A clinically convenient test to measure binocular balance across spatial frequency in amblyopia

Monocular Contrast Sensitivity Visual Perceptual Learning Rebalances Adult Amblyopes' Two Eyes

Purpose

This study investigates the effects of monocular contrast sensitivity visual perceptual learning on binocular visual functions of adults with binocular imbalances.

Methods

Sixteen adults with anisometropic amblyopia (mean age, 24.63 ± 3.56 years), 20 adults with myopic anisometropia (mean age, 24.20 ± 1.94 years), and 16 visually normal adults (mean age, 24.88 ± 1.89 years) participated in this study. Each group was evenly divided into training (anisometropic amblyopia, myopic anisometropia, normal training group) and untrained control groups (anisometropic amblyopia controls, myopic anisometropia controls, and normal controls). Training groups underwent 10 days of monocular contrast sensitivity perceptual learning (two-alternative forced-choice contrast detection task at 6 cycles per degree) using the amblyopic or nondominant eye, whereas the control groups received no intervention during the same period. Monocular visual acuity, monocular and binocular contrast sensitivity, and balance point were measured before and after the intervention.

Results

Monocular contrast sensitivity perceptual learning significantly improved both contrast sensitivity and balance point at the trained spatial frequency (6 cycles per degree) in the trained eyes of the anisometropic amblyopia training group and myopic anisometropia training group, with improvements generalizing to nearby untrained spatial frequencies. However, no significant improvements were observed in binocular summation ratios for either group. The normal training group showed modest improvements limited to the trained eye at both trained and neighboring spatial frequencies, without significant binocular or untrained eye benefits.

Conclusions

Monocular contrast sensitivity visual perceptual learning effectively enhances monocular visual performance and positively affects binocular functions across trained and nearby untrained spatial frequencies, indicating its potential clinical usefulness in improving binocular vision among adults with binocular imbalances.

Temporal Frequency Modulation of Binocular Balance in Normal and Amblyopic Vision

Purpose

To investigate how temporal frequency modulates binocular balance in normally sighted and amblyopic adults.

Methods

Twenty-three controls and 13 amblyopes participated in this study. The effects of temporal frequency differences and monocularly directed attention on binocular balance were measured using an onset binocular rivalry task with sinusoidally flickering gratings at varying temporal frequencies and static gratings with monocular attentional cues. For the flickering gratings, different combinations of temporal frequencies (2, 4, or 10 Hz in one eye vs. 2, 3, 4, 6, 10, 15, or 20 Hz in the other) were presented. Their effects were then compared, and their relationship was analyzed.

Results

There was no relationship between the shifts in balance from temporal frequency and monocularly directed attention in both controls and amblyopes. Intermediate temporal frequencies (8.9 ± 1.4 Hz) in one eye maximized its perceptual dominance, with a larger shift due to temporal frequency in amblyopes than in controls. While normally sighted observers experienced similar degrees of shift in balance from temporal frequency and attentional (active and passive) modulations, amblyopic observers experienced a larger shift from temporal frequency than from monocularly focused passive (but not active) attention.

Conclusions

Intermediate temporal frequencies in one eye, rather than a specific temporal frequency difference between both eyes, maximized its perceptual dominance in both normally sighted and amblyopic observers. This balance shift from temporal frequency modulation was larger in amblyopes than in controls. Finally, the effect of temporal frequency on balance was larger than that of monocularly directed passive attention in amblyopes.

