Influence of interocular differences in the Strehl ratio on binocular summation

Neural binocular summation and the effect of defocus on the pattern electroretinogram and visual evoked potentials for different pupil sizes

PURPOSE

To evaluate the influence of defocus and pupil size on subjective (visual acuity [VA]) and objective (electrophysiology) descriptors of human vision and their effect on binocular visual performance by means of neural binocular summation (BS).

METHODS

Fifteen healthy young subjects were recruited in this crossover study. Pattern electroretinogram (PERG) and visual evoked potentials (VEP) were measured under two levels of positive (+1.5 and +3.0 D) spherical and astigmatic defocus (axis 90°). Pupil size was controlled to reduce the inter-individual variability factor.

RESULTS

Low- and high-contrast VA showed poorer visual performance in the monocular versus the binocular condition. Positive BS (for VA) was higher with greater pupil size and higher levels of defocus. In the visual electrophysiology tests (i.e., VEP and PERG), peak time and amplitude were affected by pupil size and defocus. The increase in peak time was larger and the reduction in amplitude was more significant with greater levels of defocus and smaller pupil sizes. For the VEP, positive BS was found in all conditions, being stronger with larger amounts of defocus and pupil size (for the P100 amplitude). Significant negative correlations were observed between the P100 amplitude and VA BSs.

CONCLUSION

Smaller pupil size and levels of defocus produced greater changes in cortical activity as evidenced by both the PERG and VEP. Considering these changes and the obtained positive BS, the mechanism could be initiated as early as the retinal processing stage, then being modulated and enhanced along the visual pathway and within the visual cortex.

Effects of alcohol consumption on driving performance in the presence of interocular differences simulated by filters

The role of interocular differences simulated by filters (fog filter and Bangerter foil) on visual and driving performance in alcohol users was assessed. We found that the binocular visual function deteriorates significantly in terms of contrast sensitivity (from 6 to 18 cpd). Additionally, driving performance is significantly impaired under these conditions as evidenced by increased mean speed, standard deviation of the lateral position, distance traveled outside the lane, reaction time and number of collisions. Furthermore, we found that interocular differences due to intraocular scattering and straylight are directly related to an overall reduction in visual and driving performance. This provided a comprehensive perspective from which to understand the relationship between binocular visual function, interocular differences, and driving performance. In practice, our findings contribute to the understanding of the importance of limiting interocular differences, which can be common among presbyopes corrected using the monovision technique, as well as in cases of cataract or other ocular pathology affecting only one eye, or even in cases of cataract surgery of the first eye. These interocular differences can have an adverse impact on road safety, especially when combined with moderate alcohol consumption.

Binocular Visual Function Changes After Corneal Collagen Cross-linking in Patients With Keratoconus

PURPOSE

High-order aberrations (HOAs) have been demonstrated to profoundly affect both visual acuity and stereoacuity in patients with keratoconus. Corneal collagen cross-linking (CXL) has been proven to significantly lower HOAs in keratoconus. Yet, to the present date, no evaluation of the effect of the procedure on stereopsis has been performed. The purpose of the study was to assess differences in binocular visual performance in patients with keratoconus before and after CXL.

METHODS

Patients with keratoconus undergoing standard Dresden protocol epi-off CXL in the ophthalmology department of Policlinico Universitario Agostino Gemelli Hospital received slitlamp examination, uncorrected distance visual acuity and corrected distance visual acuity assessment, corneal tomography, Lang stereotest, TNO stereoacuity assessment, distance and near cover test, and Irvine test before surgery and 6 months after CXL. Stereopsis tests were performed with spectacle correction.

RESULTS

The analysis included 30 patients (mean age 24.63 ± 3.49 years). The Lang test improved after treatment ( P 0.027), with 6 of 30 patients (20%) showing a positive Lang test before CXL compared with 16 of 30 patients (53.3%) after treatment. Moreover, TNO stereoacuity increased after treatment ( P 0.043), and 14 of 30 patients (46.7%) manifested an improvement of at least 250″ at the TNO test after CXL. In this subgroup of patients, total root mean square values decreased after treatment ( P < 0.001), whereas patients who did not manifest a clinically significant improvement in stereoacuity did not show a reduction in total root mean square values after CXL ( P = 0.11). The results of cover test and Irvine test did not vary after CXL.

CONCLUSIONS

Patients with keratoconus showing a reduction of HOAs after CXL also manifest a significant improvement in stereoacuity after the treatment.

