Normative reference ranges for binocular summation as a function of age for low contrast letter charts

Binocular Summation With Quantitative Contrast Sensitivity Function: A Novel Parameter to Evaluate Binocular Function in Intermittent Exotropia

Purpose

Intermittent exotropia (IXT) is the most common form of strabismus. Surgery can potentially improve binocular function in patients with IXT. We aimed to evaluate binocular function using a novel parameter-binocular summation ratio (BSR), measured using quantitative contrast sensitivity function (CSF) in patients with IXT before and after surgery.

Methods

Prospective study of 63 patients with IXT and 41 healthy controls were consecutively enrolled and underwent quantitative CSF testing binocularly and monocularly. BSR was calculated by dividing the CSF of the binocular value by the better monocular value. Forty-eight patients with IXT underwent strabismus surgery. BSR, stereoacuity, fusion ability, and strabismus questionnaires were assessed pre-operatively and 2 months postoperatively.

Results

Sixty-three patients with IXT (median age = 9 years) compared with 41 healthy controls showed a worse mean BSR based on all CSF metrics at baseline (the area under the log CSF [AULCSF], spatial frequency [SF] cutoff, and contrast sensitivity at 1.0-18.0 cpd SF). All 48 patients with IXT showed successful alignment after surgery, and there were significant improvements in BSR based on the AULCSF, SF cutoff, and contrast sensitivity at 6.0, 12.0, and 18.0 cpd SF, respectively. The distance stereoacuity and fusion ability also improved after surgery, and a better BSR was associated with better stereoacuity and fusion. For strabismus questionnaires, the psychosocial subscale scores improved postoperatively, whereas the functional subscale scores did not change.

Conclusions

BSR based on quantitative CSF can characterize binocular function across a range of spatial frequencies and can be used as a supplemental measurement for monitoring binocularity in patients with IXT in clinical settings.

Binocular summation in high and low contrast letter acuities

Binocular summation, a well-known phenomenon in letter acuity measurement, refers to the improvement in visual performance when viewing with both eyes compared to one eye alone. The present study aims to assess the relationship in binocular summation between high and low contrast letter acuities, and examine whether baseline measure (binocular summation at either high or low contrast) is predictive of the change in binocular summation between contrast conditions. Corrected high and low contrast letter acuities were assessed monocularly and binocularly in 358 normal vision observers aged 18-37 years using Bailey-Lovie charts. All observers had high contrast acuities (both monocular and binocular) of 0.1 LogMAR or better and no known eye disease. Binocular summation was calculated as the difference in LogMAR between the better eye acuity and binocular acuity. We found that binocular summation was present at both contrast levels (0.044 ± 0.002 LogMAR for high and 0.069 ± 0.002 LogMAR for low contrast) with higher magnitude of summation at low contrast, and declined with increasing interocular difference. There was a correlation in binocular summation between high and low contrast. The difference in binocular summation between the two contrast levels was found to be correlated with the baseline measurement. Using common commercially available letter acuity charts, we replicated the findings on binocular acuity summation in normally sighted young adults for both high and low contrast letters. Our study revealed a positive relationship in binocular acuity summation between high and low contrast, and an association between a baseline measure and the change in binocular summation between contrast levels. These findings may serve as a reference in clinical practice and research when high and low contrast binocular summations are measured in assessing binocular functional vision.

Binocular summation in comitant exotropia: Change after surgery

Purpose:

To assess the change in binocular summation (BiS) in comitant exotropia (XT) after strabismus surgery.

Methods:

This is a prospective study on 20 patients who underwent surgery for comitant XT over a one year period. Patients with sensory exotropia and nystagmus were excluded. Best-corrected visual acuity (VA) and contrast sensitivity (CS) of both eyes separately and together (binocularly) were recorded. BiS score was calculated as binocular score minus better eye score. BiS score at the end of 3 months was compared with the preoperative data.

Results:

The mean ± SD of BiS score increased from 2.95 ± 0.88 to 4.55 ± 0.68 (P-value < 0.0001) for VA (on ETDRS letters) and from 2.75 ± 0.44 to 4.5 ± 0.76 (P-value < 0.001 for CS (on Pelli–Robson chart) after surgery.

Conclusion:

There is significant improvement in BiS in XT after strabismus surgery. Authors recommend its inclusion in evaluation of functional outcome of XT surgery.

