Treated amblyopes remain deficient in spatial vision: a contrast sensitivity and external noise study

Spatial frequency of environments and myopia: A systematic review on associated evidence and underlying mechanisms

PURPOSE

Previous animal studies have found a relationship between spatial frequency and myopia. New research in humans suggest that reduced high spatial content of the visual environment may be a contributing factor for myopia development. This study aims to review the literature and elucidate the potential biological mechanisms linking spatial frequency and myopia.

METHODS

A systematic search was conducted across PubMed and Web of Science databases. The studies published from their inception to August 2024 that have explored the connection between spatial frequency and myopia. Only full-text articles in English were included. PRISMA was used for data validity.

RESULTS

A total of 13 articles were included in this review, comprising seven animal model studies, four population-based studies, one pictorial analysis and one study on research design. Epidemiological evidence is comparatively limited and has only begun to emerge in recent years. Mid- to high spatial frequencies were found to play an important role in the emmetropization process of the eye. Low spatial frequencies can increase the risk of myopia incidence. Furthermore, the potential mechanisms of how spatial frequency affects myopia are summarized as visual information processing characteristics, eye accommodation function and eye movements, contrast sensitivity and relevant molecules involved in the pathway.

CONCLUSION

The evidence suggests that indoor spatial frequency may be related to the development of myopia. Further studies are warranted to understand if the incorporation of changes in indoor environments is helpful in the prevention and control of myopia.

Interactions between excitatory neurons and parvalbumin interneurons in V1 underlie neural mechanisms of amblyopia and visual stimulation treatment

As the main cause of visual function deficits in children and adolescents worldwide, amblyopia causes serious impairment of monocular visual acuity and stereopsis. The recovery of visual functions from amblyopia beyond the critical period is slow and incomplete due to the limited plasticity of the mature cortex; notably, visual stimulation training seems to be an effective therapeutic strategy in clinical practice. However, the precise neural basis and cellular mechanisms that underlie amblyopia and visual stimulation treatment remain to be elucidated. Using monocular deprivation in juvenile mice to model amblyopia, we employed two-photon calcium imaging and chemogenetic techniques to investigate the visual responses of individual excitatory neurons and parvalbumin (PV+) interneurons in the primary visual cortex (V1) of amblyopic mice. We demonstrate that amblyopic mice exhibit an excitation/inhibition (E/I) imbalance. Moreover, visual stimulation decreases the response of PV+ interneurons, reactivates the ocular dominance plasticity of excitatory neurons, and promotes vision recovery in adult amblyopic mice. Our results reveal a dynamic E/I balance between excitatory neurons and PV+ interneurons that may underlie the neural mechanisms of amblyopia during cortical development and visual stimulation-mediated functional recovery from adult amblyopia, providing evidence for therapeutic applications that rely on reactivating adult cortical plasticity.

Quantifying the Functional Relationship Between Visual Acuity and Contrast Sensitivity Function

Purpose

Studies have reported that individuals with certain ocular disorders may have significant decreases in contrast sensitivity function (CSF) despite having normal or near normal visual acuity (VA). This study seeks to elucidate this phenomenon by investigating the relationship between VA and CSF.

Methods

We analyzed data from 14 eyes tested with Electronic Early Treatment Diabetic Retinopathy and quantitative CSF under four Bangerter foil conditions (n = 56). From the CSF data, we estimated peak gain, peak frequency, and contrast sensitivity acuity (CSA). We explored the correlations between VA and various CSF parameters and evaluated five predictive models of VA using CSA alone and in combination with additional CSF parameters through ridge regression.

Results

We found that similar VA scores can correspond with markedly different CSFs and observed significant correlations among all CSF parameters and between VA and each CSF parameter (all P < 0.001). The most effective predictive model, incorporating CSA and peak gain, explained 90.97% of the variance with a root mean squared error of 0.0676 logMAR, which is comparable with the average standard deviation of the VA scores (0.0627 logMAR) and accounted for 38.6% of the residual variance not explained by the CSA-alone model.

Conclusions

This study offers the first empirical inference of the quantitative relationship between VA and CSF, suggesting that various CSF parameter combinations can yield identical VA. This might help to explain why some clinical populations with normal or near-normal VA exhibit significant CSF deficits and calls for further research in different clinical settings.

Lower indoor spatial frequency increases the risk of myopia in children

BACKGROUND/AIMS

Animal models have shown that the absence of high-frequency visual information can precipitate the onset of myopia, but this relationship remains unclear in humans. This study aims to explore the association between the spatial frequency content of the visual environment and myopia in children.

METHODS

Images from the rooms of children and their frequently visited outdoor areas were taken by their parents and collected by the researcher through questionnaires. The spatial frequency was quantified using Matlab. Cycloplegic refraction was used to measure the spherical equivalent (SE), and IOL Master was used to measure axial length (AL) and corneal radius (CR). AL/CR ratio was calculated.

RESULTS

The study included 566 children with an average age of (8.04±1.47) years, of which 270 were girls (47.7%), and the average SE was (0.70±1.21) D. Image analysis revealed that indoor spatial frequency slope was lower than that of the outdoor environment (-1.43±0.18 vs -1.11±0.23, p<0.001). There were 79 myopic individuals (14.0%). Images from indoor content of myopic children had a lower spatial frequency slope than non-myopic children (-1.47±0.16 vs 1.43±0.18, p=0.03) while there was no significant difference in outdoor spatial frequency slope. Regression analysis indicated that the indoor spatial frequency slope was positively associated with SE value (β=0.60, p=0.02) and inversely related to myopia (OR=0.24, p<0.05).

CONCLUSION

The spatial frequency of the outdoor environment is significantly higher than that of the indoor environment. Indoor spatial frequency is related to children's refractive status, with lower indoor spatial frequency being associated with a higher degree of myopia.

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.

Assessment of amblyopic children undergoing occlusion therapy by pattern visual evoked potentials and contrast sensitivity tests

Background: Amblyopia is a case where one or less commonly, both eyes have impaired visual performance, even with the best optical correction and no visible disease of the visual system. Objectives: To assess contrast sensitivity tests (CST) and pattern visual evoked potentials (PVEP) results in amblyopic children who have already started occlusion therapy for durations ranging from 6 to 12 months. Methods: This cross-sectional study was conducted on 200 eyes of 50 patients with monocular amblyopia and 50 age and sex matched controls. Both patients and controls underwent ophthalmological assessment, PVEP, and CST. Results: There was no statistically significant difference in the results of P100 latencies of qualitative PVEP in amblyopic eyes compared to non-amblyopic eyes and control eyes, while the qualitative CST showed a highly statistically significant difference, being affected in 98% of amblyopic eyes compared to unaffected eyes (4%) and control eyes (4%). The maximum contrast level and minimal contrast level of quantitative CST were significantly lower in amblyopic eyes compared to non-amblyopic and control eyes. The cutoff value of maximal contrast level at mean frequencies of 2.5 ± 0.9 Hz, and a range of (1.1-4.1) for amblyopic eyes is ≤21 dB, while the cutoff value of minimal contrast level at mean frequencies of 13.4 ± 2.6 Hz, and a range of (6.7-18) for amblyopic eyes is ≤12 dB. Conclusion: Detection of amblyopia by CST is a noninvasive and easy procedure, which represents a promising tool to support the diagnosis of amblyopia.

