Short exposure to telestereoscope affects the oculomotor system

Preliminary data on a novel smart glasses system for measuring the angle of deviation in strabismus

BACKGROUND/OBJECTIVES

To propose a novel smart glasses device for recording eye movement and compare its results to the prism alternate cover test (PACT).

SUBJECTS/METHODS

This method comparison study enrolled patients with strabismic conditions, who first underwent conventional strabismus evaluations (PACT in the primary position), followed by the smart glasses NeuroSpeed system (NSS) recording protocols. The video recordings were analysed using specialized software, to calculate the horizontal deviation from the primary position. The results were compared with those of the PACT using Passing‒Bablok regression and Bland‒Altman analysis.

RESULTS

This study included 70 individuals aged 4 to 80 years, of which 38 were men and 32 were women. The overall analysis of horizontal deviations using the Passing‒Bablok regression revealed a correlation coefficient (r) of 0.969, with a systemic bias of 0.00, a proportional bias of 0.809, and a perpendicular residual standard deviation of 4.134.

CONCLUSIONS

The predictive values of eye movement examinations recorded by the NSS were comparable to those of the PACT. Thus, this new system can provide additional information for ophthalmologists to aid in the diagnosis and measurement of strabismus.

The impacts of lens and stereo camera separation on perceived slant in Virtual Reality head-mounted displays

Stereoscopic AR and VR headsets have displays and lenses that are either fixed or adjustable to match a limited range of user inter-pupillary distances (IPDs). Projective geometry predicts a misperception of depth when either the displays or virtual cameras used to render images are misaligned with the eyes. However, misalignment between the eyes and lenses might also affect binocular convergence, which could further distort perceived depth. This possibility has been largely ignored in previous studies. Here, we evaluated this phenomenon in a VR headset in which the inter-lens and inter-axial camera separations are coupled and adjustable. In a baseline condition, both were matched to observers' IPDs. In two other conditions, the inter-lens and inter-axial camera separations were set to the maximum and minimum allowed by the headset. In each condition, observers were instructed to adjust a fold created by two intersecting, textured surfaces until it appeared to have an angle of 90°. The task was performed at three randomly interleaved viewing distances, monocularly and binocularly. In the monocular condition, observers underestimated the fold angle and there was no effect of viewing distance on their settings. In the binocular conditions, we found that when the lens and camera separation were less than the viewer's IPD, they exhibited compression of perceived slant relative to baseline. The reverse pattern was seen when the lens and camera separation were larger than the viewer's IPD. These results were well explained by a geometric model that considers shifts in convergence due to lens and display misalignment with the eyes, as well as the relative contribution of monocular cues.

Binocular fusion enhances the efficiency of spot-the-difference gameplay

Spot-the-difference, the popular childhood game and a prototypical change blindness task, involves identification of differences in local features of two otherwise identical scenes using an eye scanning and matching strategy. Through binocular fusion of the companion scenes, the game becomes a visual search task, wherein players can simply scan the cyclopean percept for local features that may distinctly stand-out due to binocular rivalry/lustre. Here, we had a total of 100 visually normal adult (18-28 years of age) volunteers play this game in the traditional non-fusion mode and after cross-fusion of the companion images using a hand-held mirror stereoscope. The results demonstrate that the fusion mode significantly speeds up gameplay and reduces errors, relative to the non-fusion mode, for a range of target sizes, contrasts, and chromaticity tested (all, p<0.001). Amongst the three types of local feature differences available in these images (polarity difference, presence/absence of a local feature difference and shape difference in a local feature difference), features containing polarity difference was identified as first in ~60-70% of instances in both modes of gameplay (p<0.01), with this proportion being larger in the fusion than in the non-fusion mode. The binocular fusion advantage is lost when the lustre cue is purposefully weakened through alterations in target luminance polarity. The spot-the-difference game may thus be cheated using binocular fusion and the differences readily identified through a vivid experience of binocular rivalry/lustre.

To measure the amount of ocular deviation in strabismus patients with an eye-tracking virtual reality headset

PURPOSE

To investigate the accuracy of a newly developed, eye-tracking virtual reality (VR)-based ocular deviation measurement system in strabismus patients.

METHODS

A VR-based ocular deviation measurement system was designed to simulate the alternative prism cover test (APCT). A fixation target was made to alternate between two screens, one in front of each eye, to simulate the steps of a normal prism cover test. Patient's eye movements were recorded by built-in eye tracking. The angle of ocular deviation was compared between the APCT and the VR-based system.

RESULTS

This study included 38 patients with strabismus. The angle of ocular deviation measured by the VR-based system and the APCT showed good to excellent correlation (intraclass correlation coefficient, ICC = 0.897 (range: 0.810-0.945)). The 95% limits of agreement was 11.32 PD. Subgroup analysis revealed a significant difference between esotropia and exotropia (p < 0.001). In the esotropia group, the amount of ocular deviation measured by the VR-based system was greater than that measured by the APCT (mean = 4.65 PD), while in the exotropia group, the amount of ocular deviation measured by the VR-based system was less than that of the APCT (mean = - 3.01 PD). The ICC was 0.962 (range: 0.902-0.986) in the esotropia group and 0.862 (range: 0.651-0.950) in the exotropia group. The 95% limits of agreement were 6.62 PD and 11.25 PD in the esotropia and exotropia groups, respectively.

CONCLUSIONS

This study reports the first application of a consumer-grade and commercial-grade VR-based device for assessing angle of ocular deviation in strabismus patients. This device could provide measurements with near excellent correlation with the APCT. The system also provides the first step to digitize the strabismus examination, as well as the possibility for its application in telemedicine.

How perceived egocentric distance varies with changes in tonic vergence

According to the eye muscle potentiation (EMP) hypothesis, sustained vergence leads to changes in egocentric perceived distance. This perceptual effect has been attributed to a change in the resting or tonic state of vergence. The goal of the present study was to test the EMP hypothesis by quantifying the relationship between prism-induced changes in tonic vergence and corresponding changes in perceived distance and by measuring the dynamics of changes in perceived distance. During a 10-min exposure to 5-diopter base-out prisms that increased the vergence demand, thirteen right-handed subjects pointed to visual targets located within reaching space using their left hand, without visual feedback. Pre- and post-exposure tests assessed tonic vergence through phoria measurements and egocentric distance estimate through pointing to visual targets with each hand successively, without visual feedback. Similar distance aftereffects were observed for both hands, although only the left hand was used during exposure, indicating that these aftereffects are mediated by visual processes rather than by visuomotor interactions. The distance aftereffects were significantly correlated with prism-induced changes in phoria, demonstrating a relationship between perceived distance and the level of tonic vergence. Changes in perceived distance increased monotonically across trials during prism exposure and remained stable during the post-test, indicating a long time constant for these perceptual effects, consistent with current models of the vergence control system. Overall, these results support the hypothesis that vergence plays a role in reduced-cue distance perception. They further illustrate that variations in tonic vergence influence perceived distance by altering the sensed vergence effort.