A Unified Rule for Binocular Contrast Summation Applies to Normal Vision and Common Eye Diseases

Purpose

Binocular summation refers to better visual performance with two eyes than with one eye. Little is known about the mechanism underlying binocular contrast summation in patients with common eye diseases who often exhibit binocularly asymmetric vision loss and structural changes along the visual pathway. Here we asked whether the mechanism of binocular contrast summation remains preserved in eye disease.

Methods

This study included 1035 subjects with normal ocular health, cataract, age-related macular degeneration, glaucoma, and retinitis pigmentosa. Monocular and binocular contrast sensitivity were measured by the Pelli-Robson contrast sensitivity chart. Interocular ratio (IOR) was quantified as the ratio between the poorer and better eye contrast sensitivity. Binocular summation ratio (BSR) was quantified as the ratio between binocular and better eye contrast sensitivity.

Results

All groups showed statistically significant binocular summation, with the BSR ranging from 1.25 [1.20, 1.30] in the glaucoma group to 1.31 [1.27, 1.36] in the normal vision group. There was no significant group difference in the BSR, after accounting for IOR. By fitting a binocular summation model Binocular = (Leftm + Rightm)1/m to the contrast sensitivity data, we found that the same binocular summation rule, reflected by the parameter m, applies across the five groups.

Conclusions

Cortical binocular contrast summation appears to be preserved in spite of eye diseases that can affect the two eyes differently. This finding supports the importance of assessing both monocular and binocular functions, rather than relying on a monocular assessment in the better eye as a potentially inaccurate surrogate measure.

Effect of Initial Aflibercept, Laser, or Observation on Low-Contrast Visual Acuity in Eyes With Diabetic Macular Edema and Good Vision: Ancillary Study Within a Randomized Clinical Trial

Purpose

To describe 2.5% low-contrast visual acuity (VA) among eyes with good vision despite center-involved diabetic macular edema and compare changes after initial management with aflibercept, laser, or observation.

Methods

This was an ancillary study within a multicenter randomized clinical trial (DRCR Retina Network Protocol V). Participants had diabetes and 1 study eye with center-involved diabetic macular edema and a VA of 20/25 or better randomly assigned to aflibercept (n = 112), focal/grid laser (n = 146), or observation (n = 129). Eyes in the laser and observation groups received aflibercept if VA met prespecified worsening criteria.

Results

Participants had median age of 60 years, 37% were female and 70% were non-Hispanic White. At baseline, the mean ± standard deviation (SD) high-contrast VA was 85.2 ± 3.6 letters (Snellen equivalent 20/20), mean ± SD 2.5% low-contrast VA was 47.6 ± 18.9 letters (Snellen equivalent 20/125), and low-contrast VA letter score was 2 SDs or more below the age-specific normative values in 23%. At 2 years, the mean change ± SD in low-contrast VA in the aflibercept, laser, and observation groups was 2.7 ± 20.1, –2.0 ± 19.6, and –3.1 ± 20.8 letters (adjusted difference, aflibercept vs. laser, 5.3 [95% confidence interval, –0.2 to 10.8], P = 0.06; aflibercept vs. observation, 5.5 [95% confidence interval –0.2 to 11.2], P = 0.06; and laser vs. observation, 0.2 [95% confidence interval –4.6 to 5.0], P = 0.94).

Conclusions

There was no significant difference between treatment groups in low-contrast VA change from baseline to 2 years. Considering the range of the 95% confidence intervals, however, the study may have been underpowered to detect a clinically meaningful benefit between treatment groups.

Translational Relevance

Low-contrast VA, an important visual function, is decreased in eyes with diabetic macular edema.

High- and Low-Contrast Letter Acuity Perception Matures With Age in Normally Sighted Children

BACKGROUND

High-contrast visual acuity (HCVA) changes with age, yet little is known about pediatric-specific age- and sex-normative values for low-contrast letter acuity (LCLA). We define maturational changes in monocular and binocular HCVA and LCLA in childhood and adolescence.

