Binocular contrast summation--I. Detection and discrimination

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.

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.

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.

Presbyopia correction with multifocal contact lenses: Evaluation of silent reading performance using eye movements analysis

PURPOSE

Many activities of daily living rely on reading, thus is not surprising that complaints from presbyopes originate in reading difficulties rather in visual acuity. Here, the effectiveness of presbyopia correction with multifocal contact lenses (CLs) is evaluated using an eye-fixation based method of silent reading performance. ΜETHODS: Visual performance of thirty presbyopic volunteers (age: 50 ± 5 yrs) was assessed monocularly and binocularly following 15 days of wear of monthly disposable CLs (AIR OPTIX™ plus HydraGlyde™, Alcon Laboratories) with: (a) single vision (SV) lenses - uncorrected for near (b) aspheric multifocal (MF) CLs. LogMAR acuity was measured with ETDRS charts. Reading performance was evaluated using standard IReST paragraphs displayed on a screen (0.4 logMAR print size at 40 cm distance). Eye movements were monitored with an infrared eyetracker (Eye-Link II, SR Research Ltd). Data analysis included computation of reading speed, fixation duration, fixations per word and percentage of regressions.

RESULTS

Average reading speed was 250 ± 68 and 235 ± 70 wpm, binocularly and monocularly, with SV CLs, improving statistically significantly to 280 ± 67 (p = 0.002) and 260 ± 59 wpm (p = 0.01), respectively, with MF CLs. Moreover, fixation duration, fixations per word and ex-Gaussian parameter of fixation duration, μ, showed a statistically significant improvement when reading with MF CLs, with fixation duration exhibiting the stronger correlation (r = 0.79, p < 0.001) with improvement in reading speed. The correlation between improvement in VA and reading speed was moderate (r = 0.46, p = 0.016), as was the correlation between VA and any eye fixation parameter.

CONCLUSION

Average silent reading speed in a presbyopic population was found improved with MF compared to SV CL correction and was faster with binocular compared to monocular viewing: this was mainly due to the faster average fixation duration and the lower number of fixations. Evaluating reading performance using eye fixation analysis could offer a reliable outcome of functional vision in presbyopia correction.

Binocular summation of visual acuity and contrast sensitivity in children with intermittent exotropia

PURPOSE

To investigate the binocular summation (BiS) of visual acuity (VA) and contrast sensitivity (CS) in children with intermittent exotropia (IXT) before and after surgery and to probe the relationship between the two BiS phenomena and corresponding influencing factors.

METHODS

This prospective study included 21 IXT children (11 males and 10 females; aged 6-13 years) who underwent strabismus surgery in Tianjin Eye Hospital from January to April 2022. The visual function was assessed preoperatively and 2.95 ± 0.14 months postoperatively, including monocular/ binocular visual acuity (MVA/BVA) at 100% contrast and 2.5% contrast as well as monocular/binocular contrast sensitivity (MCS/BCS), deviation, near and distant stereopsis, and fusion.

RESULTS

All patients had postoperative deviation ranging from 0 to -4 PD. Either preoperative or postoperative BVA at 2.5% contrast was superior to the MVA. The postoperative BiS at 2.5% contrast was significantly superior to the preoperative BiS for 2.5% contrast and postoperative BiS for 100% contrast (P < 0.05). Except for 3 c/d, the MCS and BCS at 6 c/d, 12 c/d and 18 c/d spatial frequencies were all notably improved postoperatively. The postoperative binocular summation ratio of CS (BSR) was highest while interocular difference ratio of CS (IOR) was the lowest at 6 c/d among 4 spatial frequencies. The deviation, distant and near stereopsis, and fusion performance were all remarkably improved after surgery (p = 0.001; p = 0.041; p = 0.000), all of which were not related to BVA at 2.5% contrast, BiS, BSC and BSR. The BCS at middle and high frequencies (6 c/ds, 12 c/ds, and 18 c/ds) was significantly negatively correlated with the BVA at 2.5% contrast, and BSR was irrelevant to the corresponding IOR across different spatial frequencies.

CONCLUSION

BVA at low contrast and BCS examinations were not equivalent to stereopsis and fusion status, which contributed to the evaluation of binocular function in the real environment and in the different aspects. BVA in 2.5% contrast is related with BCS in moderate and high spacial frequencies (especially 18c/d) but BCS in 6c/d presents more binocular summation of contrast sensitivity. MCS, BCS and the BSR persist inhibition at 3c/d after surgery. The improvement of BCS is better than that of BSR to evaluate the binouclar function in IXT. Those two methods showed different sensitivities to impairment and rehabilitation of binocular summation and inhibition.

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'.

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.

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.

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.

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.

Visual outcomes and safety of the TECNIS Symfony intraocular lens: results of a pivotal clinical trial

Randomized comparison of the TECNIS Symfony (ZXR00) and the TECNIS 1-piece monofocal (ZCB00) intraocular lenses in cataract surgery demonstrates improved intermediate and near vision with TECNIS Symfony.

To compare the effectiveness and safety of the TECNIS Symfony intraocular lens (IOL; ZXR00) with the TECNIS 1-piece monofocal IOL (ZCB00).

What is visible across the visual field?

It is sometimes claimed that because the resolution and sensitivity of visual perception are better in the fovea than in the periphery, peripheral vision cannot support the same kinds of colour and sharpness percepts as foveal vision. The fact that a scene nevertheless seems colourful and sharp throughout the visual field then poses a puzzle. In this study, I use a detailed model of human spatial vision to estimate the visibility of certain properties of natural scenes, including aspects of colourfulness, sharpness, and blurriness, across the visual field. The model is constructed to reproduce basic aspects of human contrast and colour sensitivity over a range of retinal eccentricities. I apply the model to colourful, complex natural scene images, and estimate the degree to which colour and edge information are present in the model's representation of the scenes. I find that, aside from the intrinsic drift in the spatial scale of the representation, there are not large qualitative differences between foveal and peripheral representations of 'colourfulness' and 'sharpness'.

