Modulation of mean luminance improves binocular balance across spatial frequencies 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.

Effects of Monocular Flicker on Binocular Imbalance in Amblyopic and Nonamblyopic Adults

Purpose

This study aimed to evaluate the effects of monocular flicker stimulation on binocular imbalance in both amblyopic and nonamblyopic adults.

Methods

Seven amblyopic patients (28.3 ± 3.3 years; four females) and seven normally sighted participants (27.3 ± 4.1 years; five females) participated in the study. We used liquid crystal spectacles to create externally-generated monocular flicker (4, 7, 10, 15, or 20 Hz) and used the metric of log balance point (logBP) to determine whether imposed flicker could change the eyes' equilibrium interocular contrast ratio. Flicker was applied to either the fellow eye vs. the amblyopic eye or dominant eye (DE) vs. non-DE (non-DE) of amblyopic and nonamblyopic participants, respectively. We defined a logBP of 0 to indicate complete binocular balance and an increase in logBP relative to baseline to indicate a relative strengthening of the non-DE or amblyopic eye.

Results

Monocular flicker applied to the DE or fellow eye increased logBP, whereas when applied to the non-DE or amblyopic eye, reduced the logBP. These effects were more pronounced at low temporal frequencies than that at high temporal frequencies. The interaction between eye and temporal frequency was significant in both normals, F(4, 24) = 58.082, P < 0.001, η2 = 0.906, and amblyopes, F(1.923, 11.538) = 60.555, P < 0.001, η2 = 0.91.

Conclusions

Monocular flicker diminishes the contribution of the flickered eye in binocular combination, resulting in a relative dominance of the nonflickered eye in interocular interactions. Furthermore, a more pronounced temporally modulated effect was observed at lower temporal frequencies.

Negligible contribution of adaptation of ocular opponency neurons to the effect of short-term monocular deprivation

Introduction

Modeling work on binocular rivalry has described how ocular opponency neurons represent interocular conflict. These neurons have recently been considered to mediate an ocular dominance shift to the eye that has viewed a backward movie for long during which time the other eye is presented with a regular movie. Unlike typical short-term monocular deprivation, the visual inputs are comparable across eyes in that "dichoptic-backward-movie" paradigm. Therefore, it remains unclear whether the ocular opponency neurons are also responsible for the short-term monocular deprivation effect which is prevalently explained by the homeostatic compensation theory. We designed two experiments from distinct perspectives to investigate this question.

Methods

In Experiment 1, we mitigated the imbalance in the activity of opponency neurons between the two eyes during monocular deprivation by presenting video stimuli alternately. In Experiment 2, we directly evaluated the response of opponency neurons before and after monocular deprivation using SSVEP techniques.

Results

Consistent with each other, both experiments failed to provide reliable evidence supporting the involvement of ocular opponency neurons in the short-term monocular deprivation effect.

Discussion

Our results suggest that ocular opponency neurons may not play an essential role in the short-term monocular deprivation effect, potentially due to interference from the homeostatic plasticity mechanism.

Rapid Alternate Flicker Modulates Binocular Interaction in Adults With Abnormal Binocular Vision

Purpose

The current understanding of binocular processing is primarily derived from static spatial visual perception: this leaves the role of temporal information unclear. In this study, we addressed this gap by testing the effect of alternating flicker on binocular information processing in adults with abnormal binocular vision. Our goal was to determine which temporal frequency optimally balanced input from both eyes.

Methods

We took measurements in four groups of human adults: 10 normal adults with the individual's nondominant eye covered by a 2% neutral density filter (aged 25.60 ± 1.43 years, experiment 1), 9 nonamblyopic anisometropes (aged 24.33 ± 1.66 years, experiment 2), 7 amblyopes (aged 26.5 ± 1.64 years, experiment 3), and 7 treated amblyopes (aged 24 ± 3.21 years, experiment 4). The balance point (BP), where participants' two eyes are equally effective, was measured using a binocular orientation combination task at four spatial frequencies (SFs; 0.5-4 c/d) and five temporal frequencies (TFs; baseline and 4, 7, 10, and 15 Hz). Its log transformation |logBP| was taken into further analysis.

Results

We observed clear U-shaped temporal tuning of the |logBP| for the entire range of TFs (that we measured: trough occurred at 7 Hz). This pattern occurred and was significant in all four groups (P < 0.001). In addition, the effect of SFs on |logBP| was significant in normal, amblyopic, and treated amblyopic groups (all P < 0.001) and was marginally significant in the nonamblyopic anisometropic group (P = 0.086).

Conclusions

Alternating flicker around 7 Hz may be the optimal temporal frequency for balancing eyes in human adults with binocular imbalance.

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.