Interocular suppression in children with deprivation amblyopia

Leveraging neural plasticity for the treatment of amblyopia

Amblyopia is a form of visual cortical impairment that arises from abnormal visual experience early in life. Most often, amblyopia is a unilateral visual impairment that can develop as a result of strabismus, anisometropia, or a combination of these conditions that result in discordant binocular experience. Characterized by reduced visual acuity and impaired binocular function, amblyopia places a substantial burden on the developing child. Although frontline treatment with glasses and patching can improve visual acuity, residual amblyopia remains for most children. Newer binocular-based therapies can elicit rapid recovery of visual acuity and may also improve stereoacuity in some children. Nevertheless, for both treatment modalities full recovery is elusive, recurrence of amblyopia is common, and improvements are negligible when treatment is administered at older ages. Insights derived from animal models about the factors that govern neural plasticity have been leveraged to develop innovative treatments for amblyopia. These novel therapies exhibit efficacy to promote recovery, and some are effective even at ages when conventional treatments fail to yield benefit. Approaches for enhancing visual system plasticity and promoting recovery from amblyopia include altering the balance between excitatory and inhibitory mechanisms, reversing the accumulation of proteins that inhibit plasticity, and harnessing the principles of metaplasticity. Although these therapies have exhibited promising results in animal models, their safety and ability to remediate amblyopia need to be evaluated in humans.

Recovery of node of ranvier structure in optic nerve under visual deprivation

Neural activity can increase the length of nodes of Ranvier (NOR) and slow impulse transmission; however, little is known about the biologically and clinically important recovery process. Sensory deprivation promotes neural plasticity in many phenomena, raising the question of whether recovery of NOR morphology is influenced by sensory deprivation. The results show that NOR gap length recovery in mouse optic nerve was not affected significantly by binocular visual deprivation imposed by maintaining mice in 24 hr dark for 30 days compared to mice recovering under normal visual experience. The findings provide insight into the cellular mechanism of NOR plasticity.

Harnessing brain plasticity to improve binocular vision in amblyopia: An evidence-based update

Amblyopia is a developmental visual disorder resulting from atypical binocular experience in early childhood that leads to abnormal visual cortex development and vision impairment. Recovery from amblyopia requires significant visual cortex neuroplasticity, i.e. the ability of the central nervous system and its synaptic connections to adapt their structure and function. There is a high level of neuroplasticity in early development and, historically, neuroplastic responses to changes in visual experience were thought to be restricted to a "critical period" in early life. However, as our review now shows, the evidence is growing that plasticity of the adult visual system can also be harnessed to improve vision in amblyopia. Amblyopia treatment involves correcting refractive error to ensure clear and equal retinal image formation in both eyes, then, if necessary, promoting the use of the amblyopic eye by hindering or reducing visual input from the better eye through patching or pharmacologic therapy. Early treatment in children can lead to visual acuity gains and the development of binocular vision in some cases; however, many children do not respond to treatment, and many adults with amblyopia have historically been untreated or undertreated. Here we review the current evidence on how dichoptic training can be used as a novel binocular therapeutic approach to facilitate visual processing of input from the amblyopic eye and can simultaneously engage both eyes in a training task that requires binocular integration. It is a novel and promising treatment for amblyopia in both children and adults.

Novel Quantitative Contrast Sensitivity Function Enhances the Prediction of Treatment Outcome and Recurrence in Amblyopia

Purpose

Although effective amblyopia treatments are available, treatment outcome is unpredictable, and the condition recurs in up to 25% of the patients. We aimed to evaluate whether a large-scale quantitative contrast sensitivity function (CSF) data source, coupled with machine learning (ML) algorithms, can predict amblyopia treatment response and recurrence in individuals.

Methods

Visual function measures from traditional chart vision acuity (VA) and novel CSF assessments were used as the main predictive variables in the models. Information from 58 potential predictors was extracted to predict treatment response and recurrence. Six ML methods were applied to construct models. The SHapley Additive exPlanations was used to explain the predictions.

