Visual fatigue caused by viewing stereoscopic motion images: Background, theories, and observations
Introduction
Visual fatigue is a wide range of visual symptoms, including tiredness, headaches, and soreness of the eyes [1]. Visual fatigue can be caused by demands on early visual functions such as focusing and converging the eyes on a near object and may also involve central cortical structures, for example, those involved in viewing a wide-field, high-contrast, geometric pattern [2]. Most visual tasks in everyday life can contribute in some way to visual fatigue, especially when the eyes are used for long periods. Visual fatigue may occur for example when doing fine work, when reading poorly printed texts and low-quality computer images, when reading in inadequate or intense lighting, and when exposed to flickering lights or to geometric patterns, as well as when a person has uncorrected ametropia. Visual fatigue can also arise when people are viewing stereoscopic motion images. Stereoscopic images used for entertainment must be particularly free of visual fatigue because they are not essential for our lives.
Visual fatigue as a topic of study came to the fore after the 1970s with the introduction of visual display units (VDUs) in the office environment [3]. As a result, international and domestic organizations such as ISO (International Organization for Standardizing) published recommendations for working environments using VDU, in order to reduce fatigue symptoms [4]. In spite of these measures, many people suffer from visual fatigue even today, although it is often said that the number of people reporting visual fatigue in the office environment has decreased since the recommendations were implemented.
Visual fatigue or asthenopia is sometimes referred to as eyestrain. Generally “strain” indicates the elastic distortion caused by “stress”. If the stress is too high, or if the stress continues for a long time, irreversible changes may result. A number of studies on myopia [5], [6], [7] has suggested that similar to mental strain, eyestrain is also a warning of possible irreversible health damage.
Symptoms of visual fatigue generally include eyestrain, dried mucus or tears around the eyelids, feeling of pressure in the eyes, ache around the eyes, discomfort when the eyes are open, hot eyes, difficulty in focusing or blurred vision, stiff shoulders, and headaches [1]. Very little is known about the mechanisms of fatigue, this is partly attributed to the fact that fatigue, and its symptoms are generally assessed subjectively. Although many studies have attempted to measure human stress/strain objectively as described below. Among the known studies on visual fatigue, Wilkins has published a book on visual stress [2]. He has proposed that when a person looks at a specific pattern (such as fine stripes), a part of the brain is strongly stimulated, resulting in migraine and/or fatigue. Wilkins has suggested that people who easily develop migraine also tend to be affected by eyestrain.
In this review, firstly, differences between natural depth perception and depth perception in stereoscopic images are described. Then, types of visual stress resulting from different instruments used for viewing stereoscopic images are identified. After that, methods of evaluating visual fatigue are discussed. Next, accommodation and convergence functions and their discrepancies are reviewed. Finally, the literature on changes in oculomotor balance and pupillary responses caused by viewing images are discussed.
Section snippets
Depth perception and the stereoscopic image
Depth perception uses several cues that can be categorized into the psychological and physiological realms. Psychological cues include perspective, overlap, air perspective, shadow, apparent size, texture, etc., and physiological cues which have been suggested include, binocular parallax, motion parallax, accommodation, and convergence. Binocular parallax refers to difference in image between the two eyes caused by their different location, and binocular disparity refers to retinal image
Instruments for viewing stereoscopic image
A stereoscopic image is produced by presenting one image to the left eye, and another to the right eye. The presentation of these two images may be realized by several methods [8]. In addition to the possibility of stereoscopic images causing fatigue, it is also possible that characteristics inherent in different viewing methods also cause visual stress in the viewers.
Separation by colour filters such as a pair of red and blue glasses is known as an anaglyph, and it is sometimes employed
Evaluation of visual fatigue
It is important to evaluate visual fatigue and sickness caused by viewing images using subjective methods. The simulator sickness questionnaire (SSQ) developed by Kennedy et al. [16] is a well known and a well established useful measurement tool for evaluating motion sickness caused by motion images. Kuze and Ukai [17] used a newly developed questionnaire to subjectively asses symptoms caused by viewing various types of motion images. The questionnaire included items on visual fatigue and
Accommodation and convergence discrepancy
This section focuses on the issue of the type of accommodative and convergence responses that would be evoked with an unnatural combination of stimuli such as those found in stereoscopic images. Discrepancy between accommodative and vergence stimuli is common in stereoscopic images, because accommodation should respond to the screen/image position but disparity of the two images for both eyes, vergence stimulus, varies time-to-time. Usually, accommodation and convergence responses are closely
Visual fatigue and changes in oculomotor balance
If the discrepancy is small, the accommodation and convergence systems respond correctly, i.e., differently. It is often said that, this is unnatural, results in visual stress and causes visual fatigue. Some investigators, e.g., Howarth [43], have discussed how subjective symptoms changed when subjects viewed images with various discrepancies between accommodation and convergence. The results showed that subjects’ heterophoria changed along the changes in discrepancy, and that only small
Changes in pupillary responses
Pupillary activities, such as pupil size, amplitude, and velocity of the responses to light, or other stimuli, are governed by the autonomic nervous system. Activity of the sympathetic nervous system increases pupil size and that of the parasympathetic nervous decreases it. Thus, pupil size reflects mental activity, as well as the whole body condition through the mediation of the autonomic nervous system [66]. Excitement, tiredness, sleepiness, and fatigue after viewing images can affect pupil
Conclusion and future work
In this review, an overview of health issue related to viewing stereoscopic images was presented.
Viewers should be careful to avoid viewing stereoscopic images for extended durations because visual fatigue might be accumulated [57]. They should be ready to stop immediately if fusion difficulties are experienced. Hardware/software manufacturer should avoid unnatural image presentations, such as images that diverge further than infinity, large binocular disparity in the central visual field or
Acknowledgements
This study was carried out under the Standard Authentication Research and development Programme, “Standardization of Assessment Method for Visual Image Safety,” promoted by the Ministry of Economy, Trade and Industry in Japan. This study was also supported by the Japanese Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (B), 16300034, 2004–2006, and 19300037, 2007.
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