Quantifying sighting dominance using on-display projections of monocular and binocular views

Sighting dominance is an important behavioral property which has been difficult to measure quantitatively with high precision. We developed a measurement method that is grounded in a two-camera model that satisfies these aims. Using a simple alignment task, this method quantifies sighting ocular dominance during binocular viewing, identifying each eye's relative contribution to binocular vision. The method involves placing a physical target between the viewer and a display. The viewer indicates the perceived target's projection on the display with both eyes open and with only one eye open. The relative location of the binocular projection in relation to the two monocular projections is the index of dominance. The method produces a continuous variable with robust test-retest reliability (ICC = 0.96). The unit of measurement for the computed quantity is physiologically grounded: it is proportional to the distance between the monocular projections, which we show is predictable from interpupillary distance and phoria. Comparisons with the classic 'hole in card' sighting dominance test show good agreement, but also hint at potential bias for determining right-eye dominance in the latter. Interestingly, we find that some individuals systematically demonstrate nearly balanced vision, a phenomenon previously construed as mixed dominance or noisy responses. We also present ways to quantify and mitigate sources of random noise in this measurement. Overall, this new method allows for precise estimation of sighting dominance during binocular viewing. We expect it will allow a more effective understanding of the neural basis of dominance and improved effectiveness when using sighting dominance as a covariate in more complex analyses.

Towards a principled and efficacious approach to the treatment of amblyopia. A review

There is currently a diverse array of treatment for amblyopia. In addition to the traditional penalization therapy, which has been used for over 200 years, there are not only more active treatments to recover the monocular visual loss of the amblyopic eye involving both behavioral (visual training) as well as non-invasive brain stimulation but also a variety of methods designed specifically to restore binocular function. Our understanding of visual function in general and of the etiology of the amblyopic loss in particular has progressed a great deal over the last 50 years and it is now time to take a more principled approach to how we treat, when we treat and why we treat.

Binocular imbalance measured by SSVEP predicts impaired stereoacuity in amblyopia

Purpose

The current study aims to implement steady-state visual evoked potentials (SSVEPs) in quantifying the binocular imbalance of amblyopia and to assess the predictive value of SSVEP-derived indices for amblyopic stereoacuity.

Methods

We measure frequency-tagged SSVEP responses elicited by each eye (F1 = 6 Hz through the fellow eye; F2 = 7.5 Hz through the amblyopic eye) within a binocular rivalry paradigm among a cohort of anisometropic amblyopic observers (n = 29, mean age: 12 years). Binocular suppression was quantified by assessing the disparity in SSVEP amplitudes between the eyes, while the strength of interocular interaction was evaluated through the intermodulation response at F1+F2 = 13.5 Hz. Subsequent analyses explored the associations between these neural indices and relevant behavioral metrics in amblyopia.

Results

Results reveal a significant difference in SSVEP amplitudes elicited from the fellow eye and the amblyopic eye, with the former exhibiting notably higher responses. Moreover, the fellow eye demonstrated prolonged dominance duration compared to its amblyopic counterpart. Furthermore, a negative correlation between binocular suppression and interocular interaction was observed, with stereoacuity showing a significant correlation with binocular suppression. Utilizing stepwise mulptiple linear regression analysis, we established that a predictive model combining binocular suppression and visual acuity of the amblyopic eye provided the best prediction of stereoacuity.

Conclusions

These results highlight the potential of binocular suppression, as assessed by SSVEPs within a binocular rivalry paradigm, as a promising neural predictor of stereopsis in amblyopia.

Reduced Monocular Luminance Promotes Fusion But Not Mixed Perception in Amblyopia

Purpose

The purpose of this study was to understand how monocular luminance reduction affects binocular balance and examine whether it differentially influences fusion and mixed perception in amblyopia.

Methods

Twenty-three normally sighted observers and 12 adults with amblyopia participated in this study. A novel binocular rivalry task was used to measure the phase duration of four perceptual responses (right- and left-tilts, fusion, and mixed perception) before and after a neutral density (ND) filter was applied at various levels to the dominant eye (DE) of controls and the fellow eye (FE) of patients with amblyopia. Phase durations were analyzed to assess whether the duration of fusion or mixed perception shifted after monocular luminance reduction. Moreover, we quantified ocular dominance and adjusted monocular contrast and luminance separately to investigate the relationship between changes in ocular dominance induced by the two manipulations.

