Neural markers of suppression in impaired binocular vision

Measuring contrast processing in the visual system using the steady state visually evoked potential (SSVEP)

Contrast is the currency of the early visual system. Measuring the way that the computations underlying contrast processing depend on factors such as spatial and temporal frequency, age, clinical conditions, eccentricity, chromaticity and the presence of other stimuli has been a focus of vision science for over a century. One of the most productive experimental approaches in this field has been the use of the 'steady-state visually-evoked potential' (SSVEP): a technique where contrast modulating inputs are 'frequency tagged' (presented at well-defined frequencies and phases) and the electrical signals that they generate in the brain are analyzed in the temporal frequency domain. SSVEPs have several advantages over conventional measures of visually-evoked responses: they have relatively unambiguous ouput measures, a high signal to noise ratio (SNR), and they allow us to analyze interactions between stimulus components using a convenient mathematical framework. Here we describe how SSVEPs have been used to study visual contrast over the past 70 years. Because our thinking about SSVEPs is well-described by simple mathematical models, we embed code that illustrates key steps in the modelling and analysis. This paper can therefore be used both as a review of the use of SSVEP in measuring human contrast processing, and as an interactive learning aid.

Association between suppression scotoma and visual acuity in clinical subgroups of amblyopia: a systematic review protocol

OBJECTIVE

This review will examine the association between suppression scotoma and visual acuity in clinical subgroups of amblyopia in humans to determine whether distinct causal factors of amblyopia lead to different suppression characteristics and their association with visual acuity.

INTRODUCTION

Amblyopia is a neurodevelopmental disorder that impairs visual acuity, stereo acuity, and contrast sensitivity in 1 or both eyes due to physiological alterations in the visual pathways. Numerous studies have examined the association between amblyopia and suppression. However, the relationship between suppression size, depth, and visual acuity in amblyopia remains unclear.

INCLUSION CRITERIA

This review will include randomized controlled trials, cohort studies, case-control studies, and cross-sectional studies, with participants of any age or gender, diagnosed with amblyopia and with outcome measures of visual acuity (in LogMAR or Snellen's fraction), suppression size (degree of visual angle), and suppression depth (contrast difference in dichoptic targets).

METHODS

This review will follow the JBI methodology for systematic reviews of etiology and risk. Academic and non-academic databases and gray literature will be searched, including MEDLINE (Ovid), Embase (Ovid), Cochrane Central Register of Controlled Trials (CENTRAL), Web of Science Core Collection, ProQuest Health and Medical Collection, and Google Scholar. Titles and abstracts, and later, full texts of included studies will be screened by 2 independent reviewers. Critical appraisal will be conducted using JBI checklists. Where possible, the studies will be pooled using random effects meta-analysis. If statistical pooling is not possible, the findings will be presented in narrative format, including tables and figures. Certainty of the evidence will be assessed using the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) approach.

REVIEW REGISTRATION

PROSPERO CRD42023479043.

Intermodulation frequency components in steady-state visual evoked potentials: Generation, characteristics and applications

The steady-state visual evoked potentials (SSVEPs), evoked by dual-frequency or multi-frequency stimulation, likely contains intermodulation frequency components (IMs). Visual IMs are products of nonlinear integration of neural signals and can be evoked by various paradigms that induce neural interaction. IMs have demonstrated many interesting and important characteristics in cognitive psychology, clinical neuroscience, brain-computer interface and other fields, and possess substantial research potential. In this paper, we first review the definition of IMs and summarize the stimulation paradigms capable of inducing them, along with the possible neural origins of IMs. Subsequently, we describe the characteristics and derived applications of IMs in previous studies, and then introduced three signal processing methods favored by researchers to enhance the signal-to-noise ratio of IMs. Finally, we summarize the characteristics of IMs, and propose several potential future research directions related to IMs.

Binocular imbalance measured by SSVEP predicts impaired stereoacuity in amblyopia

Purpose

The current study aims to implement steady-state visual evoked potentials (SSVEPs) in quantifying the binocular imbalance of amblyopia and to assess the predictive value of SSVEP-derived indices for amblyopic stereoacuity.

