Stereopsis and Hygiene

This book outlines the principle and display methods of stereopsis, the biological effects of image viewing, and the effects on the human body, as well as its clinical significance. The authors also present the latest research findings and future prospects for stereopsis methods. In the field of medical care, the technique is useful for the 3-dimensional identification of lesions and affected regions; however, stereoscopic images can cause unpleasant symptoms including motion sickness, headache, and visual fatigue. With increasing opportunities for using the stereoscopic viewing technique in various other fields outside medicine, it is important to resolve the underlying issues of stereoscopic viewing and improve the diagnostic accuracy, safety of surgery and reduce the stress for physicians.
Written by pioneering authors, Stereopsis and Hygiene is a valuable resource for both new and established researchers and students seeking comprehensive information on stereoscopic imaging methods as well as professionals working in environmental/occupational health and health promotion.

• Language: English
• Publisher: Springer
• Release date: Dec 11, 2018
• ISBN: 9789811316012

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© Springer Nature Singapore Pte Ltd. 2019

Hiroki Takada, Masaru Miyao and Sina Fateh (eds.)Stereopsis and HygieneCurrent Topics in Environmental Health and Preventive Medicinehttps://doi.org/10.1007/978-981-13-1601-2_1

1. Stabilometry to Evaluate Severity of Motion Sickness on Displays

Hiroki Takada¹  

(1)

Graduate School of Engineering, University of Fukui, Fukui, Japan

Hiroki Takada

Email: [e-mail]

Keywords

StabilometryBlurred imagesStereopsisVisually induced motion sickness (VIMS)Liquid crystal display (LCD)Head-mounted display (HMD)Simulator sickness questionnaire (SSQ)

1.1 Introduction

In daily life, stereoscopic image technology, such as 3D TV and movies, is used in various places. Binocular stereoscopic imaging methods markedly improved after 2009, and viewing of 3D contents for a prolonged period at home became common along with the sale of 3DTV, LCD, 3DPC, and game machines, in addition to 3D movies at movie theaters and theme parks. Symptoms due to stereopsis have been reported whereas stereoscopic imaging techniques are used not only in amusement but also in the industrial, medical care, and educational fields (see Chaps. 3 and 11), and educational fields (see Chap. 4). In the industrial field, the techniques were introduced into 3DCAD, CAM, and CAE and improved productivity and markedly shortened the development period. In the medical care field, stereoscopic images are useful to three-dimensionally identify affected regions and lesions, improving the diagnostic accuracy and safety of surgery and reducing the stress of physicians, and are applied to medical 3DLCD and 3D endoscopic surgery support systems. However, images, not limited to stereoscopic images, may exhibit unfavorable biological effects depending on the viewing and physical conditions, age, and individual differences.

Although the mechanism of the symptoms has not been elucidated and been unclear (see Sect. 1.2 and Chap. 8), stereoscopic videos utilizing binocular stereoscopic vision often cause unpleasant symptoms of asthenopia, such as headache and vomiting, depending on the audiovisual condition [1]. The methods to measure the influence of visually induced motion sickness on the body include subjective psychological methods and physiological methods concerning autonomic nerve activity. Simulator sickness questionnaire (SSQ) is the best known psychological measurement method to assess visually induced motion sickness (VIMS). This is comprised of 16 effective subjective items to assess simulator motion sickness extracted from 1119 paired data on motion sickness questionnaire (MSQ) measured before and after experiencing a simulator by factor analysis [2]. It has been reported that the SSQ score was significantly increased from the pre-resting score by setting the interval between the bilateral heels at 17 cm in the group complaining of vibration load-induced motion sickness [3]. The VIMS is also assessed by physiological measurement methods using electrocardiography, blood pressure, respiratory rate, the number of eyeblinks, electrogastrography, body sway, the resistance value of the skin, perspiration [3–6], and cerebral blood flow (see Chaps. 9 and 10). Romberg’s posture is an upright posture with the feet placed together. It is an unstable standing posture because the base of support is narrow, and so body sway becomes marked, and a reduced equilibrium function is likely to appear in stabilograms (see Sect. 1.3 and Chaps. 5 and 7).