The effect of training on sensitivity and stability of double fusion in Panum's limiting case

Panum's limiting case is a phenomenon of monocular occlusion in binocular vision. This occurs when one object is occluded by the other object for one eye, but the two objects are both visible for the other eye. Although previous studies have found that vertical gradient of horizontal disparity and cue conflict are two important factors for double fusion, the effect of training on the sensitivity and stability of Panum's limiting case remains unknown. The current study trained 26 participants for 5 days with several of Panum's configurations (Gilliam, Frisby, and Wang series). The latency and duration of double fusion were recorded to examine the effects of training on sensitivity and stability of double fusion in Panum's limiting case. For each level of vertical gradient of horizontal disparity and cue conflict, the latency of double fusion decreased and the duration of double fusion increased with each additional training session. The results showed that vertical gradient of horizontal disparity and cue conflict interacted, and the duration of high cue conflict was significantly shorter than that of medium and low cue conflict for each level of vertical gradient of horizontal disparity. The findings suggest that there is an effect of training for vertical gradient of horizontal disparity and cue conflict in Panum's limiting case, and that the three factors jointly affect the sensitivity and stability of double fusion.

Contrast sensitivity, optotype acuity and fixation eye movement abnormalities in amblyopia under binocular viewing

INTRODUCTION

Visual function deficits are seen in amblyopic subjects during fellow and binocular viewing. The purpose of the study was to examine the relationship between Fixation Eye Movement (FEM) abnormalities and binocular contrast sensitivity and optotype acuity deficits in amblyopia.

METHODS

We recruited 10 controls and 25 amblyopic subjects [Anisometropic = 6, Strabismic = 10, Mixed = 9]. We measured binocular contrast sensitivity at spatial frequencies 1,2, 4, 8, 12 and 16 and binocular and monocular optotype acuity using a staircase procedure. We recorded FEMs using high-resolution video-oculography and classified subjects as having no nystagmus(None = 9) or nystagmus without FMN(n = 7) and with Fusion Maldevelopment Nystagmus (FMN)(n = 9). We computed the fixation instability, amplitude and velocity of the fast and slow FEMs.

RESULTS

Amblyopic subjects with and without nystagmus had worse binocular contrast sensitivity at spatial frequencies 12 and 16 and binocular optotype acuity than controls. The abnormalities were most pronounced in amblyopic subjects with FMN. Fixation instability of the Fellow Eye and Amblyopic Eye and vergence instability, amplitude of fast FEMs and velocity of slow FEMs were increased with reduced binocular contrast sensitivity and reduced optotype acuity in amblyopic subjects.

CONCLUSIONS

Fixation instability of Fellow Eye and Amblyopic Eye, optotype acuity and contrast sensitivity deficits are seen under binocular viewing in amblyopic subjects with and without nystagmus but are most pronounced in those with FMN. FEMs abnormalities correlate with both lower order (contrast sensitivity) and higher order (optotype acuity) visual function impairment in amblyopia.

Effectiveness of binocular therapy as a complementary treatment of part-time patching in older amblyopic children: a randomized clinical trial

PURPOSE

To assess the effectiveness of combined use of stereoscopic 3D video movies and part-time patching in treating older amblyopic children with poor response or compliance to traditional patching treatments and comparing this combined treatment with patching alone.

METHODS

Thirty-two children aged 5-12 years with amblyopia associated with anisometropia, strabismus, or both were recruited in a randomized clinical trial. Eligible participants were assigned randomly to the combined and patching groups. Here, binocular treatment refers to using the Bangerter filter to blur the fellow eye and subsequently watching a close-up 3D movie with large parallax. The primary outcome was amblyopic eye (AE) best-corrected visual acuity (BCVA) improvement at six weeks. In addition, secondary outcomes included BCVA of AE improvement at three weeks and change of stereoacuity.

RESULTS

Of 32 participants, mean (SD) age was 6.63 (1.46) years, and 19 (59%) were female. At 6 weeks, mean (SD) amblyopic eye VA improved by 0.17 ± 0.08 logMAR (2-sided 95% CI, 0.13 to 0.22; F = 57.2, p < 0.01) and 0.05 ± 0.04 logMAR (2-sided 95% CI, 0.05 to 0.09; F = 8.73, p = 0.01) in the combined and patching groups, respectively. The difference was statistically significant (mean difference, 0.13 logMAR [1.3 line]; 95% CI, 0.08-0.17 logMAR [0.8-1.7 lines]; t25 = 5.65, p < 0 .01). After treatment, only the combined group had significantly improved stereoacuity, such as binocular function score (median [interquartile range], 2.30 [2.23 to 2.68] vs. 1.69 [1.60 to 2.30] log arcsec; paired, z = -3.53, p < 0.01), and mean stereoacuity gain was 0.47 log arcsec (± 0.22). Changes in other types of stereoacuity were similar.

CONCLUSION

Our laboratory-based binocular treatment strategy engaged a high level of compliance that led to a substantial gain in visual function after a short period of treatment for older amblyopic children having poor response or compliance to traditional patching treatments. Notably, the improving stereoacuity showed a greater advantage.

Temporal synchrony discrimination is abnormal in dichoptic but not monocular visual processing in treated anisometropic amblyopes

PURPOSE

To evaluate whether temporal synchrony processing deficits remain when normal visual acuity is restored in adults with unilateral anisometropic amblyopia.

METHODS

We recruited 14 clinically treated anisometropic amblyopes (mean age 23.17 ± 2.53 years) with best-corrected visual acuity ≤ 0.1 logMAR and 15 age-matched emmetropes (mean age 24.40 ± 1.92 years) with normal vision to participate in our experiment. We presented two pairs of flicking Gaussian dots (1 Hz) as visual stimuli: one pair of dots was synchronous (reference), and the other pair of dots was asynchronous (signal). Subjects were asked to determine the position of the asynchronous pair. We applied the constant stimuli method to measure the temporal synchrony threshold under monocular and dichoptic viewing conditions. There were eight temporal phase lags in the asynchronous pair. The minimum degree of the temporal phase at which a participant can discriminate a signal pair is defined as the temporal synchrony threshold.

RESULTS

Under monocular viewing conditions where both the reference and signal pairs were presented to one eye, the temporal synchrony thresholds of previous amblyopic eyes and fellow eyes were not significantly different (p = 0.15). Under dichoptic viewing conditions where both the reference and signal pairs were dichoptically presented to both eyes, the temporal synchrony threshold in the treated anisometropic amblyopes was significantly higher than that of the controls (119.34 ± 20.43 vs. 99.78 ± 16.60 ms, p = 0.009). There was no significant correlation between the monocular and dichoptic viewing conditions in the treated amblyopes (r = -0.22, p = 0.94).

CONCLUSIONS

Temporal synchrony discrimination is abnormal under dichoptic but not under monocular visual stimulation in treated anisometropic amblyopes with normalised visual acuity.

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.

Amblyopia: progress and promise of functional magnetic resonance imaging

Amblyopia is a neurodevelopmental disorder characterized by functional deficits in the visual cortex. Functional magnetic resonance imaging (fMRI) is the most commonly used neuroimaging technique for investigating amblyopia. Herein, we systematically searched a PubMed database from inception to December 2021 to highlight the current progress and promises about fMRI technology in amblyopia; amblyopia's neural mechanism, the comparison of different types of amblyopia, and the evaluation of the therapeutic effect were explored. Relevant articles published in English and appropriate cross-references were considered for inclusion, including basic studies, imaging techniques, clinical diagnostic and therapeutic studies, case series, and reviews.

