Keywords

1 Introduction

Recently, with the rapid progress in image processing and three-dimensional (3D) technologies, stereoscopic images are not only available on television but also in theaters, on game machines, and elsewhere. Current 3D display mechanisms include stereoscopy, integral photography, the differential binocular vision method, volumetric display [1, 2], and holography [3]. Viewing stereoscopic images may elicit adverse effects, such as asthenopia or visually-induced motion sickness (VIMS) in some individuals [4]. While the symptoms of general motion sickness include dizziness and vomiting, the phenomenon of VIMS is not fully understood. Currently, there is not enough knowledge accumulated on the effects of stereoscopic images on the living body and basic research is thus important [5].

At present, VIMS is explained by the sensory conflict theory [6]. In humans, the standing posture is maintained by the body’s balance function that is an involuntary physiological adjustment mechanism referred to as the “righting reflex”. In order to maintain the standing posture in the absence of locomotion, the righting reflex that is initiated in the vestibular system and processed in the nucleus ruber is essential in the nucleus ruber is essential. Sensory receptors, such as visual inputs, auditory and vestibular functions, and proprioceptive inputs from the skin, muscles, and joints, are required to maintain the body’s balance function [7]. According to the sensory conflict theory, motion sickness is a response to the conflict generated by a discrepancy between received and previously stored messages. Variations are thus expected that may arise from acquired experiences. Contradictory messages originating from different sensory systems, or the absence of a sensory message that is expected in a given situation, are thought to lead to the feeling of sickness. The human equilibrium system receives information input from the visual, vestibular, and somatosensory systems. The sensory conflict theory states that when the combination of information is inconsistent with previously established human experiences, spatial localization of self becomes unstable and produces discomfort. Visual input enters the brainstem from the visual and somatosensory systems and the cerebellum, in addition to the vestibular system, suggesting that the nuclei physiologically integrate this sensory information. Researchers generally agree that there is a close relationship between the vestibular and autonomic nervous systems both anatomically and electrophysiologically. This view strongly indicates that the equilibrium system is associated with the symptoms of motion sickness [8] and provides a basis for the quantitative evaluation of motion sickness based on body sway, an output of the equilibrium system.

Stabilometry is a useful test of body equilibrium for investigating the overall equilibrium function. Stabilometry methods are presented in the standards of the Japanese Society for Equilibrium Research and in international standards [9]. In Japan, devices to measure body sway are defined by the Japanese Industrial Standards (JIS) [10]. Stabilometry is a simple test in which 60 s recording starts when body sway stabilizes. Objective evaluation is possible by the computer analysis of the speed and direction of the sway, enabling diagnosis of a patient’s condition [11].

In previous studies, viewing 3D images has been shown to affect body sway [12]. However, thus far, it has not been mentioned on whether it is dependent on the viewing time period. In this study, we examined the 3D viewing effect on our equilibrium function to determine whether it is dependent on the exposure time.

2 Materials and Methods

Sixteen healthy male subjects (mean age ± standard deviation: 22.4 ± 0.8 years) participated voluntarily in the study. We ensured that the body sway was not affected by environmental conditions. We used an air conditioner to adjust the temperature at 25°C in the exercise room. The experiment was explained to all subjects and written informed consent was obtained in advance.

In this experiment, we conducted a stabilometry test with subjects viewing 2D and 3D images. The device used was a Wii Balance Board (Nintendo, Kyoto). The sampling frequency of the Wii Balance Board was 20 Hz. The subjects stood upright on the device in Romberg’s posture. We conducted two types of measurements: (I) after resting for 30 s, the body sway of each subject was measured for one minute with opened eyes and for three minutes with closed eyes consecutively, and (II) after resting for 30 s, the body sway of each subject was measured for two minutes with opened eyes and three minutes with closed eyes consecutively. However, in the two-minute measurement test with opened eyes, we also collected data for a period of one minute after the test. Experiments were performed in a dark room to avoid irritation from sources other than the video. The 2D or 3D images were shown on a 3D KDL 40HX80R display (SONY, Tokyo) placed two meters away from the subject. In the image used in the experiment, spheres were fixed at the four corners, while another sphere moved around the screen (Fig. 1). A comparison was then made with subjects who were asked to simply gaze at a point 2 m in front of them at eye level in the case where no image was displayed. The experiments were carried out in random order. Each experiment was carried out on a separate day.

Fig. 1.
figure 1

The image shown for the volunteer tests projected on a 3D display

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The x-y coordinates were recorded for each sampled time point collected in the tests that were conducted with open and closed eyes, and the quantitative indices were calculated. The data were converted to time series and included the position of the center of gravity in the x (the right direction, designated as positive) and y (the anterior direction, designated as positive) directions in each of the open and closed eye tests, the area of sway, total locus length, locus length per unit area, and density, used for subsequent evaluation. The area of sway and total locus length are analytical indices of stabilograms that were used in previous studies. We used these based on the definitions established by the Japanese Society for Equilibrium Research [13].