Influence of Interocular Differences and Alcohol Consumption on Binocular Visual Performance

The purpose of this study was to assess the influence of a moderate breath-alcohol content (BrAC of 0.40 mg/L) on binocular visual performance for different visual functions after inducing different levels of interocular differences with the use of filters. A total of 26 healthy young subjects were enrolled. The participants participated in two sessions: one without alcohol consumption and another after alcohol consumption. In each session and for the different filter conditions (subjects were wearing Bangerter foil of 0.8 and BPM2 fog filter on the dominant eye), monocular and binocular visual function was evaluated by measuring visual acuity, contrast sensitivity, visual discrimination capacity (and successively by calculating their corresponding binocular summations) and stereopsis (near and distance stereoacuity). In addition, interocular differences were calculated for different retinal-image quality and straylight parameters. All monocular and binocular visual functions were analyzed and stereopsis was significantly impaired by alcohol and filters (p < 0.05). Interocular differences for different ocular parameters and binocular summations for visual parameters were negatively affected by filters but not alcohol. Significant correlations (averaging all the experimental conditions analyzed) were found, highlighting: the higher the interocular differences, the lower the binocular summation and the poorer the stereopsis and, therefore, the worse the binocular visual performance.

Effect of interocular differences on binocular visual performance after inducing forward scattering

Purpose

To assess binocular visual performance by means of binocular summation on visual function after inducing monocular forward scattering, and to study the influence of interocular differences on ocular parameters.

Methods

Seven young healthy subjects were recruited. Four Bangerter foils and five fog filters were used to induce monocular forward scattering. To analyse the impact of the scatter, visual acuity, contrast sensitivity, visual discrimination capacity and distance stereoacuity were measured binocularly with the filter placed over the dominant eye. Additionally, interocular differences were calculated for four ocular parameters: the Objective Scatter Index (OSI), Strehl ratio (SR), modulation transfer function cut‐off (MTF cut off) and straylight (log[s]). Binocular summation was determined for these visual functions.

Results

A statistically significant deterioration in visual acuity, contrast sensitivity and stereoacuity was seen for all of the filter conditions with respect to the natural state (baseline), with the largest change being recorded for the Bangerter foils. Similarly, the interocular difference for the three retinal image quality parameters (OSI, SR and MTF cut‐off) and log(s) increased significantly for the Bangerter foil condition, but not for the fog filters (except log(s)). Binocular summation declined gradually with the Bangerter foils, but not for the fog filters. Statistically significant correlations were found, that is, the greater the interocular differences, the lower the binocular summation of the visual functions and the greater the distance stereoacuity.

Conclusion

Increased forward scattering in the dominant eye resulted in interocular differences, which reduced the overall binocular visual performance, including the binocular summation of several visual functions and distance stereoacuity. The results suggest that marked interocular differences in ocular parameters should be avoided in cases of ocular pathology, amblyopia and emmetropisation procedures (such as refractive surgery) or a monovision correction for presbyopia.

Impact on binocular visual function of small-incision lenticule extraction for high myopia

PURPOSE

To assess whether small-incision lenticule extraction (SMILE) for high myopia reduces the binocular visual function assessed by stereoacuity, binocular visual acuity, binocular contrast sensitivity, and binocular summation.

SETTING

University hospital.

DESIGN

Prospective case series.

METHODS

Patients with a myopic spherical equivalent of at least 6.00 diopters (D) scheduled for SMILE aimed at emmetropia were included. Psychophysical testing was performed with correction preoperatively but no correction postoperatively. Stereoacuity was assessed with the Randot Circles test and the near Frisby test, visual acuity (monocular and binocular) was assessed with high-contrast Early Treatment Diabetic Retinopathy Study charts, and contrast sensitivity (monocular and binocular) was assessed with the Pelli-Robson chart and the Freiburg Acuity and Contrast Test. Binocular summation was calculated by comparing the binocular score against the best monocular score.

RESULTS

A total of 138 eyes of 69 patients were included. The mean spherical equivalent changed from -7.46 D ± 1.06 (SD) to -0.23 D ± 0.40 postoperatively. Stereoacuity did not change significantly postoperatively; median change (interquartile range) was -0.32 (-6.21 to 1.55) seconds of arc with the Frisby test and 0.00 (-7.5 to 5.0) seconds of arc with the Randot test (P ≥ .06). Binocular postoperative uncorrected distance visual acuity was not different from the preoperative corrected distance visual acuity (P = .40). Contrast sensitivity declined slightly monocularly with both tests of contrast sensitivity but was unaltered binocularly (P ≥ .08). Binocular summation for visual acuity and contrast sensitivity was unaltered following surgery (P ≥ .09).

