Binocular summation revisited: Beyond √2

Surround masking reveals binocular adding and differencing channels

Recent studies suggest that binocular adding S+ and differencing S- channels play an important role in binocular vision. To test for such a role in the context of binocular contrast detection and binocular summation, we employed a surround masking paradigm consisting of a central target disk surrounded by a mask annulus. All stimuli were horizontally oriented 0.5c/d sinusoidal gratings. Correlated stimuli were identical in interocular spatial phase while anticorrelated stimuli were opposite in interocular spatial phase. There were four target conditions: monocular left eye, monocular right eye, binocular correlated and binocular anticorrelated, and three surround mask conditions: no surround, binocularly correlated and binocularly anticorrelated. We observed consistent elevation of detection thresholds for monocular and binocular targets across the two binocular surround mask conditions. In addition, we found an interaction between the type of surround and the type of binocular target: both detection and summation were relatively enhanced by surround masks and targets with opposite interocular phase relationships and reduced by surround masks and targets with the same interocular phase relationships. The data were reasonably well accounted for by a model of binocular combination termed MAX (S+S-), in which the decision variable is the probability summation of modeled S+ and S- channel responses, with a free parameter determining the relative gains of the two channels. Our results support the existence of two channels involved in binocular combination, S+ and S-, whose relative gains are adjustable by surround context.

Multisensory-inspired modeling and neural correlates for two key binocular interactions

Most binocular vision models assume that the two eyes sum incompletely. However, some facilitatory cortical neurons fire for only one eye, but amplify their firing rates if both eyes are stimulated. These 'binocular gate' neurons closely resemble subthreshold multisensory neurons. Binocular amplification for binocular gate neurons follows a power law, with a compressive exponent. Unexpectedly, this rule also applies to facilitatory true binocular neurons; although driven by either eye, binocular neurons are well modeled as gated amplifiers of their strongest monocular response, if both eyes are stimulated. Psychophysical data follows the same power law as the neural data, with a similar exponent; binocular contrast sensitivity can be modeled as a gated amplification of the more sensitive eye. These results resemble gated amplification phenomena in multisensory integration, and other non-driving modulatory interactions that affect sensory processing. Models of incomplete summation seem unnecessary for V1 facilitatory neurons or contrast sensitivity. However, binocular combination of clearly visible monocular stimuli follows Schrödinger's nonlinear magnitude-weighted average. We find that putatively suppressive binocular neurons closely follow Schrödinger's equation. Similar suppressive multisensory neurons are well documented but seldom studied. Facilitatory binocular neurons and mildly suppressive binocular neurons are likely neural correlates of binocular sensitivity and binocular appearance respectively.

Scotopic and Photopic Conventional Visual Acuity and Hyperacuity - Binocular Summation

Purpose

The purpose of this study was to determine and compare binocular summation (BiS) of conventional visual acuity (cVA) versus hyperacuity (hVA) for photopic and scotopic luminance conditions as a potential biomarker to assess the outcome of interventions on binocular function.

Methods

Sixteen young adults (age range [years] = 21-31; 8 women; cVA logMAR < 0.0) participated in this study. The Freiburg Visual Acuity Test (FrACT) was used for VA testing and retested on another day. Both cVA and hVA were determined for dark grey optotypes on light grey background. Participants underwent 40 minutes of dark adaptation prior to scotopic VA testing. Binocular and monocular VA testing was performed. The eye with better VA over the 2 days of testing was selected, the BiS was quantified (binocular VA - better monocular VA) and repeated measures ANOVAs were performed.

Results

Binocular VA exceeded monocular VA for all luminance conditions, VA-types, and sessions. We report BiS estimates for photopic and scotopic cVA and hVA, (logMAR BiS ± SEM [decimal BiS]): photopic = -0.01 ± 0.01 [1.03] and -0.06 ± 0.03 [1.15]; and scotopic = -0.05 ± 0.01 [1.12] and -0.11 ± 0.04 [1.28], respectively). Improvement for binocular vision estimates ranged from 0.01 to 0.11 logMAR. A repeated-measures ANOVA (RM ANOVA) did not reveal significant effects of LUMINANCE or VA TYPE on BiS, albeit a trend for strongest BiS for scotopic hVA (15% vs. 28%, photopic versus scotopic, respectively) and weakest for photopic cVA (3% vs. 12%, photopic versus scotopic conditions, respectively).

Conclusions

Our results indicate that BiS of VA is relevant to scotopic and photopic hVA and cVA. It appears therefore a plausible candidate biomarker to assess the outcome of retinal therapies restoring rod or cone function on binocular vision.

Translational Relevance

Binocular summation of visual acuity might serve as a clinical biomarker to monitor therapy outcome on binocular rod and cone-mediated vision.

Eight weeks of exercise intervention improves visuomotor and functional capacity, performance, and physiological profile in a patient with choroidal melanoma

The aim of this case study was to investigate the effects of an 8-wk combined exercise intervention, consisting of visual-coordinative and high-intensity interval training (HIIT), on the physical and visuomotor-functional capacity, performance, and physiological profile of a moderately active 29-yr-old man diagnosed with choroidal melanoma of the left eye. Data were collected on three occasions: at the initial diagnosis (T0), after hospitalization and radiotherapy treatment (T1), and following the recovery through the exercise intervention (T2), spanning a total of 17 wk. The primary outcome variables consisted of visuomotor and functional tests (VFTs), cardiorespiratory fitness (CRF), and microvascular circulation measured via flicker light-induced dilation (FiD). For visuomotor tests in general, a significant decline was observed between baseline T0 and T1 (by 6%-22%), followed by significant improvements at T2 (by 11%-36%), surpassing the initially observed T0 values. The cardiopulmonary exercise testing (CPET)-derived parameters exhibited a similar pattern, declining from T0 to T1 [by 8%-12% for peak V̇o2, peak power output (PO), and CPET duration, respectively], with a subsequent recovery observed in response to 8 wk of exercise training (T2), resulting in increases of 11%-25% for V̇o2, peak PO, and CPET duration. Interestingly, the dilation of both arteries and veins in response to the FiD stimulus exhibited a twofold increase compared with baseline levels. Our results suggest that the 8-wk exercise intervention improved patients' VFT and CRF profiles and exceeded baseline values. Additional investigation, particularly through randomized controlled trials, is needed to comprehensively explain changes in FiD.NEW & NOTEWORTHY Results presented here suggest that combined visual-coordinative and HIIT training improves the visual-functional capacity, performance, and physiological profile of choroidal melanoma patients during treatment recovery. This case study lays the groundwork for further research concerning exercise therapy in this unique patient population. In addition, further investigation is required to fully comprehend the combined effects of exercise and radiation therapy on vasculature and oxygenation in patients with choroidal melanoma.

Measuring the Interocular Delay and its Link to Visual Acuity in Amblyopia

Purpose

Research on interocular synchronicity in amblyopia has demonstrated a deficit in synchronization (i.e., a neural processing delay) between the two eyes. Current methods for assessing interocular delay are either costly or ineffective for assessments in severe amblyopia. In this study, we adapted a novel protocol developed by Burge and Cormack based on continuous target tracking to measure the interocular delay on a wide range of amblyopes. Our main aims were to assess the accessibility of this protocol and to investigate the relationship between interocular delay and visual acuity.