Fixation instability, astigmatism, and lack of stereopsis as factors impeding recovery of binocular balance in amblyopia following binocular therapy

Dichoptic therapy is a promising method for improving vision in pediatric and adult patients with amblyopia. However, a systematic understanding about changes in specific visual functions and substantial variation of effect among patients is lacking. Utilizing a novel stereoscopic augmented-reality based training program, 24 pediatric and 18 adult patients were trained for 20 h along a three-month time course with a one-month post-training follow-up for pediatric patients. Changes in stereopsis, distance and near visual acuity, and contrast sensitivity for amblyopic and fellow eyes were measured, and interocular differences were analyzed. To reveal what contributes to successful dichoptic therapy, ANCOVA models were used to analyze progress, considering clinical baseline parameters as covariates that are potential requirements for amblyopic recovery. Significant and lasting improvements have been achieved in stereoacuity, interocular near visual acuity, and interocular contrast sensitivity. Importantly, astigmatism, fixation instability, and lack of stereopsis were major limiting factors for visual acuity, stereoacuity, and contrast sensitivity recovery, respectively. The results demonstrate the feasibility of treatment-efficacy prediction in certain aspects of dichoptic amblyopia therapy. Furthermore, our findings may aid in developing personalized therapeutic protocols, capable of considering individual clinical status, to help clinicians in tailoring therapy to patient profiles for better outcome.

Recent Treatment Advances in Amblyopia

Occlusion therapy has a long history as the gold standard treatment for amblyopia. Over the past two decades, large multicenter randomized controlled trials and objective dose-monitoring studies have characterized the effects of refractive correction, patching, and atropine penalization, providing insights into the impact of factors such as age and treatment dose. More recent approaches, whose development has been accelerated by advances in technology, are designed to provide different stimulation to the amblyopic eye and the fellow eye. This review explores a variety of such dichoptic approaches, categorized according to whether they primarily feature requisite use of the amblyopic eye in the face of fellow-eye masking, integration of visual information from both eyes, or reduction of stimulus salience in the fellow eye. It is still unclear whether dichoptic treatments are superior to traditional, low-cost treatment methods or whether their therapeutic mechanisms are fundamentally different from those of established treatments. Expected final online publication date for the Annual Review of Vision Science, Volume 8 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Contrast Sensitivity and Stereoacuity in Successfully Treated Refractive Amblyopia

Purpose

To assess whether monocular contrast sensitivity and stereoacuity impairments remain when visual acuity is fully recovered in children with refractive amblyopia.

Methods

A retrospective review of 487 patients diagnosed with refractive amblyopia whose visual acuity improved to 0.08 logMAR or better in both eyes following optical treatment was conducted. Measurements of monocular contrast sensitivity and stereoacuity had been made when visual acuity normalized. All patients had been treated with refractive correction for approximately 2 years following diagnosis. No other treatments were provided. Monocular contrast sensitivity was measured using the CSV-1000E chart for children 6 years of age or younger and a psychophysical technique called the quick contrast sensitivity function in older children. Stereoacuity was measured using the Random Dot Test that includes monocular cues and the Randot Stereoacuity Test that does not have monocular cues.

Results

Statistically significant interocular differences in contrast sensitivity were observed. These differences tended to occur at higher spatial frequencies (12 and 18 cycles per degree). Stereoacuity within the age-specific normal range was achieved by 47.4% of patients for the Random Dot Test and only 23.1% of patients for the Randot Stereoacuity Test.

Conclusions

Full recovery of visual acuity following treatment for refractive amblyopia does not equalize interocular contrast sensitivity or restore normal stereopsis. Alternative therapeutic approaches that target contrast sensitivity and/or binocular vision are required.

Binocular visual deficits at mid to high spatial frequency in treated amblyopes

Amblyopia (lazy eye) is a neurodevelopmental disorder of vision with no ocular pathology. The loss of vision in the amblyopic eye is assumed to be the main deficit in amblyopia, which has resulted in visual acuity (VA) being the primary outcome measure for treatment. Here we used a binocular orientation combination task to quantitatively assess the binocular status by measuring the binocular balance. We set out to determine whether amblyopes who reach the acuity-based end point have a residual binocular imbalance. Our results suggest that even amblyopes who have regained normal acuity have residual binocular deficits over a wide range of spatial frequencies. A further control study suggests that these binocular deficits could not be explained by any residual contrast sensitivity deficits of the amblyopic eye. Consequently, amblyopia is not the primary problem and VA is not the appropriate end point measure.

Sensitivity and Stability of Functional Vision Tests in Detecting Subtle Changes Under Multiple Simulated Conditions

Purpose

To explore whether subtle changes in visual quality can be detected using different measures of visual function against the quick contrast sensitivity function test (quick CSF).

Methods

Sixty participants, aged 17 to 34 years, were enrolled. Participants' vision was degraded by 0.25 D undercorrection (0.25 D), 60% neutral density filter brightness reduction (60% ND), and 0.8 Bangerter foil optical diffusion (0.8BAN). Visual function tests including visual acuity and contrast sensitivity (CSV-1000E and quick CSF) were measured with participant's best-corrected vision and under simulated visual degradation conditions. Test sensitivities in detecting differences were compared.

Results

Statistically significant visual acuity degradation was observed in the 0.8BAN condition only (Pcorrected < 0.001). With CSV-1000E and outliers removed, significant CS degradation was observed in all spatial frequencies, area under log CSF (AULCSF) in the 0.8BAN condition (Pcorrected < 0.001 for all), medium and high spatial frequencies and AULCSF in the 60%ND condition (Pcorrected,6cpd = 0.002, Pcorrected,12cpd = 0.005, Pcorrected,18cpd = 0.001, Pcorrected,AULCSF < 0.001) and the 0.25 D condition (Pcorrected,6cpd = 0.011, Pcorrected,12cpd = 0.013, Pcorrected,18cpd = 0.015, Pcorrected,AULCSF < 0.001). With the quick CSF, significant CS degradation was observed in all simulated visual conditions in all spatial frequencies, cutoff frequency and AULCSF (Pcorrected < 0.001 for all). Test-retest reliability of the quick CSF method was high; coefficient of repeatability ranged from 0.14 to 0.18 logCS.

Conclusions

Compared with visual acuity and chart-based CS tests, the quick CSF method provided more reliable and sensitive measures to detect small visual changes.

Translational Relevance

The quick CSF method can provide sensitive and reliable measures to monitor disease progression and assess treatment outcomes.

Portable rotating grating stimulation for anisometropic amblyopia with 6 months training

Treatment of grating stimulation has been used in amblyopia for decades, but high dropout rate and inconvenience for daily practice occur in previous studies. We developed a home-based portable system with rotating grating stimulation on a tablet. Thirty anisometropic amblyopic children were randomly allocated into the control or Grating group. They drew contour of the picture under patch of a better eye for 6 months. Best-corrected visual acuity (BCVA), grating acuity (GA), and contrast sensitivity (CS) were assessed at the baseline, 1st, 2nd, 3rd, and 6th months of training. All participants completed the 6-month training. Patched eyes of both groups exhibited no difference. Trained eyes of the control group had significantly slight improvement in BCVA and GA. In particular, the Grating group exhibited significantly higher BCVA, GA, and CS compared with those of the control group at the 3rd and 6th months of training. Moreover, percentage of the Grating group with great improvement (BCVA ≥ 0.3 or CS ≥ 0.3) was significantly larger than those of the control group at the 3rd or 6th months of training. The portable grating stimulation system demonstrates its trainability by no dropout and effectiveness by significant improvements in all assessments through a well experimental design.

Trial Registration: ClinicalTrials.gov NCT04213066, registered 30/12/2019, https://clinicaltrials.gov/ct2/show/NCT04213066.