Methods

We measure frequency-tagged SSVEP responses elicited by each eye (F1 = 6 Hz through the fellow eye; F2 = 7.5 Hz through the amblyopic eye) within a binocular rivalry paradigm among a cohort of anisometropic amblyopic observers (n = 29, mean age: 12 years). Binocular suppression was quantified by assessing the disparity in SSVEP amplitudes between the eyes, while the strength of interocular interaction was evaluated through the intermodulation response at F1+F2 = 13.5 Hz. Subsequent analyses explored the associations between these neural indices and relevant behavioral metrics in amblyopia.

Results

Results reveal a significant difference in SSVEP amplitudes elicited from the fellow eye and the amblyopic eye, with the former exhibiting notably higher responses. Moreover, the fellow eye demonstrated prolonged dominance duration compared to its amblyopic counterpart. Furthermore, a negative correlation between binocular suppression and interocular interaction was observed, with stereoacuity showing a significant correlation with binocular suppression. Utilizing stepwise mulptiple linear regression analysis, we established that a predictive model combining binocular suppression and visual acuity of the amblyopic eye provided the best prediction of stereoacuity.

Conclusions

These results highlight the potential of binocular suppression, as assessed by SSVEPs within a binocular rivalry paradigm, as a promising neural predictor of stereopsis in amblyopia.

The relationship between visual acuity loss and GABAergic inhibition in amblyopia

Early childhood experience alters visual development, a process exemplified by amblyopia, a common neurodevelopmental condition resulting in cortically reduced vision in one eye. Visual deficits in amblyopia may be a consequence of abnormal suppressive interactions in the primary visual cortex by inhibitory neurotransmitter γ-aminobutyric acid (GABA). We examined the relationship between visual acuity loss and GABA+ in adult human participants with amblyopia. Single-voxel proton magnetic resonance spectroscopy (MRS) data were collected from the early visual cortex (EVC) and posterior cingulate cortex (control region) of 28 male and female adults with current or past amblyopia while they viewed flashing checkerboards monocularly, binocularly, or while they had their eyes closed. First, we compared GABA+ concentrations between conditions to evaluate suppressive binocular interactions. Then, we correlated the degree of visual acuity loss with GABA+ levels to test whether GABAergic inhibition could explain visual acuity deficits. Visual cortex GABA+ was not modulated by viewing condition, and we found weak evidence for a negative correlation between visual acuity deficits and GABA+. These findings suggest that reduced vision in one eye due to amblyopia is not strongly linked to GABAergic inhibition in the visual cortex. We advanced our understanding of early experience dependent plasticity in the human brain by testing the association between visual acuity deficits and visual cortex GABA in amblyopes of the most common subtypes. Our study shows that the relationship was not as clear as expected and provides avenues for future investigation.

The execution of saccadic eye movements suppresses visual processing of both color and luminance in the early visual cortex of humans

Our eyes execute rapid, directional movements known as saccades, occurring several times per second, to focus on objects of interest in our environment. During these movements, visual sensitivity is temporarily reduced. Despite numerous studies on this topic, the underlying mechanism remains elusive, including a lingering debate on whether saccadic suppression affects the parvocellular visual pathway. To address this issue, we conducted a study employing steady-state visual evoked potentials (SSVEPs) elicited by chromatic and luminance stimuli while observers performed saccadic eye movements. We also employed an innovative analysis pipeline to enhance the signal-to-noise ratio, yielding superior results compared to the previous method. Our findings revealed a clear suppression effect on SSVEP signals during saccades compared to fixation periods. Notably, this suppression effect was comparable for both chromatic and luminance stimuli. We went further to measure the suppression effect across various contrast levels, which enabled us to model SSVEP responses with contrast response functions. The results suggest that saccades primarily reduce response gain without significantly affecting contrast gain and that this reduction applies uniformly to both chromatic and luminance pathways. In summary, our study provides robust evidence that saccades similarly suppress visual processing in both the parvocellular and magnocellular pathways within the human early visual cortex, as indicated by SSVEP responses. The observation that saccadic eye movements impact response gain rather than contrast gain implies that they influence visual processing through a multiplicative mechanism.NEW & NOTEWORTHY The present study demonstrates that saccadic eye movements reduce the processing of both luminance and chromatic stimuli in the early visual cortex of humans. By modeling the contrast response function, the study further shows that saccades affect visual processing by reducing the response gain rather than altering the contrast gain, suggesting that a multiplicative mechanism of visual attenuation affects both parvocellular and magnocellular pathways.