CONCLUSIONS

SMILE for high myopia did not reduce the binocular function assessed by stereoacuity, binocular visual acuity, binocular contrast sensitivity, and binocular summation.

Optical Quality of Highly Myopic Eyes under Different Lighting Conditions and Other Relevant Parameters

Purpose: To evaluate the optical quality of highly myopic eyes under light and dark conditions and correlate it with other relevant parameters. Materials and Methods: A cross-sectional, observational study was performed on 180 eyes of 90 highly myopic patients (29 males and 61 females; mean age: 27.833 ± 0.364 years [range: 18-45 years]). The modulation transfer function (MTF) cutoff frequency, Strehl ratio, objective scattering index (OSI), and predicted visual acuity (PVA; 100%, 20%, and 9%) under different lighting conditions were measured to evaluate the optical quality, which was correlated with probable relevant factors, including, axis length, spherical equivalent (SE), uncorrected distance visual acuity (UDVA), best-corrected distance visual acuity (BDVA), astigmatism, and corneal curvature. The relationship between these visual parameters and optical quality was analyzed using a generalized linear model. Results: The OSIs were 1.241 ± 0.068 and 1.515 ± 0.076; the MTF cutoff frequencies were 37.725 ± 0.559 and 30.238 ± 0.550 cpd; the Strehl ratios were 0.219 ± 0.004 and 0.175 ± 0.003; and the PVAs at 100%, 20%, and 9% were 1.257 ± 0.019 and 1.001 ± 0.018, 0.949 ± 0.017 and 0.719 ± 0.015, and 0.579 ± 0.011 and 0.439 ± 0.010, under bright and dark conditions respectively. The optical quality parameters were correlated with age, SE, BDVA, corneal astigmatism, and curvature to different degrees under different lighting conditions. Conclusions: The optical quality declined in highly myopic eyes under both bright and dark lighting conditions. SE, BDVA, corneal astigmatism, and curvature were correlated with optical quality to different degrees under different lighting conditions.

Evaluation of visual evoked potential binocular summation after corneal refractive surgery

PURPOSE

To explore whether visual evoked binocular summation is affected in eyes with refractive errors after refractive surgery.

METHODS

Twenty participants (6M, 14F) aged 20-35 years (mean 26.7 ± 4.4) were assessed through pattern-reversal visual evoked potential viewing with their every eye (first right eye and then left eye) and then with both eyes while wearing their best correction before undergoing surgery. Also, parameters of the P100 component of pattern-reversal visual evoked potential were evaluated after 3 months of refractive surgery in two different check sizes. Monocular and binocular amplitudes and latencies of P100 wave and binocular summation index were compared between before and after surgery.

RESULTS

Monocular visual evoked potentials elicited by two different high-contrast checkerboard-patterned stimuli were significantly reduced in P100 wave amplitude (P < 0.05) after refractive surgery. However, there was no difference between the pre- and post-refractive surgery in the mean value of monocular P100 latency (P > 0.05). Similar to monocular findings of P100 wave amplitude, on binocular viewing, the mean value of pattern-reversal visual evoked potential amplitude was significantly reduced (P < 0.05), and P100 wave latency was prolonged (P < 0.05) after refractive surgery in participants. Also, the mean postoperative binocular summation index value as compared to that in the preoperative was significantly lesser (P < 0.05) for the subject in this study.

CONCLUSIONS

Refractive surgery can degrade binocular visual performance throughout the change in visual evoked potential binocular summation. However, monocular function deteriorates less than binocular function after refractive surgery.