Results

A total of 2559 consecutive records of 643 patients with amblyopia were eligible for modeling. Combining variables from VA and CSF assessments gave the highest accuracy for treatment response prediction, with the area under the receiver operating characteristic curve (AUC) of 0.863 and 0.815 for outcome predictions after 3 and 6 months, respectively. Variables from the VA assessment alone predicted the treatment response, with AUC values of 0.723 and 0.675 after 3 and 6 months, respectively. Variables from the CSF assessment gave rise to an AUC of 0.909 for recurrence prediction compared to 0.539 for VA assessment alone, and adding VA variables did not improve predictive performance. The interocular differences in CSF features are significant contributors to recurrence risk.

Conclusions

Our models showed CSF data could enhance treatment response prediction and accurately predict amblyopia recurrence, which has the potential to guide amblyopia management by enabling patient-tailored decision making.

Associations between the cause of amblyopia and pre-treatment contrast sensitivity, stereoacuity, fixation, and nystagmus

Purpose

To explore the association between the cause of amblyopia and pre-treatment contrast sensitivity, stereoacuity, fixation and nystagmus.

Design

Retrospective cohort study.

Methods

A retrospective review was conducted for 3408 patients with amblyopia who had not yet started amblyopia treatment utilizing a large amblyopia patient database maintained at Zhongshan Ophthalmic Centre. Six amblyogenic factor subtypes were identified: anisometropia, isoametropia, strabismus, anisometropia and strabismus, monocular visual deprivation, and binocular visual deprivation amblyopia. Monocular best corrected visual acuity (BCVA), the contrast sensitivity function (CSF), fixation, and stereopsis were compared between the subtypes before and after propensity score matching (PSM) for age and sex.

Results

The two deprivation groups had poorer BCVA and CSF than the other groups. There were no systematic differences in CSF between the non-deprivation groups. Nystagmus was more common in the bilateral amblyopia groups compared to the monocular amblyopia groups. Eccentric fixation was uncommon with the exception of the anisometropia and strabismus group which had an eccentric fixation rate of 20%. Distance stereoacuity measured without monocular cues was absent for almost all patients. The results were consistent when analyzed using PSM.

Conclusion

Visual deprivation causes more severe amblyopia than other amblyogenic factors. For non-deprivation amblyopia subtypes, individual differences such as variation in the severity of the amblyogenic factor might be more important in determining pre-treatment vision than whether amblyopia was caused by refractive error, strabismus or both.

Motion-Defined Form Perception in Deprivation Amblyopia

Purpose

The purpose of this study was to assess motion-defined form perception, including the association with clinical and sensory factors that may drive performance, in each eye of children with deprivation amblyopia due to unilateral cataract.

Methods

Coherence thresholds for orientation discrimination of motion-defined form were measured using a staircase procedure in 30 children with deprivation amblyopia and 59 age-matched controls. Visual acuity, stereoacuity, fusion, and interocular suppression were also measured. Fixation stability and fellow-eye global motion thresholds were measured in a subset of children.

Results

Motion-defined form coherence thresholds were elevated in 90% of children with deprivation amblyopia when viewing with the amblyopic eye and in 40% when viewing with the fellow eye. The deficit was similar in children with a cataract that had been visually significant at birth (congenital) and in children for whom the cataract appeared later in infancy or childhood (developmental). Poorer motion-defined form perception in amblyopic eyes was associated with poorer visual acuity, poorer binocular function, greater interocular suppression, and the presence of nystagmus. Fellow-eye deficits were not associated with any of these factors, but a temporo-nasal asymmetry for global motion perception in favor of nasalward motion suggested a general disruption in motion perception.

Conclusions

Deficits in motion-defined form perception are common in children with deprivation amblyopia and may reflect a problem in motion processing that relies on binocular mechanisms.

Leber Hereditary Optic Neuropathy (LHON) in Patients with Presumed Childhood Monocular Amblyopia

BACKGROUND

Most Leber hereditary optic neuropathy (LHON) cases are bilateral and sequential; however, there are rare unilateral examples, or those in which the delay of onset of vision loss between one and the other eye is longer. In the case of presumed childhood amblyopia in one eye, vision loss in the good eye may be the only symptom of bilateral disease, which was unnoticed in the previously amblyopic eye, or a preexisting episode of LHON in the "amblyopic" eye. The clinical decision in such cases may be difficult and suggestive of other forms of atypical optic neuropathy until confirmed by genetic testing.