Peripheral Binocular Imbalance in Anisometropic and Strabismic Amblyopia

Purpose

Individuals with amblyopia experience central vision deficits, including loss of visual acuity, binocular vision, and stereopsis. In this study, we examine the differences in peripheral binocular imbalance in children with anisometropic amblyopia, strabismic amblyopia, and typical binocular vision to determine if there are systematic patterns of deficits across the visual field.

Methods

This prospective cohort study recruited 12 participants with anisometropic amblyopia, 10 with strabismic amblyopia, and 10 typically sighted controls (age range, 5-18 years). Binocular imbalance was tested at 0°, 4°, and 8° eccentricities (4 angular locations each) using band-pass filtered Auckland optotypes (5 cycles per optotype) dichoptically presented with differing contrast to each eye. The interocular contrast ratio was adjusted until the participant reported each optotype with equal frequency.

Results

Participants with anisometropic and strabismic amblyopia had a more balanced contrast ratio, or decreased binocular imbalance, at 4° and 8° eccentricities as compared with central vision. Participants with strabismic amblyopia had significantly more binocular imbalance in the periphery as compared with individuals with anisometropic amblyopia or controls. A linear mixed effects model showed a main effect for strabismic amblyopia and eccentricity on binocular imbalance across the visual field.

Conclusions

There is evidence of decreased binocularity deficits, or interocular suppression, in the periphery in anisometropic and strabismic amblyopia as compared with controls. Notably, those with strabismic amblyopia exhibited more significant peripheral binocular imbalance. These variations in binocularity across the visual field among different amblyopia subtypes may necessitate tailored approaches for dichoptic treatment.

Reduced interocular suppression after inverse patching in anisometropic amblyopia

Purpose

Recent investigations observed substantial enhancements in binocular balance, visual acuity, and stereovision among older children and adults with amblyopia by patching the amblyopic eye (i.e., inverse patching) for 2 h daily over 2 months. Despite these promising findings, the precise neural mechanisms underlying inverse patching remain elusive. This study endeavors to delve deeper into the neural alterations induced by inverse patching, focusing on steady-state visual evoked potentials (SSVEPs). We specifically investigate the changes in SSVEPs following monocular deprivation of either the fellow eye or the amblyopic eye in older amblyopic children and adults.

Method

Ten participants (17.60 ± 2.03 years old; mean ± SEM), clinically diagnosed with anisometropic amblyopia, were recruited for this study. Each participant underwent a 120 min patching session on their fellow eye on the first day, followed by a similar session on their amblyopic eye on the second day. Baseline steady-state visual evoked potentials (SSVEPs) measurements were collected each day prior to patching, with post-patching SSVEPs measurements obtained immediately after the patching session. The experimental design incorporated a binocular rivalry paradigm, utilizing SSVEPs measurements.

Results

The results revealed that inverse patching induced a heightened influence on neural plasticity, manifesting in a reduction of interocular suppression from the fellow eye to the amblyopic eye. In contrast, patching the fellow eye demonstrated negligible effects on the visual cortex. Furthermore, alterations in interocular suppression subsequent to inverse patching exhibited a correlation with the visual acuity of the amblyopic eye.

Conclusion

Inverse patching emerges as a promising therapeutic avenue for adolescents and adults grappling with severe anisometropic amblyopia that proves refractory to conventional interventions. This innovative approach exhibits the potential to induce more robust neural plasticity within the visual cortex, thereby modulating neural interactions more effectively than traditional amblyopia treatments.

Digital Therapy for Visual Acuity and Binocular Function in Children with Anisometropic Amblyopia

Amblyopia affects development of children's monocular vision and binocular function and becomes a largely intractable problem with increasing aging. This study is to investigate the binocular function and evaluate efficacy of digital therapy in children 8-13 years of age with anisometropic amblyopia. The patients in the digital therapy group performed the training with the digital amblyopia therapeutic software. The visual acuity and binocular function (perceptual eye position [PEP], suppression, and stereopsis) were examined at the first visit and 3-month post-treatment. Twenty-three cases in the control group and 25 cases in the digital therapy group were enrolled. The results revealed that 3-month digital therapy can effectively improve corrected distance visual acuity (CDVA) and improve the binocular function, including PEP, suppression, and second-order stereopsis in children with anisometropic amblyopia, 8-13 years of age. Digital therapy for amblyopia can effectively improve monocular CDVA of amblyopic eyes and binocular function in older children with anisometropic amblyopia.