The Curve Visible on the Campbell-Robson Chart Is Not the Contrast Sensitivity Function

The Campbell-Robson chart is a highly popular figure used in psychophysics and visual perception textbooks to illustrate the Contrast Sensitivity Function (CSF). The chart depicts a grating which varies logarithmically in spatial frequency (SF) from left to right and in contrast from bottom to top. Campbell and Robson’s (1964) intuition was that the boundary between the grating and the homogeneous gray area (below threshold) would trace the shape of the observer’s own CSF. In this paper, we tested this intuition. A total of 170 participants (96 adults and 74 children) adjusted the four parameters of a truncated log-parabola directly onto a Campbell-Robson chart rendition and completed a gold-standard CSF evaluation. We hoped that this procedure which requires a mere three clicks on the computer mouse, would speed up the measurement of the CSF to under a minute. Unfortunately, the only parameter of the truncated log-parabola fitted to the gold-standard CSF data that could be predicted from the Campbell-Robson chart data was the peak sensitivity for the adult participants. We conclude that the curve visible on the Campbell-Robson chart cannot be used practically to measure the CSF.

Alternative Flicker Glass: a New Anti-suppression Approach to the Treatment of Anisometropic Amblyopia

INTRODUCTION

Amlyopia always presents with monocular and binocular dysfunction. In this study, we aim to investage the efficacy of alternative occlusion using liquid crystal glasses versus continuous occlusion therapy using traditional patches for treating amblyopia.

METHODS

Eligible subjects with anisometropic amblyopia were randomized into two groups: alternative flicker glass (AFG) or patching group. In the AFG group, subjects were instructed to wear the flicker glasses for 1 hour a day. The AFG is a lightweight spectacle frame with liquid crystal lenses that provide direct square-wave alternating occlusion, which were pre-programmed at temporal frequency of 7Hz. In the patching group, the patients were prescribed to wear traditional patches for 2 hours a day. The best corrected visual acuity (BCVA), contrast sensitivity function (CSF) and stereoacuity were measured at the baseline, 3 and 12 weeks.

RESULTS

In this pilot study, a total of forty children were recruited, with twenty in the AFG group. Mean BCVA improved by 0.17±0.14logMAR (95% CI=0.10 to 0.23) in the AFG group, while 0.18±0.18logMAR (95% CI=0.09 to 0.26) in the patching group from baseline to 12 weeks. The improvement of BCVA in both groups were significant (both P＜0.01), while no significant difference between the groups (P=0.82). The CSF of both low and high spatial frequencies exhibited significant improvement at 12 weeks in the AFG group (P＜0.01, respectively), while just have a significant improvement at low spatial frequency in the patching group (P＜0.01). The stereoacuity significantly improved by 504.00±848.00 (95% CI= -900.88 to -107.12) arc seconds in the AFG group (P＜0.05), while 263.50± 639.55 (95% CI=-562.82 to 35.82) arc seconds in the patching group (P＞0.05).

CONCLUSION

Alternative flicker glass was effective in improving both monocular and binocular function, which was most likely achieved by reducing the suppression and promoting binocular fusion. This therapy exhibited promise as an alternative method for amblyopia treatment.

Multi-Stage Cortical Plasticity Induced by Visual Contrast Learning

Perceptual learning, the improved sensitivity via repetitive practice, is a universal phenomenon in vision and its neural mechanisms remain controversial. A central question is which stage of processing is changed after training. To answer this question, we measured the contrast response functions and electroencephalography (EEG) before and after ten daily sessions of contrast detection training. Behavioral results showed that training substantially improved visual acuity and contrast sensitivity. The learning effect was significant at the trained condition and partially transferred to control conditions. Event-related potential (ERP) results showed that training reduced the latency in both early and late ERPs at the trained condition. Specifically, contrast-gain-related changes were observed in the latency of P1, N1-P2 complex, and N2, which reflects neural changes across the early, middle, and high-level sensory stages. Meanwhile, response-gain-related changes were found in the latency of N2, which indicates stimulus-independent effect in higher-level stages. In sum, our findings indicate that learning leads to changes across different processing stages and the extent of learning and transfer may depend on the specific stage of information processing.

A New Dichoptic Training Strategy Leads to Better Cooperation Between the Two Eyes in Amblyopia

Recent clinical trials failed to endorse dichoptic training for amblyopia treatment. Here, we proposed an alternative training strategy that focused on reducing signal threshold contrast in the amblyopic eye under a constant and high noise contrast in the fellow eye (HNC), and compared it to a typical dichoptic strategy that aimed at increasing the tolerable noise contrast in the fellow eye (i.e., TNC strategy). We recruited 16 patients with amblyopia and divided them into two groups. Eight patients in Group 1 received the HNC training, while the other eight patients in Group 2 performed the TNC training first (Phase 1) and then crossed over to the HNC training (Phase 2). We measured contrast sensitivity functions (CSFs) separately in the amblyopic and fellow eyes when the untested eye viewed mean luminance (monocularly unmasked) or noise stimuli (dichoptically masked) before and after training at a particular frequency. The area under the log contrast sensitivity function (AULCSF) of masked and unmasked conditions, and dichoptic gain (the ratio of AULCSF of masked to unmasked condition) were calculated for each eye. We found that both dichoptic training paradigms substantially improved masked CSF, dichoptic gain, and visual acuity in the amblyopic eye. As opposed to the TNC paradigm, the HNC training produced stronger effects on masked CSFs, stereoacuity, dichoptic gain, and visual acuity in the amblyopic eye. Interestingly, the second-phase HNC training in Group 2 also induced further improvement in the masked contrast sensitivity and AULCSF in the amblyopic eye. We concluded that the HNC training strategy was more effective than the TNC training paradigm. Future design for dichoptic training should not only focus on increasing the tolerable noise contrast in the fellow eye but should also "nurture" the amblyopic eye under normal binocular viewing conditions and sustained interocular 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.

Increased Equivalent Input Noise in Glaucomatous Central Vision: Is it Due to Undersampling of Retinal Ganglion Cells?

Purpose

Recent evidence shows that macular damage is common even in early stages of glaucoma. Here we investigated whether contrast sensitivity loss in the central vision of glaucoma patients is due to an increase in equivalent input noise (Neq), a decrease in calculation efficiency, or both. We also examined how retinal undersampling resulting from loss of retinal ganglion cells (RGCs) may affect Neq and calculation efficiency.

Methods

This study included 21 glaucoma patients and 23 age-matched normally sighted individuals. Threshold contrast for orientation discrimination was measured with a sinewave grating embedded in varying levels of external noise. Data were fitted to the linear amplifier model (LAM) to factor contrast sensitivity into Neq and calculation efficiency. We also correlated macular RGC counts estimated from structural (spectral-domain optical coherence tomography) and functional (standard automated perimetry Swedish interactive thresholding algorithm 10-2) data with either Neq or efficiency. Furthermore, using analytical and computer simulation approach, the relative effect of retinal undersampling on Neq and efficiency was evaluated by adding the RGC sampling module into the LAM.

Results

Compared with normal controls, glaucoma patients exhibited a significantly larger Neq without significant difference in efficiency. Neq was significantly correlated with Pelli-Robson contrast sensitivity and macular RGC counts. The results from analytical derivation and model simulation further demonstrated that Neq can be expressed as a function of internal noise and retinal sampling.

Conclusions

Our results showed that equivalent input noise is significantly elevated in glaucomatous vision, thereby impairing foveal contrast sensitivity. Our findings further elucidated how undersampling at the retinal level may increase equivalent input noise.

Effect of age and refractive error on quick contrast sensitivity function in Chinese adults: a pilot study

PURPOSE

To evaluate the potential effect of age and refractive error on visual acuity (VA) performance and quick contrast sensitivity function (qCSF) in normal Chinese adults.