CASE SERIES

We present three genetically confirmed (MT-ND1:m.3700G>A, MT-ND6:m14484 T>C, and MT-ND4:m.11778G>A) patients with subacute vision loss in the previously good eye, with the other eye believed to be amblyopic from childhood and their features different from what would be expected in true amblyopia. In all, electrophysiology testing showed a bilaterally reduced amplitude of PERG with low VEP P100 wave amplitudes and prolonged peak time in both eyes, also unusual for amblyopia. During follow-up, the pallor of the optic discs progressed in all eyes. Significant thinning of the peripapillary retinal nerve fiber layer (pRNFL; retinal nerve fiber layer around the optic disc) and ganglion cell complex (GCC) in the macular region was present. All three patients had a peculiar history. The first patient was treated for presumed hyperopic amblyopia that did not improve since childhood, experienced visual loss in the good eye at the age of 17, and was negative for the three typical LHON mutations. Extended testing confirmed an atypical pathogenic variant MT-ND1:m.3700G>A in homoplasmy. The second patient with presumed strabismic amblyopia had an unusual presentation of vision loss only at the age of 61, and after the exclusion of other causes, a typical MT-ND4:m.11778G>A pathogenic variant was found in homoplasmy. The third case was peculiar as he had presumed strabismic amblyopia since childhood and had some degree of disc pallor in the amblyopic eye upon presenting with loss of vision in the good eye at the age of 21, and a typical pathogenic variant m14484 T>C, p.Met64Val was subsequently confirmed. However, one year after disease onset, he started to experience significant spontaneous functional improvement in the non-amblyopic up to 1.0 Snellen whilst improvement in the presumed amblyopic eye was modest, suggesting preexisting amblyopia. This interestingly extensive improvement was carefully followed by electrophysiology as well as visual acuity and fields.

CONCLUSIONS

This report shows three different scenarios of presentation of LHON in patients with presumed uniocular amblyopia from childhood. In such cases, the diagnosis may be difficult, and detailed structural and functional evaluation of the optic nerve head is necessary to assess whether an earlier LHON episode was misdiagnosed as amblyopia or whether LHON presented bilaterally on both eyes whilst only being noticed in the previously good eye.

Human deprivation amblyopia: treatment insights from animal models

Amblyopia is a common visual impairment that develops during the early years of postnatal life. It emerges as a sequela to eye misalignment, an imbalanced refractive state, or obstruction to form vision. All of these conditions prevent normal vision and derail the typical development of neural connections within the visual system. Among the subtypes of amblyopia, the most debilitating and recalcitrant to treatment is deprivation amblyopia. Nevertheless, human studies focused on advancing the standard of care for amblyopia have largely avoided recruitment of patients with this rare but severe impairment subtype. In this review, we delineate characteristics of deprivation amblyopia and underscore the critical need for new and more effective therapy. Animal models offer a unique opportunity to address this unmet need by enabling the development of unconventional and potent amblyopia therapies that cannot be pioneered in humans. Insights derived from studies using animal models are discussed as potential therapeutic innovations for the remediation of deprivation amblyopia. Retinal inactivation is highlighted as an emerging therapy that exhibits efficacy against the effects of monocular deprivation at ages when conventional therapy is ineffective, and recovery occurs without apparent detriment to the treated eye.