Dissociable Neural Mechanisms Underlie the Effects of Attention on Visual Appearance and Response Bias

A prominent theoretical framework spanning philosophy, psychology, and neuroscience holds that selective attention penetrates early stages of perceptual processing to alter the subjective visual experience of behaviorally relevant stimuli. For example, searching for a red apple at the grocery store might make the relevant color appear brighter and more saturated compared with seeing the exact same red apple while searching for a yellow banana. In contrast, recent proposals argue that data supporting attention-related changes in appearance reflect decision- and motor-level response biases without concurrent changes in perceptual experience. Here, we tested these accounts by evaluating attentional modulations of EEG responses recorded from male and female human subjects while they compared the perceived contrast of attended and unattended visual stimuli rendered at different levels of physical contrast. We found that attention enhanced the amplitude of the P1 component, an early evoked potential measured over visual cortex. A linking model based on signal detection theory suggests that response gain modulations of the P1 component track attention-induced changes in perceived contrast as measured with behavior. In contrast, attentional cues induced changes in the baseline amplitude of posterior alpha band oscillations (∼9-12 Hz), an effect that best accounts for cue-induced response biases, particularly when no stimuli are presented or when competing stimuli are similar and decisional uncertainty is high. The observation of dissociable neural markers that are linked to changes in subjective appearance and response bias supports a more unified theoretical account and demonstrates an approach to isolate subjective aspects of selective information processing.SIGNIFICANCE STATEMENT Does attention alter visual appearance, or does it simply induce response bias? In the present study, we examined these competing accounts using EEG and linking models based on signal detection theory. We found that response gain modulations of the visually evoked P1 component best accounted for attention-induced changes in visual appearance. In contrast, cue-induced baseline shifts in alpha band activity better explained response biases. Together, these results suggest that attention concurrently impacts visual appearance and response bias, and that these processes can be experimentally isolated.

Laplacian reference is optimal for steady-state visual-evoked potentials

Steady-state visual-evoked potentials (SSVEPs) are widely used in human neuroscience studies and applications such as brain-computer interfaces (BCIs). Surprisingly, no previous study has systematically evaluated different reference methods for SSVEP analysis, despite that signal reference is crucial for the proper assessment of neural activities. In the present study, using four datasets from our previous SSVEP studies (Chen J, Valsecchi M, Gegenfurtner KR. J Neurophysiol 118: 749-754, 2017; Chen J, Valsecchi M, Gegenfurtner KR. Neuropsychologia 102: 206-216, 2017; Chen J, McManus M, Valsecchi M, Harris LR, Gegenfurtner KR. J Vis 19: 8, 2019) and three public datasets from other studies (Baker DH, Vilidaite G, Wade AR. PLoS Comput Biol 17: e1009507, 2021; Lygo FA, Richard B, Wade AR, Morland AB, Baker DH. NeuroImage 230: 117780, 2021; Vilidaite G, Norcia AM, West RJH, Elliott CJH, Pei F, Wade AR, Baker DH. Proc R Soc B 285: 20182255, 2018), we compared four reference methods: monopolar reference, common average reference, averaged-mastoids reference, and Laplacian reference. The quality of the resulting SSVEP signals was compared in terms of both signal-to-noise ratios (SNRs) and reliability. The results showed that Laplacian reference, which uses signals at the maximally activated electrode after subtracting the average of the nearby electrodes to reduce common noise, gave rise to the highest SNRs. Furthermore, the Laplacian reference resulted in SSVEP signals that were highly reliable across recording sessions or trials. These results suggest that Laplacian reference is optimal for SSVEP studies and applications. Laplacian reference is especially advantageous for SSVEP experiments where short preparation time is preferred as it requires only data from the maximally activated electrode and a few surrounding electrodes.NEW & NOTEWORTHY The present study provides a comprehensive evaluation of the use of different reference methods for steady-state visual-evoked potentials (SSVEPs) and has found that Laplacian reference increases signal-to-noise ratios (SNRs) and enhances reliabilities of SSVEP signals. Thus, the results suggest that Laplacian reference is optimal for SSVEP analysis.

