Absolute Not Relative Interocular Luminance Modulates Sensory Eye Dominance Plasticity in Adults

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.

The duration effect of short-term monocular deprivation measured by binocular rivalry and binocular combination

The ocular dominance shift observed after short-term monocular deprivation is a widely used measure of visual homeostatic plasticity in adult humans. Binocular rivalry and binocular combination techniques are used interchangeably to characterize homeostatic plasticity, sometimes leading to contradictory results. Here we directly compare the effect of short-term monocular deprivation on ocular dominance measured by either binocular rivalry or binocular combination and its dependence on the duration of deprivation (15 or 120 min) in the same group of participants. Our results show that both binocular rivalry and binocular combination provide reliable estimates of ocular dominance, which are strongly correlated across techniques both before and after deprivation. Moreover, while 15 min of monocular deprivation induce a larger shift of ocular dominance when measured using binocular combination compared to binocular rivalry, for both techniques, the shift in ocular dominance exhibits a strong dependence on the duration of monocular deprivation, with longer deprivation inducing a larger and longer-lasting shift in ocular dominance. Taken together, our results indicate that both binocular rivalry and binocular combination offer very consistent and reliable measurements of both ocular dominance and the effect short-term monocular deprivation.

Sensory eye dominance following surgically correction for acute acquired concomitant esotropia of adulthood

BACKGROUND

Acute acquired concomitant esotropia (AACE) is an unusual presentation characterized by acute onset of esotropia after infancy. For patients with AACE of adulthood, the outcome of surgery, which is a common treatment, often shows successful recovery of stereopsis. However, whether surgically corrected patients with AACE of adulthood achieved balanced eyes is yet unclear.

METHODS

Here, we used a binocular phase combination paradigm to quantitatively assess the ocular dominance of 22 surgically aligned patients with AACE of adulthood, which all had regained normal stereopsis after the surgery and 14 adult controls with normal vision. The sensory eye dominance was quantified as the interocular contrast ratio, termed balance point, at which each eye contributed equally to the perception of cyclopean grating.

RESULTS

We found that, normal controls had a mean balance point value close to unity (0.96 ± 0.01), whereas adult AACE patients exhibited apparent interocular imbalance (0.76 ± 0.04), which was significantly different from control group (Mann-Whitney U = 135, P < 0.001, two tailed). In addition, the balance point of adults with AACE didn't correlate with the interval between onset of esotropia and the surgery (r =  - 0.262, p = 0.239), or the length of postoperative follow-up period (r = 0.127, p = 0.575).

CONCLUSION

Our results suggest that, for patients with AACE of adulthood whose eyes had been straightened, there is still residual sensory imbalance which may be a potential risk factor for AACE of adulthood.

A brief light reduction induces a significant delay in the previously dimmed eye

PURPOSE

We investigated how a short-term luminance reduction in one eye can influence temporal processing of that eye after luminance is restored by measuring the relative delay between the eyes.

METHODS

A paradigm based on the Pulfrich effect, which is a visual illusion of depth when no depth cue is present, was used to measure relative delay in visual processing between the eyes. We deprived the monocular luminance in adults with normal vision across different intensities. In the first experiment, the ratio of the light level between the eyes stayed constant, whereas the absolute value was allowed to vary. In the second experiment, both the ratio and the absolute light level stayed constant, by controlling the environmental light level. In both experiments, we measured the changes in relative delay before and after 60 min of light deprivation.

RESULTS

Our results indicated that short-term monocular deprivation of luminance slows the processing in the previously dimmed eye and that the magnitude of the delay is correlated with the degree of luminance reduction. In addition, we observed that the absolute luminance difference, rather than the absolute luminance levels seen by the dimmed eye, is important in determining the magnitude of delay in the previously dimmed eye. These findings differ from what has been reported previously for the monocular deprivation of contrast.