A general stereoscopic view is obtained by using the binocular parallax. The images are composed of photographs taken by two cameras or by computer graphics (CG). Camera axes are fixed and crossed at the point of the virtual image at which the creator expects viewers to gaze. That is, viewers would suffer from finding the anomalous vergence if they looked at the other elements in a stereoscopic flame. Our previous study showed an increase in sway values while viewing video clips blurred on the liquid crystal display (LCD) compared to those on a cathode-ray tube (CRT) in a dark room <100 lx (see Chap. 7) [7–10]. The increase has been also seen in stereo vision on an LCD compared to the static standing in the dark room [11] and the head-mounted display (HMD) (see Sects 1.4 and 1.5) [12]. We also investigated the correlation between head acceleration and the body sway [13, 14] and the appropriate posture to measure degree of the severity of the VIMS in accordance with Scibora et al. [3]. The increase has been suppressed in a new type of the stereo vision, which is called POWER 3D™, compared to the static standing with the wide stance in the dark room [12, 15]. This new technology to construct stereoscopic video clips has been proposed by Nishihara and Tahara (2003) [16] that set each camera axis as well as human beings that change the vergence angle corresponding to the visual distance of subjects for photography. Viewers might not feel a sense of incongruity if they gazed at any elements in the flame. Hence, the new 3D video clip would reduce the body sway. Chapter 6 is well acquainted with the survey of the references in this field.

The increase has been seen in peripheral vision for the conventional 3D video clip compared to that for the 2D in the dark room <10 lx [17]. This result was supported by our subjective evaluation [18]. Especially in the background (BG), there is a large difference between human binocular image and artificial stereoscopic image to which our convergence corresponding to depth cues is not accommodated (Chaps. 5 and 8). That is why equilibrium function affects from peripheral viewing. In Chap. 5, we examine the effect of the exposure to stereoscopic video clips without the background on our equilibrium function.

1.2 Bio–System Involved in the Motion Sickness

When humans maintain an upright posture, the body always sways. The base supporting the body is a narrow area comprising the bilateral feet. To ensure a stable posture, it is necessary to control the feet along a spatial perpendicular line from the center of gravity (CoG) within the narrow base of support [19]. Although the floor reaction to actually support the body is provided by the bilateral legs, since the CoG of the head, upper limbs, and trunk, accounting for 2/3 of the body weight, is present at 2/3 of the height from the floor surface, the CoG constantly sways in the space, and balance is maintained by controlling the relationship with the center of pressure (CoP), serving as a fulcrum to support the body, within the base of support [20, 21]. The reflex to return a swaying body to its original position is termed the righting reflex, which uses a combination of visual system, vestibular, and somatosensory inputs to make postural adjustments. Our brain corrects for the difference between expected posture and perceived posture, which is made comparisons in the cerebellum (Fig. 1.1). Physiologically, it is a body equilibrium function controlled by an involuntary regulatory system [22]. The reflex can be affected by various types of balance disorders. Elucidation of the function is essential to diagnose symptoms accompanying equilibrium disorders, such as progressive cerebellar degeneration, basal ganglia disorder, and Parkinson’s disease [23]. The last patients impaired function of the dopaminergic system suffer from all categories for movements [24, 25] such as voluntary/involuntary processes and emotional behaviors because the dopaminergic system plays an important role in the function of the basal ganglia [26].

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Fig. 1.1

A body equilibrium function controlled by an involuntary regulatory system (postural reflex). Inhibitory control by the cerebellum is marked with dashed line. Lateral pyramidal tract and rubrospinal tract voluntarily control distal muscles of the pupil, and we can conduct skill movement: vestibular-ocular reflex (VO reflex), vestibulospinal reflex (VS reflex)

Reflections caused by appropriate stimuli for the proprioceptor are called proprioceptive reflex, which is involved in muscle tension and maintaining upright posture. Muscle spindle, Golgi tendon organs, joint receptors, and vestibular labyrinth receptors are located in the muscles, tendons, joints, and vestibular labyrinths, respectively. Bio-signal from these proprioceptors provides information on body position and movement [27]. Especially, the latter uses the vestibular organs (semicircular canals, otolith, etc.) as well as the skeletal muscle in order to maintain posture in a gravitational environment. The sensory inputs stimulate the vestibular nerve as well as the vestibular nucleus that has outputs to the thalamus, interstitial nuclei of the medial longitudinal fasciculus, vestibulospinal tract, center of the autonomic nervous system, and vestibulo-cerebellum through a vestibular nucleus neuron [22]. Among these, details of the vestibular-autonomic nervous interaction are not known for sure, i.e., whether it directly reflects to Bötzinger complex in the ventrolateral medulla (reticular formation). This complex is regarded as a center of vomiting pattern generator [28], which may also conduct the behavior caused by the other pathologies such as the chemoreceptor trigger zone (CTZ) and the gastrointestinal tract [29–33]. Also, it directly reflects to the vestibulo-cerebellum (flocculonodular lobe, uvula of vermis, and fastigial nucleus in the cerebellum) as shown in Fig. 1.1, reticular formation [34], and abducens nucleus.