Binocular cross-correlation analyses of the effects of high-order aberrations on the stereoacuity of eyes with keratoconus

Stereoacuity losses are induced by increased magnitudes and interocular differences in high-order aberrations (HOAs). This study used keratoconus as a model to investigate the impact of HOAs on disparity processing and stereoacuity. HOAs and stereoacuity were quantified in subjects with keratoconus (n = 21) with HOAs uncorrected (wearing spectacles) or minimized (wearing rigid gas-permeable contact lenses) and in control subjects without keratoconus (n = 5) for 6-mm pupil diameters. Disparity signal quality was estimated using metrics derived from binocular cross-correlation functions of stereo pairs convolved with point-spread functions from these HOAs. Metrics computed for all subjects were compared with stereoacuities. The effects of contrast losses and phase shifts on disparity signal quality were studied independently by manipulating the amplitude and phase components of optical transfer functions. The magnitudes, orientations, interocular relationships in magnitude, and shape of the point-spread function affected the cross-correlation metrics that determine disparity signal quality. Stereoacuity covaries strongly with cross-correlation metrics and moderately with image-quality metrics. Both phase distortions and contrast losses due to HOAs significantly influence computations of binocular disparity. HOA-induced stereoacuity reductions are attributable to disparity blur and noise from image properties that reduce the height and kurtosis of the peak stimulus disparity match of the cross-correlation. Phase distortions and contrast losses due to HOAs are both partly responsible for the greater stereoacuity losses seen with spectacles compared to rigid gas-permeable contact lenses in keratoconus.

Retinal-image quality and contrast sensitivity function in eyes with epiretinal membrane: a cross-sectional observational clinical study

Background

To investigate the effect of idiopathic epiretinal membrane (ERM) on the retinal-image quality and psychophysical contrast sensitivity function (CSF).

Methods

Forty-four subjects with diagnosis of idiopathic unilateral ERM were enrolled in this cross-sectional observational clinical study. The fellow unaffected eyes were set as the control group. For retinal-image quality assessment, an Optical Quality Analysis System (OQAS) based on double-pass technique was used to evaluate objective scatter index (OSI) and Strehl ratio. For visual performance, the CSF under photopic condition was measured.

Results

For retinal-image quality, the result of double-pass device revealed a significant lower Strehl ratio and larger OSI in the ERM eyes compared to the fellow eyes (all P < 0.05). For visual performance, the CSF at all spatial frequencies under photopic condition were also significantly degraded in the ERM eyes compared to the fellow eyes (all P < 0.05). For the ERM eyes, the reduction of Strehl ratio and CSF was 29.41 and 54.39%, respectively, and the increase of OSI was 164.10% compared to the fellow eyes. Besides, BCVA significantly correlated to the total CSF (ERM eyes, r = − 0.53, P < 0.001; the fellow eyes, r = − 0.467, P = 0.002) and Strehl ratio (ERM eyes, r = − 0.485, P = 0.001; the fellow eyes, r = − 0.311, P = 0.043) in both of the ERM and the fellow eyes.

Conclusion

Eyes affected with ERM showed poorer retinal-image quality and visual performance than the normal eyes. Retinal-image quality measured by OQAS based on double-pass technique could be useful for assessing the retinal-image quality for ERM-affected eyes, in which retinal scattering was significantly increased.

Electronic supplementary material

The online version of this article (10.1186/s12886-018-0957-1) contains supplementary material, which is available to authorized users.

Binocular summation revisited: Beyond √2

Our ability to detect faint images is better with two eyes than with one, but how great is this improvement? A meta-analysis of 65 studies published across more than 5 decades shows definitively that psychophysical binocular summation (the ratio of binocular to monocular contrast sensitivity) is significantly greater than the canonical value of √2. Several methodological factors were also found to affect summation estimates. Binocular summation was significantly affected by both the spatial and temporal frequency of the stimulus, and stimulus speed (the ratio of temporal to spatial frequency) systematically predicts summation levels, with slow speeds (high spatial and low temporal frequencies) producing the strongest summation. We furthermore show that empirical summation estimates are affected by the ratio of monocular sensitivities, which varies across individuals, and is abnormal in visual disorders such as amblyopia. A simple modeling framework is presented to interpret the results of summation experiments. In combination with the empirical results, this model suggests that there is no single value for binocular summation, but instead that summation ratios depend on methodological factors that influence the strength of a nonlinearity occurring early in the visual pathway, before binocular combination of signals. Best practice methodological guidelines are proposed for obtaining accurate estimates of neural summation in future studies, including those involving patient groups with impaired binocular vision. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Stereopsis Simulating Small-Aperture Corneal Inlay and Monovision Conditions

PURPOSE

To analyze stereoscopic vision under a broad range of experimental conditions simulating small-aperture corneal inlay and monovision conditions.

