Kazuhiko Ukai

Visual fatigue caused by viewing stereoscopic motion images: Background, theories, and observations

The background, theories, and observations on visual stress possibly caused by viewing stereoscopic motion images are reviewed. Visual fatigue caused by stereoscopic images is a safety issue. Fatigue is possible caused by the discrepancy between accommodative and convergence stimuli that are included in the image. Studies on accommodation and convergence are surveyed and an explanation regarding the characteristics of these functions is offered. Studies in the literature on changes in oculomotor function after viewing stereoscopic images, including changes in pupillary responses, are discussed. Evaluation of visual fatigue, particularly in relation to different methods of viewing stereoscopic displays is described.

Subjective evaluation of visual fatigue caused by motion images

A questionnaire was developed to subjectively assess visual fatigue caused by viewing various types of motion images. The questionnaire was evaluated using four types of moving images; playing a TV game using an HMD or a TV, viewing images with and without stabilization of camera shake, viewing a movie with and without colour break-up and viewing either a stereoscopic movie (anaglyph method) or a nonstereoscopic movie. Factor analysis revealed five factors: (1) Eye Strain, (2) General Discomfort, (3) Nausea, (4) Focusing Difficulty and (5) Headache, which were effective for classifying motion images.

Target spatial frequency determines the response to conflicting defocus- and convergence-driven accommodative stimuli

Asthenopia, or visual fatigue, is a frequent complaint from observers of stereoscopic three-dimensional displays. It has been proposed that asthenopia is a consequence of anomalous oculomotor responses generated by conflict between accommodative and convergence stimuli. The hypothesis was examined by measuring accommodation and convergence continuously with a Shin-Nippon SRW5000 infrared autorefractor and a limbus tracking device. Subjects viewed a high contrast Maltese Cross target at three levels of Gaussian filter target blur under conditions of relatively low- and high-conflict between accommodation and convergence stimuli, the latter inducing the sensation of stereopsis. Under the low-conflict conditions accommodation was stable, but convergence-driven accommodation was dominant when the target was extremely blurred. Under the high-conflict conditions the role of convergence-driven accommodation increased systematically with the degree of target blur. It is proposed that defocus-driven accommodation becomes weak when the target comprises low spatial frequency components. Large accommodative overshoots to step stimuli that are not blurred or only mildly blurred were consistently observed and are attributed to the initial accommodative response being convergence-driven. Whereas the possibility that high-conflict conditions are a cause of asthenopia has been previously reported, this is the first evidence that they specifically affect accommodative responses while viewing stereoscopic displays.

The use of video refraction to measure the dynamic properties of the near triad in observers of a 3-D display.

Abstract Objective: Recording of the dynamic behaviour of the near triad during viewing of a three‐dimensional (3‐D) display is attempted. This may then be used to identify the possible origin of asthenopia and other ocular problems which occur whilst viewing such displays.

Spatial pattern as a stimulus to the pupillary system

Pupillary responses to the contrast reversal of a checkerboard pattern of variable check size and contrast were measured. The results show that the pupillary system is responsive to the contrast reversal. It is argued that this response is the light reflex caused by the local luminance change. The summation characteristics of the pupillary afferent are considered. It has also been found that the pupil size, without any influence from contrast reversal, systematically changes with the changes in the check size. Whether this pupillary change is caused by the variation in accommodative performance with changing spatial factors of visual stimulus is discussed.

A QUASI‐STATIC STUDY OF ACCOMMODATION IN AMBLYOPIA*

Abstract— A method named quasi‐static recording of accommodation is introduced. While accommodative stimulus (AS) is slowly changed over a wide range with a constant velocity (0.2 D s‐1), accommodative response (AR) is continuously measured. AS and AR, respectively, are recorded on the abscissa and the ordinate of an x ‐ y recorder to produce a “quasi‐static” response/stimulus curve.

The relationship between CA/C ratio and individual differences in dynamic accommodative responses while viewing stereoscopic images.