Methods

This protocol, which consists of tracking a target undergoing random lateral motion with the mouse cursor, is performed both binocularly and monocularly. The processing speed of a given eye is computed by comparing the changes in velocity of the target and mouse via cross-correlation. The difference in processing speed between the eyes defines the interocular delay.

Results

Cross-correlations revealed that the amblyopic eye tends to be delayed in time compared with the fellow eye. Interocular delays fell in the range of 0.6 to 114.0 ms. The magnitude of the delay was positively correlated with differences in interocular visual acuity (R2 = 0.484; P = 0.0002).

Conclusions

These results demonstrate the accessibility of this new protocol and further support the link between interocular synchronicity and amblyopia. Furthermore, we determine that the interocular delay in amblyopia is best explained by a deficit in the temporal integration of the amblyopic eye.

Binocular Summation With Quantitative Contrast Sensitivity Function: A Novel Parameter to Evaluate Binocular Function in Intermittent Exotropia

Purpose

Intermittent exotropia (IXT) is the most common form of strabismus. Surgery can potentially improve binocular function in patients with IXT. We aimed to evaluate binocular function using a novel parameter-binocular summation ratio (BSR), measured using quantitative contrast sensitivity function (CSF) in patients with IXT before and after surgery.

Methods

Prospective study of 63 patients with IXT and 41 healthy controls were consecutively enrolled and underwent quantitative CSF testing binocularly and monocularly. BSR was calculated by dividing the CSF of the binocular value by the better monocular value. Forty-eight patients with IXT underwent strabismus surgery. BSR, stereoacuity, fusion ability, and strabismus questionnaires were assessed pre-operatively and 2 months postoperatively.

Results

Sixty-three patients with IXT (median age = 9 years) compared with 41 healthy controls showed a worse mean BSR based on all CSF metrics at baseline (the area under the log CSF [AULCSF], spatial frequency [SF] cutoff, and contrast sensitivity at 1.0-18.0 cpd SF). All 48 patients with IXT showed successful alignment after surgery, and there were significant improvements in BSR based on the AULCSF, SF cutoff, and contrast sensitivity at 6.0, 12.0, and 18.0 cpd SF, respectively. The distance stereoacuity and fusion ability also improved after surgery, and a better BSR was associated with better stereoacuity and fusion. For strabismus questionnaires, the psychosocial subscale scores improved postoperatively, whereas the functional subscale scores did not change.

Conclusions

BSR based on quantitative CSF can characterize binocular function across a range of spatial frequencies and can be used as a supplemental measurement for monitoring binocularity in patients with IXT in clinical settings.

Acquisition of colour categories through learning: Differences between hue and lightness

Colour categories are acquired through learning, but the nature of this process is not fully understood. Some category distinctions are defined by hue (e.g. red/purple) but other by lightness (red/pink). The aim of this study was to investigate if the acquisition of key information for making accurate cross-boundary discriminations poses different challenges for hue-defined as opposed to lightness-defined boundaries. To answer this question, hue- and lightness-learners were trained on a novel category boundary within the GREEN region of colour space. After training, hue- and lightness-learners as well as untrained controls performed delayed same-different discrimination for lightness and hue pairs. In addition to discrimination data, errors during learning and category-labelling strategies were examined. Errors during learning distributed non-uniformly and in accordance with the Bezold-Brücke effect, which accounts for darker colours at the green-blue boundary appearing greener and lighter colours appearing bluer. Only hue-learners showed discrimination improvements due to category boundary acquisition. Thus, acquisition is more efficient for hue-category compared to lightness-category boundaries. Almost all learners reported using category-labelling strategies, with hue-learners almost exclusively using 'green'/'blue' and lightness learners using a wider range of labels, most often 'light'/'dark'. Thus, labels play an important role in colour category learning and such labelling does not conform to everyday naming: here, the label 'blue' is used for exemplars that would normally be named 'green'. In conclusion, labelling serves the purpose of highlighting key information that differentiates exemplars across the category boundary, and basic colour terms may be particularly effective in facilitating such attentional guidance.

Space, time, and dynamics of binocular interactions

Binocular summation (BS), defined as the superiority of binocular over monocular visual performance, shows that thresholds are about 40% (a factor of 1.4) better in binocular than in monocular viewing. However, it was reported that different amounts of BS exist in a range from 1.4 to 2 values because BS is affected by the spatiotemporal parameters of the stimulus. Lateral interactions can be defined as the neuron's ability to affect the neighboring neurons by either inhibiting or exciting their activity. We investigated the effect of the spatial and temporal domains on binocular interactions and BS under the lateral masking paradigm and how BS would be affected by lateral interactions via a lateral masking experiment. The two temporal alternative forced-choice (2TAFC) method was used. The stimuli consisted of a central vertically oriented Gabor target and high-contrast Gabor flankers positioned in two configurations (orthogonal or collinear) with target-flanker separations of either 2 or 3 wavelengths (λ), presented at 4 different presentation times (40, 80, 120, and 200 ms) using a different order of measurements across the different experiments. Opaque lenses were used to control the monocular and binocular vision. BS is absent at close distances (2λ), depending on the presentation time's order, for the collinear but not for the orthogonal configuration. However, BS exists at more distant flankers (collinear and orthogonal, 3λ). BS is not uniform (1.4); it depends on the stimulus condition, the presentation times, the order, and the method that was used to control the monocular and binocular vision.

Different rules for binocular combination of luminance flicker in cortical and subcortical pathways

How does the human brain combine information across the eyes? It has been known for many years that cortical normalization mechanisms implement 'ocularity invariance': equalizing neural responses to spatial patterns presented either monocularly or binocularly. Here, we used a novel combination of electrophysiology, psychophysics, pupillometry, and computational modeling to ask whether this invariance also holds for flickering luminance stimuli with no spatial contrast. We find dramatic violations of ocularity invariance for these stimuli, both in the cortex and also in the subcortical pathways that govern pupil diameter. Specifically, we find substantial binocular facilitation in both pathways with the effect being strongest in the cortex. Near-linear binocular additivity (instead of ocularity invariance) was also found using a perceptual luminance matching task. Ocularity invariance is, therefore, not a ubiquitous feature of visual processing, and the brain appears to repurpose a generic normalization algorithm for different visual functions by adjusting the amount of interocular suppression.

Characteristics and Related Parameters of Quick Contrast Sensitivity Function in Chinese Ametropia Children

PURPOSE

To investigate the characteristics of quick contrast sensitivity function (qCSF) and its related parameters in Chinese ametropia children.

METHODS

This case series study enrolled 106 eyes of 53 children (male/female=29/24, age: 9.04±2.06 years). Examinations included manifest refraction, axial length, corneal curvature, and monocular and binocular qCSF readings without refractive correction (area under log CSF [AULCSF], CSF acuity, and contrast sensitivity [CS] at 1.0 to 18.0 cpd). The subjects were divided into groups according to age and refractive parameters for analysis.

RESULTS

The mean spherical equivalent (SE), AULCSF, and CSF acuity of the test eyes were -0.94±1.53 D, 0.44±0.33, and 8.50±5.97 cpd, respectively. In the monocular qCSF comparison, the refraction sphere (RS) was the major factor correlated with qCSF readings (B=0.186, P =0.009 for AULCSF; B=0.543, P =0.019 for CSF acuity; generalized linear model). The three groups stratified by RS/SE (<-1.00D, -1.00D to 0D, and >0D) showed significant differences in CS at medium spatial frequencies (3.0 and 6.0 cpd; all P <0.05). In the low RS/SE group (within -1 to 0 D), the CS at 12.0 cpd was significantly lower than that in the hyperopia group (all P <0.05). Binocular qCSF readings were significantly correlated with those of the eyes with lower RS (all P <0.05).