Strabismus and amblyopia in Africa - a systematic review and meta-analysis

The objective of this review was to estimate the prevalence of strabismus and amblyopia in Africa. A systematic online literature search was conducted for articles on strabismus and amblyopia in Africa. Meta-analysis was performed, using the Freeman-Tukey double arcsine transformation, to estimate the prevalence of strabismus and amblyopia in Africa. Subgroup analyses were conducted according to age, gender, study year, and type of amblyopia. Meta-regression was used to evaluate the influence of predetermined factors on the prevalence of amblyopia. 8 (1 population-based & 7 school-based) and 21 (3 population-based & 18 school-based) studies on strabismus and amblyopia with sample sizes of 22,355 and 46,841, respectively, were included in the review. Overall prevalence of strabismus in Africa was estimated to be 0.8% (95% CI: 0.4% - 1.4%); exotropia was 0.2% (95% CI: 0.1% - 0.5%) and esotropia was 0.5% (95% CI: 0.1% - 1.2%). Overall prevalence of amblyopia was estimated to be 0.6% (95% CI: 0.3% - 0.9%); refractive and strabismic amblyopia were 1.1% (95% CI: 0.2% - 2.5%) and 0.4% (95% CI: 0.2% - 0.6%), respectively. Prevalence estimate of amblyopia in males was 1.8% (95% CI: 0.7% - 3.3%) and in females was 1.3% (95% CI: 0.4% - 2.6%). There was a significant association between the prevalence of amblyopia and the type of amblyopia (p = .007) and the study year (p = .006). Although there appears to be a relatively low prevalence of strabismus and amblyopia in Africa, there is a dearth of well-designed population-based studies on strabismus and amblyopia in Africa, resulting in the lack of epidemiological information on strabismus and amblyopia within the general African population. Information about the prevalence of strabismus and amblyopia across Africa can inform policy making and design and implementation of public health intervention program.

Childhood-Onset Leber Hereditary Optic Neuropathy-Clinical and Prognostic Insights

PURPOSE

To investigate the clinical and molecular genetic features of childhood-onset Leber hereditary optic neuropathy (LHON) to gain a better understanding of the factors influencing the visual outcome in this atypical form of the disease.

DESIGN

Retrospective cohort study.

METHODS

We retrospectively included 2 cohorts of patients with LHON with onset of visual loss before the age of 12 years from Italy and the United Kingdom. Ophthalmologic evaluation, including best-corrected visual acuity, orthoptic evaluation, slit-lamp biomicroscopy, visual field testing, and optical coherence tomography, was considered. Patients were classified based on both the age of onset and the pattern of visual loss.

RESULTS

A total of 68 patients were stratified based on the age of onset of visual loss: group 1 (<3 years): 14 patients (20.6%); group 2 (≥3 to <9 years): 27 patients (39.7%); and group 3 (≥9 to ≤12 years): 27 patients (39.7%). Patients in group 2 achieved a better visual outcome than those in group 3. Patients in groups 1 and 2 had better mean deviation on visual field testing than those in group 3. The mean ganglion cell layer thickness on optical coherence tomography in group 2 was higher than those in groups 1 and 3. Patients were also categorized based on the pattern of visual loss as follows: Subacute Bilateral: 54 patients (66.7%); Insidious Bilateral: 14 patients (17.3%); Unilateral: 9 patients (11.1%); and Subclinical Bilateral: 4 patients (4.9%).

CONCLUSIONS

Children who lose vision from LHON before the age of 9 years have a better visual prognosis than those who become affected in later years, likely representing a "form frustre" of the disease.

The Study of Short-Term Plastic Visual Perceptual Training Based on Virtual and Augmented Reality Technology in Amblyopia

Backgrounds. The treatment for amblyopia can have a substantial impact on quality of life. Conventional treatments for amblyopia have some limitations, then we try to explore a new and effective method to treat amblyopia. This study aimed to determine the potential effect of short-term plastic visual perceptual training based on VR and AR platforms in amblyopic patients. Methods. All observers were blinded to patient groupings. A total of 145 amblyopic children were randomly assigned into 2 groups: VR group (71 patients) and AR group (74 patients). In the VR group, each subject underwent a 20-min short-term plastic visual perceptual training based on a VR platform, and in the AR group, based on an AR platform. The best-corrected visual acuity (BCVA), fine stereopsis, and contrast sensitivity function (CSF) were measured before and after training. Results. The BCVA (P < 0.001) and fine stereopsis (P < 0.05) were improved significantly both in VR and AR group after training. Moreover, in the AR group, the CSF showed the value of all spatial frequencies had a statistically significant improvement after training (P < 0.05), while in the VR group, only the value of spatial frequency 12 improved significantly (P = 0.008). Conclusions. This study showed that the short-term plastic visual perceptual training based on VR and AR technology can improve BCVA, fine stereopsis and CSF of refractive amblyopia. It was suggested that the visual perceptual training based on the VR and AR platforms may be potentially applied in treatment for amblyopia and provided a high-immersing alternative.