The oculomotor synergy as expressed by the CA/C and AC/A ratios was investigated to examine its influence on our previous observation that whereas convergence responses to stereoscopic images are generally stable, some individuals exhibit significant accommodative overshoot. Using a modified video refraction unit while viewing a stereoscopic LCD, accommodative and convergence responses to balanced and unbalanced vergence and focal stimuli (BVFS and UBVFS) were measured. Accommodative overshoot of at least 0.3 D was found in 3 out of 8 subjects for UBVFS. The accommodative response differential (RD) was taken to be the difference between the initial response and the subsequent mean static steady-state response. Without overshoot, RD was quantified by finding the initial response component. A mean RD of 0.11 +/- 0.27 D was found for the 1.0 D step UBVFS condition. The mean RD for the BVFS was 0.00 +/- 0.17 D. There was a significant positive correlation between CA/C ratio and RD (r = +0.75, n = 8, p < 0.05) for only UBVFS. We propose that inter-subject variation in RD is influenced by the CA/C ratio as follows: an initial convergence response, induced by disparity of the image, generates convergence-driven accommodation commensurate with the CA/C ratio; the associated transient defocus subsequently decays to a balanced position between defocus-induced and convergence-induced accommodations.

Visual fatigue in congenital nystagmus caused by viewing images of color sequential projectors

Color breakup is the perceived splitting of the white portions of an image into its red, green, and blue components when the image is projected with the color sequential method and the viewer is moving his or her eyes. This study aims to evaluate how color breakup affects symptoms of visual fatigue in people with congenital nystagmus. The eyes of people with congenital nystagmus continuously oscillate leading to color breakup without pause. One in every 1 500 persons is afflicted with congenital nystagmus. Many sufferers have almost no symptoms in daily life except for a mild deterioration of visual acuity. Five subjects with congenital nystagmus were shown a 15-min portion of a movie projected with three video projectors (one liquid crystal display (LCD) projector and two single-chip digital light processing (DLP) projectors). They were subjectively evaluated both pre- and post-viewing with a questionnaire listing visual fatigue symptoms. One subject was tested in an additional experiment using six more projectors. Results indicated that subjects with congenital nystagmus felt severe visual fatigue after they viewed images produced by color sequential projectors. Mechanism of the cause of visual fatigue is not clear in general and in color breakup in congenital nystagmus, however, it was clear that people with nystagmus felt continuing color breakup as a flickering image. Flickering light is a major cause of visual fatigue. Color sequential projectors are best avoided in public settings, such as classrooms, lecture theaters and conference sites.

Effects of visually simulated roll motion on vection and postural stabilization

BackgroundVisual motion often provokes vection (the induced perception of self-motion) and postural movement. Postural movement is known to increase during vection, suggesting the same visual motion signal underlies vection and postural control. However, self-motion does not need to be consciously perceived to influence postural control. Therefore, visual motion itself may affect postural control mechanisms. The purpose of the present study was to investigate the effects of visual motion and vection on postural movements during and after exposure to a visual stimulus motion.MethodsEighteen observers completed four experimental conditions, the order of which was counterbalanced across observers. Conditions corresponded to the four possible combinations of rotation direction of the visually simulated roll motion stimulus and the two different visual stimulus patterns. The velocity of the roll motion was held constant in all conditions at 60 deg/s. Observers assumed the standard Romberg stance, and postural movements were measured using a force platform and a head position sensor affixed to a helmet they wore. Observers pressed a button when they perceived vection. Postural responses and psychophysical parameters related to vection were analyzed.ResultsDuring exposure to the moving stimulus, body sway and head position of all observers moved in the same direction as the stimulus. Moreover, they deviated more during vection perception than no-vection-perception, and during no-vection-perception than no-visual-stimulus-motion. The postural movements also fluctuated more during vection-perception than no-vection-perception, and during no-vection-perception than no-visual-stimulus-motion, both in the left/right and anterior/posterior directions. There was no clear habituation for vection and posture, and no effect of stimulus type.ConclusionOur results suggested that visual stimulus motion itself affects postural control, and supported the idea that the same visual motion signal is used for vection and postural control. We speculated that the mechanisms underlying the processing of visual motion signals for postural control and vection perception operate using different thresholds, and that a frame of reference for body orientation perception changed along with vection perception induced further increment of postural sway.

Binocular Rivalry Alternation Rate Declines with Age

The effect of aging on the time of spontaneous perceptual alternation in binocular rivalry was examined in 59 subjects. An earlier study reported the change of alternation time by comparing middle-age and elderly subjects. We also observed age-related prolongation in alternation time by comparing subjects in a lower age group (20–34 years) with those in both a middle-age group (35–49 years) and a higher age group (50–64 years). Aging of visual optical functions such as presbyopia or the reduction of contrast sensitivity has an accelerating effect over age and may not be related to the age-associated monotonic prolongation of alternation time in binocular rivalry. The origin of aging in binocular rivalry is still unclear but a neural interval generator for perceptual switching is suggested.