CONCLUSIONS

Taken together, these findings support the view that short-term deprivation of visual information could affect two distinct mechanisms (contrast gain and temporal dynamics) of neural plasticity.

The Mechanism of Short-Term Monocular Pattern Deprivation-Induced Perceptual Eye Dominance Plasticity

Previously published studies have reported that 150 min of short-term monocular deprivation temporarily changes perceptual eye dominance. However, the possible mechanisms underlying monocular deprivation-induced perceptual eye dominance plasticity remain unclear. Using a binocular phase and contrast co-measurement task and a multi-pathway contrast-gain control model (MCM), we studied the effect of 150 min of monocular pattern deprivation (MPD) in normal adult subjects. The perceived phase and contrast varied significantly with the interocular contrast ratio, and after MPD, the patched eye (PE) became dominant. Most importantly, we focused on the potential mechanisms of the deprivation effect. The data of an averaged subject was best fitted by a model, which assumed a monocular signal enhancement of the PE after the MPD. The present findings might have important implications for investigations of binocular vision in both normal and amblyopic populations.

Exercise alone impacts short-term adult visual neuroplasticity in a monocular deprivation paradigm

Adult homeostatic visual plasticity can be induced by short-term patching, heralded by a shift in ocular dominance in favor of the deprived eye after monocular occlusion. The potential to boost visual neuroplasticity with environmental enrichment such as exercise has also been explored; however, the results are inconsistent, with some studies finding no additive effect of exercise. Studies to date have only considered the effect of patching alone or in combination with exercise. Whether exercise alone affects typical outcome measures of experimental estimates of short-term visual neuroplasticity is unknown. We therefore measured binocular rivalry in 20 healthy young adults (20–34 years old) at baseline and after three 2-hour interventions: patching (of the dominant eye) only, patching with exercise, and exercise only. Consistent with previous work, the patching interventions produced a shift in ocular dominance toward the deprived (dominant) eye. Mild- to moderate-intensity exercise in the absence of patching had several effects on binocular rivalry metrics, including a reduction in the dominant eye percept. The proportion of mixed percept and the time to first switch (onset rivalry) did not change from baseline across all interventions. Thus, we demonstrate that exercise alone can impact binocular rivalry outcomes measures. We did not observe a synergistic effect between patching and exercise in our data.

Attentional eye selection modulates sensory eye dominance

Brief periods of monocular deprivation significantly modify binocular visual processing. For example, patching one eye for a few hours alters the inter-ocular balance, with the previously patched eye becoming dominant once the patch is removed. However, the contribution of higher-level visual processing to this phenomenon is still unclear. Here, we compared changes in sensory eye dominance produced by three types of monocular manipulations in adult participants with normal binocular vision. One eye was covered for 150 min using either an opaque patch, a diffusing lens, or a prism that inverted the image. All three manipulations altered dominance duration and predominance during binocular rivalry (BR) in favour of the treated eye and the time courses of the changes were similar. These results indicate that modifications of luminance or contrast are not strictly necessary to drive shifts in eye dominance, as both were unaltered in the prism condition. Next, we found that shifts in eye dominance were dependent on attentional demands during the monocular treatment period, providing support for the role of attentional eye selection in modulating eye dominance. Finally, we found relatively rapid build-up of the ocular dominance shift after the onset of monocular treatment. Taken together, our results suggest that modifications to monocular input alter inter-ocular balance via selective attentional mechanisms that bias output towards the deprived eye. Eye-based attention may play an important role in conditions where normal input to one eye is disrupted, such as childhood amblyopia.

Can Short-Term Ocular Dominance Plasticity Provide a General Index to Visual Plasticity to Personalize Treatment in Amblyopia?

Purpose

Recently, Lunghi et al. (2016) showed that amblyopic eye’s visual acuity per se after 2 months of occlusion therapy could be predicted by a homeostatic plasticity, that is, the temporary shift of perceptual eye dominance observed after a 2-h monocular deprivation, in children with anisometropic amblyopia. In this study, we assess whether the visual acuity improvement of the amblyopic eye measured after 2 months of occlusion therapy could be predicted by this plasticity.