METHODS

Normally sighted youth (ages 5-20 years, without neurologic or ophthalmologic disease and best-corrected HCVA of 20/25 or better in each eye) were recruited. Mean monocular and binocular scores using Early Treatment Diabetic Retinopathy Study (for HCVA) and 2.5% and 1.25% Sloan (for LCLA) charts and the magnitude of binocular summation were calculated using 2-year bins. Relationships between scores and age were explored using scatterplots with Locally Weighted Scatterplot Smoothing (LOWESS) and analysis of variance that accounts for intereye correlation, followed by test of linear trend for age effect.

RESULTS

Among 101 (202 eyes) healthy participants (mean age 13 years, 42% males), monocular and binocular scores varied by age, with highest mean scores achieved in the 13 to 14-year age group for both HCVA and LCLA. Between the ages of 5 and 14.9 years, monocular scores increased linearly with age (0.76 letter/year for HCVA, 1.11 letters/year for 2.5% LCLA, and 0.97 letter/year for 1.25% LCLA; all P < 0.0001). Binocular HCVA scores also increased with age between 5 and 14.9 years (0.71 letters/year, P < 0.0001). The magnitude of binocular summation for HCVA or LCLA did not change with age.

CONCLUSIONS

HCVA and LCLA abilities mature into adolescence, peak between 13 and 14.9 years of age, and then plateau into adulthood. Evaluation of patients with visual deficits should consider age-expected normal visual acuity.

Binocular Summation and Suppression of Contrast Sensitivity in Strabismus, Fusion and Amblyopia

Purpose: Amblyopia and strabismus affect 2%–5% of the population and cause a broad range of visual deficits. The response to treatment is generally assessed using visual acuity, which is an insensitive measure of visual function and may, therefore, underestimate binocular vision gains in these patients. On the other hand, the contrast sensitivity function (CSF) generally takes longer to assess than visual acuity, but it is better correlated with improvement in a range of visual tasks and, notably, with improvements in binocular vision. The present study aims to assess monocular and binocular CSFs in amblyopia and strabismus patients.

Methods: Both monocular CSFs and the binocular CSF were assessed for subjects with amblyopia (n = 11), strabismus without amblyopia (n = 20), and normally sighted controls (n = 24) using a tablet-based implementation of the quick CSF, which can assess a full CSF in <3 min. Binocular summation was evaluated against a baseline model of simple probability summation.

Results: The CSF of amblyopic eyes was impaired at mid-to-high spatial frequencies compared to fellow eyes, strabismic eyes without amblyopia, and control eyes. Binocular contrast summation exceeded probability summation in controls, but not in subjects with amblyopia (with or without strabismus) or strabismus without amblyopia who were able to fuse at the test distance. Binocular summation was less than probability summation in strabismic subjects who were unable to fuse.

Conclusions: We conclude that monocular and binocular contrast sensitivity deficits define important characteristics of amblyopia and strabismus that are not captured by visual acuity alone and can be measured efficiently using the quick CSF.

Binocular summation for reflexive eye movements

Psychophysical studies and our own subjective experience suggest that, in natural viewing conditions (i.e., at medium to high contrasts), monocularly and binocularly viewed scenes appear very similar, with the exception of the improved depth perception provided by stereopsis. This phenomenon is usually described as a lack of binocular summation. We show here that there is an exception to this rule: Ocular following eye movements induced by the sudden motion of a large stimulus, which we recorded from three human subjects, are much larger when both eyes see the moving stimulus, than when only one eye does. We further discovered that this binocular advantage is a function of the interocular correlation between the two monocular images: It is maximal when they are identical, and reduced when the two eyes are presented with different images. This is possible only if the neurons that underlie ocular following are sensitive to binocular disparity.

Identification and treatment of the visual processing asymmetry in MS patients with optic neuritis: The Pulfrich phenomenon

BACKGROUND

The Pulfrich phenomenon (PF) is the illusory perception that an object moving linearly along a 2-D plane appears to instead follow an elliptical 3-D trajectory, a consequence of inter-eye asymmetry in the timing of visual object identification in the visual cortex; with optic neuritis as a common etiology.

OBJECTIVE

We have designed an objective method to identify the presence and magnitude of the PF, in conjunction with a cooresponding strategy by which to abolish the effect; with monocular application of neutral density filters to the less affected fellow eye, in patients with MS and a history of optic neuropathy (e.g. related to acute optic neuritis or subclinical optic neuropathy).