METHOD

Ninety-two subjects with normal best corrected distance VA (BCDVA) were enrolled in this pilot study. Measurements included BCDVA, best corrected near VA (BCNVA), unaided VA (UNVA), habitual spectacle-corrected near VA (SCNVA) and qCSF. For analyses, subjects were categorized into three age groups (20~40 years, 41~60 year and >60 years) and four refractive groups (hyperopia, emmetropia, myopia and high myopia). Relationships between age, refractive error, types of VA and qCSF were tested using simple and multiple linear regressions analyses.

RESULT

Mean age and refractive error of the study participants were 44.04 ± 12.68 years and -1.86 ± 2.91D, respectively. Among the stratified age groups, a hyperopic shift of refraction (-3.24 ± 2.88D vs. -1.24 ± 2.64D vs. 0.39 ± 1.42D, respectively; P < 0.001) and a reduction in BCNVA (P = 0.014), SCNVA (P < 0.001) and cut-off spatial frequency (SF) (P = 0.032) were found with increasing age. Among the four refractive groups, the SCNVA and cut-off SF of hyperopia were worse compared to the other refractive statuses (all P < 0.05). Age was significantly associated with cut-off SF (standardized β = -0.29, P = 0.005) after adjustment for SER, gender and all types of VA.

CONCLUSION

For normal Chinese adults with normal BCDVA, age was the main factor associated with CSF, which may be independent of refractive error.

Effects of Monocular Perceptual Learning on Binocular Visual Processing in Adolescent and Adult Amblyopia

Re-establishing normal binocular visual processing is the key to amblyopia recovery beyond the critical period of visual development. Here, by combining perceptual learning, behavioral testing, and steady-state visually evoked potentials (SSVEPs), we examined how monocular perceptual learning in the amblyopic eye could change binocular visual processing in the adolescent and adult amblyopic visual system. We found that training reduced the interocular difference between amblyopic and fellow eyes and increased the amplitude of a binocular SSVEP component, with a significant negative correlation between the two measures. Our results demonstrate that training in the amblyopic eye primarily improves binocular rather than monocular visual processing in the amblyopic visual system, suggesting that behavioral training could potentially address key neural deficits in adolescent and adult amblyopia.

Perceptual Learning at Higher Trained Cutoff Spatial Frequencies Induces Larger Visual Improvements

It is well known that extensive practice of a perceptual task can improve visual performance, termed perceptual learning. The goal of the present study was to evaluate the dependency of visual improvements on the features of training stimuli (i.e., spatial frequency). Twenty-eight observers were divided into training and control groups. Visual acuity (VA) and contrast sensitivity function (CSF) were measured and compared before and after training. All observers in the training group were trained in a monocular grating detection task near their individual cutoff spatial frequencies. The results showed that perceptual learning induced significant visual improvement, which was dependent on the cutoff spatial frequency, with a greater improvement magnitude and transfer of perceptual learning observed for those trained with higher spatial frequencies. However, VA significantly improved following training but was not related to the cutoff spatial frequency. The results may broaden the understanding of the nature of the learning rule and the neural plasticity of different cortical areas.

Binocular Summation and Suppression of Contrast Sensitivity in Strabismus, Fusion and Amblyopia

Purpose: Amblyopia and strabismus affect 2%–5% of the population and cause a broad range of visual deficits. The response to treatment is generally assessed using visual acuity, which is an insensitive measure of visual function and may, therefore, underestimate binocular vision gains in these patients. On the other hand, the contrast sensitivity function (CSF) generally takes longer to assess than visual acuity, but it is better correlated with improvement in a range of visual tasks and, notably, with improvements in binocular vision. The present study aims to assess monocular and binocular CSFs in amblyopia and strabismus patients.

Methods: Both monocular CSFs and the binocular CSF were assessed for subjects with amblyopia (n = 11), strabismus without amblyopia (n = 20), and normally sighted controls (n = 24) using a tablet-based implementation of the quick CSF, which can assess a full CSF in <3 min. Binocular summation was evaluated against a baseline model of simple probability summation.

Results: The CSF of amblyopic eyes was impaired at mid-to-high spatial frequencies compared to fellow eyes, strabismic eyes without amblyopia, and control eyes. Binocular contrast summation exceeded probability summation in controls, but not in subjects with amblyopia (with or without strabismus) or strabismus without amblyopia who were able to fuse at the test distance. Binocular summation was less than probability summation in strabismic subjects who were unable to fuse.

Conclusions: We conclude that monocular and binocular contrast sensitivity deficits define important characteristics of amblyopia and strabismus that are not captured by visual acuity alone and can be measured efficiently using the quick CSF.

Visual Search in Amblyopia: Abnormal Fixational Eye Movements and Suboptimal Sampling Strategies

Purpose

Microsaccades shift the image on the fovea and counteract visual fading. They are also thought to serve as an optimal sampling strategy while viewing complex visual scenes. The goal of our study was to assess visual search in amblyopic children.

Methods

Twenty-one amblyopic children with varying severity of amblyopia and 10 healthy controls were recruited. Eye movements were recorded using infrared video-oculography during amblyopic and fellow eye viewing while the subjects performed (1) visual fixation, (2) exploration of a blank scene, and (3) visual search task (spot the difference between two images). The number of correctly identified picture differences and reaction time were recorded. Microsaccade, saccades, and intersaccadic drifts were analyzed in patients without latent nystagmus (LN). Slow phase velocities were computed for patients with LN.

Results

Both patients with and without LN were able to spot the same number of differences but took longer during fellow eye viewing compared to controls. The ability to identify differences was diminished during amblyopic eye viewing particularly those with LN and severe amblyopia. We found reduced frequencies of microsaccades and saccades in both amblyopic and fellow eyes during fixation and visual search but not during exploration of blank scene. Across all tasks, amblyopes with LN had increased intersaccadic drifts.

Conclusions

Our findings suggest that deficient microsaccade and saccadic activity contributes to poorer sampling strategy in amblyopia, which is seen in both amblyopic and fellow eye. These deficits are more notable among subjects who experienced binocular decorrelation earlier in life, with subsequent development of LN.

Oral fluoxetine in the management of amblyopic patients aged between 10 and 40 years old: a randomized clinical trial

OBJECTIVE

The objective of this study is to assess the efficacy of oral fluoxetine therapy in improving the visual function of amblyopic patients aged between 10 and 40 years old.

METHODS

In this double-blinded, randomized, controlled trial (IRCT2016052428046N1; registered retrospectively), 40 eligible participants with anisometropic or mixed amblyopia were randomly assigned to either fluoxetine or placebo groups. Participants with anisometropia and logMAR best spectacle-corrected visual acuity (BSCVA) worse than 0.2 logMAR in the amblyopic eye or at least a two-line of difference in the BSCVA between the fellow eyes were included. Participants with significant ocular or systemic diseases were excluded. In both groups, the better eye of each patient was patched for 4-6 h a day during the study period. Participants in the treatment group were treated with oral fluoxetine for 3 months. Change in the Snellen BSCVA (after 3 months) was regarded as the primary outcome measure.

RESULTS

Data from 20 participants in the fluoxetine group and 15 participants from the placebo group were analyzed (aged 11-37 years). The magnitude of improvement in visual acuity (from baseline to 3 months after treatment) was significantly higher in the fluoxetine group (0.240 ± 0.068 logMAR; 2.4 line-gain) compared with the control group (0.120 ± 0.086 logMAR; 1.2 line-gain).

CONCLUSIONS

This study suggests beneficial effects of fluoxetine in the management of adult and adolescent amblyopia.

Measuring the Contrast Sensitivity Function Using the qCSF Method With 10 Digits

PURPOSE

The Bayesian adaptive quick contrast sensitivity function (qCSF) method with a 10-letter identification task provides an efficient CSF assessment. However, large populations are unfamiliar with letters and cannot benefit from this test. To overcome the barrier, we conducted this study.