CONCLUSION

RS and SE are the major contributing factors of qCSF without refractive correction in children. The CS at medium spatial frequencies decrease significantly as the RS/SE increase. In low myopia children, the CS at medium and high spatial frequencies are significantly decreased, providing practical value in visual function screening in children.

Perceptual learning based on a temporal stimulus enhances visual function in adult amblyopic subjects

Studies have shown that Perceptual Learning (PL) can lead to enhancement of spatial visual functions in amblyopic subjects. Here we aimed to determine whether a simple flickering stimulus can be utilized in PL to enhance temporal function performance and whether enhancement will transfer to spatial functions in amblyopic subjects. Six adult amblyopic and six normally sighted subjects underwent an evaluation of their performance of baseline psychophysics spatial functions (Visual acuity (VA), contrast sensitivity (CS), temporal functions (critical fusion frequency (CFF) test), as well as a static and flickering stereopsis test, and an electrophysiological evaluation (VEP). The subjects then underwent 5 training sessions (on average, a total of 150 min over 2.5 weeks), which included a task similar to the CFF test using the method of constant stimuli. After completing the training sessions, subjects repeated the initial performance evaluation tasks. All amblyopic subjects showed improved temporal visual performance (CFF) in the amblyopic eye (on average, 17%, p << 0.01) following temporal PL. Generalization to spatial, spatio-temporal, and binocular tasks was also found: VA increased by 0.12 logMAR (p = 0.004), CS in backward masking significantly increased (by up to 19%, p = 0.003), and flickering stereopsis increased by 85 arcsec (p = 0.048). These results were further electrophysiologically manifested by an increase in VEP amplitude (by 43%, p = 0.03), increased Signal-to-Noise ratio (SNR) (by 39%, p = 0.024) to levels not different from normally sighted subjects, along with an improvement in inter-ocular delay (by 5.8 ms, p = 0.003). In contrast, no significant effect of training was found in the normally sighted group. These results highlight the potential of PL based on a temporal stimulus to improve the temporal and spatial visual performance in amblyopes. Future work is needed to optimize this method for clinical applications.

The role of binocular vision in the control and development of visually guided upper limb movements

Vision provides a key sensory input for the performance of fine motor skills, which are fundamentally important to daily life activities, as well as skilled occupational and recreational performance. Binocular visual function is a crucial aspect of vision that requires the ability to combine inputs from both eyes into a unified percept. Summation and fusion are two aspects of binocular processing associated with performance advantages, including more efficient visuomotor control of upper limb movements. This paper uses the multiple processes model of limb control to explore how binocular viewing could facilitate the planning and execution of prehension movements in adults and typically developing children. Insight into the contribution of binocularity to visuomotor control also comes from examining motor performance in individuals with amblyopia, a condition characterized by reduced visual acuity and poor binocular function. Overall, research in this field has advanced our understanding of the role of binocular vision in the development and performance of visuomotor skills, the first step towards developing assessment tools and targeted rehabilitation for children with neurodevelopment disorders at risk of poor visuomotor outcomes. This article is part of a discussion meeting issue 'New approaches to 3D vision'.

Temporal synchronization elicits enhancement of binocular vision functions

Integration of information over the CNS is an important neural process that affects our ability to perceive and react to the environment. The visual system is required to continuously integrate information arriving from two different sources (the eyes) to create a coherent percept with high spatiotemporal precision. Although this neural integration of information is assumed to be critical for visual performance, it can be impaired under some pathological or developmental conditions. Here we took advantage of a unique developmental condition, amblyopia ("lazy eye"), which is characterized by an impaired temporal synchronization between the two eyes, to meticulously study the effect of synchronization on the integration of binocular visual information. We measured the eyes' asynchrony and compensated for it (with millisecond temporal resolution) by providing time-shifted stimuli to the eyes. We found that the re-synchronization of the ocular input elicited a significant improvement in visual functions, and binocular functions, such as binocular summation and stereopsis, were regained. This phenomenon was also evident in neurophysiological measures. Our results can shed light on other neural processing aspects and might also have translational relevance for the field of training, rehabilitation, and perceptual learning.

Mirror symmetry and aging: The role of stimulus figurality and attention to colour

Symmetry perception studies have generally used two stimulus types: figural and dot patterns. Here, we designed a novel figural stimulus-a wedge pattern-made of centrally aligned pseudorandomly positioned wedges. To study the effect of pattern figurality and colour on symmetry perception, we compared symmetry detection in multicoloured wedge patterns with nonfigural dot patterns in younger and older adults. Symmetry signal was either segregated or nonsegregated by colour, and the symmetry detection task was performed under two conditions: with or without colour-based attention. In the first experiment, we compared performance for colour-symmetric patterns that varied in the number of wedges (24 vs. 36) and number of colours (2 vs. 3) and found that symmetry detection was facilitated by attention to colour when symmetry and noise signals were segregated by colour. In the second experiment, we compared performance for wedge and dot patterns on a sample of younger and older participants. Effects of attention to colour in segregated stimuli were magnified for wedge compared with dot patterns, with older and younger adults showing different effects of attention to colour on performance. Older adults significantly underperformed on uncued wedge patterns compared with dot patterns, but their performance improved greatly through colour cueing, reaching performance levels similar to young participants. Thus, while confirming the age-related decline in symmetry detection, we found that this deficit could be alleviated in figural multicoloured patterns by attending to the colour that carries the symmetry signal.

Effect of monocular sensitivity on binocular summation of luminance-modulated flicker

Binocular summation of luminance contrast signals in the spatial domain has been investigated in many studies, but less attention has been paid to the analogous interactions in the temporal domain. The present study determined the impact of monocular sensitivity on the binocular detection of luminance-modulated flickering stimuli. Binocular summation ratios (BSRs) were determined in 13 visually-normal adults for a range of monocular flicker modulation thresholds (FMTs), generated by changing stimulus size (7'- 60') and luminance (mesopic and photopic). Monocular and binocular FMTs were measured at the point of regard and in each of the four quadrants at 5° eccentricity for each target size and luminance using the Flicker-Plus test. Monocular and binocular FMT's increased with decreasing target size for all retinal locations (p<0.001), and were overall larger for mesopic than for photopic condition (p<0.001). BSRs for mesopic (mean±SD: 1.50±0.21) and photopic (1.60±0.24) stimuli were greater than unity (p<0.001), with the latter showing larger estimates than former (p<0.001). BSRs showed no significant trend across target sizes for both luminance conditions (p>0.12). The results demonstrate that the visual system successfully summates inputs from the two eyes to enhance flicker detection, independent of their absolute monocular detection thresholds. These findings may serve as a predictive baseline for further experiments designed to determine how other stimulus properties and interocular differences in monocular thresholds may affect the binocular perception of flicker.