METHODS

A total of 10 patients participated in the study. The anisocoria generated by corneal inlays was simulated by using a small-aperture contact lens fitted on the non-dominant eye. Visual acuity, contrast sensitivity function, visual discrimination capacity (halo perception), and stereoacuity tests were performed under natural conditions or when the non-dominant eye wore the small-aperture contact lens under two conditions: induced anisocoria and induced anisocoria combined with monovision (using two add-powers: +0.75 and +1.25 diopters [D]). Stereoscopic vision and binocular summation were analyzed under those experimental conditions and for far, intermediate, and near vision.

RESULTS

The results showed a deterioration in stereoacuity with respect to natural conditions, being significant mainly for near and intermediate distances. Results with other visual functions showed a deterioration of the binocular summation and therefore of visual performance, although not in a generalized way for all distances. Binocular visual acuity under the conditions that used the small-aperture contact lens was comparable to that of the natural eye.

CONCLUSIONS

The deterioration of stereopsis and other binocular functions found in this study may be acceptable for patients who had this surgical technique because it is similar to the deterioration found in other surgical procedures of emmetropization. [J Refract Surg. 2018;34(7):482-488.].

Binocular summation and visual function with induced anisocoria and monovision

The advances in new techniques for correcting presbyopia, such as a small aperture combined with monovision, require an in-depth study of binocular aspects. In this work, we have studied binocular visual performance of 12 subjects after inducing different degrees of anisocoria combined with two different add powers in the non-dominant eye. We have analysed visual performance in terms of the visual-discrimination capacity (a function to evaluate the strength of bothersome halos) and the contrast-sensitivity. The results show a deterioration of the binocular vision when inducing anisocoria and with any add power, with a higher perception of halos, a lower contrast sensitivity and poorer binocular summation of these visual functions on increasing anisocoria. This deterioration is clinically acceptable in the case of low add power, since positive binocular summation is maintained in contrast sensitivity, and visual discrimination is not altered.

Bilateral symmetry in vision and influence of ocular surgical procedures on binocular vision: A topical review

We analyze the role of bilateral symmetry in enhancing binocular visual ability in human eyes, and further explore how efficiently bilateral symmetry is preserved in different ocular surgical procedures. The inclusion criterion for this review was strict relevance to the clinical questions under research. Enantiomorphism has been reported in lower order aberrations, higher order aberrations and cone directionality. When contrast differs in the two eyes, binocular acuity is better than monocular acuity of the eye that receives higher contrast. Anisometropia has an uncommon occurrence in large populations. Anisometropia seen in infancy and childhood is transitory and of little consequence for the visual acuity. Binocular summation of contrast signals declines with age, independent of inter-ocular differences. The symmetric associations between the right and left eye could be explained by the symmetry in pupil offset and visual axis which is always nasal in both eyes. Binocular summation mitigates poor visual performance under low luminance conditions and strong inter-ocular disparity detrimentally affects binocular summation. Considerable symmetry of response exists in fellow eyes of patients undergoing myopic PRK and LASIK, however the method to determine whether or not symmetry is maintained consist of comparing individual terms in a variety of ad hoc ways both before and after the refractive surgery, ignoring the fact that retinal image quality for any individual is based on the sum of all terms. The analysis of bilateral symmetry should be related to the patients' binocular vision status. The role of aberrations in monocular and binocular vision needs further investigation.

Corneal Transplantation in Disease Affecting Only One Eye: Does It Make a Difference to Habitual Binocular Viewing?

BACKGROUND

Clarity of the transplanted tissue and restoration of visual acuity are the two primary metrics for evaluating the success of corneal transplantation. Participation of the transplanted eye in habitual binocular viewing is seldom evaluated post-operatively. In unilateral corneal disease, the transplanted eye may remain functionally inactive during binocular viewing due to its suboptimal visual acuity and poor image quality, vis-à-vis the healthy fellow eye.