METHOD

A new font for digits (0∼9) was created. The digits were then filtered with a raised cosine filter, rescaled to different sizes to cover spatial frequencies from 0.5 to 16 cycles per degree (cpd), and used as stimuli in a 10-alternative forced choice (10AFC) digit identification task. With the 10AFC digit identification task, the CSFs of five young and five old observers were measured using the qCSF and Psi methods. The estimates from the latter served as reference.

RESULTS

The new digit font showed significantly improved similarity structure, Levene's test, F(1, 88) = 6.36, P = 0.014. With the 10-digit identification task, the CSFs obtained with the qCSF method matched well with those obtained with the Psi method (root mean square error [RMSE] = 0.053 log10 units). With approximately 30 trials, the precision of the qCSF method reached 0.1 log10 units. With approximately 75 trials, the precision of the CSFs obtained with the qCSF was comparable to that of the CSFs measured by the Psi method in 150 trials.

CONCLUSIONS

The qCSF with the 10 digit identification task is validated for both young and old observers.

TRANSLATIONAL RELEVANCE

The qCSF method with the 10-digit identification task provides an efficient and precise CSF test especially for people who are unfamiliar with letters.

A complete investigation of monocular and binocular functions in clinically treated amblyopia

The gold standard of a successful amblyopia treatment is full recovery of visual acuity (VA) in the amblyopic eye, but there has been no systematic study on both monocular and binocular visual functions. In this research, we aimed to quantify visual qualities with a variety of perceptual tasks in subjects with treated amblyopia. We found near stereoacuity and pAE dominance in binocular rivalry in "treated" amblyopia were largely comparable to those of normal subjects. CSF of the pAE remained deficient in high spatial frequencies. The binocular contrast summation ratio is significantly lower than normal standard. The interocular balance point is 34%, indicating that contrast in pAE is much less effective as the same contrast in pFE in binocular phase combination. Although VA, stereoacuity and binocular rivalry at low spatial frequency in treated amblyopes were normal or nearly normal, the pAE remained "lazy" in high frequency domain, binocular contrast summation, and interocular phase combination. Our results suggest that structured monocular and binocular training are necessary to fully recover deficient functions in amblyopia.

The effects of monocular training on binocular functions in anisometropic amblyopia

Intensive monocular perceptual learning can improve visual acuity, contrast sensitivity, and vernier acuity in the amblyopic eye in adults with amblyopia. It is however not clear how much monocular training can enhance binocular visual functions. In the current study, we aimed to evaluate effects of monocular training on a variety of binocular functions. Nineteen anisometropic amblyopes (18.5±1.26yrs, mean±s.e.) were trained in a grating contrast detection task near each individual's cutoff spatial frequency for 6-10days (630 trials/day). Visual acuity, stereoacuity, monocular and binocular contrast sensitivity functions (CSF), binocular phase combination and binocular rivalry were tested before and after training. Although monocular training can improve visual acuity and contrast sensitivity and eye dominance of the amblyopic eye, the magnitudes of improvements did not correlate with each other; the impact of monocular training on binocular phase combination was not significant. The results strongly suggest that structured monocular and binocular training is needed to fully recover deficient visual functions in anisometropic amblyopia.

Functional visual acuity in patients with successfully treated amblyopia: a pilot study

PURPOSE

The aim of this study was to use conventional visual acuity measurements to quantify the functional visual acuity (FVA) in eyes with successfully treated amblyopia, and to compare the findings with those for contralateral normal eyes.

METHODS

Nineteen patients (7 boys, 12 girls; age 7.5 ± 2.2 years) with successfully treated unilateral amblyopia and the same conventional decimal visual acuity in both eyes (better than 1.0) were enrolled. FVA, the visual maintenance ratio (VMR), maximum and minimum visual acuity, and the average response time were recorded for both eyes of all patients using an FVA measurement system. The differences in FVA values between eyes were analyzed.

RESULTS

The mean LogMAR FVA scores, VMR (p < 0.001 for both), and the LogMAR maximum (p < 0.005) and minimum visual acuity (p < 0.001) were significantly poorer for the eyes with treated amblyopia than for the contralateral normal eyes. There was no significant difference in the average response time.

CONCLUSIONS

Our results indicate that FVA and VMR were poorer for eyes with treated amblyopia than for normal eyes, even though the treatment for amblyopia was considered successful on the basis of conventional visual acuity measurements. These results suggest that visual function is impaired in eyes with amblyopia, regardless of treatment success, and that FVA measurements can provide highly valuable diagnosis and treatment information that is not readily provided by conventional visual acuity measurements.

Sensory Eye Dominance in Treated Anisometropic Amblyopia

Amblyopia results from inadequate visual experience during the critical period of visual development. Abnormal binocular interactions are believed to play a critical role in amblyopia. These binocular deficits can often be resolved, owing to the residual visual plasticity in amblyopes. In this study, we quantitatively measured the sensory eye dominance in treated anisometropic amblyopes to determine whether they had fully recovered. Fourteen treated anisometropic amblyopes with normal or corrected to normal visual acuity participated, and their sensory eye dominance was assessed by using a binocular phase combination paradigm. We found that the two eyes were unequal in binocular combination in most (11 out of 14) of our treated anisometropic amblyopes, but none of the controls. We concluded that the treated anisometropic amblyopes, even those with a normal range of visual acuity, exhibited abnormal binocular processing. Our results thus suggest that there is potential for improvement in treated anisometropic amblyopes that may further enhance their binocular visual functioning.

Contrast Sensitivity in Microtropic and Anisometropic Eyes of Successfully Treated Amblyopes

Objectives:

To assess and compare contrast sensitivity function in the previously amblyopic and non-amblyopic “normal” eyes of patients with microtropia and anisometropia who achieved 20/20 visual acuity after occlusion therapy.

Materials and Methods:

Contrast sensitivity was tested monocularly on both eyes of 34 successfully treated microtropic and 15 anisometropic subjects (visual acuity 20/20 in both eyes). Contrast sensitivity function was evaluated by CSV-1000E and age-matched nomograms were used (spatial frequencies of 3, 6, 12, and 18 cycles per degree [cpd]) for comparison.

Results:

The mean age of subjects was 11.2±1.3 years in the microtropic group, 9.8±1.7 years in the anisometropic group (7-12 years); the mean follow-up time was 16.4±3.2 months (12 to 92) in the microtropic group and 27.7±1.8 months (12-84) in the anisometropic group. Statistical comparison of the microtropic amblyopic eyes versus non-microtropic eyes showed significant differences at spatial frequencies of 3, 12 and 18 cpd (3 cpd, t=2.8, p=0.007; 6 cpd, t=1.1 p=0.261; 12 cpd, t=2.2, p=0.033; 18 cpd, t=2.2, p=0.030). When anisometropic eyes were compared with non-anisometropic eyes, there was a significant difference only at 12 cpd (t=2.1 p=0.049). The comparison of non-amblyopic eyes versus age-matched nomograms revealed no differences at any of the spatial frequencies (p>0.05 for all).

Conclusion:

Contrast sensitivity was decreased in patients with amblyopia, especially in the microtropic group. The assessment of contrast sensitivity function may serve as a new parameter for termination of occlusion therapy.