Central Visual Field Sensitivity With and Without Background Light Given to the Nontested Fellow Eye in Glaucoma Patients

PRECIS

This study showed the difference of monocular visual sensitivity between with and without background light given to the nontested fellow eyes in glaucoma patients. Monocular sensitivity measurements of the worse eyes with fellow eye's background light conditions should carefully be considered when assessing the impact of functional impairment in glaucoma patients.

PURPOSE

The purpose of this study was to investigate the difference between monocular sensitivities measured with and without background light given to the nontested fellow eye in glaucoma patients using a new perimeter named "imo."

METHODS

In this cross-sectional study, we examined 102 eyes of 51 patients (mean age, 65.1±14.9 y) with open-angle glaucoma who were affected with at least 1 significant point in the central 10 degrees. We conducted a routine ophthalmic examination and visual field testing using the Humphrey Field Analyzer 24-2 and 10-2 programs. The eyes were assigned to "better" and "worse" categories based on the visual acuity and central visual thresholding. Subsequently, we compared the central visual sensitivities with and without background light given to the nontested fellow eye.

RESULTS

The mean sensitivity (MS) in the central 5 points of the visual field of the worse eyes was better when measured with background light than without background light (P=0.037) given to the nontested fellow eye. No significant difference was seen among the MS in the visual field of the better eyes. After dividing the patients into low (n=25) and high sensitivity (n=26) groups, only the low sensitivity group in worse eyes had higher MS with background light than without background light (P<0.05) given to the nontested fellow eye.

CONCLUSIONS

Monocular sensitivities measured with background light given to the nontested fellow eye were higher than those without background light in the worse eye group of glaucoma patients. Monocular MS measurements of the worse eyes with fellow eye's background light conditions and their related indices should thus carefully be considered when assessing the impact of functional impairment in glaucoma patients.

Perceptual enhancement of suprathreshold luminance modulation in stereoscopic patterns

Previous studies have shown that the binocular summation of luminance contrast signals depends on the parameters involved in stereopsis when the luminance contrast is at the detection threshold. However, less attention has been paid to the perception of luminance modulation in stereoscopic patterns at suprathreshold contrast. To address this issue, we determined the contrast of stereoscopic patterns at the perceptual match to a standard contrast as a function of binocular disparity. The matched contrast was close to the standard contrast at 0 degrees disparity, but decreased as disparity deviated from 0 degrees, suggesting that sufficient disparity perceptually enhances luminance contrast. The reduction of matched contrast was more evident for uncrossed disparities than for crossed disparities, which almost disappeared when the contrast was near the threshold and also occurred when vertical disparity was introduced. We argue that the perceptual enhancement of the luminance contrast is due to the weaker interocular suppression for stimuli with large disparities.

Monocular and binocular thresholds for abruptly and gradually presented illusory contours

BACKGROUND

In this study, monocular thresholds and binocular summation for abrupt onset/offset versus gradually revealed phantom letter E (illusory contours) stimuli are compared to determine the suitability of these stimuli for assessment of the integrity of two of the major retinal streams: the magnocellular and the parvocellular pathways. Such tests are important in progressive retinal disease where disease severity may differ between the classes of retinal ganglion cells and between the two eyes. Abrupt onset phantom contours have long been considered to activate the magnocellular visual pathway and we propose that gradually revealed high contrast ramped onset/offset stimuli are more likely to promote the more sustained processing of the parvocellular stream.

METHODS

Contrast discrimination thresholds for monocular and binocular viewing were compared in a counter-balanced order in 70 young normal subjects, using tests of contrast threshold for a flicker-defined letter E produced by alternation of light and dark dots. Three onset/offset conditions were used - abrupt onset that was maintained for 34 milliseconds (four frames of 8.5 milliseconds) then discontinued, ramped onset over 34 milliseconds (four frames) with offset over 34 milliseconds and ramped onset over 85 milliseconds (10 frames) with offset over 85 milliseconds.

RESULTS

Contrast thresholds for identification of the orientation of the E, when presented with four frames ramped onset and offset when compared to the four frames abrupt onset/offset were three times higher, irrespective of monocular or binocular viewing conditions. Threshold contrasts were seven times higher when the 10 frames ramped onset/offset stimuli were compared to abrupt four frames onset/offset. Binocular contrast thresholds were reduced by approximately 40 per cent compared to monocular thresholds for all conditions. The binocular increase in contrast sensitivity is approximately equal for abrupt transiently presented stimuli and for gradually presented more sustained stimuli.

DISCUSSION

The results indicate that the same mechanisms of monocular processing and binocular summation are used for identification of a flickering contrast-defined phantom contour under presentation conditions, which are characteristic of the temporal and contrast preferences of the primate magnocellular and parvocellular visual pathways. This suggests that the phantom contour E test may be useful for clinical differentiation of the integrity of the M and P retinal ganglion-derived visual pathways, regardless of whether it is applied monocularly or binocularly.

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.

Reduced surround suppression in monocular motion perception

Motion discrimination of large stimuli is impaired at high contrast and short durations. This psychophysical result has been linked with the center-surround suppression found in neurons of area MT. Recent physiology results have shown that most frontoparallel MT cells respond more strongly to binocular than to monocular stimulation. Here we measured the surround suppression strength under binocular and monocular viewing. Thirty-nine participants took part in two experiments: (a) where the nonstimulated eye viewed a blank field of the same luminance (n = 8) and (b) where it was occluded with a patch (n = 31). In both experiments, we measured duration thresholds for small (1 deg diameter) and large (7 deg) drifting gratings of 1 cpd with 85% contrast. For each subject, a Motion Suppression Index (MSI) was computed by subtracting the duration thresholds in logarithmic units of the large minus the small stimulus. Results were similar in both experiments. Combining the MSI of both experiments, we found that the strength of suppression for binocular condition (MSIbinocular = 0.249 ± 0.126 log10 (ms)) is 1.79 times higher than under monocular viewing (MSImonocular = 0.139 ± 0.137 log10 (ms)). This increase is too high to be explained by the higher perceived contrast of binocular stimuli and offers a new way of testing whether MT neurons account for surround suppression. Potentially, differences in surround suppression reported in clinical populations may reflect altered binocular processing.