  • Area of sway: Area of a region surrounded (enveloped) by the circumferential line of sway on the x-y coordinates. An increase in the value represents a more unstable sway;

  • Total locus length: Total extended distance of movement of the center of gravity within the measurement time period. An increase in the value represents a more unstable sway;

  • Locus length per unit area: Value calculated by dividing the total locus length by the area of sway. A decrease in the value represents a more unstable sway;

  • Sparse Density (SpD): Frequency of passage by the center of foot pressure at each fraction established by dividing the stabilogram into squares (See Appendix). The value gets closer to unity when sway is small, i.e., in a region with a high local density. Inversely, the value increases when the sway is scattered.

The area of the sway, the total locus length, and the SpD were obtained from stabilograms recorded when the volunteers had their eyes open and closed. Solidity of the subjects’ vision (2D/3D) and persistency of this influence were assumed to be important affecting factors on which a two-way analysis of variance (ANOVA) was conducted accounting for the number of repetitions. In addition to this two-way ANOVA for each stabilogram index, post hoc comparisons were employed at a significance level of 0.05.

3 Results

We compared stabilograms measured before exposure to the stereoscopic film with those after the exposure (Fig. 2). We also calculated the area of sway, the total locus length, and the locus length per unit area for each subject studied (Fig. 3).

Fig. 2.
figure 2

Typical stabilograms (a, one min recordings with opened eyes, without viewing an image; b, one min recordings with closed eyes, without viewing an image; c, one min recordings with opened eyes, without viewing 2D images; d, one min recordings with closed eyes, without viewing 2D images; e, one min recordings with opened eyes, without viewing 3D images; f, one min recordings with closed eyes, without viewing 3D images).

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Fig. 3.
figure 3

Result of the area of sway, total locus length, locus length per unit area, and density (a, area of sway, viewing 3D image; b, area of sway, viewing 2D image; c, total locus length, viewing 3D image; d, total locus length, viewing 2D image; e, locus length per unit area, viewing 3D images; f, locus length per unit area, viewing 2D images; g, density, viewing 3D images; h, density, viewing 2D images).

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In accordance to the two-way ANOVA results, there was no interaction between the abovementioned two factors. When we viewed the video clips recorded over a period of one min, there was a main effect of the locus length per unit area on the solidity. Except for the locus length per unit area, there was no significant difference between the sway values estimated during the open eye test and those estimated during the closed eye test in terms of multiple comparisons.

On the other hand, when viewing a 3D video clip for two min, the total locus length for the open eye test was significantly larger than that for the closed eye test (after been viewed for two min) (p < 0.01). Moreover, when we viewed a 3D video clip for one min, the area of sway, the total locus length, and the SpD for the open eye test were significantly larger than those for the closed eye test (after been viewed for one min), respectively (p < 0.01). When viewing video clips for two min, the area of sway, the total locus length, and the SpD for the open eye test were significantly larger than those for the closed eye test (after been viewed for one, and two min, respectively) (p < 0.01). When a video clip was viewed for two min, the locus length per unit area for the open eye test was significantly smaller than that for the closed eye test (after been viewed for two min) (p < 0.01). When a 3D video clip was viewed for one min, the area of sway, the total locus length, and the SpD for the open eye test were significantly larger than those for the closed eye test (after been viewed for three min), respectively (p < 0.05). When a 3D video clip was viewed for one min, the locus length per unit area for the open eye test was significantly smaller than that for the closed eye test (after been viewed for three min), respectively (p < 0.05).

Furthermore, when a 2D image was viewed of two min, the area of sway and the total locus length for the open eye test were significantly larger than those for the closed eye test (after been viewed for 1 min), respectively (p < 0.05). The area of sway and the total locus length for the open eye test were significantly larger than those for the closed eye test (after been viewed for two min), respectively (p < 0.01). The area of sway and the total locus length for the open eye test were significantly larger than those for the closed eye test (after been viewed for three min), respectively (p < 0.05).

4 Discussion

When viewing a 2D image, sway is increased depending on the viewing time. Moreover, when viewing an image for two min, the area of sway and the total locus length were significantly larger for the open eye test compared to the values elicited when the image had been viewed two min after the test. Therefore, we considered that after the image had been viewed, equilibrium function still remained. Moreover, regardless of the 3D or 2D images, the area of sway and the total locus length were significantly larger in the open eye test compared to the values elicited in the case where these images were viewed three min after the test. The results of the control were the same as these. Accordingly, the reason for the sway increase in the case when the images were viewed three min after the test is not attributed to the effects of VIMS but to the effects of fatigue. Therefore, we considered that the change in viewing time affected the equilibrium function system, and that viewing a 3D image for two min affected the equilibrium function system for a period of two min after the images had been viewed.