METHODS AND FINDINGS

This study prospectively quantified the contribution of the transplanted eye towards habitual binocular viewing in 25 cases with unilateral transplants [40 yrs (IQR: 32-42 yrs) and 25 age-matched controls [30 yrs (25-37 yrs)]. Binocular functions including visual field extent, high-contrast logMAR acuity, suppression threshold and stereoacuity were assessed using standard psychophysical paradigms. Optical quality of all eyes was determined from wavefront aberrometry measurements. Binocular visual field expanded by a median 21% (IQR: 18-29%) compared to the monocular field of cases and controls (p = 0.63). Binocular logMAR acuity [0.0 (0.0-0.0)] almost always followed the fellow eye's acuity [0.00 (0.00 --0.02)] (r = 0.82), independent of the transplanted eye's acuity [0.34 (0.2-0.5)] (r = 0.04). Suppression threshold and stereoacuity were poorer in cases [30.1% (13.5-44.3%); 620.8 arc sec (370.3-988.2 arc sec)] than in controls [79% (63.5-100%); 16.3 arc sec (10.6-25.5 arc sec)] (p<0.001). Higher-order wavefront aberrations of the transplanted eye [0.34 μ (0.21-0.51 μ)] were higher than the fellow eye [0.07 μ (0.05-0.11 μ)] (p<0.001) and their reduction with RGP contact lenses [0.09 μ (0.08-0.12 μ)] significantly improved the suppression threshold [65% (50-72%)] and stereoacuity [56.6 arc sec (47.7-181.6 arc sec)] (p<0.001).

CONCLUSIONS

In unilateral corneal disease, the transplanted eye does participate in gross binocular viewing but offers limited support to fine levels of binocularity. Improvement in the transplanted eye's optics enhances its participation in binocular viewing. Current metrics of this treatment success can expand to include measures of binocularity to assess the functional benefit of the transplantation process in unilateral corneal disease.

Reproducibility of optical quality parameters measured at objective and subjective best focuses in a double-pass system

AIM

To evaluate intra-session repeatability and reproducibility of optical quality parameters measured at objective and subjective best focuses in a double-pass system.

METHODS

Thirty Chinese healthy adults (19 to 40 years old) meeting our inclusion criterion were enrolled in the study. After a basic eye examination, two methods of optical quality measurement, based on subjective and objective best focuses were performed using the Optical Quality Analysis System (OQAS) with an artificial pupil diameter of 4.0 mm.

RESULTS

With each method, three consecutive measurements of the following parameters: the modulation transfer function cutoff frequency (MTFcutoff), the Strehl(2D) ratio, the OQAS values (OVs) at contrasts of 100%, 20%, 9% and the objective scatter index (OSI) were performed by an experienced examiner. The repeatability of each method was evaluated by the repeatability limit (RL) and the coefficient of repeatability (COR). Reproducibility of the two methods was evaluated by intra-class correlation coefficient (ICC) and the 95% limits of agreement (Bland and Altman analysis). Thirty subjects, seven females and twenty three males, of whom 15 right eyes and 15 left eyes were selected randomly for recruitment in the study. The RLs (percentage) for the six parameters measured at objective focus and subjective focus ranged from 8.44% to 15.13% and 10.85% to 16.26%, respectively. The CORs for the two measurement methods ranged from 8.27% to 14.83% and 10.63% to 15.93%, respectively. With regard to reproducibility, the ICCs for the six parameters of OQAS ranged from 0.024 to 0.276. The 95% limits of agreement obtained for the six parameters (in comparison of the two methods) ranged from -0.57 to 42.18 (MTFcutoff), -0.01 to 0.23 (Strehl(2D) ratio), -0.02 to 1.40 (OV100%), -0.10 to 1.75 (OV20%), -0.14 to 1.80 (OV9%) and -1.46 to 0.18 (OSI).

CONCLUSION

Measurements provided by OQAS with either method showed a good repeatability. However, the results obtained from the two different measurement methods showed a poor reproducibility. These findings suggest that it might be best to evaluate patients' optical quality by OQAS using the best focus as chosen automatically by the instrument.

Comparison of binocular through-focus visual acuity with monovision and a small aperture inlay

Corneal small aperture inlays provide extended depth of focus as a solution to presbyopia. As this procedure is becoming more popular, it is interesting to compare its performance with traditional approaches, such as monovision. Here, binocular visual acuity was measured as a function of object vergence in three subjects by using a binocular adaptive optics vision analyzer. Visual acuity was measured at two luminance levels (photopic and mesopic) under several optical conditions: 1) natural vision (4 mm pupils, best corrected distance vision), 2) pure-defocus monovision ( + 1.25 D add in the nondominant eye), 3) small aperture monovision (1.6 mm pupil in the nondominant eye), and 4) combined small aperture and defocus monovision (1.6 mm pupil and a + 0.75 D add in the nondominant eye). Visual simulations of a small aperture corneal inlay suggest that the device extends DOF as effectively as traditional monovision in photopic light, in both cases at the cost of binocular summation. However, individual factors, such as aperture centration or sensitivity to mesopic conditions should be considered to assure adequate visual outcomes.