Results

In line with previous studies, binocular balance shifted in favor of the brighter eye in both normal adults and patients with amblyopia. As a function of the ND filter's density, the duration of fusion and mixed perception decreased in normal controls, whereas that of fusion but not mixed perception increased significantly in patients with amblyopia. In addition, changes in binocular balance from luminance reduction were more significant in more balanced amblyopes or normal observers. Furthermore, shifts in binocular balance after contrast and luminance modulation were correlated in both normal and amblyopic observers.

Conclusions

The duration of fusion but not mixed perception increased in amblyopia after monocular luminance reduction in the FE. Moreover, our findings demonstrate that changes in ocular dominance from contrast-modulation and luminance-modulation are correlated in both normal and amblyopic observers.

Reduced interocular suppression after inverse patching in anisometropic amblyopia

Purpose

Recent investigations observed substantial enhancements in binocular balance, visual acuity, and stereovision among older children and adults with amblyopia by patching the amblyopic eye (i.e., inverse patching) for 2 h daily over 2 months. Despite these promising findings, the precise neural mechanisms underlying inverse patching remain elusive. This study endeavors to delve deeper into the neural alterations induced by inverse patching, focusing on steady-state visual evoked potentials (SSVEPs). We specifically investigate the changes in SSVEPs following monocular deprivation of either the fellow eye or the amblyopic eye in older amblyopic children and adults.

Method

Ten participants (17.60 ± 2.03 years old; mean ± SEM), clinically diagnosed with anisometropic amblyopia, were recruited for this study. Each participant underwent a 120 min patching session on their fellow eye on the first day, followed by a similar session on their amblyopic eye on the second day. Baseline steady-state visual evoked potentials (SSVEPs) measurements were collected each day prior to patching, with post-patching SSVEPs measurements obtained immediately after the patching session. The experimental design incorporated a binocular rivalry paradigm, utilizing SSVEPs measurements.

Results

The results revealed that inverse patching induced a heightened influence on neural plasticity, manifesting in a reduction of interocular suppression from the fellow eye to the amblyopic eye. In contrast, patching the fellow eye demonstrated negligible effects on the visual cortex. Furthermore, alterations in interocular suppression subsequent to inverse patching exhibited a correlation with the visual acuity of the amblyopic eye.

Conclusion

Inverse patching emerges as a promising therapeutic avenue for adolescents and adults grappling with severe anisometropic amblyopia that proves refractory to conventional interventions. This innovative approach exhibits the potential to induce more robust neural plasticity within the visual cortex, thereby modulating neural interactions more effectively than traditional amblyopia treatments.

Characterizing amblyopic perception under non-rivalrous viewing conditions

Current assessments of interocular interactions in amblyopia often use rivalrous stimuli, with conflicting stimuli in each eye, which does not reflect vision under typical circumstances. Here we measure interocular interactions in observers with amblyopia, strabismus with equal vision, and controls using a non-rivalrous stimulus. Observers used a joystick to continuously report the perceived binocular contrast of dichoptic grating stimuli, identical except that the stimulus was contrast-modulated independently in each eye over time. Consistent with previous studies, a model predicting the time-course of perceived contrast found increased amblyopic eye attenuation, and reduced contrast normalization of the fellow eye by the amblyopic eye, in amblyopic participants compared to controls. However, these suppressive interocular effects were weaker than those found in previous studies, suggesting that rivalrous stimuli may overestimate the effects of amblyopia on interocular interactions during naturalistic viewing conditions.

The shift in sensory eye dominance from short-term monocular deprivation exhibits no dependence on test spatial frequency

Background

Studies have shown that short-term monocular deprivation induces a shift in sensory eye dominance in favor of the deprived eye. Yet, how short-term monocular deprivation modulates sensory eye dominance across spatial frequency is not clear. To address this issue, we conducted a study to investigate the dependence of short-term monocular deprivation effect on test spatial frequency.