METHODS

Twenty-three MS patients with a history of acute unilateral or bilateral optic neuritis, and ten healthy control subjects (HC) were recruited to participate in a pilot study to assess our strategy. Subjects were asked to indicate whether a linearly moving pendulum ball followed a linear 2-D path versus an illusory 3-D elliptical object-motion trajectory, by reporting the ball's approximation to one of nine horizontally-oriented colored wires that were positioned parallel to one another and horizontal to the linear pendulum path. Perceived motion of the bob that moved along wires behind or in front (along the 'Z' plane) of the middle reference wire indicated an illusory elliptical trajectory of ball motion consistent with the PF.

RESULTS

When the neutral density filter titration was applied to the fellow eye the severity of the PF decreased, eventually being fully abolished in all but one patient. The magnitude of neutral density filtering required correlated to the severity of the patient's initial PF magnitude (p < 0.001).

CONCLUSIONS

We ascertained the magnitude of the visual illusion associated with the PF, and the corresponding magnitude of neutral density filtering necessary to abolish it.

Binocular Summation and Control of Intermittent Exotropia

BACKGROUND

Binocular summation (BiS), or improvement in binocular vision exceeding the better eye alone, is affected by strabismus. Being easily measured, BiS may be a useful indicator for subjective outcomes like stereopsis in strabismus. This study aims to investigate the relationship between BiS and measures of control of intermittent exotropia (IXT).

METHODS

Patients with IXT were recruited before undergoing strabismus surgery and underwent tests of binocular and monocular high- and low-contrast visual acuity, stereopsis at distance and near, and Newcastle Control Score (NCS), a score developed by incorporating home control and clinic control criteria into a control rating scale. BiS was calculated using high-contrast Early Treatment of Diabetic Retinopathy Study (ETDRS) and Sloan low-contrast acuity charts (LCA) at 2.5% and 1.25% contrast as the difference between the binocular score and that of the better eye. The relationship between BiS and measures of IXT control (NCS and distance near stereoacuity disparity) was evaluated using a correlation analysis by Spearman correlation coefficients and the Kruskal-Wallis test.

RESULTS

Thirty-four patients were included (mean [± standard deviation (SD)] age 19±16 years) having a mean (±SD) of 26±16Δ IXT at distance and 20±16Δ at near. Mean (±SD) BiS for ETDRS and Sloan LCA at 2.5% and 1.25% was 0.8±3.6, 1.9±6.0, and -2.3±7.2, respectively. The Spearman correlation coefficient of BiS and NCS was -0.53 (95% CI -0.85 to -0.25) for 2.5% LCA and -0.43 (95% CI -0.77 to -0.13) for 1.25% LCA. BiS at 2.5% LCA (P=0.006) and at 1.25% LCA (P=0.029) significantly differed between the groups based on NCS score groupings (1-3, 4-6, and 7-9), with patients who had better control scores having higher levels of BiS. BiS did not differ significantly between patients grouped according to the difference between stereoacuity measured at near versus distance.

CONCLUSION

Significantly lower low-contrast BiS in patients with higher NCS may suggest that decreased BiS is associated with less control in IXT. This finding suggests that BiS may reflect control in IXT across a population of patients with IXT.

Decreased Binocular Summation in Strabismic Amblyopes and Effect of Strabismus Surgery

PURPOSE

Binocular summation (BiS) occurs when binocular visual function surpasses that of the better eye alone. We sought to evaluate whether strabismic amblyopia reduces BiS more than strabismus alone, and determine whether BiS improves in strabismic amblyopes after strabismus surgery.

METHODS

We prospectively recruited 15 patients with strabismic amblyopia who then underwent strabismus surgery. Thirty age-matched normal subjects and 30 non-amblyopic strabismic patients served as controls. Subjects underwent binocular and monocular visual acuity testing on high-contrast Early Treatment Diabetic Retinopathy Study (ETDRS) as well as 2.5% and 1.25% Sloan low contrast acuity (LCA) charts. BiS was calculated as the difference between better eye and binocular scores.

RESULTS

Strabismic amblyopes and strabismic controls did not significantly differ in preoperative BiS, but both had subnormal BiS preoperatively on LCA charts. Among 11 strabismic amblyopes with preoperative and postoperative BiS measurements, average postoperative BiS was not significantly different from preoperative. Improved LCA BiS postoperatively occurred in some patients and was associated with measurable preoperative stereoacuity (P=0.02), older age at strabismus onset (P=0.02), and larger preoperative angle of strabismus (P=0.0043).

