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

The Interaction between Vergence and Accommodation Cues in the Assessment of Fusional Vergence Range

Fusional vergence range tests are commonly used in optometric practice. The aim of this study was to investigate the possible contribution of CA/C, AC/A, and proximal cues (PCT) to the magnitude and presence of blur and recovery during the measurement of fusional vergence ranges and to determine whether the occurrence of blur is influenced by these vergence and accommodation cues. A total of 27 participants with normal binocular vision were included and AC/A, CA/C, and PCT ratios were evaluated. Blur, break, and recovery values in convergence and divergence were assessed with base-out and base-in prisms, respectively. No statistical correlations were found between AC/A, CA/C, and PCT ratios and the magnitude of blur, break, and recovery values in neither far, near, convergence, nor divergence testing conditions. However, better near point of convergence values were related to higher break values in convergence at far distances, but not at near distances. In addition, for convergence and far distance, a statistical difference was found between groups reporting and not reporting blur in AC/A stimulus and PCT ratios. The present results cannot confirm whether vergence and accommodation cues, such as AC/A, CA/C, and PCT ratios, may play an active role during the assessment of fusional vergence range.

Study of the Immediate Effects of Autostereoscopic 3D Visual Training on the Accommodative Functions of Myopes

Purpose

Stereoscopic viewing has an impact on ocular dynamics, but its effects on accommodative functions are not fully understood, especially for autostereoscopic viewing. This study aimed to investigate the changes in dynamic accommodative response, accommodative amplitude, and accommodative facility of myopes after autostereoscopic visual training.

Methods

We enrolled 46 adults (men = 22 and women = 24; age = 21.5 ± 2.5 [range = 18–25] years, spherical equivalent: −4.52 ± 1.89 [−8.88 to −1.75] diopters [D]) who visited the Eye & ENT Hospital of Fudan University. The study population was randomly divided into three-dimensional (3D) and two-dimensional (2D) viewing groups to watch an 11-minute training video displayed in 3D or 2D mode. Dynamic accommodative response, accommodative facility, and accommodative amplitude were measured before, during, and immediately after the training. Accommodative lag and the variability of accommodation were also analyzed. Visual fatigue was evaluated subjectively using a questionnaire.

Results

Accommodative lag decreased from 0.54 ± 0.29 D to 0.42 ± 0.32 D (P = 0.004), whereas accommodative facility increased from 10.83 ± 4.55 cycles per minute (cpm) to 13.15 ± 5.25 cpm (P < 0.001) in the 3D group. In the 2D group, there was no significant change in the accommodative lag (P = 0.163) or facility (P = 0.975), but a decrease in accommodative amplitude was observed (from 13.88 ± 3.17 D to 12.71 ± 2.23 D, P = 0.013). In the 3D group, the accommodative response changed with the simulated target distance. Visual fatigue was relatively mild in both groups.

Conclusions

The immediate impact of autostereoscopic training included a decrease in the accommodative lag and an increase in the accommodative facility. However, the long-term effects of autostereoscopic training require further exploration.

Effect of Illumination on Ocular Status Modifications Induced by Short-Term 3D TV Viewing

Objectives. This study aimed to compare changes in ocular status after 3D TV viewing under three modes of illumination and thereby identify optimal illumination for 3D TV viewing. Methods. The following measures of ocular status were assessed: the accommodative response, accommodative microfluctuation, accommodative facility, relative accommodation, gradient accommodative convergence/accommodation (AC/A) ratio, phoria, and fusional vergence. The observers watched 3D television for 90 minutes through 3D shutter glasses under three illumination modes: A, complete darkness; B, back illumination (50 lx); and C, front illumination (130 lx). The ocular status of the observers was assessed both before and after the viewing. Results. After 3D TV viewing, the accommodative response and accommodative microfluctuation were significantly changed under illumination Modes A and B. The near positive fusional vergence decreased significantly after the 90-minute 3D viewing session under each illumination mode, and this effect was not significantly different among the three modes. Conclusions. Short-term 3D viewing modified the ocular status of adults. The least amount of such change occurred with front illumination, suggesting that this type of illumination is an appropriate mode for 3D shutter TV viewing.

Are high lags of accommodation in myopic children due to motor deficits?