Quantitative interocular suppression in children with intermittent exotropia

Purpose

We have demonstrated that the depth of unbalanced interocular suppression can be quantified by balancing the interocular luminance differences required when both eyes are viewing simultaneously. In this study, we aimed to investigate the applicability of this method in children with intermittent exotropia (IXT), offering a quantitative assessment of interocular suppression in individuals with binocular imbalance. Additionally, we evaluated its association with the clinical characteristics of IXT.

Methods

Interocular suppression in IXT was quantitatively measured using a polarizer and neutral-density (ND) filters. The density of the ND filter was adjusted incrementally from 0.3ND to 3ND, with a step size of 0.3ND (a total of 10 levels). Our prospective study involved 46 patients with IXT (mean age: 10.12 ± 4.89 years; mean ± SD) and 24 normal observers (mean age: 7.88 ± 1.83 years).

Results

The suppression test exhibited good test-retest reliability, supported by statistical analysis. We observed more pronounced interocular suppression in individuals with IXT compared to controls. Notably, the magnitude of suppression during distant and near viewing significantly differed in IXT (1.55 ± 0.93 vs. 0.57 ± 0.64; Z = 4.764, p < 0.001). Furthermore, we identified a positive correlation between interocular suppression and data obtained from the Worth-4-Dot test. Additionally, interocular suppression showed a significant association with distance control scores.

Conclusion

Our novel test offers a convenient and reliable means to quantify interocular suppression in patients with IXT. The quantitative assessment of interocular suppression provides a sensitive tool to evaluate the clinical characteristics of IXT.

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.

Evaluation of the Efficacy of a New Dichoptic Digital Platform to Treat the Anisometropic and Isometropic Amblyopia

The aim of the current study was to evaluate the results of a novel dichoptic training program using an online platform in a group of subjects with refractive amblyopia, performing a comparative analysis of unilateral and bilateral amblyopic cases. For this purpose, a retrospective study analysis of data of 161 children (4−13 years) who underwent dichoptic treatment with the Bynocs® platform (Kanohi Eye Pvt. Ltd., Mumbai, India) was performed. In all cases, the therapy protocol consisted of sessions of training of 30 min daily 5 times a week for 6 weeks. Best corrected visual acuity (BCVA) in the non-dominant eye improved significantly with the treatment, with a mean change of 0.39 logMAR in the whole sample (p < 0.001). Regarding binocularity, the binocular function (BF) score also experienced a significant improvement (p < 0.001), with a mean change of 1.55 with therapy in the whole sample. The BCVA of the dominant eye only improved significantly (p < 0.001) in the isometropic amblyopic subgroup. In conclusion, the use of the dichoptic therapy with the digital platform evaluated allows an effective restoration of visual acuity and binocular function in children with anisometropic and isometropic amblyopia.

A Dichoptic Optokinetic Nystagmus Paradigm for Interocular Suppression Quantification in Intermittent Exotropia

Purposes: To investigate the effectiveness of a dichoptic optokinetic nystagmus (dOKN) test to objectively quantify interocular suppression in intermittent exotropia (IXT) patients during the states of orthotropia and exodeviation.

Methods: The OKN motion in subjects (15 controls and 59 IXT subjects) who viewed dichoptic oppositely moving gratings with different contrast ratios was monitored and recorded by an eye tracker. Interocular suppression in control subjects was induced using neutral density (ND) filters. The OKN direction ratios were fitted to examine the changes of interocular suppression in subjects under different viewing states. Two established interocular suppression tests (phase and motion) were conducted for a comparative study.

Results: The dOKN test, which requires a minimal response from subjects, could accurately quantify the interocular suppression in both IXT and control subjects, which is in line with the established interocular suppression tests. Overall, although comparative, the strength of interocular suppression detected by the dOKN test (0.171 ± 0.088) was stronger than those of the phase (0.293 ± 0.081) and the motion tests (0.212 ± 0.068) in the control subjects with 1.5 ND filters. In IXT patients, when their eyes kept aligned, the dOKN test (0.58 ± 0.09) measured deeper visual suppression compared with the phase (0.73 ± 0.17) or the motion test (0.65 ± 0.14). Interestingly, strong interocular suppression (dOKN: 0.15 ± 0.12) was observed in IXT subjects during the periods of exodeviation, irrespective of their binocular visual function as measured by synoptophore.

Conclusion: The dOKN test provides efficient and objective quantification of interocular suppression in IXT, and demonstrates how it fluctuates under different eye positions.