Binocular fusion disorders impair basic visual processing

In an era of increasing screen consumption, the requirement for binocular vision is demanding, leading to the emergence of syndromes such as the computer vision syndrome (CVS) or visual discomfort reported by virtual reality (VR) users. Heterophoria (phoria) is a latent eye misalignment (with a prevalence up to 35%) that appears in conditions that disrupt binocular vision and may affect the quality of binocular fusion. Collinear facilitation (CF), the mechanism for grouping contour elements, is a process that reveals lateral interactions by improving the visibility of a target by flankers placed collinearly. An abnormal pattern of CF has been observed in strabismic amblyopia. We hypothesize that phoria may affect CF in the horizontal meridian (HM) due to latent eye misalignment and its impact on binocular fusion. Fully corrected participants (phoria group and controls) completed a standard CF experiment for horizontal and vertical meridians during binocular and monocular viewing. Phoric observers exhibited (1) an asymmetry and an abnormal pattern of CF only for the HM, during both monocular and binocular viewing, (2) poor binocular summation between the monocular inputs, and (3) no binocular advantage of the CF. Phoria affects the CF in a way that is reminiscent of meridional amblyopia without being attributed to abnormal refraction. The abnormal pattern of CF in monocular viewing suggests that phoria could be a binocular developmental disorder that affects monocular spatial interactions. We suggest that the results could contribute to explain the visual discomfort experienced with VR users or symptoms when presenting CVS.

Interocular Transfer: The Dichoptic Flash-Lag Effect in Controls and Amblyopes

Purpose

The mammalian brain can take into account the neural delays in visual information transmission from the retina to the cortex when accurately localizing the instantaneous position of moving objects by motion extrapolation. In this study, we wanted to investigate whether such extrapolation mechanism operates in a comparable fashion between the eyes in normally sighted and amblyopic observers.

Methods

To measure interocular extrapolation, we adapted a dichoptic version of the flash-lag effect (FLE) paradigm, in which a flashed bar is perceived to lag behind a moving bar when their two positions are physically aligned. Twelve adult subjects with amblyopia and 12 healthy controls participated in the experiment. We measured the FLE magnitude of the subjects under binocular, monocular, and dichoptic conditions.

Results

In controls, the FLE magnitude of binocular condition was significantly smaller than that of monocular conditions (P ≤ 0.023), but there was no difference between monocular and dichoptic conditions. Subject with amblyopia exhibited a smaller FLE magnitude in the dichoptic condition when the moving bar was presented to the amblyopic eye and the flash to the fellow eye (DA condition) compared to the opposite way around (DF condition), consistent with a delay in the processing of the amblyopic eye (P = 0.041).

Conclusions

Our observations confirm that trajectory extrapolation mechanisms transfer between the eyes of normal observers. However, such transfer may be impaired in amblyopia. The smaller FLE magnitude in DA compared to DF in patients with amblyopia could be due to an interocular delay in the amblyopic visual system. The observation that normal controls present a smaller FLE in binocular conditions raises the question whether a larger FLE is or is not an indicator of better motion processing and extrapolation.

Impact of Degraded Optics on Monocular and Binocular Vision: Lessons from Recent Advances in Highly-Aberrated Eyes

PURPOSE

Optical imperfections of the eye, characterized by higher-order wavefront aberrations, are exaggerated in corneal disease (e.g., keratoconus) and iatrogeny (e.g., keratorefractive surgery for myopia correction, keratoplasty for optical clarity restoration). This article reviews the recent advances on this topic for a comprehensive understanding of how optical degradations in disease models impact retinal image quality and monocular and binocular visual performance.

METHODS

Published literature over the last decade on retinal image quality and/or monocular and binocular visual functions with corneal irregularity was reviewed based on their relevance to the current topic, study population and strength of study design. The literature was summarized into four themes: 1) wavefront errors and retinal image quality of highly aberrated eyes, 2) monocular and binocular vision loss consequent to degraded optics and visual strategies to optimize performance, 3) impact of optical correction modalities on visual performance and 4) implications for clinical management of patients.

RESULTS

Across the 46 articles reviewed, the results clearly indicated that an increase in higher-order aberrations across these conditions had a significant negative impact on the patient's retinal image quality, and monocular and binocular visual functions. Interocular differences in retinal image quality deteriorated visual performance more than an overall worsening of image quality bilaterally. Minimizing optical degradation using rigid contact lenses and adaptive optics technology significantly improves retinal image quality and monocular and binocular vision, but performance remains sub-optimal relative to age-similar healthy controls.

CONCLUSION

Corneal disease and iatrogeny are useful models to understand the impact of optical degradation on retinal image quality and visual performance. Clinical management will greatly benefit from equalizing retinal image quality of both eyes of these patients. Future studies that deepen our understanding of the structure-function relation in these conditions are desirable for advancing vision science in this area and for developing novel clinical management strategies.

Binocular Integration in the Primate Primary Visual Cortex

binocular vision, binocular fusion, binocular combination, LGN, V1.

Stimulating both eyes with matching stimuli enhances V1 responses

Neurons in the primary visual cortex (V1) of primates play a key role in combining monocular inputs to form a binocular response. Although much has been gleaned from studying how V1 responds to discrepant (dichoptic) images, equally important is to understand how V1 responds to concordant (dioptic) images in the two eyes. Here, we investigated the extent to which concordant, balanced, zero-disparity binocular stimulation modifies V1 responses to varying stimulus contrast using intracranial multielectrode arrays. On average, binocular stimuli evoked stronger V1 activity than their monocular counterparts. This binocular facilitation scaled most proportionately with contrast during the initial transient. As V1 responses evolved, additional contrast-mediated dynamics emerged. Specifically, responses exhibited longer maintenance of facilitation for lower contrast and binocular suppression at high contrast. These results suggest that V1 processes concordant stimulation of both eyes in at least two sequential steps: initial response enhancement followed by contrast-dependent control of excitation.

Luminance Contrast Drives Interactions between Perception and Working Memory

Visual working memory (WM) enables the use of past sensory experience in guiding behavior. Yet, laboratory tasks commonly evaluate WM in a way that separates it from its sensory bottleneck. To understand how perception interacts with visual memory, we used a delayed shape recognition task to probe how WM may differ for stimuli that bias processing toward different visual pathways. Luminance compared with chromatic signals are more efficient in driving the processing of shapes and may thus also lead to better WM encoding, maintenance, and memory recognition. To evaluate this prediction, we conducted two experiments. In the first psychophysical experiment, we measured contrast thresholds for different WM loads. Luminance contrast was encoded into WM more efficiently than chromatic contrast, even when both sets of stimuli were equated for discriminability. In the second experiment, which also equated stimuli for discriminability, early sensory responses in the EEG that are specific to luminance pathways were modulated by WM load and thus likely reflect the neural substrate of the increased efficiency. Our results cannot be accounted for by simple saliency differences between luminance and color. Rather, they provide evidence for a direct connection between low-level perceptual mechanisms and WM by showing a crucial role of luminance for forming WM representations of shape.

Interocular Phase Disparity Tuning of Binocular Contrast Summation Depends on Carrier Spatial Frequency and Orientation

SIGNIFICANCE

Binocular summation is a sensitive metric of binocular integration. As such, characterization of the mechanisms underlying binocular summation is a key step in translating and applying this knowledge to abnormal binocular systems afflicted with strabismus and amblyopia.

PURPOSE

Computational models of binocular summation have advocated the operation of mechanisms sensitive to the interocular phase disparity of first order carrier gratings. This study investigated if such generalization depended on carrier spatial frequency and orientation.

