about perceived symptoms during and after viewing. In addition, they responded to the immersion questionnaire to assess their perceived immersion during movie viewing. Finally, an e-mail was sent to them next morning to ask subjects to report any remaining viewing symptoms, as described in the visual and physical discomfort questionnaire.
It took about 2 h for participants to complete the viewing session. They did not receive any monetary compensation other than free movie viewing.
Participants were grouped based on display dimension (2D vs. 3D), their age (13–23, 24–34, 35–45, and 46+ years), gender, and seating position. Responses were categorized into groups in which the symptoms increased from baseline, decreased from baseline, or remained the same based on the 5-point scale. A McNemar method was used to compute the odds ratios.27 For this, the ratio of responses with increased symptoms to those with decreased symptoms was defined as the “odds ratio,” as shown in Table 1. Natural log odds ratios and their 95% confidence intervals were constructed for graphical representation of the relative impact of display dimension (2D vs. 3D) on symptoms. Natural log transformation was adopted to transform the asymmetric range of odds ratio (from 0 to infinity, and a 1.0 midpoint) to a symmetric one (between −1 and 1, with 0 as the midpoint). Confidence intervals of the log odds that do not overlap zero demonstrate a significant increase or decrease of symptoms from baseline. Non-overlapping confidence intervals for 2D and 3D indicate significant difference between these two conditions. The width of our confidence intervals was adjusted so that non-overlap with 0 was equivalent to a criterion p = 0.017 to acknowledge the number of comparisons among baseline (0), 2D, and 3D for each measurement.
Principal components factor analysis was used to combine symptoms into internal ocular (eyes sore, pain inside, pulling sensation), external ocular (dry, watery, sandy, irritated, burning), and physical (neck, shoulder, back) symptom scores, based on our previous exploratory analysis on the co-occurrence of these symptoms.23 Their internal consistency was evaluated with Cronbach α correlation. Analysis of covariance (ANCOVA) was conducted to determine the effect of display dimension (2D vs. 3D), age, gender, and seating position on the magnitude of composite symptoms. The magnitude of composite symptoms measured before movie viewing was used as a covariate to control for pre-experiment symptoms.
Visual symptoms (blurred vision and double vision) and those related to motion sickness symptoms (dizziness, nausea, and disorientation) were entered into ANCOVA separately. Display type was treated as repeated measures, and age and gender independent measures. This allows better assessment of individual symptoms' magnitude during and after movie viewing. Questions for perceived immersion were analyzed with analysis of variance (ANOVA), as no previewing response was taken to serve as the covariate. Note that the application of ANOVA/ANCOVA on ordinal measurements is desirable because of the investigation of multiple independent variables (within- and between-subject) for immersion and viewing symptoms and because of the need of analysis of covariate (for viewing symptoms).
To correlate prior visual experiences/symptoms and perceived immersion with measurements of the above five composite symptom categories (internal ocular, external ocular, physical, visual [blurred vision and double vision], and motion sickness symptoms [dizziness, nausea, and disorientation]), Bivariate Pearson correlation was computed and subjected 2-tailed testing with α < 0.01 to reflect the large number of tested correlation values. Symptoms related to vision and motion sickness were submitted to principle component factor analysis to generate composite scores for the correlational analysis to reduce the size of correlational matrix.
Frequencies of Changes in Pre- and Post-Viewing Symptoms
Table 1 shows the frequencies of decreased and increased symptoms in comparing symptoms reported before movie viewing to those during and after movie viewing. Analysis of the frequency in symptom changes reveals that participants in the 2D group had only a slightly greater number of increased symptoms compared with the number of decreased symptoms (187 vs. 172); those in the 3D group reported a greater number of increased symptoms compared with decreased symptoms (302 vs. 143). Overall, 12% and 20% of 2D and 3D viewers reported an increase in any of measured symptoms. For 3D viewing, symptoms with the greatest increase of report frequency in heightened severity (odds ratio >4) include pain inside the eyes, pulling sensation inside the eye, blurred vision (during and after), double image (during and after), dizziness (during), and disorientation (during and after).
Fig. 3 shows the significance of frequency change for ocular (internal and external) and physical symptoms from pre- to post-viewing for 2D and 3D groups, expressed as log odds ratios. Note that subjects were only asked to report these symptoms after movie viewing not during movie viewing.
