PURPOSE:Stereoscopic displays challenge the neural cross-coupling between accommodation and vergence by inducing a constant accommodative demand and a varying vergence demand. Stereoscopic viewing calls for a decrease in the gain of vergence accommodation, which is the accommodation caused by vergence, quantified by using the convergence-accommodation to convergence (CA/C) ratio. However, its adaptability is still a subject of debate.METHODS:Cross-coupling (CA/C and AC/A ratios) and tonic components of vergence and accommodation were assessed in 12 participants (27.5 ± 5 years, stereoacuity better than 60 arc seconds, 6/6 acuity with corrected refractive error) before and after a 20-minute exposure to stereoscopic viewing. During stimulation, vergence demand oscillated from 1 to 3 meter angles along a virtual sagittal line in sinusoidal movements, while accommodative demand was fixed at 1.5 diopters.RESULTS:Results showed a decreased CA/C ratio (-10.36%, df = 10, t = 2.835, P = 0.018), with no change in the AC/A ratio (P = 0.090), tonic vergence (P = 0.708), and tonic accommodation (P = 0.493).CONCLUSIONS:These findings demonstrated that the CA/C ratio can exhibit adaptive adjustments. The observed nature and amount of the oculomotor modification failed to compensate for the stereoscopic constraint.

BACKGROUND:Several tests are available to optometrists for investigating accommodation and vergence. This study sought to investigate the agreement between clinical and laboratory methods and to clarify which components are actually measured when tonic and cross-link of accommodation and vergence are assessed.METHODS:Tonic vergence, tonic accommodation, accommodative vergence (AC/A) and vergence accommodation (CA/C) were measured using several tests. Clinical tests were compared to the laboratory assessment, the latter being regarded as an absolute reference. The repeatability of each test and the degree of agreement between the tests were quantified using Bland-Altman analysis.RESULTS:The values obtained for each test were found to be stable across repetitions; however, in most cases, significant differences were observed between tests supposed to measure the same oculomotor component. Tonic and cross-link components cannot be easily assessed because proximal and instrumental responses interfere with the assessment. Other components interfere with oculomotor assessment. Specifically, accommodative divergence interferes with tonic vergence estimation and the type of accommodation considered in the AC/A ratio affects its magnitude. Results on clinical tonic accommodation and clinical CA/C show that further investigation is needed to clarify the limitations associated with the use of difference of Gaussian as visual targets to open the accommodative loop.CONCLUSIONS:Although different optometric tests of accommodation and vergence rely on the same basic principles, the results of this study indicate that clinical and laboratory methods actually involve distinct components. These differences, which are induced by methodological choices, must be taken into account, when comparing studies or when selecting a test to investigate a particular oculomotor component.

Purpose Telestereoscopic (or hyperstereo-) viewing which increases effective interocular distance has been a subject of interest since Helmholtz’s pioneering work. Increased disparities and vergence alter 3D-space perception. In particular, apparent egocentric distances decrease, owing to the increased vergence (Priot et al., 2010; Rogers, 2011). This study sought to investigate how the enrichment of stimulus information influences its perceived distance. Methods We assessed distance perception through open-loop pointing before and during exposure to a telestereoscope in three stimulus viewing conditions: i) a red pinpoint of light (pinhole) providing vergence cue only; ii) a red cross with randomized diameter preventing size cue, providing vergence and some accommodative cues and iii) a real-size coin slide providing vergence, accommodation, relative- and familiar-size cues. Results Before exposure (normal viewing), we observed an improved perceived-distance accuracy induced by enriched stimulus information. During exposure (with vergence discrepancy), we found that, on average: i) telestereoscopic viewing altered perceived distance; ii) this effect was modulated by the stimulus viewing condition; iii) the mean slope of the perceived distance of the cross stimulus vs. its physical distance was not different from that predicted by vergence changes according to Helmholtz’s scaling theory; and iv) the hyperstereo effect was attenuated by the enriched stimulus information. Conclusion We ascribe the attenuation of hyperstereopsis by the addition of secondary visual cues to a down-weighting of vergence contribution relative to those of the other cues.

Oculomotor conflict induced between the accommodative and vergence components in stereoscopic displays represents an unnatural viewing condition. There is now some evidence that stereoscopic viewing may induce discomfort and changes in oculomotor parameters. The present study sought to measure oculomotor performance during stereoscopic viewing. Using a 3D stereo setup and an eye-tracker, vergence responses were measured during 20-min exposure to a virtual visual target oscillating in depth, which participants had to track. The results showed a significant decline in the amplitude of the in-depth oscillatory vergence response over time. We propose that eye-tracking provides a useful tool to objectively assess the timevarying alterations of the vergence system when using stereoscopic displays.

The modification of binocular cues in some stereoscopic-display designs potentially alters apparent distances. Although further work is needed to completely clarify the determinants of adaptation to stereoscopic displays, our results reveal that such adaptation can occur, and involves oculomotor, perceptual and motor changes.