Methods

Seven children (6.86 ± 1.46 years old; SD) with anisometropic amblyopia participated in this study. All patients were newly diagnosed and had no treatment history before participating in our study. They finished 2 months of refractive adaptation and then received a 4-h daily fellow eye patching therapy with an opaque patch for a 2-month period. Best-corrected visual acuity of the amblyopic eye was measured before and after the patching therapy. The homeostatic plasticity was assessed by measuring the temporary shift of perceptual eye dominance from 2-h occlusion of the amblyopic eye before treatment. A binocular phase combination paradigm was used for this study.

Results

We found that there was no significant correlation between the temporary shift of perceptual eye dominance observed after 2-h occlusion of the amblyopic eye and the improvement in visual acuity in the amblyopic eye from 2 months of classical patching therapy. This result, although in disagreements with the conclusions of Lunghi et al. involving the short-term patching of the amblyopic eye, is in fact consistent with a reanalysis of Lunghi and colleagues’ data.

Conclusion

The short-term changes in perceptual eye dominance as a result of short-term monocular deprivation do not provide an index of cortical plasticity in the general sense such that they are able to predict acuity outcomes from longer-term classical patching.

Short-term monocular deprivation reduces inter-ocular suppression of the deprived eye

The adult visual system was traditionally thought to be relatively hard-wired, but recent studies have challenged this view by demonstrating plasticity following short-term monocular deprivation. Depriving one eye of spatial information for 2-3 h increased subsequent sensory dominance of that eye. However, the mechanism underlying this phenomenon is unclear. The present study sought to address this issue and determine the consequences of short-term monocular deprivation on inter-ocular suppression of each eye. Sensory eye dominance was examined before and after depriving an eye of all input using an opaque patch for 2.5 h, in six adult participants with normal binocular vision. We used a percept tracking task during binocular rivalry (BR) to assess the relative eye dominance, and an objective probe detection task under continuous flash suppression (CFS) to quantify each eye's susceptibility to inter-ocular suppression. The monocular contrast increment threshold of each eye was also measured using the probe task to ascertain if the altered eye dominance is accompanied by changes in monocular perception. Our BR results replicated previous findings of a shift of relative dominance towards the eye that has been deprived of form information. More crucially, using CFS we demonstrated reduced inter-ocular suppression of the deprived eye with no complementary changes in the other eye, and no monocular changes in increment threshold. These findings imply that short-term monocular deprivation alters binocular interactions. The differential effect on inter-ocular suppression between eyes may have important implications for the use of patching as a therapy to recover visual function in amblyopia.

Natural-scene-based Steady-state Visual Evoked Potentials Reveal Effects of Short-term Monocular Deprivation

Ocular dominance plasticity beyond the critical period has been demonstrated in adult humans in recent investigations of short-term monocular deprivation (MD). To our knowledge, all previous research adopted non-natural synthetic stimuli in testing perceptual ocular dominance before and after the MD. However, it is recognized that complex natural stimuli may engage cortical mechanisms substantially different from simple synthetic stimuli. Therefore, it remains largely unknown whether reshaping of ocular dominance following MD could be observed during perception of natural scene stimuli without conspicuous interocular competition. Here we used the steady-state visual evoked potential (SSVEP) technique to measure the ocular-specific neural effects of MD with natural scene stimuli where the two eyes' images were tagged with different frequencies. Two hours of MD boosted the neural gain for the deprived eye. During the course of MD, the SSVEP amplitude ratio for the deprived eye compared to the non-deprived eye increased significantly over time, indicating a progressive increase of neural gain for the deprived eye. These findings demonstrate that the effects of short-term MD can manifest when viewing natural scenes, providing a natural case in support of the homeostatic compensation theory of MD. Our work also indicates that the technique of natural-scene-based SSVEP could be particularly useful for future work exploring the neural dynamics during adaptation to natural stimuli.