Bayesian Inference of Two-Dimensional Contrast Sensitivity Function from Data Obtained with Classical One-Dimensional Algorithms Is Efficient

The contrast sensitivity function that spans the two dimensions of contrast and spatial frequency is crucial in predicting functional vision both in research and clinical applications. In this study, the use of Bayesian inference was proposed to determine the parameters of the two-dimensional contrast sensitivity function. Two-dimensional Bayesian inference was extensively simulated in comparison to classical one-dimensional measures. Its performance on two-dimensional data gathered with different sampling algorithms was also investigated. The results showed that the two-dimensional Bayesian inference method significantly improved the accuracy and precision of the contrast sensitivity function, as compared to the more common one-dimensional estimates. In addition, applying two-dimensional Bayesian estimation to the final data set showed similar levels of reliability and efficiency across widely disparate and established sampling methods (from classical one-dimensional sampling, such as Ψ or staircase, to more novel multi-dimensional sampling methods, such as quick contrast sensitivity function and Fisher information gain). Furthermore, the improvements observed following the application of Bayesian inference were maintained even when the prior poorly matched the subject's contrast sensitivity function. Simulation results were confirmed in a psychophysical experiment. The results indicated that two-dimensional Bayesian inference of contrast sensitivity function data provides similar estimates across a wide range of sampling methods. The present study likely has implications for the measurement of contrast sensitivity function in various settings (including research and clinical settings) and would facilitate the comparison of existing data from previous studies.

Covert spatial attention is functionally intact in amblyopic human adults

Certain abnormalities in behavioral performance and neural signaling have been attributed to a deficit of visual attention in amblyopia, a neurodevelopmental disorder characterized by a diverse array of visual deficits following abnormal binocular childhood experience. Critically, most have inferred attention's role in their task without explicitly manipulating and measuring its effects against a baseline condition. Here, we directly investigate whether human amblyopic adults benefit from covert spatial attention-the selective processing of visual information in the absence of eye movements-to the same degree as neurotypical observers. We manipulated both involuntary (Experiment 1) and voluntary (Experiment 2) attention during an orientation discrimination task for which the effects of covert spatial attention have been well established in neurotypical and special populations. In both experiments, attention significantly improved accuracy and decreased reaction times to a similar extent (a) between the eyes of the amblyopic adults and (b) between the amblyopes and their age- and gender-matched controls. Moreover, deployment of voluntary attention away from the target location significantly impaired task performance (Experiment 2). The magnitudes of the involuntary and voluntary attention benefits did not correlate with amblyopic depth or severity. Both groups of observers showed canonical performance fields (better performance along the horizontal than vertical meridian and at the lower than upper vertical meridian) and similar effects of attention across locations. Despite their characteristic low-level vision impairments, covert spatial attention remains functionally intact in human amblyopic adults.

A hierarchical Bayesian approach to adaptive vision testing: A case study with the contrast sensitivity function

Measurement efficiency is of concern when a large number of observations are required to obtain reliable estimates for parametric models of vision. The standard entropy-based Bayesian adaptive testing procedures addressed the issue by selecting the most informative stimulus in sequential experimental trials. Noninformative, diffuse priors were commonly used in those tests. Hierarchical adaptive design optimization (HADO; Kim, Pitt, Lu, Steyvers, & Myung, 2014) further improves the efficiency of the standard Bayesian adaptive testing procedures by constructing an informative prior using data from observers who have already participated in the experiment. The present study represents an empirical validation of HADO in estimating the human contrast sensitivity function. The results show that HADO significantly improves the accuracy and precision of parameter estimates, and therefore requires many fewer observations to obtain reliable inference about contrast sensitivity, compared to the method of quick contrast sensitivity function (Lesmes, Lu, Baek, & Albright, 2010), which uses the standard Bayesian procedure. The improvement with HADO was maintained even when the prior was constructed from heterogeneous populations or a relatively small number of observers. These results of this case study support the conclusion that HADO can be used in Bayesian adaptive testing by replacing noninformative, diffuse priors with statistically justified informative priors without introducing unwanted bias.

Perceptual learning improves neural processing in myopic vision

Visual performance is jointly determined by the quality of optical transmission of the eye and neural processing in the visual system. An open question is: Can effects of optical defects be compensated by perceptual learning in neural processing? To address this question, we conducted a perceptual learning study on 23 observers with myopic vision, targeting high frequency deficits by training them in a monocular grating detection task in the non-dominant eye near their individual cutoff spatial frequencies. The contrast sensitivity function and visual acuity in both eyes (without optical correction) were assessed for all the observers in the training group before and after training, and for all the observers in the control group twice with a 10-day interval between the tests. In addition, the threshold versus external noise contrast function was measured for five observers in the training group before and after training. We found that (a) training significantly improved contrast sensitivity at the trained spatial frequency, visual acuity, and contrast sensitivity over a wide range of spatial frequencies in both eyes; (b) training did not lead to any significant refractive changes; (c) the mechanism of improvements was a combination of internal additive noise reduction and external noise exclusion; and (d) the improvements in visual acuity and contrast sensitivity were almost fully retained for at least four months in the three observers tested. These results suggest that perceptual learning may provide a potential noninvasive procedure to compensate for optical defects in mild to modest myopia.

Evaluating the performance of the quick CSF method in detecting contrast sensitivity function changes

The contrast sensitivity function (CSF) has shown promise as a functional vision endpoint for monitoring the changes in functional vision that accompany eye disease or its treatment. However, detecting CSF changes with precision and efficiency at both the individual and group levels is very challenging. By exploiting the Bayesian foundation of the quick CSF method (Lesmes, Lu, Baek, & Albright, 2010), we developed and evaluated metrics for detecting CSF changes at both the individual and group levels. A 10-letter identification task was used to assess the systematic changes in the CSF measured in three luminance conditions in 112 naïve normal observers. The data from the large sample allowed us to estimate the test-retest reliability of the quick CSF procedure and evaluate its performance in detecting CSF changes at both the individual and group levels. The test-retest reliability reached 0.974 with 50 trials. In 50 trials, the quick CSF method can detect a medium 0.30 log unit area under log CSF change with 94.0% accuracy at the individual observer level. At the group level, a power analysis based on the empirical distribution of CSF changes from the large sample showed that a very small area under log CSF change (0.025 log unit) could be detected by the quick CSF method with 112 observers and 50 trials. These results make it plausible to apply the method to monitor the progression of visual diseases or treatment effects on individual patients and greatly reduce the time, sample size, and costs in clinical trials at the group level.

Tilt after-effect from high spatial-frequency patterns in the amblyopic eye of adults with anisometropic amblyopia

With abnormal visual cortical development, amblyopia is generally associated with high spatial frequency deficits in spatial vision. In this study, we aim to answer a critical question: How much high spatial frequency information is available to the amblyopic visual system? We measured the tilt after-effect following adaption to perceptually resolvable and unresolvable sinewave gratings, and showed that gratings with spatial frequency up to 1.5 times the cutoff frequency in grating orientation identification can still produce significant tilt after-effects in adults with amblyopia. Our results suggest that neural connections in the amblyopic visual cortex, at least in V1, may have profoundly developed to represent high spatial frequency information. The demonstration of extant neural connections for high spatial frequencies may have important implications for the development of training protocols for amblyopia treatment. Our paradigm may also serve as a non-invasive probe to diagnose the status of neural connections in other visual deficits.

Broad bandwidth of perceptual learning in second-order contrast modulation detection

Comparing characteristics of learning in first- and second-order systems might inform us about different neural plasticity in the two systems. In the current study, we aim to determine the properties of perceptual learning in second-order contrast modulation detection in normal adults. We trained nine observers to detect second-order gratings at an envelope modulation spatial frequency of 8 cycles/° with their nondominant eyes. We found that, although training generated the largest improvements around the trained frequency, contrast sensitivity over a broad range of spatial frequencies also improved, with a 4.09-octave bandwidth of perceptual learning, exhibiting specificity to the trained spatial frequency as well as a relatively large degree of generalization. The improvements in the modulation sensitivity function (MSF) were not significantly different between the trained and untrained eyes. Furthermore, training did not significantly change subjects' ability in detecting first-order gratings. Our results suggest that perceptual learning in second-order detection might occur at the postchannel level in binocular neurons, possibly through reducing the internal noise of the visual system.