Fig. 3 reveals the 2D group had a significantly increased frequency of heightened gritty or sandy eyes, pulling sensation, neck ache, and backache after the movie. The 3D group had a significantly increased frequency of heightened burning on the surface of the eye, eye ache, pain inside the eye, pulling sensation, and neck ache; however, there were more frequent decreases in the severity of tiredness/sleepiness.
In addition, 3D viewing resulted in a significantly increased frequency of greater eye sore/pain, and there were more frequent decreases in sandy eyes and backache symptoms compared with 2D viewing.
Fig. 4 presents the frequency of change in visual and motion sickness symptoms from pre- to during or post-movie viewing for 2D and 3D groups. For visual symptoms, the 2D group had significant decreases in the frequency of symptom of seeing double images during the movie, but significant increase in the frequency of symptom of seeing double images after movie viewing. The 3D group had significantly more increases in the severity of blurred vision and double vision during and after the movie. The 3D group also had significantly more increases in blurred vision and double vision than 2D.
For motion sickness symptoms, the 2D group had significantly more decreases in dizziness and disorientation during the movie but not after the movie. The 3D group had significantly more increases in dizziness and disorientation during and after the movie, and nausea after the movie. Comparisons between 2D and 3D symptoms reveal more increases in the frequency of dizziness and disorientation during and after 3D viewing.
Ocular and Physical Symptoms
To determine the change in the magnitude of viewing symptoms, confirmatory principle component analysis was conducted to compute composite scores for five latent variables: internal ocular, external ocular, physical, visual, and motion sickness symptoms. These variables show high interval consistency, as determined by their Cronbach α coefficients of 0.79, 0.63, 0.73, 0.76, and 0.74, respectively. Visual and motion sickness composite variables were done for both during viewing and post-viewing symptoms, and the other three for after viewing symptoms.
ANCOVAs were conducted for the first three composite latent variables, with their corresponding previewing symptom scores serving as covariates. ANOVAs were conducted for individual immersion responses. A summary of ANCOVA and ANOVA results, as expressed by the magnitude of p values, is provided in Table 2. Note that p values for main effects are not reported here if there was a corresponding interaction for the same independent variables. Means and p values are reported for different levels of significant interactions.
For internal ocular symptoms, there was a significant effect of display dimension (p = 0.010). Viewing the 3D movie (1.23) led to a greater increase in internal ocular symptoms compared with 2D viewing (1.02). For external symptoms, there was a significant interaction between seat position and display dimension (p = 0.028). Viewing the 3D movie resulted in stronger external ocular symptoms than 2D viewing for those sitting at slight rightward (seat 4: 1.32) and farther positions (seat 6: 1.12, p = 0.023). There was also a marginally significant dimension by gender interaction (p = 0.045). Women (1.21) had stronger external ocular symptoms than men (1.05, p = 0.042) in 2D viewing. In addition, there was a marginally significant interaction between display dimension and age (p = 0.046). Participants of ages 13 to 24 had stronger external ocular symptoms in 3D viewing (1.56) than 2D viewing (1.13, p = 0.043), whereas the 45+ group had stronger external ocular symptoms in 2D viewing (1.32) than 3D viewing (1.09, p = 0.048). There was no effect of display dimension, age, gender, and seating position on physical symptoms.
Visual and Motion Sickness Symptoms
For visual and motion symptoms, separate ANCOVAs were conducted for individual symptoms in relation to display dimension, age, gender, and seating position. Corresponding previewing symptoms were entered as covariates. Results of ANCOVAs were also reported in Table 2. Significant effects are discussed later in the text, with measured effects on symptoms during and after movie viewing reported separately.
During Movie Viewing
For blurred vision, there were effects of display dimension and age. Blurred vision during 2D viewing (1.09) was lower than during 3D viewing (1.48). Participants in 13 to 23 (1.37) and 24 to 34 (1.39) age-groups reported a greater increase in the severity of blurred vision than those in 35 to 45 age-group (1.10).
There was also an interaction between age and display dimension (p = 0.048). Fig. 5 depicts this interaction between display dimension and age. Although there was no effect of age on blurred vision for 2D viewing, there was significantly greater blurred vision for younger participants (13–23 and 24–34) than older ones (35–45 and 46+) for 3D viewing.