Children with a progressing myopia exhibit an abnormal pattern of high accommodative lags coupled with high accommodative convergence (AC/A) and high accommodative adaptation. This is not predicted by the current models of accommodation and vergence. Reduced accommodative plant gain and reduced sensitivity to blur have been suggested as potential causes for this abnormal behavior. These etiologies were tested by altering parameters (sensory, controller and plant gains) in the Simulink model of accommodation. Predictions were then compared to the static and dynamic blur accommodation (BA) measures taken using a Badal optical system on 12 children (6 emmetropes and 6 myopes, 8-13years) and 6 adults (20-35years). Other critical parameters such as CA/C, AC/A, and accommodative adaptation were also measured. Usable BA responses were classified as either typical or atypical. Typical accommodation data confirmed the abnormal pattern of myopia along with an unchanged CA/C. Main sequence relationship remained invariant between myopic and nonmyopic children. An overall reduction was noted in the response dynamics such as peak velocity and acceleration with age. Neither a reduced plant gain nor reduced blur sensitivity could predict the abnormal accommodative behavior. A model adjustment reflecting a reduced accommodative sensory gain (ASG) coupled with an increased AC cross-link gain and reduced vergence adaptive gain does predict the empirical findings. Empirical measures also showed a greater frequency of errors in accommodative response generation (atypical responses) in both myopic and control children compared to adults.

3D Displays

Creating realistic three-dimensional (3D) experiences has been a very active area of research and development, and this article describes progress and what remains to be solved. A very active area of technical development has been to build displays that create the correct relationship between viewing parameters and triangulation depth cues: stereo, motion, and focus. Several disciplines are involved in the design, construction, evaluation, and use of 3D displays, but an understanding of human vision is crucial to this enterprise because in the end, the goal is to provide the desired perceptual experience for the viewer. In this article, we review research and development concerning displays that create 3D experiences. And we highlight areas in which further research and development is needed.

Stereoscopic 3D Visual Discomfort Prediction: A Dynamic Accommodation and Vergence Interaction Model

The human visual system perceives 3D depth following sensing via its binocular optical system, a series of massively parallel processing units, and a feedback system that controls the mechanical dynamics of eye movements and the crystalline lens. The process of accommodation (focusing of the crystalline lens) and binocular vergence is controlled simultaneously and symbiotically via cross-coupled communication between the two critical depth computation modalities. The output responses of these two subsystems, which are induced by oculomotor control, are used in the computation of a clear and stable cyclopean 3D image from the input stimuli. These subsystems operate in smooth synchronicity when one is viewing the natural world; however, conflicting responses can occur when viewing stereoscopic 3D (S3D) content on fixed displays, causing physiological discomfort. If such occurrences could be predicted, then they might also be avoided (by modifying the acquisition process) or ameliorated (by changing the relative scene depth). Toward this end, we have developed a dynamic accommodation and vergence interaction (DAVI) model that successfully predicts visual discomfort on S3D images. The DAVI model is based on the phasic and reflex responses of the fast fusional vergence mechanism. Quantitative models of accommodation and vergence mismatches are used to conduct visual discomfort prediction. Other 3D perceptual elements are included in the proposed method, including sharpness limits imposed by the depth of focus and fusion limits implied by Panum's fusional area. The DAVI predictor is created by training a support vector machine on features derived from the proposed model and on recorded subjective assessment results. The experimental results are shown to produce accurate predictions of experienced visual discomfort.

Depth-of-Focus Affects 3D Perception in Stereoscopic Displays

Stereoscopic systems present binocular images on planar surface at a fixed distance. They induce cues to flatness, indicating that images are presented on a unique surface and specifying the relative depth of that surface. The center of interest of this study is on a second problem, arising when a 3D object distance differs from the display distance. As binocular disparity must be scaled using an estimate of viewing distance, object depth can thus be affected through disparity scaling. Two previous experiments revealed that stereoscopic displays can affect depth perception due to conflicting accommodation and vergence cues at near distances. In this study, depth perception is evaluated for farther accommodation and vergence distances using a commercially available 3D TV. In Experiment I, we evaluated depth perception of 3D stimuli at different vergence distances for a large pool of participants. We observed a strong effect of vergence distance that was bigger for younger than for older participants, suggesting that the effect of accommodation was reduced in participants with emerging presbyopia. In Experiment 2, we extended 3D estimations by varying both the accommodation and vergence distances. We also tested the hypothesis that setting accommodation open loop by constricting pupil size could decrease the contribution of focus cues to perceived distance. We found that the depth constancy was affected by accommodation and vergence distances and that the accommodation distance effect was reduced with a larger depth-of-focus. We discuss these results with regard to the effectiveness of focus cues as a distance signal. Overall, these results highlight the importance of appropriate focus cues in stereoscopic displays at intermediate viewing distances.

