The most striking result of our analysis is the high occurrence of symptoms reported by spectators during and after the vision of a 3D movie in a cinema. Two thirds of individuals reported 1 or more symptoms during the movie, more than one third right after the movie and one quarter still after 2
hours. However, it should be remarked that most of the individuals reported mild symptoms that are barely clinically relevant and that disappeared quickly.
The frequency of spectators reporting symptoms in our study is roughly similar to those observed by others in controlled experimental settings on a smaller number of participants [15
] and support their findings. Polonen and colleagues[15
] used the simulator sickness questionnaire [20
] and a visual strain questionnaire to quantify the level of visual comfort of spectators before and after the movie “U2 3-D”, one of the first filmed with the 2 parallel digital camera system. In their study (sample size
84 people), 50% of the people had some sickness symptoms after the movie, but in general, symptom severity was relatively low and often related to visual discomfort. Only two individuals experienced strong signs of visually induced motion sickness and 4 other people had a large increase in symptom severity after the movie [15
In our study, we did not ask to participants to rate the intensity of the symptoms suffered but it is reasonable to think that individuals still reporting symptoms after 2
hours (25.3% in our study) were those with most severe sickness.
Most symptoms experienced by 3D movie spectators disappeared as soon as the movie ended (Figure ). The rapid dissipation of symptoms agrees with previous findings on the effects of head mounted displays, that disappeared within 10
min from the end of their use [14
Although the low number of cases reported at 2
hours hindered a proper analysis of association with characteristics of individuals, it seems that some people recover slower than other after the intense visual stimulations experienced during a 3D movie. Research should be directed towards identify the reasons of this behavior.
The highest frequency of symptoms in our study refers to visual fatigue (tired eyes, double vision, headache). This result resembles earlier observation that the prolonged use of 3D displays produces visual fatigue and discomfort in the viewers [21
]. A possible explanation is connected to the vergence-accomodation conflict [24
]. The causal mechanism relays on the fact that the eye focus mechanisms (accomodation and blur in the retinal image) specify the depth of the display rather than the depth of the movie scene. Thus the uncoupling of vergence and accommodation required by 3D displays may reduce the ability to fuse the binocular stimulus and causes discomfort and fatigue in the viewers [24
]. For example, Emoto and collegaues [21
] reported symptoms of visual fatigue when the observers viewed stimuli with a larger conflict between the vergence and the focal distances. Kuze and Ukai [8
] showed a significant increase of individuals reporting eyestrain, general discomfort and focusing difficulty when viewing a stereoscopic movie compared to a conventional 2D movies. This might also relate to a more intense eye movements in viewers of 3D movies compared to 2D movies [25
]. In their recent study Hakkinen and colleagues found that in a 2D movie viewers tend to focus at the actors while the eye movement patterns of 3D viewers were more widely distributed to other targets such as complex stereoscopic structures and objects nearer than the actors [25
]. This behavior might increase the vergence-accomodation mismatch, increasing the visual stress on 3D spectators.
Besides visual fatigue symptoms, a portion of viewers reported dizziness and/or nausea. Nausea, dizziness and vertigo relate to vestibular disturbance and may be due to a mismatch between the sensed and the expected vertical position of the body that have been recently suggested as responsible mechanism for causing VIMS [11
]. In Japan Ujike [26
] studied the Matsue movie sickness incident, when 36 out of 294 junior high school students were treated at the hospital for symptoms of motion sickness after watching a hand camera made movie characterized by unexpected whole image motion and vibration. Interestingly, the students who reported greater severity were distributed around the front row and center positions, and those who reported less severity were distributed around the backmost row. For the authors this may indicate that the critical visual angle, above which the risk of visually induced motion sickness noticeably increase, may be between the visual angles obtained in the front row and at the end of the row, at least in the conditions of the incident [26
Regarding the associations between symptom occurrence and individual characteristics, we found a significant association between the number of individuals reporting visually induced symptoms during the movie and history of headache, car sickness, dizziness/vertigo but not with the use of computer and/or videogame console for more than 5
hours per day. The above mentioned associations were not surprising as the relationships between motion sickness, vertigo, dizziness and migraine is well documented [27
]. Also the association between high level of (perceived) anxiety and headache after the movie is probably connected to individual stress or tension, a typical trigger of headache [30
Symptoms like tired eyes and double vision could be associated to anxiety because of the worsening of a pre-existing physiological condition, as can occur in a psychosomatic process.
Among movie vision characteristics, low enjoyment of the movie was associated with several visually induced symptoms. However, it is not possible to infer if the low enjoyment was the result of the symptom onset or a predictor of them. Animation 3D movies seems to be protective for tired eyes but not for disorientation type symptoms. Notably showtime, sitting close to the screen during projection and movie duration all had little and not significant effects. The only exception was palpitation during the movie that resulted associated with several individual and movie vision characteristics. However, the large confidence intervals of OR resulting from low reporting in several factor per symptom combinations hindered a proper interpretation of those results.
This study has also several limitations that are relevant for a thorough evaluation of its findings. Most notably, all results are based on data collected through a self-administered questionnaire to not primed individuals. Some people might have been not familiar with the descriptions of symptoms in the questionnaire, although we did not choose to use more complex and long instruments (like the SSQ [20
]) to avoid such problem. It is also possible that only people who remembered to have suffered of some health problem during or after the 3D movie actually agreed in answering the questionnaire while those who did not have problems, did not. The lack of a control group is also a limitation of this study as some individuals might be sensitive to triggers of symptoms beyond the 3D movie vision. For example, visual stimuli like flickering lights, striped patterns, TV and 2D movie could also act as triggers of migraine [17
] or videoterminal prolonged use may cause visual fatigue [10
]. However, given the cross sectional nature of this research, we did not have among the aims the investigation of possible causal relationships and a control group was not strictly necessary. Additionally, although we posed as inclusion criteria to have seen the 3D movie within 6
months from questionnaire administration, and a large portion of participants (N
417; 46%) declared to have seen a 3D movie within a month from the questionnaire administration, some answers might be affected by recall bias. Nevertheless, the assessment of recall bias on a subsample of individuals gave satisfactory results indicating that the bias should be minimal at least for those that administered the questionnaire within 3
weeks from the day of the movie.