METHODS

Monocular and binocular contrast detection thresholds were measured in nine observers with normal binocular vision. Stimuli comprised Gabor targets presented with one of three spatial frequencies (1, 3, and 9 cpd), two orientations (horizontal and vertical) and five interocular phase disparities (0, +/- pi/2, +/- pi radians). Horizontal and vertical fixation disparities were measured for each binocular threshold condition. Binocular summation ratios (BSR) were computed by dividing the mean monocular detection threshold by the respective binocular detection threshold.

RESULTS

BSR varied significantly with interocular phase disparity for the 1 cpd and 3 cpd horizontal and vertical gratings. Phase dependency was reduced with the 9 cpd horizontal grating, and absent for the 9 cpd vertical grating, even though BSR exceeded predictions of probability summation. Computational modelling that incorporated the variability of fixation disparity into a vector summation model, predicted a reduction in peak BSR with increasing carrier spatial frequency, but did not account for the reduction of phase sensitivity noted with the 9 cpd stimulus.

CONCLUSIONS

Binocular summation magnitude is less dependent on interocular phase disparities as carrier spatial frequencies exceed 3 cpd, especially with vertical gratings. While vergence variability due to fixation disparities contribute to the overall reduction in binocular summation magnitude with increasing carrier spatial frequency; however, it does not provide a complete account for the lack of interocular phase disparity tuning noted with high grating spatial frequencies.

Exploring the effects of degraded vision on sensorimotor performance

PURPOSE

Many people experience unilateral degraded vision, usually owing to a developmental or age-related disorder. There are unresolved questions regarding the extent to which such unilateral visual deficits impact on sensorimotor performance; an important issue as sensorimotor limitations can constrain quality of life by restricting 'activities of daily living'. Examination of the relationship between visual deficit and sensorimotor performance is essential for determining the functional implications of ophthalmic conditions. This study attempts to explore the effect of unilaterally degraded vision on sensorimotor performance.

METHODS

In Experiment 1 we simulated visual deficits in 30 participants using unilateral and bilateral Bangerter filters to explore whether motor performance was affected in water pouring, peg placing, and aiming tasks. Experiment 2 (n = 74) tested the hypothesis that kinematic measures are associated with visuomotor deficits by measuring the impact of small visual sensitivity decrements created by monocular viewing on sensorimotor interactions with targets presented on a planar surface in aiming, tracking and steering tasks.

RESULTS

In Experiment 1, the filters caused decreased task performance-confirming that unilateral (and bilateral) visual loss has functional implications. In Experiment 2, kinematic measures were affected by monocular viewing in two of three tasks requiring rapid online visual feedback (aiming and steering).

CONCLUSIONS

Unilateral visual loss has a measurable impact on sensorimotor performance. The benefits of binocular vision may be particularly important for some groups (e.g. older adults) where an inability to complete sensorimotor tasks may necessitate assisted living. There is an urgent need to develop rigorous kinematic approaches to the quantification of the functional impact of unilaterally degraded vision and of the benefits associated with treatments for unilateral ophthalmic conditions to enable informed decisions around treatment.

Laboratory Investigation of Preclinical Visual-Quality Metrics and Halo-Size in Enhanced Monofocal Intraocular Lenses

Introduction

This study aims to compare preclinical visual-quality metrics and halo size of intraocular lenses (IOL) with enhanced intermediate vision to a standard monofocal lens.

Methods

Three monofocal- IOL models with an extended-depth-of-focus (EDoF) intended for monocular implantation (Tecnis ICB00, AE2UV/ZOE, and IsoPure) and one for monovision (RayOne EMV) were compared against a standard monofocal lens (Tecnis ZCB00). An optical-metrology station was used in the assessment of IOLs' optical quality in polychromatic light. The imaging quality was compared with metrics derived from the optical transfer function. Halo size was estimated from the projection of the point spread function under scotopic pupil.

Results

The monofocal IOL showed the highest image quality at the far focus. The ICB00’s, the AE2UV/ZOE’s, and the IsoPure’s performance at − 1D was superior to that of the monofocal lens. The monocular defocus tolerance of the RayOne EMV was comparable with that of the ZCB00. The RayOne EMV’s intermediate range was improved in a monovision configuration (− 1D offset). This approach, however, yielded the largest halo area, i.e., 53% of the ZCB00’s halo, compared to 34% for the IsoPure, 14% for the AE2UV/ZOE, and 8% for the ICB00.

Conclusion

The mono-EDoF models have a clear advantage over the standard monofocal lens by expanded imaging capability beyond − 0.5D. Although the RayOne EMV provided the largest (binocular) visual-range extension, it was at the expense of monocular vision and higher susceptibility to halo. The ICB00’s and the AE2UV/ZOE’s halo-profile was similar to that of the ZCB00, indicating their low potential to induce photic phenomena.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40123-021-00411-9.

A Unified Rule for Binocular Contrast Summation Applies to Normal Vision and Common Eye Diseases

Purpose

Binocular summation refers to better visual performance with two eyes than with one eye. Little is known about the mechanism underlying binocular contrast summation in patients with common eye diseases who often exhibit binocularly asymmetric vision loss and structural changes along the visual pathway. Here we asked whether the mechanism of binocular contrast summation remains preserved in eye disease.

Methods

This study included 1035 subjects with normal ocular health, cataract, age-related macular degeneration, glaucoma, and retinitis pigmentosa. Monocular and binocular contrast sensitivity were measured by the Pelli-Robson contrast sensitivity chart. Interocular ratio (IOR) was quantified as the ratio between the poorer and better eye contrast sensitivity. Binocular summation ratio (BSR) was quantified as the ratio between binocular and better eye contrast sensitivity.

Results

All groups showed statistically significant binocular summation, with the BSR ranging from 1.25 [1.20, 1.30] in the glaucoma group to 1.31 [1.27, 1.36] in the normal vision group. There was no significant group difference in the BSR, after accounting for IOR. By fitting a binocular summation model Binocular = (Leftm + Rightm)1/m to the contrast sensitivity data, we found that the same binocular summation rule, reflected by the parameter m, applies across the five groups.

Conclusions

Cortical binocular contrast summation appears to be preserved in spite of eye diseases that can affect the two eyes differently. This finding supports the importance of assessing both monocular and binocular functions, rather than relying on a monocular assessment in the better eye as a potentially inaccurate surrogate measure.

Binocular Integration of Perceptually Suppressed Visual Information in Amblyopia

Purpose

The purpose of this study was to assess whether motion information from suppressed amblyopic eyes can influence visual perception.

Methods

Participants with normal vision (n = 20) and with amblyopia (n = 20; 11 anisometropic and 9 strabismic/mixed) viewed dichoptic, orthogonal drifting gratings through a mirror stereoscope. Participants continuously reported form and motion percepts as gratings rivaled for 60 seconds. Responses were binned into categories ranging from binocular integration to complete suppression. Periods when the grating presented to the nondominant/amblyopic eye was suppressed were analyzed further to determine the extent of binocular integration of motion.

Results

Individuals with amblyopia experienced longer periods of non-preferred eye suppression than controls. When the non-preferred eye grating was suppressed, binocular integration of motion occurred 48.1 ± 6.2% and 31.2 ± 5.8% of the time in control and amblyopic participants, respectively. Periods of motion integration from the suppressed eye were significantly non-zero for both groups.

Conclusions

Visual information seen only by a suppressed amblyopic eye can be binocularly integrated and influence the overall visual percept. These findings reveal that visual information subjected to interocular suppression can still contribute to binocular vision and suggest the use of appropriate optical correction for the amblyopic eye to improve image quality for binocular combination.