Management of amblyopia in pediatric patients: Current insights

Amblyopia is a cause of significant ocular morbidity in pediatric population and may lead to visual impairment in future life. It is caused due to formed visual deprivation or abnormal binocular interactions. Several risk factors in pediatric age group may lead to this disease. Author groups have tried managing different types of amblyopia, like anisometropic amblyopia, strabismic amblyopia and combined mechanism amblyopia, with optical correction, occlusion therapy, penalization, binocular therapy and surgery. We review historical and current management strategies of different types of amblyopia affecting children and outcomes in terms of visual acuity, binocularity and ocular deviation, highlighting evidence from recent studies. Literature searches were performed through Pubmed. Risk factors for amblyopia need to be identified in pediatric population as early in life as possible and managed accordingly, as visual outcomes in amblyopia are best if treated at the earliest. Although, monocular therapies like occlusion or penalization have been shown to be quite beneficial over the years, newer concepts related to binocular vision therapy are still evolving.

A pilot randomized trial of contrast-rebalanced binocular treatment for deprivation amblyopia

BACKGROUND

Binocular neural architecture may be preserved in children with deprivation amblyopia due to unilateral cataract. The purpose of this study was to investigate whether a contrast-rebalanced binocular treatment, recently used with success to treat the interocular suppression and amblyopia in strabismic and anisometropic children, can contribute to rehabilitation of visual acuity in children with deprivation amblyopia secondary to monocular cataract.

METHODS

In a pilot randomized trial, 15 children (4-13 years of age) were enrolled and randomized to continue with their current treatment only (n = 7) or to continue with their current treatment and add contrast-rebalanced binocular iPad game play 5 hours/week for 4 weeks (n = 8). The primary outcome was change in visual acuity at 4 weeks.

RESULTS

Although 10 of 15 participants were patching, there was little change in visual acuity during the 3 months prior to enrollment. At the 4-week primary outcome visit, the mean improvement in visual acuity for the binocular game group was significantly greater than that for the current-treatment group (0.08 ± 0.10 logMAR vs -0.03 ± 0.05 logMAR [t_10.2 = 2.53, P = 0.03]). None of the children who had dense congenital cataract achieved improved visual acuity with binocular treatment.

CONCLUSIONS

In this study cohort, visual acuity improved over 8 weeks in children with unilateral deprivation amblyopia who played a binocular contrast-rebalanced binocular iPad game.

Quantifying Nasotemporal Asymmetry of Interocular Suppression in Alternating Strabismus After Correction

Purpose

This study identifies and characterizes the nasotemporal hemifield difference of interocular suppression in subjects who have been successfully treated for strabismus.

Methods

Interocular suppression in the nasal and temporal hemifields were measured using two methods, namely, binocular phase combination and dichoptic motion coherence, both entailed suprathreshold stimuli. We tested 29 clinical subjects, who had strabismus (19 with exotropia and 10 with esotropia) but regained good ocular alignment (within 10 prism diopters) after surgical or refractive correction, and 10 control subjects.

Results

Both the hemifield binocular phase combination and the hemifield dichoptic motion coherence tests revealed similar nasotemporal asymmetry of interocular suppression. Subjects with previous exotropia showed deeper suppression in the nasal hemifield, and those with previous esotropia showed deeper suppression in the temporal hemifield. This finding was consistent with the hemifield suppression theory. Furthermore, there was deeper suppression but less imbalance of nasotemporal asymmetry in the hemifield dichoptic motion coherence test. Finally, clinical stereopsis and the nasotemporal asymmetry of suppression (P < 0.05 in both tests) were negatively correlated in subjects with previous exotropia and measurable stereopsis.

Conclusions

Hemifield asymmetry of interocular suppression in corrected strabismus can be measured by using static and dynamic suprathreshold stimuli. Thus, the evaluation of binocular vision in strabismus should focus on both the magnitude and the pattern of interocular suppression.

