Interocular suppressive interactions in amblyopia depend on spatial frequency

Longstanding effects of continuous theta burst stimulation in adult amblyopes

CLINICAL RELEVANCE

Continuous theta burst stimulation may be an important tool in the therapeutic management of amblyopia, when trying to correct the established neuronal imbalance. It is important to understand whether two sessions of continuous theta burst stimulation produce greater and longstanding changes in visual acuity and suppressive imbalance than one session of continuous theta burst stimulation.

BACKGROUND

We hypothesise that through the usage of continuous theta burst stimulation (cTBS) it is possible to change cortical excitability in a situation where visual impairment is present.

METHODS

We selected 22 adult amblyopes, 18 females and 4 males, with an age range of 20-59 years. They were randomised into two groups: group A with 10 amblyopes was submitted to one session of cTBS and group B with 12 amblyopes submitted to two sessions of cTBS. Visual acuity (VA) and suppressive imbalance (SI) were evaluated immediately before and after stimulation in both groups A and B. A follow-up was done in both groups.

RESULTS

For both group A and B, the VA improvements were significant after cTBS (p = 0.005 and p = 0.003, respectively). Regarding SI, both group A and B had significant improvements after cTBS (p = 0.03 and p = 0.005, respectively). Comparing groups, A and B no significant differences were found with regard to the results obtained both for VA (p = 0.72) and SI (p = 0.24). However, significant differences were found between group A and B with regard to the duration of stimulation effect for VA (p = 0.049) and SI (p = 0.03).

CONCLUSION

We conclude that two sessions of cTBS do not produce better results than one session of stimulation. However, it seems that two sessions of cTBS produce longstanding effects in VA and SI.

Spatial frequency channels depend on stimulus bandwidth in normal and amblyopic vision: an exploratory factor analysis

The Contrast Sensitivity Function (CSF) is the measure of an observer's contrast sensitivity as a function of spatial frequency. It is a sensitive measure to assess visual function in fundamental and clinical settings. Human contrast sensitivity is subserved by different spatial frequency channels. Also, it is known that amblyopes have deficits in contrast sensitivity, particularly at high spatial frequencies. Therefore, the aim of this study was to assess whether the contrast sensitivity function is subtended by the same spatial frequency channels in control and amblyopic populations. To determine these spatial frequency channels, we performed an exploratory factor analysis on five datasets of contrasts sensitivity functions of amblyopic and control participants measured using either gratings or noise patches, taken from our previous studies. In the range of 0.25-10 c/d, we identified two spatial frequency channels. When the CSF was measured with noise patches, the spatial frequency channels presented very similar tuning in the amblyopic eye and the fellow eye and were also similar to what was observed in controls. The only major difference was that the weight attributed to the high frequency channel was reduced by approximately 50% in the amblyopic eye. However, when the CSF was measured using gratings, the spatial frequency channels of the amblyopic eye were tuned toward lower spatial frequencies. These findings suggest that there is no mechanistic deficit for contrast sensitivity in amblyopia and that amblyopic vision may just be subjected to excessive internal noise and attenuation at higher spatial frequencies, thereby supporting the use of therapeutic strategies that involve rebalancing contrast.

The Suppressive Basis of Ocular Dominance Changes Induced by Short-Term Monocular Deprivation in Normal and Amblyopic Adults

Purpose

We aimed to study the effect of short-term monocular deprivation on the suppressive interocular interactions in normals and amblyopes by using a dichoptic masking paradigm.

Methods

Nine adults with anisometropic or mixed amblyopia and 10 control adults participated in our study. The contrast sensitivity in discriminating a target Gabor dichoptically masked was measured before and after 2 hours of monocular deprivation. The mask consisted of bandpass-filtered noise. Both the target and the mask were horizontally oriented at the spatial frequency of 1.31 cpd. Deprivation was achieved using an opaque patch on the amblyopic eye of amblyopes and the dominant eye of controls.

Results

Results were similar in both controls and amblyopes. After 2 hours of monocular deprivation, the previously patched eye showed a significant increase in contrast sensitivity under dichoptic masking, which also suggested reduced suppressive effect from the nonpatched eye. Meanwhile, the contrast sensitivity of the nonpatched eye remained almost unchanged under dichoptic masking.

Conclusions

We demonstrate that the ocular dominance changes induced by short-term monocular deprivation-namely, the strengthening of the deprived eye's contribution-are associated with the unilateral and asymmetric changes in suppressive interaction. The suppression from the nondeprived eye is reduced after short-term monocular deprivation. This provides a better understanding of how inverse patching (patching of the amblyopic eye) could, by reducing the suppressive drive from the normally sighted (nondeprived) eye, form the basis of a new treatment for the binocular deficit in amblyopia.

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 Orientation Selectivity of Dichoptic Masking Suppression is Contrast Dependent in Amblyopia

Purpose

We aimed to study the effect of stimulus contrast on the orientation selectivity of interocular interaction in amblyopia using a dichoptic masking paradigm.

Methods

Eight adults with anisometropic or mixed amblyopia and 10 control adults participated in our study. The contrast threshold in discriminating a target Gabor in the tested eye was measured with mean luminance in the untested eye, as well as with a bandpass oriented filtered noise in the other eye at low spatial frequency (0.25 c/d). Threshold elevation, which represents interocular suppression, was assessed using a the dichoptic masking paradigm (i.e. the contrast threshold difference between the target only and masked conditions), for each eye. Orientation selectivity of the interocular suppression as reflected by dichoptic masking was quantified by the difference between the parallel and orthogonal masking configurations. Two levels of mask's contrast (3 times or 10 times that of an individual's contrast threshold) were tested in this study.