Comparison of central visual sensitivity between monocular and binocular testing in advanced glaucoma patients using imo perimetry

BACKGROUND/AIM

This study aimed to compare central visual sensitivity under monocular and binocular conditions in patients with glaucoma using the new imo static perimetry.

METHODS

Fifty-one consecutive eyes of 51 patients with open-angle glaucoma who were affected with at least one significant point in the central 10° were examined in this cross-sectional study. Monocular and binocular random single-eye tests were performed using the imo perimeter and the Humphrey field analyser (HFA) 24-2 and 10-2 tests. The eyes were assigned to 'better' and 'worse' categories based on the visual acuity and central visual thresholding. Central visual sensitivity results obtained by monocular, binocular random single-eye tests and binocular simultaneous both eye test were compared.

RESULTS

The average mean deviation with the HFA 24-2 was -5.5 (-1.5, -14.6) dB (median, (IQR)) in the better eyes and -18.0 (-12.9, -23.8) dB in the worse eyes. The mean sensitivity in the central 4 points of the visual field (VF) of the worse eyes was lower when measured under the binocular eye condition than under the monocular condition. Conversely, this value of the better eyes was greater when measured under the binocular eye condition than under the monocular condition.

CONCLUSIONS

The central sensitivity of the better eyes was better and that of the worse eyes poorer with binocular testing than with monocular testing in patients with glaucoma. Although monocular VF testing is still the most straightforward means to monocularly monitor glaucoma at clinical settings, binocular testing, such as provided with imo perimetry, may be a useful clinical tool to predict the effect of VF impairments on a patient's quality of visual life.

Contrast Normalization Accounts for Binocular Interactions in Human Striate and Extra-striate Visual Cortex

During binocular viewing, visual inputs from the two eyes interact at the level of visual cortex. Here we studied binocular interactions in human visual cortex, including both sexes, using source-imaged steady-state visual evoked potentials over a wide range of relative contrast between two eyes. The ROIs included areas V1, V3a, hV4, hMT⁺, and lateral occipital cortex. Dichoptic parallel grating stimuli in each eye modulated at distinct temporal frequencies allowed us to quantify spectral components associated with the individual stimuli from monocular inputs (self-terms) and responses due to interaction between the inputs from the two eyes (intermodulation [IM] terms). Data with self-terms revealed an interocular suppression effect, in which the responses to the stimulus in one eye were reduced when a stimulus was presented simultaneously to the other eye. The suppression magnitude varied depending on visual area, and the relative contrast between the two eyes. Suppression was strongest in V1 and V3a (50% reduction) and was least in lateral occipital cortex (20% reduction). Data with IM terms revealed another form of binocular interaction, compared with self-terms. IM response was strongest at V1 and was least in hV4. Fits of a family of divisive gain control models to both self- and IM-term responses within each cortical area indicated that both forms of binocular interaction shared a common gain control nonlinearity. However, our model fits revealed different patterns of binocular interaction along the cortical hierarchy, particularly in terms of excitatory and suppressive contributions.SIGNIFICANCE STATEMENT Using source-imaged steady-state visual evoked potentials and frequency-domain analysis of dichoptic stimuli, we measured two forms of binocular interactions: one is associated with the individual stimuli that represent interocular suppression from each eye, and the other is a direct measure of interocular interaction between inputs from the two eyes. We demonstrated that both forms of binocular interactions share a common gain control mechanism in striate and extra-striate cortex. Furthermore, our model fits revealed different patterns of binocular interaction along the visual cortical hierarchy, particularly in terms of excitatory and suppressive contributions.

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

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

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

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

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

Modeling visual performance differences 'around' the visual field: A computational observer approach

Visual performance depends on polar angle, even when eccentricity is held constant; on many psychophysical tasks observers perform best when stimuli are presented on the horizontal meridian, worst on the upper vertical, and intermediate on the lower vertical meridian. This variation in performance 'around' the visual field can be as pronounced as that of doubling the stimulus eccentricity. The causes of these asymmetries in performance are largely unknown. Some factors in the eye, e.g. cone density, are positively correlated with the reported variations in visual performance with polar angle. However, the question remains whether these correlations can quantitatively explain the perceptual differences observed 'around' the visual field. To investigate the extent to which the earliest stages of vision-optical quality and cone density-contribute to performance differences with polar angle, we created a computational observer model. The model uses the open-source software package ISETBIO to simulate an orientation discrimination task for which visual performance differs with polar angle. The model starts from the photons emitted by a display, which pass through simulated human optics with fixational eye movements, followed by cone isomerizations in the retina. Finally, we classify stimulus orientation using a support vector machine to learn a linear classifier on the photon absorptions. To account for the 30% increase in contrast thresholds for upper vertical compared to horizontal meridian, as observed psychophysically on the same task, our computational observer model would require either an increase of ~7 diopters of defocus or a reduction of 500% in cone density. These values far exceed the actual variations as a function of polar angle observed in human eyes. Therefore, we conclude that these factors in the eye only account for a small fraction of differences in visual performance with polar angle. Substantial additional asymmetries must arise in later retinal and/or cortical processing.

Binocular summation revisited: Beyond √2

Our ability to detect faint images is better with two eyes than with one, but how great is this improvement? A meta-analysis of 65 studies published across more than 5 decades shows definitively that psychophysical binocular summation (the ratio of binocular to monocular contrast sensitivity) is significantly greater than the canonical value of √2. Several methodological factors were also found to affect summation estimates. Binocular summation was significantly affected by both the spatial and temporal frequency of the stimulus, and stimulus speed (the ratio of temporal to spatial frequency) systematically predicts summation levels, with slow speeds (high spatial and low temporal frequencies) producing the strongest summation. We furthermore show that empirical summation estimates are affected by the ratio of monocular sensitivities, which varies across individuals, and is abnormal in visual disorders such as amblyopia. A simple modeling framework is presented to interpret the results of summation experiments. In combination with the empirical results, this model suggests that there is no single value for binocular summation, but instead that summation ratios depend on methodological factors that influence the strength of a nonlinearity occurring early in the visual pathway, before binocular combination of signals. Best practice methodological guidelines are proposed for obtaining accurate estimates of neural summation in future studies, including those involving patient groups with impaired binocular vision. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

How Ocular Dominance and Binocularity Are Reflected by the Population Receptive Field Properties

Purpose

The neural substrate of binocularity and sighting ocular dominance in humans is not clear. By utilizing the population receptive field (pRF) modeling technique, we explored whether these phenomena are associated with amplitude and pRF size differences.