CONCLUSIONS

In this preliminary study, strabismic amblyopes experienced subnormal BiS, but amblyopia generally did not further impair BiS beyond that due to strabismus alone. Some strabismic amblyopes experienced improved low-contrast BiS after strabismus surgery. This suggests that further investigation in larger groups of patients should be undertaken to analyze a previously unrecognized functional benefit of strabismus surgery in strabismic amblyopes.

Quantitative visual tests after poorly recovered optic neuritis due to multiple sclerosis

BACKGROUND

Visual dysfunction in MS can be quantified using a variety of tests. Many vision tests have not been formally evaluated among MS patients with existing visual dysfunction.

OBJECTIVE

Evaluate several versions of visual acuity and contrast sensitivity tests, measures of central and peripheral vision, retina structure, electrophysiologic function, and quality of life among MS patients with moderate/severe visual dysfunction.

METHODS

Cross-sectional study of 46 patients with stable, incompletely recovered optic neuritis. Testing included Snellen eye charts, several Sloan low contrast charts, Pelli Robson (PR) contrast sensitivity charts, optical coherence tomography, visual fields, Farnsworth Munsell 100-hue test, visual evoked potentials (VEP), and visual function quality of life (VFQ-25) testing.

RESULTS

98% of eyes could read two lines of the PR chart, while only 43% read the 2.5% contrast chart. Low contrast tests correlated strongly with each other and with retinal nerve fiber layer (RNFL) thickness, visual fields, and color vision but not with VEPs. For patients with RNFL <75µm, VFQ-25 scores dropped by approximately 2 points for every 1µm decrease in RNFL.

CONCLUSION

Among MS patients with visual impairment due to optic neuritis, PR contrast sensitivity could be utilized as a single chart. Visual quality of life was associated with RNFL thinning below 75µm.

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.

Improvement in binocular summation after strabismus surgery

IMPORTANCE

Binocular summation (BiS), or improvement in visual acuity using binocular vision compared with the better eye alone, is diminished in patients with strabismus. However, it is still not known how strabismus surgery affects BiS.

OBJECTIVE

To determine whether BiS improves after strabismus surgery.

DESIGN, SETTING, AND PARTICIPANTS

Prospective study of 97 patients undergoing strabismus surgery between September 1, 2011, and January 31, 2014, comparing preoperative and postoperative measures of BiS. Patients were recruited within 1 month before undergoing strabismus surgery. The study took place at an academic pediatric ophthalmology and strabismus practice.

INTERVENTION

Strabismus surgery.

MAIN OUTCOMES AND MEASURES

All patients underwent high- and low-contrast visual acuity testing binocularly and monocularly at preoperative and 2-month postoperative visits. Binocular summation was calculated for high-contrast Early Treatment of Diabetic Retinopathy Study charts and Sloan low-contrast acuity charts at 2.5% and 1.25% contrast as the difference between the binocular score and that of the better eye. Preoperative and postoperative values were compared.

RESULTS

There was an improvement in BiS at the 2 low-contrast levels for all patients and for all contrast levels in the 75 patients in whom surgery successfully restored binocular alignment. For low-contrast acuity, the proportion of patients with a BiS score of at least 5 letters postoperatively was almost twice that of preoperatively (21% to 30% and 13% to 26% for 2.5% contrast and 1.25% contrast, respectively). Similarly, the proportion of patients with binocular inhibition (BiS score worse by at least 5 letters than the better eye score) was decreased postoperatively at all contrast levels (from 22% to 14% for 1.25% contrast). Thirty-one percent of patients experienced improvement in BiS scores postoperatively at the lowest contrast level.

CONCLUSIONS AND RELEVANCE

Binocular summation scores improved postoperatively in most patients undergoing strabismus surgery. This occurred most frequently at the lowest contrast level. These findings suggest that improved BiS could represent a newly recognized functional benefit from the surgical correction of strabismus. Further studies evaluating the correlation of BiS with stereopsis, visual field expansion, and quality of life will be necessary to fully evaluate the role that improved BiS has in improving binocularity postoperatively.