Stereoscopic videos utilizing binocular stereoscopic vision often cause unpleasant symptoms of asthenopia, such as headache and vomiting, depending on the audiovisual condition [1]. Ataxia in simulator-induced sickness has been also reported. These autonomic symptoms are caused by the motion sickness [35–40] and can also be seen during the attack of the inner ear vertigo as the Meniere disease. The cause of the motion sickness has been unknown; however, there are some hypotheses: overstimulation theory and sensory conflict theory. The former has been obviously contradicted by the space motion sickness [41–46] and the simulator sickness with no vestibular stimulation. Along with the latter [36, 37, 39, 40, 47], disagreement between the convergence and accommodation has been pointed out as a cause of visually induced motion sickness (VIMS) with stereopsis (see Chap. 2). As mentioned above, stimulus in the inner ear is transformed to the vestibular nucleus through the vestibular nerve. Generally speaking, the body balance is adjusted by vestibular-ocular reflex and vestibulospinal reflex.

The vestibular-ocular reflex and the vestibulospinal reflex are known as a static reaction. Also, it is said that inhibitory control is conducted by the cerebellum (Fig. 1.1), and the body movement is modified in general. Signals for the modification are provided to this vestibular nucleus, reticular formation, nucleus ruber, and thalamus [27]. The outputs of the former two (tractus vestibulospinalis, tractus reticulospinalis) have a lot of involvement in the postural reflex in addition to the tractus tectospinalis and tractus corticospinalis originating from supplementary motor area and premotor area [48]. This ipsilateral motor system involuntarily controls proximal muscles in the limbs and antigravity muscles in the trunk. Whereas this postural reaction consists of the compensatory postural adjustment and the anticipatory postural adjustment (APA) [49, 50], the former compensates the body balance for the disturbance. Interestingly, APA precedes target voluntary movement by 100 ms. But again, these reactions also affect to the body sway during upright posture.

1.3 Stabilometry

Stabilometry is known to be a test for the equilibrium function, which is useful for overall evaluation of the stability in the standing posture and diagnosis of the equilibrium function disorder. In the usual Romberg’s posture in stabilometry, deterioration in the equilibrium function can be detected because this posture is unstable with a small support area. In this study, we investigate the control system under unstable conditions. We have developed an apparatus for adjusting the tilt angle on the slope on which the body sway is recorded, continuously, with eyes open for 60 s and with eyes closed for 60 s.

Motion process of the CoP is important not only on clinical diagnosis but also elucidation of the human system to control a standing upright as a two-leg robot [51, 52]. The application of this research is also valuable to these studies. A body equilibrium function test procedure, termed stabilometry, is considered useful in comprehensively evaluating the equilibrium function. Stabilometry is typically performed with a subject standing in Romberg’s posture, in which the feet are together and the eyes are open or closed. Sways of the CoP are measured for 60 s at a time. Measurement methods and analytical indices of stabilograms have been proposed to increase the diagnostic value of stabilometry [53]. The analytical indices include the total length of body sway and the locus length per unit area. The latter is considered to represent microchanges in postural control and to serve as a scale of proprioceptive postural control. Romberg’s posture is an upright posture with the feet placed together. It is an unstable standing position because the base of support is narrow, so body sway becomes marked, and a reduced equilibrium function is likely to be evident in stabilograms.

1.4 Instability

When users viewed moving pictures on liquid crystal displays (LCDs), they experienced a visually induced motion sickness that was caused by a disagreement between the visual stimulation and the stimulation of the inner ear [54]. The blurred images on the LCDs sometimes induced image sickness in viewers, which is an unpleasant feeling that is similar to motion sickness. Significant increases in the postural sway were observed during the image sickness induced by simulator [55]. We have studied the following two contents of the instability of human control system at least.

1.4.1 Blurred Images on LCD

Many researches have been conducted previously for obtaining legible character displays on the screen. The optimal brightness ratios required for displaying characters as well as the background were obtained from ergonomic experiments [56, 57]. Scharff et al. also examined the legibility of the colors in the characters [58]. In comparison with backlit LCDs, it has been observed that nonbacklit LCDs reduce the focusing speed among young subjects and the reading performance among middle-aged subjects [59]. With regard to the readability of sufficiently large characters, no significant difference was observed between the high-resolution and standard-resolution video display terminals (VDTs). However, for very small characters, a higher resolution was found to improve the readability [60]. Omori et al. and Hasegawa et al. stochastically discussed the aspect ratio involved in the readability of characters on the LCDs of mobile phones [61, 62].

On the other hand, optokinetic stimulation (OKS) is known to trigger motion sickness [63]. Anterior displacement of the center of gravity was observed during the body sway. In particular, the displacement increased when random dots were rotated vertically at a speed of 40–60 °/s as an OKS to the subjects. However, there has been no study to evaluate the LCDs viewed by subjects using the data obtained from stabilograms. By the way, the center of gravity could be measured in accordance with stabilometry in which many of the earlier studies limited the analysis of the plots to