Methods

The visual field maps of 13 subjects were scanned (3-T Skyra) while viewing drifting bar stimuli. Both eyes (binocular condition), the dominant eye and the nondominant eye (two monocular conditions) were stimulated in separate sessions. For each condition, pRF size and amplitude were assessed. Binocular summation ratios were calculated by dividing binocular by mean monocular values (amplitude and pRF size).

Results

No differences in pRF size were seen between the viewing conditions within each region, that is, either between binocular and monocular or between dominant and nondominant viewing conditions. Binocular amplitudes were higher than the monocular amplitudes, but similar among the dominant and nondominant eyes. Binocular summation ratios derived from amplitudes were significantly higher than one (∼1.2), while those ratios derived from pRF size were not. These effects were found in all studied areas along the visual hierarchy, starting in V1.

Conclusions

Neither the amplitude nor the pRF size show intereye difference and therefore cannot explain the different roles of the dominant and the nondominant eyes. Binocular, as compared to monocular vision, resulted in higher amplitudes, while receptive fields' sizes were similar, suggesting increased binocular response intensity as the basis for the binocular summation phenomenon. Our results could be applicable in imaging studies of monocular disease and studies that deal with nondisparity binocularity effects.

Binocular contrast summation and inhibition depends on spatial frequency, eccentricity and binocular disparity

PURPOSE

When central vision is compromised, visually-guided behaviour becomes dependent on peripheral retina, often at a preferred retinal locus (PRL). Previous studies have examined adaptation to central vision loss with monocular 2D paradigms, whereas in real tasks, patients make binocular eye movements to targets of various sizes and depth in 3D environments.

METHODS

We therefore examined monocular and binocular contrast sensitivity functions with a 26-AFC (alternate forced choice) band-pass filtered letter identification task at 2° or 6° eccentricity in observers with simulated central vision loss. Binocular stimuli were presented in corresponding or non-corresponding stereoscopic retinal locations. Gaze-contingent scotomas (0.5° radius disks of pink noise) were simulated independently in each eye with a 1000 Hz eye tracker and 120 Hz dichoptic shutter glasses.

RESULTS

Contrast sensitivity was higher for binocular than monocular conditions, but only exceeded probability summation at low-mid spatial frequencies in corresponding retinal locations. At high spatial frequencies or non-corresponding retinal locations, binocular contrast sensitivity showed evidence of interocular suppression.

CONCLUSIONS

These results suggest that binocular vision deficits may be underestimated by monocular vision tests and identify a method that can be used to select a PRL based on binocular contrast summation.

Binocular summation for reflexive eye movements

Psychophysical studies and our own subjective experience suggest that, in natural viewing conditions (i.e., at medium to high contrasts), monocularly and binocularly viewed scenes appear very similar, with the exception of the improved depth perception provided by stereopsis. This phenomenon is usually described as a lack of binocular summation. We show here that there is an exception to this rule: Ocular following eye movements induced by the sudden motion of a large stimulus, which we recorded from three human subjects, are much larger when both eyes see the moving stimulus, than when only one eye does. We further discovered that this binocular advantage is a function of the interocular correlation between the two monocular images: It is maximal when they are identical, and reduced when the two eyes are presented with different images. This is possible only if the neurons that underlie ocular following are sensitive to binocular disparity.

Subnormal Binocular Contrast Sensitivity Summation in Patients with Intermittent Exotropia

BACKGROUND

To evaluate binocular summation ratio using contrast sensitivity (CS) testing and correlation between binocular summation and stereoacuity, and control scale in intermittent exotropia (IXT).

METHODS

We conducted a prospective case-control study. Thirty-seven IXT and 41 controls were evaluated with both monocular and binocular CS testing. We compared the binocular summation ratio of IXT to that of controls. Near and distance stereoacuity was assessed and office-based control scale was evaluated. We investigated correlation between binocular CS summation ratio and stereoacuity, and control scale in IXT, respectively.

RESULTS

IXT had lower binocular CS summation ratio than controls at 1.5 and 3.0 cycles/degree (1.01 ± 1.02 vs. 1.62 ± 1.88 and 1.17 ± 0.96 vs. 1.86 ± 1.75, Both P < 0.05). We found significant correlation between binocular CS summation ratio at 3.0 cycles/degree and both near and distance stereoacuity (r = -0.411, P = 0.012 and r = -0.624, P = 0.005), and ratio at 1.5 cycles/degree also correlated significantly with distance stereoacuity (r = -0.397, P = 0.034) in the IXT. Binocular CS summation ratio was correlated to control scale at 1.5 and 3.0 cycles/degree (r = -0.327, P = 0.041 and r = -0.418, P = 0.028), and the ratio significantly differed in control scale groupings analysis at the same frequencies (Both P < 0.05).

CONCLUSION

Our findings of subnormal binocular CS summation ratio in IXT had correlation with stereoacuity and control scale suggest that binocular CS testing may be a useful method in assessing binocular visual function in IXT.