The Modulation of Laser Refractive Surgery on Sensory Eye Dominance of Anisometropia

Purpose

To evaluate the effect of laser refractive surgery on sensory eye dominance of anisometropia.

Methods

A total of 156 subjects with nonanisometropic myopia and 70 subjects with anisometropic myopia were enrolled in the first part of the study. The dichoptic motion coherence threshold technique was applied to collect the normal dataset and distribution of sensory eye dominance. The second part of the study included 40 subjects with nonanisometropic myopia and 40 subjects with anisometropic myopia who received the femtosecond laser-assisted in situ keratomileusis (Fs-LASIK). A comprehensive ophthalmologic evaluation was performed with particular attention to sensory eye dominance preoperatively and one-week and one-month postoperatively. The ocular dominance index (ODI) was applied to evaluate the subject's overall degree of sensory ocular dominance. Visual acuity, sighting eye dominance, and stereo acuity were also accessed.

Results

In experiment one, the mean ODI in the nonanisometropic group and the anisometropic group was 1.48 ± 0.63 and 1.95 ± 1.07, respectively. The ODI values of the anisometropic group were significantly higher than those of the nonanisometropic group (Mann–Whitney U test, P < 0.001). The demographics information and the distribution of ODI values in both groups are summarized in tables and figures. In experiment two, all LASIK procedures were uneventful and no postoperative complications were observed during the postoperative follow-up. Preoperatively, the ODI values of the anisometropic LASIK group were significantly higher than those of the nonanisometropic LASIK group, which was consistent with the results of part 1. However, one week after operation, the mean ODI values of the anisometropic LASIK group had significantly decreased from 1.89 ± 1.09 to 1.39 ± 0.44. And, the mean ODI values slightly increased to 1.65 ± 0.61 one-month postoperatively. In the nonanisometropic LASIK group, there were no statistically significant differences of ODI changes among preoperative, post-one-week and post-one-month visits. The demographics information and the changes of ODI of both LASIK groups are summarized in tables and figures.

Conclusion

Stronger sensory eye dominance is seen in the subjects with anisometropic myopia compared to subjects with nonanisometropic myopia. The strong sensory dominance of anisometropia becomes more balanced at one week of postoperation but returns to the preoperative level after one month. Laser refractive surgery had a short-term modulation of sensory eye dominance.

Inverse Occlusion: A Binocularly Motivated Treatment for Amblyopia

Recent laboratory findings suggest that short-term patching of the amblyopic eye (i.e., inverse occlusion) results in a larger and more sustained improvement in the binocular balance compared with normal controls. In this study, we investigate the cumulative effects of the short-term inverse occlusion in adults and old children with amblyopia. This is a prospective cohort study of 18 amblyopes (10-35 years old; 2 with strabismus) who have been subjected to 2 hours/day of inverse occlusion for 2 months. Patients who required refractive correction or whose refractive correction needed updating were given a 2-month period of refractive adaptation. The primary outcome measure was the binocular balance which was measured using a phase combination task; the secondary outcome measures were the best-corrected visual acuity which was measured with a Tumbling E acuity chart and converted to logMAR units and the stereoacuity which was measured with the Random-dot preschool stereogram test. The average binocular gain was 0.11 in terms of the effective contrast ratio (z = -2.344, p = 0.019, 2-tailed related samples Wilcoxon Signed Rank Test). The average acuity gain was 0.13 logMAR equivalent (t(17) = 4.76, p < 0.001, 2-tailed paired samples t-test). The average stereoacuity gain was 339 arc seconds (z = -2.533, p = 0.011). Based on more recent research concerning adult ocular dominance plasticity, we conclude that inverse occlusion in adults and old children with amblyopia does produce long-term gains to binocular balance and that acuity and stereopsis can improve in some subjects.