Binocular Interactions in Glaucoma Patients With Nonoverlapping Visual Field Defects: Contrast Summation, Rivalry, and Phase Combination

Purpose

In glaucoma, visual field defects in the left and right eye may be non-overlapping, resulting in an intact binocular visual field. In clinical management, these patients are often considered to have normal vision. However, visual performance also relies on binocular processing. The aim of this study was to evaluate binocular visual functions in glaucoma patients with intact binocular visual field, normal visual acuity, and stereoscopy.

Methods

We measured in 10 glaucoma patients and 12 age-similar controls: (1) monocular and binocular contrast sensitivity functions (CSF) using a modified quick CSF test to assess binocular contrast summation, (2) dominance during rivalry, and (3) contrast ratio at balance point with a binocular phase combination test. A mirror stereoscope was used to combine the left and right eye image (each 10° horizontally by 12° vertically) on a display.

Results

Area under the monocular and binocular CSF was lower in glaucoma compared to healthy (P < 0.001), but the binocular contrast summation ratio did not differ (P = 0.30). For rivalry, the percentage of time of mixed percept was 9% versus 18% (P = 0.056), the absolute difference of the percentage of time of dominance between the two eyes 19% versus 10% (P = 0.075), and the rivalry rate 8.2 versus 12.1 switches per minute (P = 0.017) for glaucoma and healthy, respectively. Median contrast ratio at balance point was 0.66 in glaucoma and 1.03 in controls (P = 0.011).

Conclusions

Binocular visual information processing deficits can be found in glaucoma patients with intact binocular visual field, normal visual acuity, and stereoscopy.

Perceptual and cognitive processes in augmented reality - comparison between binocular and monocular presentations

In the present study, we investigated the difference between monocular augmented reality (AR) and binocular AR in terms of perception and cognition by using a task that combines the flanker task with the oddball task. A right- or left-facing arrowhead was presented as a central stimulus at the central vision, and participants were instructed to press a key only when the direction in which the arrowhead faced was a target. In a small number of trials, arrowheads that were facing in the same or opposite direction (flanker stimuli) were presented beside the central stimulus binocularly or monocularly as an AR image. In the binocular condition, the flanker stimuli were presented to both eyes, and, in the monocular condition, only to the dominant eye. The results revealed that participants could respond faster in the binocular condition than in the monocular one; however, only when the flanker stimuli were in the opposite direction was the response faster in the monocular condition. Moreover, the results of event-related brain potentials (ERPs) showed that all stimuli were processed in both the monocular and the binocular conditions in the perceptual stage; however, the influence of the flanker stimuli was attenuated in the monocular condition in the cognitive stage. The influence of flanker stimuli might be more unstable in the monocular condition than in the binocular condition, but more precise examination should be conducted in a future study.

Nearby contours abolish the binocular advantage

That binocular viewing confers an advantage over monocular viewing for detecting isolated low luminance or low contrast objects, has been known for well over a century; however, the processes involved in combining the images from the two eyes are still not fully understood. Importantly, in natural vision, objects are rarely isolated but appear in context. It is well known that nearby contours can either facilitate or suppress detection, depending on their distance from the target and the global configuration. Here we report that at close distances collinear (but not orthogonal) flanking contours suppress detection more under binocular compared to monocular viewing, thus completely abolishing the binocular advantage, both at threshold and suprathreshold levels. In contrast, more distant flankers facilitate both monocular and binocular detection, preserving a binocular advantage up to about four times the detection threshold. Our results for monocular and binocular viewing, for threshold contrast discrimination without nearby flankers, can be explained by a gain control model with uncertainty and internal multiplicative noise adding additional constraints on detection. However, in context with nearby flankers, both contrast detection threshold and suprathreshold contrast appearance matching require the addition of both target-to-target and flank-to-target interactions occurring before the site of binocular combination. To test an alternative model, in which the interactions occur after the site of binocular combination, we performed a dichoptic contrast matching experiment, with the target presented to one eye, and the flanks to the other eye. The two models make very different predictions for abutting flanks under dichoptic conditions. Interactions after the combination site predict that the perceived contrast of the flanked target will be strongly suppressed, while interactions before the site predict the perceived contrast will be more or less veridical. The data are consistent with the latter model, strongly suggesting that the interactions take place before the site of binocular combination.

Binocular Vision and Stereopsis Across the Animal Kingdom

Most animals have at least some binocular overlap, i.e., a region of space that is viewed by both eyes. This reduces the overall visual field and raises the problem of combining two views of the world, seen from different vantage points, into a coherent whole. However, binocular vision also offers many potential advantages, including increased ability to see around obstacles and increased contrast sensitivity. One particularly interesting use for binocular vision is comparing information from both eyes to derive information about depth. There are many different ways in which this might be done, but in this review, I refer to them all under the general heading of stereopsis. This review examines the different possible uses of binocular vision and stereopsis and compares what is currently known about the neural basis of stereopsis in different taxa. Studying different animals helps us break free of preconceptions stemming from the way that stereopsis operates in human vision and provides new insights into the different possible forms of stereopsis. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Binocular vision supports the development of scene segmentation capabilities: Evidence from a deep learning model

The application of deep learning techniques has led to substantial progress in solving a number of critical problems in machine vision, including fundamental problems of scene segmentation and depth estimation. Here, we report a novel deep neural network model, capable of simultaneous scene segmentation and depth estimation from a pair of binocular images. By manipulating the arrangement of binocular image pairs, presenting the model with standard left-right image pairs, identical image pairs or swapped left-right images, we show that performance levels depend on the presence of appropriate binocular image arrangements. Segmentation and depth estimation performance are both impaired when images are swapped. Segmentation performance levels are maintained, however, for identical image pairs, despite the absence of binocular disparity information. Critically, these performance levels exceed those found for an equivalent, monocularly trained, segmentation model. These results provide evidence that binocular image differences support both the direct recovery of depth and segmentation information, and the enhanced learning of monocular segmentation signals. This finding suggests that binocular vision may play an important role in visual development. Better understanding of this role may hold implications for the study and treatment of developmentally acquired perceptual impairments.

Power contours: Optimising sample size and precision in experimental psychology and human neuroscience

When designing experimental studies with human participants, experimenters must decide how many trials each participant will complete, as well as how many participants to test. Most discussion of statistical power (the ability of a study design to detect an effect) has focused on sample size, and assumed sufficient trials. Here we explore the influence of both factors on statistical power, represented as a 2-dimensional plot on which iso-power contours can be visualized. We demonstrate the conditions under which the number of trials is particularly important, that is, when the within-participant variance is large relative to the between-participants variance. We then derive power contour plots using existing data sets for 8 experimental paradigms and methodologies (including reaction times, sensory thresholds, fMRI, MEG, and EEG), and provide example code to calculate estimates of the within- and between-participants variance for each method. In all cases, the within-participant variance was larger than the between-participants variance, meaning that the number of trials has a meaningful influence on statistical power in commonly used paradigms. An online tool is provided (https://shiny.york.ac.uk/powercontours/) for generating power contours, from which the optimal combination of trials and participants can be calculated when designing future studies. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Binocular Enhancement of Multisensory Temporal Perception

Purpose

The goal of this study was to examine the behavioral effects and to suggest possible underlying mechanisms of binocularity on audiovisual temporal perception in normally-sighted individuals.