Using 10AFC to further improve the efficiency of the quick CSF method

The contrast sensitivity function (CSF) provides a fundamental characterization of spatial vision, important for basic and clinical applications, but its long testing times have prevented easy, widespread assessment. The original quick CSF method was developed using a two-alternative forced choice (2AFC) grating orientation identification task (Lesmes, Lu, Baek, & Albright, 2010), and obtained precise CSF assessments while reducing the testing burden to only 50 trials. In this study, we attempt to further improve the efficiency of the quick CSF method by exploiting the properties of psychometric functions in multiple-alternative forced choice (m-AFC) tasks. A simulation study evaluated the effect of the number of alternatives m on the efficiency of the sensitivity measurement by the quick CSF method, and a psychophysical study validated the quick CS method in a 10AFC task. We found that increasing the number of alternatives of the forced-choice task greatly improved the efficiency of CSF assessment in both simulation and psychophysical studies. The quick CSF method based on a 10-letter identification task can assess the CSF with an averaged standard deviation of 0.10 decimal log unit in less than 2 minutes.

The external noise normalized gain profile of spatial vision

The contrast sensitivity function (CSF), a measure of visual sensitivity to a wide range of spatial frequencies, has been widely used as the gain profile of the front-end filter of the visual system to predict how we perceive spatial patterns. However, the CSF itself is determined by the gain profile and other processing inefficiencies of the visual system; it may be problematic to use the CSF as the gain profile in observer models. Here, we applied the external noise paradigm and the perceptual template model (PTM) to characterize several major properties of the visual system. With the external noise normalized gain profile, nonlinearity, and internal additive and multiplicative noises, the PTM accounted for 92.8% of the variance in the experiment data measured in a wide range of conditions and revealed the major processing components that determine the CSF. Unlike the CSF, the external noise normalized gain profile of the visual system is relatively flat across a wide range of spatial frequencies. The results may have major implications for understanding normal and abnormal spatial vision.

Binocular vision in amblyopia: structure, suppression and plasticity

The amblyopic visual system was once considered to be structurally monocular. However, it now evident that the capacity for binocular vision is present in many observers with amblyopia. This has led to new techniques for quantifying suppression that have provided insights into the relationship between suppression and the monocular and binocular visual deficits experienced by amblyopes. Furthermore, new treatments are emerging that directly target suppressive interactions within the visual cortex and, on the basis of initial data, appear to improve both binocular and monocular visual function, even in adults with amblyopia. The aim of this review is to provide an overview of recent studies that have investigated the structure, measurement and treatment of binocular vision in observers with strabismic, anisometropic and mixed amblyopia.

Effect of luminance noise on the object frequencies mediating letter identification

Purpose: To determine if the same object frequency information mediates letter contrast threshold in the presence and absence of additive luminance noise (i.e., “noise-invariant processing”) for letters of different size.

Methods: Contrast thresholds for Sloan letters ranging in size from 0.9 to 1.8 log MAR were obtained from three visually normal observers under three paradigms: (1) high- and low-pass Gaussian filtered letters were presented against a uniform adapting field; (2) high- and low-pass Gaussian filtered letters were presented in additive white luminance noise; and (3) unfiltered letters were presented in high- and low-pass Gaussian filtered luminance noise. A range of high- and low-pass filter cutoffs were used to limit selectively the object frequency content of the letters (paradigms 1 and 2) or noise (paradigm 3). The object frequencies mediating letter identification under each paradigm were derived from plots of log contrast threshold vs. log filter cutoff frequency.

Results: The object frequency band mediating letter identification systematically shifted to higher frequencies with increasing log MAR letter size under all three paradigms. However, the relationship between object frequency and letter size depended on the paradigm under which the measurements were obtained. The largest difference in object frequency among the paradigms was observed at 1.8 log MAR, where the addition of white noise nearly doubled the center frequency of the band of object frequencies mediating letter identification, compared to measurements made in the absence of noise.

Conclusion: Noise can affect the object frequency band mediating letter contrast threshold, particularly for large letters, an effect that is likely due to strong masking of the low frequency letter components by low frequency noise checks. This finding indicates that noise-invariant processing cannot necessarily be assumed for large letters presented in white noise.

Broadband noise masks suppress neural responses to narrowband stimuli

White pixel noise is widely used to estimate the level of internal noise in a system by injecting external variance into the detecting mechanism. Recent work (Baker and Meese, 2012) has provided psychophysical evidence that such noise masks might also cause suppression that could invalidate estimates of internal noise. Here we measure neural population responses directly, using steady-state visual evoked potentials, elicited by target stimuli embedded in different mask types. Sinusoidal target gratings of 1 c/deg flickered at 5 Hz, and were shown in isolation, or with superimposed orthogonal grating masks or 2D white noise masks, flickering at 7 Hz. Compared with responses to a blank screen, the Fourier amplitude at the target frequency increased monotonically as a function of target contrast when no mask was present. Both orthogonal and white noise masks caused rightward shifts of the contrast response function, providing evidence of contrast gain control suppression. We also calculated within-observer amplitude variance across trials. This increased in proportion to the target response, implying signal-dependent (i.e., multiplicative) noise at the system level, the implications of which we discuss for behavioral tasks. This measure of variance was reduced by both mask types, consistent with the changes in mean target response. An alternative variety of noise, which we term zero-dimensional noise, involves trial-by-trial jittering of the target contrast. This type of noise produced no gain control suppression, and increased the amplitude variance across trials.

Development of sampling efficiency and internal noise in motion detection and discrimination in school-aged children

The aim of this study was to use an equivalent noise paradigm to investigate the development and maturation of motion perception, and how the underlying limitations of sampling efficiency and internal noise effect motion detection and direction discrimination in school-aged children (5-14 years) and adults. Contrast energy thresholds of a 2c/deg sinusoidal grating drifting at 1.0 or 6.0 Hz were measured as a function of added dynamic noise in three tasks: detection of a drifting grating; detection of the sum of two oppositely drifting gratings and direction discrimination of oppositely drifting gratings. Compared to the ideal observer, in both children and adults, the performance for all tasks was limited by reduced sampling efficiency and internal noise. However, the thresholds for discrimination of motion direction and detection of moving gratings show very different developmental profiles. Motion direction discrimination continues to improve after the age of 14 years due to an increase in sampling efficiency that differs with speed. Motion detection and summation were already mature at the age of 5 years, and internal noise was the same for all tasks. These findings were confirmed in a 1-year follow-up study on a group of children from the initial study. The results support suggestions that the detection of a moving pattern and discriminating motion direction are processed by different systems that may develop at different rates.

What is the primary cause of individual differences in contrast sensitivity?

One of the primary objectives of early visual processing is the detection of luminance variations, often termed image contrast. Normal observers can differ in this ability by at least a factor of 4, yet this variation is typically overlooked, and has never been convincingly explained. This study uses two techniques to investigate the main source of individual variations in contrast sensitivity. First, a noise masking experiment assessed whether differences were due to the observer's internal noise, or the efficiency with which they extracted information from the stimulus. Second, contrast discrimination functions from 18 previous studies were compared (pairwise, within studies) using a computational model to determine whether differences were due to internal noise or the low level gain properties of contrast transduction. Taken together, the evidence points to differences in contrast gain as being responsible for the majority of individual variation across the normal population. This result is compared with related findings in attention and amblyopia.