Quantitative measurement of eyestrain on 3D stereoscopic display considering the eye foveation model and edge information

We propose a new method for measuring the degree of eyestrain on 3D stereoscopic displays using a glasses-type of eye tracking device. Our study is novel in the following four ways: first, the circular area where a user's gaze position exists is defined based on the calculated gaze position and gaze estimation error. Within this circular area, the position where edge strength is maximized can be detected, and we determine this position as the gaze position that has a higher probability of being the correct one. Based on this gaze point, the eye foveation model is defined. Second, we quantitatively evaluate the correlation between the degree of eyestrain and the causal factors of visual fatigue, such as the degree of change of stereoscopic disparity (CSD), stereoscopic disparity (SD), frame cancellation effect (FCE), and edge component (EC) of the 3D stereoscopic display using the eye foveation model. Third, by comparing the eyestrain in conventional 3D video and experimental 3D sample video, we analyze the characteristics of eyestrain according to various factors and types of 3D video. Fourth, by comparing the eyestrain with or without the compensation of eye saccades movement in 3D video, we analyze the characteristics of eyestrain according to the types of eye movements in 3D video. Experimental results show that the degree of CSD causes more eyestrain than other factors.

Developmental changes in the balance of disparity, blur, and looming/proximity cues to drive ocular alignment and focus

Accurate coordination of accommodation and convergence is necessary to view near objects and develop fine motor coordination. We used a remote haploscopic videorefraction paradigm to measure longitudinal changes in simultaneous ocular accommodation and vergence to targets at different depths, and to all combinations of blur, binocular disparity, and change-in-size ('proximity') cues. Infants were followed longitudinally and compared with older children and young adults, with the prediction that sensitivity to different cues would change during development. Mean infant responses to the most naturalistic condition were similar to those of adults from 6-7 weeks (accommodation) and 8-9 weeks (vergence). Proximity cues influenced responses most in infants of less than 14 weeks of age, but sensitivity declined thereafter. Between 12 and 28 weeks of age infants were equally responsive to all three cues, while in older children and adults manipulation of disparity resulted in the greatest changes in response. Despite rapid development of visual acuity (thus increasing availability of blur cues), responses to blur were stable throughout development. Our results suggest that, during much of infancy, vergence and accommodation responses are not dependent on the development of specific depth cues, but make use of any cues available to drive appropriate changes in response.

Empirical Evaluation of Visual Fatigue from Display Alignment Errors Using Cerebral Hemodynamic Responses

The purpose of this study was to investigate the effect of stereoscopic display alignment errors on visual fatigue and prefrontal cortical tissue hemodynamic responses. We collected hemodynamic data and perceptual ratings of visual fatigue while participants performed visual display tasks on 8 ft × 6 ft NEC LT silver screen with NEC LT 245 DLP projectors. There was statistical significant difference between subjective measures of visual fatigue before air traffic control task (BATC) and after air traffic control task (ATC 3), (P < 0.05). Statistical significance was observed between left dorsolateral prefrontal cortex oxygenated hemoglobin (l DLPFC-HbO₂), left dorsolateral prefrontal cortex deoxygenated hemoglobin (l DLPFC-Hbb), and right dorsolateral prefrontal cortex deoxygenated hemoglobin (r DLPFC-Hbb) on stereoscopic alignment errors (P < 0.05). Thus, cortical tissue oxygenation requirement in the left hemisphere indicates that the effect of visual fatigue is more pronounced in the left dorsolateral prefrontal cortex.

Evidence that convergence rather than accommodation controls intermittent distance exotropia

PURPOSE

This study considered whether vergence drives accommodation or accommodation drives vergence during the control of distance exotropia for near fixation. High accommodative convergence to accommodation (AC/A) ratios are often used to explain this control, but the role of convergence to drive accommodation (the CA/C relationship) is rarely considered. Atypical CA/C characteristics could equally, or better, explain common clinical findings.

METHODS

Nineteen distance exotropes, aged 4-11 years, were compared while controlling their deviation with 27 non-exotropic controls aged 5-9 years. Simultaneous vergence and accommodation responses were measured to a range of targets incorporating different combinations of blur, disparity and looming cues at four fixation distances between 2 m and 33 cm. Stimulus and response AC/A and CA/C ratios were calculated.

RESULTS

Accommodation responses for near targets (p = 0.017) and response gains (p = 0.026) were greater in the exotropes than in the controls. Despite higher clinical stimulus AC/A ratios, the distance exotropes showed lower laboratory response AC/A ratios (p = 0.02), but significantly higher CA/C ratios (p = 0.02). All the exotropes, whether the angle changed most with lenses ('controlled by accommodation') or on occlusion ('controlled by fusion'), used binocular disparity not blur as their main cue to target distance.

CONCLUSIONS

Increased vergence demand to control intermittent distance exotropia for near also drives significantly more accommodation. Minus lens therapy is more likely to act by correcting overaccommodation driven by controlling convergence, rather than by inducing blur-driven vergence. The use of convergence as a major drive to accommodation explains many clinical characteristics of distance exotropia, including apparently high near stimulus AC/A ratios.