Binocular vision adaptively suppresses delayed monocular signals

A neutral density filter placed before one eye will produce a dichoptic imbalance in luminance, which attenuates responses to visual stimuli and lags neural signals from retina to cortex in the filtered eye. When stimuli are presented to both the filtered and unfiltered eye (i.e., binocularly), neural responses show little attenuation and no lag compared with their baseline counterpart. This suggests that binocular visual mechanisms must suppress the attenuated and delayed input from the filtered eye; however, the mechanisms involved remain unclear. Here, we used a Steady-State Visual Evoked Potential (SSVEP) technique to measure neural responses to monocularly and binocularly presented stimuli while observers wore an ND filter in front of their dominant eye. These data were well-described by a binocular summation model, which received the sinusoidal contrast modulation of the stimulus as input. We incorporated the influence of the ND filter with an impulse response function, which adjusted the input magnitude and phase in a biophysically plausible manner. The model captured the increase in attenuation and lag of neural signals for stimuli presented to the filtered eye as a function of filter strength, while also generating the filter phase-invariant responses from binocular presentation for EEG and psychophysical data. These results clarify how binocular visual mechanisms-specifically interocular suppression-can suppress the delayed and attenuated signals from the filtered eye and maintain normal neural signals under imbalanced luminance conditions.

Binocular contrast-gain control for natural scenes: Image structure and phase alignment

In the context of natural scenes, we applied the pattern-masking paradigm to investigate how image structure and phase alignment affect contrast-gain control in binocular vision. We measured the discrimination thresholds of bandpass-filtered natural-scene images (targets) under various types of pedestals. Our first experiment had four pedestal types: bandpass-filtered pedestals, unfiltered pedestals, notch-filtered pedestals (which enabled removal of the spatial frequency), and misaligned pedestals (which involved rotation of unfiltered pedestals). Our second experiment featured six types of pedestals: bandpass-filtered, unfiltered, and notch-filtered pedestals, and the corresponding phase-scrambled pedestals. The thresholds were compared for monocular, binocular, and dichoptic viewing configurations. The bandpass-filtered pedestal and unfiltered pedestals showed classic dipper shapes; the dipper shapes of the notch-filtered, misaligned, and phase-scrambled pedestals were weak. We adopted a two-stage binocular contrast-gain control model to describe our results. We deduced that the phase-alignment information influenced the contrast-gain control mechanism before the binocular summation stage and that the phase-alignment information and structural misalignment information caused relatively strong divisive inhibition in the monocular and interocular suppression stages. When the pedestals were phase-scrambled, the elimination of the interocular suppression processing was the most convincing explanation of the results. Thus, our results indicated that both phase-alignment information and similar image structures cause strong interocular suppression.

Identification and treatment of the visual processing asymmetry in MS patients with optic neuritis: The Pulfrich phenomenon

BACKGROUND

The Pulfrich phenomenon (PF) is the illusory perception that an object moving linearly along a 2-D plane appears to instead follow an elliptical 3-D trajectory, a consequence of inter-eye asymmetry in the timing of visual object identification in the visual cortex; with optic neuritis as a common etiology.

OBJECTIVE

We have designed an objective method to identify the presence and magnitude of the PF, in conjunction with a cooresponding strategy by which to abolish the effect; with monocular application of neutral density filters to the less affected fellow eye, in patients with MS and a history of optic neuropathy (e.g. related to acute optic neuritis or subclinical optic neuropathy).

METHODS

Twenty-three MS patients with a history of acute unilateral or bilateral optic neuritis, and ten healthy control subjects (HC) were recruited to participate in a pilot study to assess our strategy. Subjects were asked to indicate whether a linearly moving pendulum ball followed a linear 2-D path versus an illusory 3-D elliptical object-motion trajectory, by reporting the ball's approximation to one of nine horizontally-oriented colored wires that were positioned parallel to one another and horizontal to the linear pendulum path. Perceived motion of the bob that moved along wires behind or in front (along the 'Z' plane) of the middle reference wire indicated an illusory elliptical trajectory of ball motion consistent with the PF.

RESULTS

When the neutral density filter titration was applied to the fellow eye the severity of the PF decreased, eventually being fully abolished in all but one patient. The magnitude of neutral density filtering required correlated to the severity of the patient's initial PF magnitude (p < 0.001).

CONCLUSIONS

We ascertained the magnitude of the visual illusion associated with the PF, and the corresponding magnitude of neutral density filtering necessary to abolish it.

Binocular combination of luminance profiles

We develop and test a new two-dimensional model for binocular combination of the two eyes' luminance profiles. For first-order stimuli, the model assumes that one eye's luminance profile first goes through a luminance compressor, receives gain-control and gain-enhancement from the other eye, and then linearly combines the other eye's output profile. For second-order stimuli, rectification is added in the signal path of the model before the binocular combination site. Both the total contrast and luminance energies, weighted sums over both the space and spatial-frequency domains, were used in the interocular gain-control, while only the total contrast energy was used in the interocular gain-enhancement. To challenge the model, we performed a binocular brightness matching experiment over a large range of background and target luminances. The target stimulus was a dichoptic disc with a sharp edge that has an increment or decrement luminance from its background. The disk's interocular luminance ratio varied from trial to trial. To refine the model we tested three luminance compressors, five nested binocular combination models (including the Ding–Sperling and the DSKL models), and examined the presence or absence of total luminance energy in the model. We found that (1) installing a luminance compressor, either a logarithmic luminance function or luminance gain-control, (2) including both contrast and luminance energies, and (3) adding interocular gain-enhancement (the DSKL model) to a combined model significantly improved its performance. The combined model provides a systematic account of binocular luminance summation over a large range of luminance input levels. It gives a unified explanation of Fechner's paradox observed on a dark background, and a winner-take-all phenomenon observed on a light background. To further test the model, we conducted two additional experiments: luminance summation of discs with asymmetric contour information (Experiment 2), similar to Levelt (1965) and binocular combination of second-order contrast-modulated gratings (Experiment 3). We used the model obtained in Experiment 1 to predict the results of Experiments 2 and 3 and the results of our previous studies. Model simulations further refined the contrast space weight and contrast sensitivity functions that are installed in the model, and provide a reasonable account for rebalancing of imbalanced binocular vision by reducing the mean luminance in the dominant eye.