Occlusion for stimulus deprivation amblyopia

BACKGROUND

Stimulus deprivation amblyopia (SDA) develops due to an obstruction to the passage of light secondary to a condition such as cataract. The obstruction prevents formation of a clear image on the retina. SDA can be resistant to treatment, leading to poor visual prognosis. SDA probably constitutes less than 3% of all amblyopia cases, although precise estimates of prevalence are unknown. In high-income countries, most people present under the age of one year; in low- to middle-income countries, people are likely to be older at the time of presentation. The mainstay of treatment is correction of the obstruction (e.g., removal of the cataract) and then occlusion of the better-seeing eye, but regimens vary, can be difficult to execute, and traditionally are believed to lead to disappointing results.

OBJECTIVES

To evaluate the effectiveness of occlusion therapy for SDA in an attempt to establish realistic treatment outcomes and to examine evidence of any dose-response effect and assess the effect of the duration, severity, and causative factor on the size and direction of the treatment effect.

SEARCH METHODS

We searched CENTRAL (2018, Issue 12), which contains the Cochrane Eyes and Vision Trials Register; Ovid MEDLINE; Embase.com; and five other databases. We used no date or language restrictions in the electronic searches. We last searched the databases on 12 December 2018.

SELECTION CRITERIA

We planned to include randomized controlled trials (RCTs) and controlled clinical trials of participants with unilateral SDA with visual acuity worse than 0.2 LogMAR or equivalent. We specified no restrictions for inclusion based upon age, gender, ethnicity, comorbidities, medication use, or the number of participants.

DATA COLLECTION AND ANALYSIS

We used standard Cochrane methodology.

MAIN RESULTS

We identified no trials that met the inclusion criteria specified in the protocol for this review.

AUTHORS' CONCLUSIONS

We found no evidence from RCTs or quasi-randomized trials on the effectiveness of any treatment for SDA. RCTs are needed in order to evaluate the safety and effectiveness of occlusion, duration of treatment, level of vision that can be realistically achieved, effects of age at onset and magnitude of visual defect, optimum occlusion regimen, and factors associated with satisfactory and unsatisfactory outcomes with the use of various interventions for SDA.

Orientation Tuning and Contrast Dependence of Continuous Flash Suppression in Amblyopia and Normal Vision

Purpose

Suppression in amblyopia may be an unequal form of normal interocular suppression or a distinct pathophysiology. To explore this issue, we examined the orientation tuning and contrast dependence of continuous flash suppression (CFS) in adults with amblyopia and visually normal controls.

Methods

Nine patients (mean age, 26.9 ± SD 4.7 years) and 11 controls (mean age, 24.8 ± SD 5.3 years) participated. In the CFS paradigm, spatially one-dimensional noise refreshing at 10 Hz was displayed in one eye to induce suppression of the other eye, and suppression strength was measured by using a grating contrast increment detection task. In experiment 1, noise contrast was fixed and the orientation difference between the noise and the grating was varied. In experiment 2, noise and grating orientations were identical and noise contrast was varied.

Results

Suppression patterns varied in both groups. In experiment 1, controls showed consistently orientation-tuned CFS (mean half-height bandwidth, 35.8° ± SD 21.5°) with near-equal strength between eyes. Five of nine patients with amblyopia exhibited orientation-independent CFS. Eight patients had markedly unequal suppression between eyes. Experiment 2 found that increasing the noise contrast to the amblyopic eye may produce suppression of the fellow eye, but suppression remained unequal between eyes.

Conclusions

Our data revealed that orientation specificity in CFS was very broad or absent in some patients with amblyopia, which could not be predicted by clinical measures. Suppression was unbalanced across the entire contrast range for most patients. This suggests that abnormal early visual experience disrupts the development of interocular suppression mechanisms.

Phenomenological approach to childhood cataract treatment in New Zealand using semi-structured interviews: how might we improve provision of care

PURPOSE

To understand how we might improve the provision of medical care for children with cataracts.