Methods

Participants performed two audiovisual simultaneity judgment tasks-one using simple flashes and beeps and the other using audiovisual speech stimuli-with the left eye, right eye, and both eyes. Two measures, the point of subjective simultaneity (PSS) and the temporal binding window (TBW), an index for audiovisual temporal acuity, were derived for each viewing condition, stimulus type, and participant. The data were then modeled using causal inference, allowing us to determine whether binocularity affected low-level unisensory mechanisms (i.e., sensory noise level) or high-level multisensory mechanisms (i.e., prior probability of interring a common cause, pC=1).

Results

Whereas for the PSS there was no significant effect of viewing condition, for the TBW, a significant interaction between stimulus type and viewing condition was found. Post hoc analyses revealed a significantly narrower TBW during binocular than monocular viewing (average of left and right eyes) for the flash-beep condition but no difference between the viewing conditions for the speech stimuli. Modeling results showed no significant difference in pC=1 but a significant reduction in sensory noise during binocular performance on flash-beep trials.

Conclusions

Binocular viewing was found to enhance audiovisual temporal acuity as indexed by the TBW for simple low-level audiovisual stimuli. Furthermore, modeling results suggest that this effect may stem from enhanced sensory representations evidenced as a reduction in sensory noise affecting the measurement of physical asynchrony during audiovisual temporal perception.

Binocular summation is affected by crowding and tagging

In perceptual crowding, a letter easily recognized on its own, becomes unrecognizable if it is surrounded by other letters, an effect that confers a limit on the visual processing. Models assume that crowding is a hallmark of the periphery but that it is almost absent in the fovea. However, recently it was shown that crowding occurs in the fovea of people with an abnormal development of functional vision (amblyopia), when the stimulus is presented for a very short time. When targets and flankers are dissimilar, the crowding is reduced (tagging). Since a combination of binocular inputs increases the processing load, we investigated whether color tagging the target reduces crowding in the fovea of subjects with normal vision and determined how crowding is combined with binocular vision. The crowding effect at the fovea was significantly reduced by tagging with a color target. Interestingly, whereas binocular summation for a single letter was expected to be about 40%, it was significantly reduced and almost absent under crowding conditions. Our results are consistent with the notion that the crowding effect produces a high processing load on visual processing, which interferes with other processes such as binocular summation. We assume that the tagging effect in our experiment improved the subject's abilities (sensitivity and RT) by creating a "segmentation", i.e., a visual simulated separation between the target letter and the background. Interestingly, tagging the target with a distinct color can eliminate or reduce the crowding effect and consequently, binocular summation recovers.

Asymmetries in visual acuity around the visual field

Human vision is heterogeneous around the visual field. At a fixed eccentricity, performance is better along the horizontal than the vertical meridian and along the lower than the upper vertical meridian. These asymmetric patterns, termed performance fields, have been found in numerous visual tasks, including those mediated by contrast sensitivity and spatial resolution. However, it is unknown whether spatial resolution asymmetries are confined to the cardinal meridians or whether and how far they extend into the upper and lower hemifields. Here, we measured visual acuity at isoeccentric peripheral locations (10 deg eccentricity), every 15° of polar angle. On each trial, observers judged the orientation (± 45°) of one of four equidistant, suprathreshold grating stimuli varying in spatial frequency (SF). On each block, we measured performance as a function of stimulus SF at 4 of 24 isoeccentric locations. We estimated the 75%-correct SF threshold, SF cutoff point (i.e., chance-level), and slope of the psychometric function for each location. We found higher SF estimates (i.e., better acuity) for the horizontal than the vertical meridian and for the lower than the upper vertical meridian. These asymmetries were most pronounced at the cardinal meridians and decreased gradually as the angular distance from the vertical meridian increased. This gradual change in acuity with polar angle reflected a shift of the psychometric function without changes in slope. The same pattern was found under binocular and monocular viewing conditions. These findings advance our understanding of visual processing around the visual field and help constrain models of visual perception.

Binocular summation and efficient coding

Two eyes are better than one at detecting a pattern, an advantage termed binocular summation. It is widely believed that binocular summation is mediated by neurons that sum the two eyes' inputs. Here we suggest an alternative model based on a model of binocular interactions proposed by Cohn, Leong & Lasley (Vision Research, 1981, 21, 1017-1023) and further motivated by the efficient coding framework proposed by Li & Atick (Network: Computation in Neural Systems, 1994, 5, 157-174). In the model, termed MAX(S+S-), binocular summation is mediated by channels that compute the sum, S+, and difference, S-, of the two eyes' monocular signals. The S+ and S- signals are assumed to be perturbed by independent noise, have independent gains and contribute independently to detection via the MAX rule. To test the model we measured binocular summation for horizontally-oriented Gabor patches at a range of spatial-frequencies and bandwidths, at both contrast detection threshold and for increment thresholds on binocular pedestals at contrasts set to 10x detection threshold. The model's performance was compared to that of two conventional models of binocular summation, one in which the two eyes' signals remain separate at the decision stage, termed MAX(LR), the other in which the two eye's signals are summed by a single channel, termed B+, with both models incorporating interocular inhibition. The MAX(S+S-) model gave as good a performance as the other two models. Together with the evidence for the involvement of separately gain controlled S+ and S- signals underpinning a wide range of binocular behaviors, we conclude that the MAX(S+S-) model can and should be considered as a viable model for binocular summation.

Distorted optical input affects human perception

Collinear facilitation, the mechanism for grouping contour elements, is a process involving lateral interactions that improve the detectability of a target by the presence of collinear flankers. It was shown that the development of collinear facilitation is experience dependent and that it may be impaired when the visual input is distorted in one meridian (meridional amblyopia). In oblique astigmatism, the blurring is on the opposite oblique meridian in both eyes, resulting in two conflicting images, which may affect the development of binocular vision. We hypothesized that the collinear facilitation of adults with oblique astigmatism is reminiscent of the abnormal development of the lateral facilitation of meridional amblyopia. We explored the perception of binocular vision and collinear facilitation in cases of both distorted and non-distorted vision. Fully corrected participants that tested for the target contrast detection of Gabor patches and two collinear flankers, presented for 80 ms, were positioned at different orientations (0° (180°), 45°, 90°, and 135°) and for different eyes (monocular, binocular). The results show a significant anisotropy for monocular collinear facilitation between the blured and the clear meridians, being lower in the blurriest meridian than in the clearest meridian, resembling the meridional amblyopia results. Collinear facilitation results in poor binocular summation between the monocular channels. Our results indicate that the perceptual behavior was similar to that of meridional amblyopic subjects having an anisotropy of collinear facilitation between cardinal meridians in oblique astigmatic subjects.

Interocular difference thresholds are mediated by binocular differencing, not summing, channels

Patterns in the two eyes' views that are not identical in hue or contrast often elicit an impression of luster, providing a cue for discriminating them from perfectly matched patterns. Here we attempt to determine the mechanisms for detecting interocular differences in luminance contrast, in particular in relation to the possible contributions of binocular differencing and binocular summing channels. Test patterns were horizontally oriented multi-spatial-frequency luminance-grating patterns subject to variable amounts of interocular difference in grating phase, resulting in varying degrees of local interocular contrast difference. Two types of experiment were conducted. In the first, subjects discriminated between a pedestal with an interocular difference that ranged upward from zero (i.e., binocularly correlated) and a test pattern that contained a bigger interocular difference. In the second type of experiment, subjects discriminated between a pedestal with an interocular difference that ranged downward from a maximum (i.e., binocularly anticorrelated) and a test pattern that contained smaller interocular difference. The two types of task could be mediated by a binocular differencing and a binocular summing channel, respectively. However, we found that the results from both experiments were well described by a simpler model in which a single, linear binocular differencing channel is followed by a standard nonlinear transducer that is expansive for small signals but strongly compressive for large ones. Possible reasons for the lack of involvement of a binocular summing channel are discussed in the context of a model that incorporates the responses of both monocular and binocular channels.