Blur perception throughout the visual field in myopia and emmetropia

We evaluated the ability of emmetropic and myopic observers to detect and discriminate blur across the retina under monocular or binocular viewing conditions. We recruited 39 young (23-30 years) healthy adults (n = 19 myopes) with best-corrected visual acuity 0.0 LogMAR (20/20) or better in each eye and no binocular or accommodative dysfunction. Monocular and binocular blur discrimination thresholds were measured as a function of pedestal blur using naturalistic stimuli with an adaptive 4AFC procedure. Stimuli were presented in a 46° diameter window at 40 cm. Gaussian blur pedestals were confined to an annulus at either 0°, 4°, 8°, or 12° eccentricity, with a blur increment applied to only one quadrant of the image. The adaptive procedure efficiently estimated a dipper shaped blur discrimination threshold function with two parameters: intrinsic blur and blur sensitivity. The amount of intrinsic blur increased for retinal eccentricities beyond 4° (p < 0.001) and was lower in binocular than monocular conditions (p < 0.001), but was similar across refractive groups (p = 0.47). Blur sensitivity decreased with retinal eccentricity (p < 0.001) and was highest for binocular viewing, but only for central vision (p < 0.05). Myopes showed worse blur sensitivity than emmetropes monocularly (p < 0.05) but not binocularly (p = 0.66). As expected, blur perception worsens in the visual periphery and binocular summation is most evident in central vision. Furthermore, myopes exhibit a monocular impairment in blur sensitivity that improves under binocular conditions. Implications for the development of myopia are discussed.

High and Low Contrast Visual Acuity Are Not Affected in Amyotrophic Lateral Sclerosis

The afferent visual system may be affected by neuro-degeneration in amyotrophic lateral sclerosis (ALS) based on observations of visual function impairment and retinal inclusions on histopathology in ALS patients. To test the hypothesis that visual acuity is impaired in ALS, we compared three measures of visual acuity in ALS patients (n = 25) attending a multidisciplinary ALS clinic and age matched control subjects (n = 25). Bilateral monocular and binocular visual acuities were assessed using high contrast (black letters on white background) and low contrast (2.5%, 1.25% grey letters on white background) visual acuity charts under controlled lighting conditions following refraction. Binocular summation was calculated as the difference between binocular and best monocular acuity scores. There were no associations between binocular or monocular high contrast visual acuity or low contrast visual acuity and amyotrophic lateral sclerosis diagnosis (generalized estimating equation models accounting for age). Binocular summation was similar in both amyotrophic lateral sclerosis and control subjects. There was a small magnitude association between increased duration of ALS symptoms and reduced 1.25% low contrast visual acuity. This study does not confirm prior observations of impaired visual acuity in patients with amyotrophic lateral sclerosis and does not support this particular measure of visual function for use in broad scale assessment of visual pathway involvement in ALS patients.

Binocular combination of stimulus orientation

When two sine waves that differ slightly in orientation are presented to the two eyes separately, a single cyclopean sine wave is perceived. However, it is unclear how the brain calculates its orientation. Here, we used a signal detection rating method to estimate the perceived orientation when the two eyes were presented with Gabor patches that differed in both orientation and contrast. We found a nearly linear combination of orientation when both targets had the same contrast. However, the binocular percept shifted away from the linear prediction towards the orientation with the higher contrast, depending on both the base contrast and the contrast ratio. We found that stimuli that differ slightly in orientation are combined into a single percept, similarly for monocular and binocular presentation, with a bias that depends on the interocular contrast ratio. Our results are well fitted by gain-control models, and are consistent with a previous study that favoured the DSKL model that successfully predicts binocular phase and contrast combination and binocular contrast discrimination. In this model, the departures from linearity may be explained on the basis of mutual suppression and mutual enhancement, both of which are stronger under dichoptic than monocular conditions.

Binocular functional architecture for detection of contrast-modulated gratings

Combination of signals from the two eyes is the gateway to stereo vision. To gain insight into binocular signal processing, we studied binocular summation for luminance-modulated gratings (L or LM) and contrast-modulated gratings (CM). We measured 2AFC detection thresholds for a signal grating (0.75c/deg, 216ms) shown to one eye, both eyes, or both eyes out-of-phase. For LM and CM, the carrier noise was in both eyes, even when the signal was monocular. Mean binocular thresholds for luminance gratings (L) were 5.4dB better than monocular thresholds - close to perfect linear summation (6dB). For LM and CM the binocular advantage was again 5-6dB, even when the carrier noise was uncorrelated, anti-correlated, or at orthogonal orientations in the two eyes. Binocular combination for CM probably arises from summation of envelope responses, and not from summation of these conflicting carrier patterns. Antiphase signals produced no binocular advantage, but thresholds were about 1-3dB higher than monocular ones. This is not consistent with simple linear summation, which should give complete cancellation and unmeasurably high thresholds. We propose a three-channel model in which noisy monocular responses to the envelope are binocularly combined in a contrast-weighted sum, but also remain separately available to perception via a max operator. Vision selects the largest of the three responses. With in-phase gratings the binocular channel dominates, but antiphase gratings cancel in the binocular channel and the monocular channels mediate detection. The small antiphase disadvantage might be explained by a subtle influence of background responses on binocular and monocular detection.

Binocular contrast discrimination needs monocular multiplicative noise

The effects of signal and noise on contrast discrimination are difficult to separate because of a singularity in the signal-detection-theory model of two-alternative forced-choice contrast discrimination (Katkov, Tsodyks, & Sagi, 2006). In this article, we show that it is possible to eliminate the singularity by combining that model with a binocular combination model to fit monocular, dichoptic, and binocular contrast discrimination. We performed three experiments using identical stimuli to measure the perceived phase, perceived contrast, and contrast discrimination of a cyclopean sine wave. In the absence of a fixation point, we found a binocular advantage in contrast discrimination both at low contrasts (<4%), consistent with previous studies, and at high contrasts (≥34%), which has not been previously reported. However, control experiments showed no binocular advantage at high contrasts in the presence of a fixation point or for observers without accommodation. We evaluated two putative contrast-discrimination mechanisms: a nonlinear contrast transducer and multiplicative noise (MN). A binocular combination model (the DSKL model; Ding, Klein, & Levi, 2013b) was first fitted to both the perceived-phase and the perceived-contrast data sets, then combined with either the nonlinear contrast transducer or the MN mechanism to fit the contrast-discrimination data. We found that the best model combined the DSKL model with early MN. Model simulations showed that, after going through interocular suppression, the uncorrelated noise in the two eyes became anticorrelated, resulting in less binocular noise and therefore a binocular advantage in the discrimination task. Combining a nonlinear contrast transducer or MN with a binocular combination model (DSKL) provides a powerful method for evaluating the two putative contrast-discrimination mechanisms.