For double vision, there was a main effect of display dimension (p = 0.008). 2D viewing (1.02) resulted in weaker double vision symptoms than 3D viewing (1.33). For dizziness, there was no effect of display dimension, but an interaction between dimension and age (p = 0.013). Fig. 6 shows that younger age-groups (13–23 and 24–34) reported greater dizziness in the 3D condition, whereas older groups (35–45 and 46+) reported slightly greater dizziness in the 2D condition. For nausea and disorientation, there was no effect of display dimension, age, gender, and seating position.
After Movie Viewing
For blurred vision, there was a marginal effect of dimension (p = 0.048), but no interaction. Viewing the movie in 3D (1.30) led to slightly greater blurred vision than 2D viewing (1.14). For double vision, there was no significant effect of display dimension, age, gender, and seating position.
For dizziness, there was an interaction between dimension and age (p = 0.019). Dizziness symptom was stronger in 3D viewing (1.22) than in 2D viewing (1.04, p = 0.024) for the 24 to 34 age-group, but the opposite for the 46+ age-group (2D: 1.22, 3D: 1.03, p = 0.038).
For nausea, there was an effect of gender (p = 0.014) and interactions between dimension and seating position (p = 0.035) and between dimension and age (p = 0.002). Women (1.24) reported stronger nausea symptom than men (1.03). Participants reported stronger nausea symptoms in 3D viewing when seated directly in front of the TV (seats 3: 3D, 1.23; 2D, 1.02, p = 0.008).
Fig. 7 shows that younger participants (24–34) reported stronger nausea symptoms after 3D viewing than 2D viewing; for older participants (46+), the opposite was observed.
For disorientation, there was a marginal interaction between display dimension and age (p = 0.042). Again, for younger participants (24–34), greater nausea symptoms were reported after 3D viewing (1.22) than 2D viewing (1.00, p = 0.036); for older participants (46+), 2D viewing (1.12) resulted in slightly greater disorientation than 3D viewing (1.04, p = 0.046).
For perceiving objects as moving in space, there were effects of dimension (p < 0.001), seating position (p = 0.010), gender (p = 0.048), but no interaction among them. The perception of objects moving in space was higher in 3D viewing (4.11) than 2D viewing (3.28). Women (3.72) reported slightly greater sense of object motion than men (3.45). Sitting at the center (seat 3: 3.91) resulted in greater perceived object motion than 45° away from it (seat 5: 2.90, p = 0.032), and farther and 22.5° to the right (seat 6: 2.56, p = 0.041).
For perceiving oneself moving through the space, there was an effect of dimension (p = 0.003). The perception of oneself moving through space was higher in 3D viewing (2.94) than 2D viewing (2.30). For perceived self involvement in the movie, there was effects of gender (p = 0.001) and seating position. There was an interaction between dimension and seating position (p = 0.024). Women (3.61) reported greater involvement in the story than men (3.26). Sitting at the center and closer to the screen (seat 3: 3.74) resulted in greater sense of involvement than sitting at the right but farther away (seat 6: 3.03, p = 0.038).
For interaction between display dimension and seating position, Fig. 8 shows that 3D viewing at central seats (seats 3 and 6) resulted in greater involvement than seats with greater viewing angles (seats 1 and 5), whereas in 2D viewing, the opposite was observed. For the disruptive effect of display quality on viewing enjoyment and for losing track of time, there was no effect of display dimension, age, gender, and seating position.
Correlation Between Previous Experiences, Discomfort, and Immersion
Table 3 shows the correlation between prior viewing symptoms, perceived immersion, age, and the five composite viewing symptoms. Participants in the 2D group who reported higher prior viewing symptoms in movie and TV viewing had greater external ocular (r = 0.34) and physical symptoms (r = 0.30). Those with higher prior symptoms in utilizing PC reported higher external ocular (r = 42), internal ocular (0.29), and physical (r = 0.40) symptoms after viewing, and visual symptoms during viewing (r = .33). For 2D group, motion sickness is positively correlated to visual disruption in viewing the display (r = 0.26).
For participants in 3D group, there were negative correlations between viewer involvement and external ocular (r = −0.31) as well as internal ocular (r = −0.27) symptoms after viewing; there were also negative correlations between viewer involvement and visual symptoms during (r = −0.27) and after viewing (r = −0.30). Those considering 3D display disruptive to view also reported higher visual symptoms during (r = 0.26) and after viewing (r = 0.31).