Monocular and Binocular Temporal Visual Perception of Infantile Nystagmus

Contrast sensitivity is mostly used as a tool for testing aspects of visual functions. Infantile nystagmus is a pathological phenomenon that affects the spatial-temporal visual functions due to spontaneous oscillating movements of the eyes. We examined the spatial-temporal aspects of nystagmus perception, aiming to investigate the mechanisms underlying the deterioration of their visual performance. We tested the monocular and binocular contrast sensitivity of nystagmus and normally sighted subjects by measuring contrast detection of a Gabor target with spatial frequencies slightly above the cutoff threshold of each subject (nystagmus ~3; controls = 9cpd; presentation times 60–480 ms). The dominant eye of nystagmus revealed large differences over the non-dominant eye, highlighting the superiority of the dominant over the non-dominant eye in nystagmus. In addition, binocular summation mechanism was impaired in majority of the nystagmus subjects. Furthermore, these differences are not attributed to differences in visual acuity. Moreover, the visual performance in nystagmus continue to improve for longer presentation time compared with controls and was longer in the poor eye. Since the results are not due to differences in eye movements and strabismus, we suggest that the differences are due to developmental impairment in the visual system during the critical period.

Binaural summation of amplitude modulation involves weak interaural suppression

The brain combines sounds from the two ears, but what is the algorithm used to achieve this summation of signals? Here we combine psychophysical amplitude modulation discrimination and steady-state electroencephalography (EEG) data to investigate the architecture of binaural combination for amplitude-modulated tones. Discrimination thresholds followed a ‘dipper’ shaped function of pedestal modulation depth, and were consistently lower for binaural than monaural presentation of modulated tones. The EEG responses were greater for binaural than monaural presentation of modulated tones, and when a masker was presented to one ear, it produced only weak suppression of the response to a signal presented to the other ear. Both data sets were well-fit by a computational model originally derived for visual signal combination, but with suppression between the two channels (ears) being much weaker than in binocular vision. We suggest that the distinct ecological constraints on vision and hearing can explain this difference, if it is assumed that the brain avoids over-representing sensory signals originating from a single object. These findings position our understanding of binaural summation in a broader context of work on sensory signal combination in the brain, and delineate the similarities and differences between vision and hearing.

Human visual steady-state responses to amplitude-modulated flicker: Latency measurement

The response latency of steady-state visually evoked potentials (SSVEPs) is a sensitive measurement for investigating visual functioning of the human brain, specifically in visual development and for clinical evaluation. This latency can be measured from the slope of phase versus frequency of responses by using multiple frequencies of stimuli. In an attempt to provide an alternative measurement of this latency, this study utilized an envelope response of SSVEPs elicited by amplitude-modulated visual stimulation and then compared with the envelope of the generating signal, which was recorded simultaneously with the electroencephalography recordings. The advantage of this measurement is that it successfully estimates the response latency based on the physiological envelope in the entire waveform. Results showed the response latency at the occipital lobe (Oz channel) was approximately 104.55 ms for binocular stimulation, 97.14 ms for the dominant eye, and 104.75 ms for the nondominant eye with no significant difference between these stimulations. Importantly, the response latency at frontal channels (125.84 ms) was significantly longer than that at occipital channels (104.11 ms) during binocular stimulation. Together with strong activation of the source envelope at occipital cortex, these findings support the idea of a feedforward process, with the visual stimuli propagating originally from occipital cortex to anterior cortex. In sum, these findings offer a novel method for future studies in measuring visual response latencies and also potentially shed a new light on understanding of how long collective neural activities take to travel in the human brain.

Unraveling nonlinear electrophysiologic processes in the human visual system with full dimension spectral analysis

Natural sensory signals have nonlinear structures dynamically composed of the carrier frequencies and the variation of the amplitude (i.e., envelope). How the human brain processes the envelope information is still poorly understood, largely due to the conventional analysis failing to quantify it directly. Here, we used a recently developed method, Holo-Hilbert spectral analysis, and steady-state visually evoked potential collected using electroencephalography (EEG) recordings to investigate how the human visual system processes the envelope of amplitude-modulated signals, in this case with a 14 Hz carrier and a 2 Hz envelope. The EEG results demonstrated that in addition to the fundamental stimulus frequencies, 4 Hz amplitude modulation residing in 14 Hz carrier and a broad range of carrier frequencies covering from 8 to 32 Hz modulated by 2 Hz amplitude modulation are also found in the two-dimensional frequency spectrum, which have not yet been recognized before. The envelope of the stimulus is also found to dominantly modulate the response to the incoming signal. The findings thus reveal that the electrophysiological response to amplitude-modulated stimuli is more complex than could be revealed by, for example, Fourier analysis. This highlights the dynamics of neural processes in the visual system.

Binocular facilitation in light-mediated melatonin suppression?

In humans, the production of melatonin is suppressed by light exposure. This effect is mediated by a retinal pathway involving the melanopsin-containing intrinsincally photosensitive retinal ganglion cells (ipRGCs), which exhibit maximum sensitivity to short-wavelength light. Here, based on extant and published data, we examine how signals from the two eyes are integrated in driving the suppression of melatonin by light. We find that melatonin suppression by light exposure to two eyes corresponds to a sensitivity shift by about 1.2 log units (factor ~17.4).

Evaluation of visual evoked potential binocular summation after corneal refractive surgery

PURPOSE

To explore whether visual evoked binocular summation is affected in eyes with refractive errors after refractive surgery.

METHODS

Twenty participants (6M, 14F) aged 20-35 years (mean 26.7 ± 4.4) were assessed through pattern-reversal visual evoked potential viewing with their every eye (first right eye and then left eye) and then with both eyes while wearing their best correction before undergoing surgery. Also, parameters of the P100 component of pattern-reversal visual evoked potential were evaluated after 3 months of refractive surgery in two different check sizes. Monocular and binocular amplitudes and latencies of P100 wave and binocular summation index were compared between before and after surgery.

RESULTS

Monocular visual evoked potentials elicited by two different high-contrast checkerboard-patterned stimuli were significantly reduced in P100 wave amplitude (P < 0.05) after refractive surgery. However, there was no difference between the pre- and post-refractive surgery in the mean value of monocular P100 latency (P > 0.05). Similar to monocular findings of P100 wave amplitude, on binocular viewing, the mean value of pattern-reversal visual evoked potential amplitude was significantly reduced (P < 0.05), and P100 wave latency was prolonged (P < 0.05) after refractive surgery in participants. Also, the mean postoperative binocular summation index value as compared to that in the preoperative was significantly lesser (P < 0.05) for the subject in this study.

CONCLUSIONS

Refractive surgery can degrade binocular visual performance throughout the change in visual evoked potential binocular summation. However, monocular function deteriorates less than binocular function after refractive surgery.