Orientation tuning of binocular summation: a comparison of colour to achromatic contrast

A key function of the primary visual cortex is to combine the input from the two eyes into a unified binocular percept. At low, near threshold, contrasts a process of summation occurs if the visual inputs from the two eyes are similar. Here we measure the orientation tuning of binocular summation for chromatic and equivalent achromatic contrast. We derive estimates of orientation tuning by measuring binocular summation as a function of the orientation difference between two sinusoidal gratings presented dichoptically to different eyes. We then use a model to estimate the orientation bandwidth of the neural detectors underlying the binocular combination. We find that orientation bandwidths are similar for chromatic and achromatic stimuli at both low (0.375 c/deg) and mid (1.5 c/deg) spatial frequencies, with an overall average of 29 ± 3 degs (HWHH, s.e.m). This effect occurs despite the overall greater binocular summation found for the low spatial frequency chromatic stimuli. These results suggest that similar, oriented processes underlie both chromatic and achromatic binocular contrast combination. The non-oriented detection process found in colour vision at low spatial frequencies under monocular viewing is not evident at the binocular combination stage.

Chromatic detection from cone photoreceptors to V1 neurons to behavior in rhesus monkeys

Chromatic sensitivity cannot exceed limits set by noise in the cone photoreceptors. To determine how close neurophysiological and psychophysical chromatic sensitivity come to these limits, we developed a parameter-free model of stimulus encoding in the cone outer segments, and we compared the sensitivity of the model to the psychophysical sensitivity of monkeys performing a detection task and to the sensitivity of individual V1 neurons. Modeled cones had a temporal impulse response and a noise power spectrum that were derived from in vitro recordings of macaque cones, and V1 recordings were made during performance of the detection task. The sensitivity of the simulated cone mosaic, the V1 neurons, and the monkeys were tightly yoked for low-spatiotemporal-frequency isoluminant modulations, indicating high-fidelity signal transmission for this class of stimuli. Under the conditions of our experiments and the assumptions for our model, the signal-to-noise ratio for these stimuli dropped by a factor of ∼3 between the cones and perception. Populations of weakly correlated V1 neurons narrowly exceeded the monkeys' chromatic sensitivity but fell well short of the cones' chromatic sensitivity, suggesting that most of the behavior-limiting noise lies between the cone outer segments and the output of V1. The sensitivity gap between the cones and behavior for achromatic stimuli was larger than for chromatic stimuli, indicating greater postreceptoral noise. The cone mosaic model provides a means to compare visual sensitivity across disparate stimuli and to identify sources of noise that limit visual sensitivity.

Positive and negative polarity contrast sensitivity measuring app

Contrast sensitivity (CS) quantifies an observer's ability to detect the smallest (threshold) luminance difference between a target and its surrounding. In clinical settings, printed letter contrast charts are commonly used, and the contrast of the letter stimuli is specified by the Weber contrast definition. Those paper-printed charts use negative polarity contrast (NP, dark letters on bright background) and are not available with positive polarity contrast (PP, bright letters on dark background), as needed in a number of applications. We implemented a mobile CS measuring app supporting both NP and PP contrast stimuli that mimic the paper charts for NP. A novel modified Weber definition was developed to specify the contrast of PP letters. The validity of the app is established in comparison with the paper chart. We found that our app generates more accurate and a wider range of contrast stimuli than the paper chart (especially at the critical high CS, low contrast range), and found a clear difference between NP and PP CS measures (CSNP>CSPP) despite the symmetry afforded by the modified Weber contrast definition. Our app provides a convenient way to measure CS in both lighted and dark environments.

Distance and near visual performance in pseudophakic eyes with simulated spherical and astigmatic blur

PURPOSE

Increased depth-of-focus after cataract surgery is dependent on various ocular characteristics, including refractive astigmatism. This work studied the influence of meridional blur produced by astigmatism, and spherical blur, on distance and near visual performance in pseudophakic eyes.

METHODS

Visual performance was assessed monocularly in 59 pseudophakes (mean ± standard deviation, 66.9 ± 7.53 years) at distance (3.0 m) and near (0.33 m) using high- (HC: 94%) and low-contrast (LC: 10%) visual acuity (VA) charts. Reading performance (maximum reading speed [MRS], threshold print size [TPS] and reading acuity [RA]) was evaluated at near using a reading chart. Four refractive conditions at distance and near were monocularly simulated using the following trial lenses: distance (in-focus: ± 0.00 DS, with-the-rule [WTR] astigmatism: +2.00 × 180; against-the-rule [ATR] astigmatism: +2.00 × 90; spherical defocus: +1.00 DS) and near (in-focus: +3.00 DS, WTR astigmatism: +2.00 × 180; ATR astigmatism: +2.00 × 90, spherical blur: ±0.00 DS).

RESULTS

Distance high- and low-contrast VA were degraded by spherical and astigmatic blur in comparison to the distance in-focus condition (p < 0.001 for all), with astigmatism in either of the forms being more harmful than spherical blur (p < 0.001 for all). At near, best HC- and LCVA were attained with full near correction (p < 0.001 for all) followed by the ATR and WTR astigmatism. For all conditions, the VA measured was independent of whether the astigmatism was WTR or ATR. However, MRS was higher with +3.00 DS and ATR astigmatism compared to the other conditions. RA and TPS significantly strengthened the weak pattern seen with HCVA at near, with ATR astigmatism allowing significantly better RA and TPS than WTR astigmatism (p < 0.001 for all).