Impact on stereo-acuity of two presbyopia correction approaches: monovision and small aperture inlay

Some of the different currently applied approaches that correct presbyopia may reduce stereovision. In this work, stereo-acuity was measured for two methods: (1) monovision and (2) small aperture inlay in one eye. When performing the experiment, a prototype of a binocular adaptive optics vision analyzer was employed. The system allowed simultaneous measurement and manipulation of the optics in both eyes of a subject. The apparatus incorporated two programmable spatial light modulators: one phase-only device using liquid crystal on silicon technology for wavefront manipulation and one intensity modulator for controlling the exit pupils. The prototype was also equipped with a stimulus generator for creating retinal disparity based on two micro-displays. The three-needle test was programmed for characterizing stereo-acuity. Subjects underwent a two-alternative forced-choice test. The following cases were tested for the stimulus placed at distance: (a) natural vision; (b) 1.5 D monovision; (c) 0.75 D monovision; (d) natural vision and small pupil; (e) 0.75 D monovision and small pupil. In all cases the standard pupil diameter was 4 mm and the small pupil diameter was 1.6 mm. The use of a small aperture significantly reduced the negative impact of monovision on stereopsis. The results of the experiment suggest that combining micro-monovision with a small aperture, which is currently being implemented as a corneal inlay, can yield values of stereoacuity close to those attained under normal binocular vision.

Adaptive Optics for Visual Simulation

A revision of the current state-of-the-art adaptive optics technology for visual sciences is provided. The human eye, as an optical system able to generate images onto the retina, exhibits optical aberrations. Those are continuously changing with time, and they are different for every subject. Adaptive optics is the technology permitting the manipulation of the aberrations, and eventually their correction. Across the different applications of adaptive optics, the current paper focuses on visual simulation. These systems are capable of manipulating the ocular aberrations and simultaneous visual testing though the modified aberrations on real eyes. Some applications of the visual simulators presented in this work are the study of the neural adaptation to the aberrations, the influence of aberrations on accommodation, and the recent development of binocular adaptive optics visual simulators allowing the study of stereopsis.

New testing software for quantifying discrimination capacity in subjects with ocular pathologies

We develop a new visual test, designed as software for quantifying discrimination capacity under low-illumination conditions. This is an important task in the presence of visual disturbances, such as those perceived by subjects with some ocular pathologies. For this purpose, we propose a visual-disturbance index, checking the test with two groups of observers having different ocular pathologies: a group with unilateral keratitis and another group affected with age-related macular degeneration (ARMD). To compare the test results to objective data, we use a double-pass device to measure the Strehl ratio, a parameter that quantifies the retinal-image quality, taking into account aberrations, retinal reflection, and intraocular scattering working jointly. Diseased eyes present higher disturbance indexes and a lower Strehl ratio compared to their healthy fellow eyes, registering a significant descending correlation between the disturbance index and the Strehl ratio. The lower the Strehl ratio is, the higher the disturbance index for the eyes studied. Therefore, in keratitis and ARMD eyes, our results demonstrate a deterioration in the retinal-image quality and a lower discrimination capacity to peripheral stimuli, reducing visual performance. The test presented here could be useful for the study and time course in different eye diseases, especially those involving an increase in scattered light or alterations in the ocular media, as shown in this work.

Adaptive optics binocular visual simulator to study stereopsis in the presence of aberrations

A binocular adaptive optics visual simulator has been devised for the study of stereopsis and of binocular vision in general. The apparatus is capable of manipulating the aberrations of each eye separately while subjects perform visual tests. The correcting device is a liquid-crystal-on-silicon spatial light modulator permitting the control of aberrations in the two eyes of the observer simultaneously in open loop. The apparatus can be operated as an electro-optical binocular phoropter with two micro-displays projecting different scenes to each eye. Stereo-acuity tests (three-needle test and random-dot stereograms) have been programmed for exploring the performance of the instrument. As an example, stereo-acuity has been measured in two subjects in the presence of defocus and/or trefoil, showing a complex relationship between the eye's optical quality and stereopsis. This instrument might serve for a better understanding of the relationship of binocular vision and stereopsis performance and the eye's aberrations.