Methods

Ten healthy young adults (age: 24.7 ± 1.7 years, four males) with normal vision participated. We deprived their dominant eye with a translucent patch for 2.5 h. The interocular contrast ratio (dominant eye/non-dominant eye, i.e., the balance point [BP]), which indicates the contribution that the two eyes make to binocular combination, was measured using a binocular orientation combination task. We assessed if BPs at 0.5, 4 or 6 cycles/degree (c/d) change as a result of monocular deprivation. Different test spatial frequency conditions were conducted on three separate days in a random fashion.

Results

We compared the BPs at 0.5, 4 and 6 c/d before and after monocular deprivation. The BPs were found to be significantly affected by deprivation, where sensory eye dominance shift to the deprived eye (F_{1.86, 16.76} = 33.09, P < 0.001). The changes of BP were consistent at 0.5, 4, and 6 c/d spatial frequencies (F_2,18 = 0.15, P = 0.57).

Conclusion

The sensory eye dominance plasticity induced by short-term deprivation is not dependent on test spatial frequency, suggesting it could provide a practical solution for amblyopic therapy that was concerned with the binocular outcome.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40662-022-00303-4.

Issues Revisited: Shifts in Binocular Balance Depend on the Deprivation Duration in Normal and Amblyopic Adults

INTRODUCTION

Recent studies indicate that short-term monocular deprivation increases the deprived eye's contribution to binocular fusion in both adults with normal vision and amblyopia. In this study, we investigated whether the changes in visual plasticity depended on the duration of deprivation in normal and amblyopic adults.

METHODS

Twelve anisometropia amblyopic observers (aged 24.8 ± 2.3 years) and 12 age-matched normal observers (aged 23.9 ± 1.2 years) participated in the study. The non-dominant eye of normal observers or amblyopic eye of amblyopic observers was deprived for 30, 120, and 300 min in a randomized order. Their eye balance was measured with a phase combination task, which is a psychophysical test, before and after the deprivation. This design enabled us to measure changes induced in binocular balance as an index visual plasticity due to monocular deprivations.

RESULTS

By comparing the ocular dominance changes as a result of monocular deprivation with different deprivation durations, we found evidence that the ocular dominance changes are slightly larger after longer deprivations in both normal and amblyopic observers, albeit with a statistical significance. The changes from 120-min were significantly greater than those from 30-min deprivation in both groups. The magnitude of changes in sensory eye balance was significantly larger in normal observers than that in the amblyopic observers; however, the longevity of changes in visual plasticity was found to be more long-lasting in amblyopic observers than the normal counterparts.

CONCLUSIONS

The duration of deprivation matters in both normal and amblyopic observers. Ocular dominance imbalance that is typically observed in amblyopia can be more ameliorated with a longer duration of deprivation.

Modulation of mean luminance improves binocular balance across spatial frequencies in amblyopia

Amblyopia is a visual impairment that perturbs binocular balance at high spatial frequencies in favor of the fellow eye. Studies reveal that amblyopes who had been treated with monocular therapies still show imbalance. Binocular balance is achieved when both eyes’ inputs are weighed equally. A reduced light can diminish the dimmed eye's weight in binocular combination. In this study, we examined if binocular balance across spatial frequencies could be improved by reducing the luminance of the fellow eye in adult amblyopes. By doing so, we relieved their binocular imbalance across spatial frequencies. Also, normal observers showed amblyopic binocular imbalance when the dominant eye’s light level was dimmed. Therefore, reducing the luminance in the unaffected eye in amblyopia mitigated the binocular imbalance, whereas doing so in normal adults simulated the amblyopic imbalance across spatial frequencies.

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Binocular balance is impaired in amblyopia

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Reduced luminance of the fellow eye can improve it across spatial frequencies in amblyopia

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Reduced luminance of one eye from normal observers simulates amblyopic imbalance