The present results show that participants in both 2D and 3D groups reported similar increases of some ocular and physical symptoms, including tension/pulling sensation in the eye, neck ache, and seeing double images after movie viewing. Their history of symptoms when viewing 2D movies and TV predicted external ocular and physical symptoms in 2D viewing but less so in 3D viewing (weaker correlation to physical symptoms and no correlation to external ocular symptom). These ocular symptoms likely reflect the stress on external ocular factors such as reduced eye blinks (causing sandy/gritty eyes) and internal ocular factors such as prolonged squinting and muscle tension (causing pain inside the eye and pulling sensation).28 The common physical symptoms (neck ache and backache) likewise reflect the effect of poor posture from sustained movie viewing.
Comparisons between 2D and 3D viewing reveal that 3D viewing resulted in more frequent increases in some visual and motion sickness symptoms than 2D viewing during the movie. These include pain inside eyes, blurred vision (during and after), seeing double images (during and after), dizziness (during and after), and disorientation (during and after). These symptoms were very moderate, generally no >2 on a 5-point scale.
The observed double images in 3D viewing could have resulted from image cross talk because of poor synchronization of active shutter glasses.29 Alternatively, it could have resulted from inappropriate/inadequate vergence responses due to excessive vergence demand.30 Contribution of these factors to double vision can only be addressed by careful manipulation and measurement of cross talk in rendered images, or by comparing different rendering methods and display devices, while monitoring vergence responses.
S3D viewing also led to a greater sense of immersion, with greater sense of object motion and motion of the viewer in space than 2D viewing. In 3D viewing, an inverse relationship between immersion and motion sickness symptoms was found, with weaker immersion accompanied by more severe symptoms. Participants in central and/or closer seating positions reported greater immersion (self involvement), but also greater symptoms related to motion sickness (nausea) during 3D viewing.
The lower perceived immersion accompanied by higher viewing symptom can be explained by the attenuated visuo-ocular responses to the 3D stimulus because of the accomodative-convergence conflict between the actual viewing angle and the angle suggested by the image. Moreover, too large a viewing angle may prevent successful 3D viewing because of the loss of fusion for the two separate images. Sitting farther away and with greater viewing angle also resulted in heightened symptoms in 2D viewing, which could have resulted from the reduced luminance and contrast level (42% reduction in luminance as measured from a white screen in the present testing setup)31 or from image distortion (30% of foreshortening horizontally).
Older participants reported greater dizziness and nausea in 2D viewing, especially with greater viewing angles and farther distance, whereas younger participants reported greater blurred and double vision, dizziness, and nausea in 3D viewing, especially when seated closer and with more direct viewing angles. Our findings suggest older adults are less susceptible to greater vergence demand, the likely source of blurred and double vision. These are consistent with the clinical knowledge that older individuals suffer from presbyopia and have significantly reduced or absent ability to mobilize accommodation.32–34 This decoupled accommodative–vergence relationship enables older viewers to converge without causing accommodative (focus) changes, likely explaining why the older age-group reported lower symptoms of blurred vision during 3D viewing. Conversely, younger individuals typically have more closely linked accommodative and vergence processes, and this might lead to more accommodation- and/or vergence-related symptoms in 3D viewing than older adults. Because of the lack of accommodation ability, older viewers likely incur greater symptoms in 2D viewing because of the need to maintain the same level of vergence or squinting responses.
Woman reported not only greater motion sickness symptom but also greater immersion in 3D viewing. As there is no known gender difference in related visual abilities, these likely resulted from their greater engagement in 3D viewing.
In summary, the present study revealed several new findings. First, symptoms in 3D viewing result from different causative factors. Movie viewing itself likely causes general ocular and physical symptoms associated with performing sustained visual tasks. However, 3D viewing is specific in causing blurred vision and double vision, and the resultant symptoms are greater for younger adults. Second, 3D viewing results in a greater sense of immersion but also more severe motion sickness symptoms, and this is especially so for younger adults and when viewing from a closer distance and a more direct angle. Finally, in 3D viewing, weaker perceived immersion was accompanied by greater symptoms, indicating that greater viewing symptoms might reduce viewer's immersive experience or that elements of 3D viewing might cause lower immersion and also greater symptoms, such as having too high degree of disparity or too much perceived object or self-motion in the movie.
Pacific University College of Optometry
2043 College Way
Forest Grove, Oregon 97006
This study is funded by a grant from the User Experience Research Group at the Intel Corporation.
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stereoscopic display; visual discomfort; motion sickness; 3D; immersion