DESIGN

A phenomenological design was employed. Semi-structured interviews were conducted to capture rich descriptions of the phenomena. Our goal in the interview and the analysis was to understand the sources of distress associated with treatment for cataract and deprivation amblyopia which (1) could be addressed by the medical community and (2) related to treatment adherence.

SETTING

Interviews were conducted by a non-clinician researcher in New Zealand (NZ) in a location chosen by informants. In NZ, the red reflex screening test is performed shortly after birth, and surgery to remove paediatric cataracts is publicly funded.

PARTICIPANTS

Families of children who had a history of cataract in Auckland, NZ were posted an invitation to participate. Twenty families were interviewed.

RESULTS

Our analysis illustrated that informants described a wide range of experiences, from declined cataract surgery to full adherence to medical advice including years of patching for more than 4 hours a day. Across these experiences, we identified three relevant themes; timing of diagnosis, communication between the parent and clinician, and parental social support networks.

CONCLUSION

The medical community may be better placed to support families dealing with childhood cataract by improving detection of childhood cataract, building appropriate communication pathways and promoting social support, with an emphasis on empathetic, individualised care.

Long-Range Interocular Suppression in Adults with Strabismic Amblyopia: A Pilot fMRI Study

Interocular suppression plays an important role in the visual deficits experienced by individuals with amblyopia. Most neurophysiological and functional MRI studies of suppression in amblyopia have used dichoptic stimuli that overlap within the visual field. However, suppression of the amblyopic eye also occurs when the dichoptic stimuli do not overlap, a phenomenon we refer to as long-range suppression. We used functional MRI to test the hypothesis that long-range suppression reduces neural activity in V1, V2 and V3 in adults with amblyopia, indicative of an early, active inhibition mechanism. Five adults with amblyopia and five controls viewed monocular and dichoptic quadrant stimuli during fMRI. Three of five participants with amblyopia experienced complete perceptual suppression of the quadrants presented to their amblyopic eye under dichoptic viewing. The blood oxygen level dependant (BOLD) responses within retinotopic regions corresponding to amblyopic and fellow eye stimuli were analyzed for response magnitude, time to peak, effective connectivity and stimulus classification. Dichoptic viewing slightly reduced the BOLD response magnitude in amblyopic eye retinotopic regions in V1 and reduced the time to peak response; however, the same effects were also present in the non-dominant eye of controls. Effective connectivity was unaffected by suppression, and the results of a classification analysis did not differ significantly between the control and amblyopia groups. Overall, we did not observe a neural signature of long-range amblyopic eye suppression in V1, V2 or V3 using functional MRI in this initial study. This type of suppression may involve higher level processing areas within the brain.

A comparison of tests for quantifying sensory eye dominance

Clinicians rely heavily on stereoacuity to measure binocular visual function, but stereo-vision represents only one aspect of binocularity. Lab-based tests of sensory eye dominance (SED) are commonplace, but have not been translated to wider clinical practice. Here we compare several methods of quantifying SED in a format suitable for clinical use. We tested 30 participants with ostensibly normal vision on eight tests. Seven tests (#1-7) were designed to quantify SED in the form of an interocular balance-point (BP). In tests #1-6, we estimated a contrast-BP, the interocular difference in contrast required for observers to be equally likely to base their judgement on either eye, whereas in test #7 we measured binocular rivalry (interocular ratio of sensory dominance duration). We compare test-retest reliability (intra-observer consistency) and test-validity (inter-observer discriminatory power) and compare BP to stereoacuity (test #8). The test that best preserved inter-observer differences in contrast balance while maintaining good test-retest reliability was a polarity judgement using superimposed opposite-contrast polarity same-identity optotypes. A reliable and valid measure of SED can be obtained rapidly (20 trials) using a simple contrast-polarity judgement. Tests that use polarity-rivalrous stimuli elicit more reliable judgments than those that do not. SIGNIFICANCE STATEMENT: Although sensory eye dominance is central to understanding normal and disordered binocular vision, there is currently no consensus as to the best way to measure it. Here we compare several candidate measures of sensory eye dominance and conclude that a reliable measure of SED can be achieved rapidly using a judgement of stimulus contrast-polarity.