Results

The strength of dichoptic masking suppression was stronger at high, rather than low mask contrast in both amblyopic and control subjects. Normal controls showed orientation-dependent dichoptic masking suppression both under high and low contrast levels. However, amblyopes showed orientation-tuned dichoptic masking suppression only under the high contrast level, but untuned under the low contrast level.

Conclusions

We demonstrate that interocular suppression assessed by dichoptic masking is contrast-dependent in amblyopia, being orientation-tuned only at high suprathreshold contrast levels of the mask.

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

Amblyopia is a visual disorder that originates from the brain. It exhibits no pathology in the eye. Studies have shown that measuring both visual acuity and binocular balance for assessing amblyopia could be more helpful. However, tests that measure binocular balance are time-consuming, often exceeding 30 min. Their long test durations prevent them from being used in the clinic. For this reason, we have developed a quick (i.e., about 7 min) and precise tool that quantitatively measures binocular balance of patients with amblyopia. The new test can capture binocular imbalance that is typically exhibited at high spatial frequency in amblyopes. In addition, it has an excellent test-retest reliability and repeatability between two experimental sessions. We hope that our newly developed test can pave the road for physicians and researchers to better assess and diagnose amblyopia and other visual disorders that disrupt binocular balance beyond the laboratory.

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Measuring binocular balance of amblyopes is difficult and time-consuming (>30 min)

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We introduce a psychophysical test that is reliable and quick (7 min)

Stimulus dependence of interocular suppression

Interocular suppression is the phenomenon in which the signal from one eye inhibits the other eye in the presence of dissimilar images. Various clinical and laboratory-based tests have been used to assess suppression, which vary in color, contrast, and stimulus size. These stimulus variations may yield different spatial extents of suppression, which makes it difficult to compare the outcomes. To evaluate the role of stimulus characteristics, we measured the suppression zone using a binocular rivalry paradigm in normally-sighted observers by systematically varying the stimulus parameters. The stimuli consist of a constantly visible horizontal reference seen by one eye while two vertical suppressors were presented to the other eye. With a keypress, the suppressors appeared for 1 s, to induce a transient suppression zone in the middle part of the reference. Subjects adjusted the width between the suppressors to determine the zone. The zone decreased significantly with increasing spatial frequency and lower contrast. The width was 1.4 times larger than the height. The zone was smaller with negative compared to positive contrast polarity but independent of eye dominance, luminance, and colored filters. A departure from scale invariance was captured with a model suggesting a stimulus-dependent and a small fixed non-stimulus-dependent portion.

Interocular Suppression as Revealed by Dichoptic Masking Is Orientation-Dependent and Imbalanced in Amblyopia

Purpose

We investigate the orientation tuning of interocular suppression using a dichoptic masking paradigm in adult controls and amblyopes.

Methods

Fourteen adults with anisometropic or mixed amblyopia and 10 control adults participated in our study. Contrast sensitivity was measured by presenting a target Gabor in the tested eye and mean luminance in the untested eye (monocular) and by presenting a target in the tested eye and a bandpass oriented filtered noise in the other eye (masked). Interocular suppression was defined as the thresholds difference between the monocular and masked conditions for each eye. Interocular suppression was measured under parallel and orthogonal suppression configurations. The peak spatial frequency of the target and mask was 0.25 c/d in experiment 1 (low), 1.31 c/d in experiment 2 (mid), and 6.87 c/d in experiment 3 (high).

Results

The masking suppression induced by the amblyopic eye was less strong than that induced by the fellow eye. The suppression from the fellow eye was similar to that observed in the controls. Interocular suppression under parallel configuration was less strong than under orthogonal configuration in amblyopes at low and mid spatial frequency, but not at high spatial frequency.

Conclusions

We demonstrate that the abnormal interocular masking in amblyopia displays the expected characteristic of orientation selectivity expected of normal controls at low and mid spatial frequency, but not at high spatial frequency. The dichoptic masking imbalance between the eyes of amblyopes results in a net suppression of the amblyopic eye during binocular viewing, modeling clinical suppression.

Dichoptic Spatial Contrast Sensitivity Reflects Binocular Balance in Normal and Stereoanomalous Subjects

Purpose

To study binocular balance by comparing dichoptic and standard monocular contrast sensitivity function (CSF) in stereonormal and stereoanomalous/stereoblind amblyopic subjects.

Methods

Sixteen amblyopes and 17 controls participated. Using the capability of the passive three-dimensional display, we measured their CSF both monocularly and dichoptically at spatial frequencies 0.5, 1, 2, 4, and 8 cpds using achromatic Gabor patches on a luminance noise background. During monocular stimulation, the untested eye was covered, while for the dichoptic stimulation the untested eye viewed background noise. Dichoptic CSF of both eyes was acquired within one block.

Results

In patients with central fixation, dichoptic viewing had a large negative impact on the CSF of the amblyopic eye, although it hardly affected that of the dominant eye. In contrast, dichoptic viewing had a small but significant effect on both eyes for controls. In addition, all participants lay along a continuum in terms of how much their two eyes were affected by dichoptic stimulation: by using two predefined contrast sensitivity ratios, namely, amblyopic sensitivity decrement and dichoptic sensitivity decrement, not only did we find a significant correlation between these variables among all participants, but also the two groups were identified with minimum error using a cluster analysis.

Conclusions

Dichoptic CSF may be considered to measure visual performance in patients with altered binocular vision, because it better reflects the visual capacity of the amblyopic eye than the standard monocular examinations. It may also be a more reliable parameter to assess the efficacy of modern approaches to treat amblyopia.