CONCLUSIONS

Simple myopic astigmatism improved near visual performance in pseudophakic eyes at the expense of some deterioration in distance performance. ATR astigmatism degraded VA at distance marginally more than WTR astigmatism and provided a marginally better VA at near. However, the benefit at near was more explicit when measured by reading performance, confirming the role of blur orientation on visual performance.

Binocular summation of chance decisions

Seeing with two eyes usually helps one respond faster. Here we show that with ambiguous stimuli, binocular viewing can paradoxically slow down reaction time. This is explained by the observers basing their decision on a noisy neuronal representation within the visual system, with the added noise breaking the symmetry between the two possible interpretations. Binocular integration improves the representation by reducing the noise, increasing ambiguity, and decision time. The neuronal Accumulator (Race) model is applied to quantify the underlying binocular integration. The model accounts for the distributions of reaction times, and predicts suboptimal integration between eyes. We conclude that under ambiguous stimulation neuronal noise within the visual system determines responses.

Index of contrast sensitivity (ICS) in pseudophakic eyes with different intraocular lens designs

PURPOSE

To evaluate the index of contrast sensitivity (ICS) in eyes after cataract surgery with various intraocular lens designs and to compare with the area under log contrast sensitivity curve (AULCSF).

METHODS

The study comprised 395 eyes of 198 patients in the age of 73.1 ± 7.86 years receiving 11 different aspheric IOL designs (aberration-free and correcting) and a spherical (IOL) as control group. Follow-up examination after bilateral cataract surgery was completed within 71 ± 21.4 days after second IOL implantation. Patients underwent complete examination and biometry before surgery. The follow-up examination included visual acuity, pupil diameter, residual spherical aberration and mesopic as well as photopic contrast sensitivity (CS) measured with the Optec 6500 Functional Vision Analyzer. From the contrast sensitivity, we calculated the ICS according to Haughom and Strand.

RESULTS

The median mesopic ICS was -144, -131 and -85, and the median photopic ICS was -289, -285 and -212 for the spherical, aberration-free and aberration-correcting IOL group, respectively. While we could not detect a significant difference between the aberration groups in some spatial frequencies, the ICS showed a significant difference between the aberration-correcting and the aberration-free or the spherical group, respectively. No significant difference was found between the aberration-free and the spherical group.

CONCLUSIONS

The ICS is a useful index for evaluation of overall CS and comparison of different patient groups. With aberration-correcting IOLs, ICS was statistically better than with aberration-free or spherical IOLs, whereas the latter two showed no significant difference.

Clinicopathologic report of ocular involvement in ALS patients with C9orf72 mutation

Our objective was to present clinicopathologic evidence of anterior visual pathway involvement in patients with amyotrophic lateral sclerosis (ALS) secondary to a C9orf72 mutation. Two related patients from an extended pedigree with ALS and GGGGCC hexanucleotide repeat expansion in the C9orf72 gene (C9-ALS) underwent neuro-ophthalmologic examination. Following death and tissue donation of the younger ALS patient, histopathologic examination of the retina, optic nerve and central nervous system (CNS) was performed. Ophthalmologic examination revealed contrast sensitivity impairment in the younger C9-ALS patient. Immunohistochemistry performed on this patient's donor tissue demonstrated p62-positive, pTDP43-negative perinuclear inclusions in the inner nuclear layer of the retina and CNS. Further colocalization with GLT-1 and recoverin suggested that the majority of retinal p62-positive inclusions are found within cone bipolar cells as well as some amacrine and horizontal cells. In conclusion, this is the first report that identifies disease-specific pathologic inclusions in the anterior visual pathway of a patient with a C9orf72 mutation. Cone bipolar cell involvement within the inner nuclear layer of the retina may explain the observed subtle visual function deficiencies in this patient. Further clinical and histopathologic studies are needed to fully characterize a larger population of C9-ALS patients and explore these findings in other forms of ALS.

Normative reference ranges for binocular summation as a function of age for low contrast letter charts

PURPOSE

Binocular summation (BiS), defined as the superiority of binocular over monocular viewing on visual threshold tasks, is most often studied in laboratory settings. Few studies have evaluated BiS with readily available clinical tools. Low contrast acuity (LCA) charts are increasingly popular in clinical research, yet their utility in detecting BiS has not been evaluated.

METHODS

129 normal subjects aged 3 to 85 years were prospectively enrolled and underwent monocular and binocular testing using 2.5% and 1.25% Sloan LCA charts and Pelli-Robson (PR) contrast sensitivity (CS) charts at an academic institution. Subjects also underwent similar testing with Early Treatment Diabetic Retinopathy Study (ETDRS) VA charts. BiS was calculated as the difference between the better eye and binocular scores.

RESULTS

Monocular and binocular scores decreased with increasing age for all metrics. The mean (±SD) BiS scores for 2.5% and 1.25% Sloan LCA were 6±4.5 and 3±5 letters, respectively. BiS score was 4.5±7 letters for PR charts and 2±3 letters for ETDRS VA. There was a significant effect of age on BiS for the low contrast metrics (P≤0.001 for all), but not for high-contrast ETDRS VA. Linear regression revealed significant associations between increased interocular difference (IOD) in acuity and decreased BiS for all tests, and associations between increasing age and decreased BiS for the LCA tests.

CONCLUSION

Of the clinical tests evaluated, 2.5% and 1.25% Sloan LCA charts most readily demonstrated BiS in young normal subjects. BiS declined with increasing age and increased IOD. Median values presented in this study may be useful for future clinical studies utilizing LCA.

To characterize contrast detection, noise should be extended, not localized

Adding noise to a stimulus is useful to characterize visual processing. To avoid triggering a processing strategy shift between the processing in low and high noise, Allard and Cavanagh (2011) recommended using noise that is extended as a function of all dimensions such as space, time, frequency and orientation. Contrariwise, to avoid cross-channel suppression affecting contrast detection, Baker and Meese (2012) suggested using noise that is localized as a function of all dimensions, namely "0D noise," which basically consists in randomly jittering the target contrast (and, for blank intervals or catch trials, jittering the contrast of an identical zero-contrast signal). Here we argue that contrast thresholds in extended noise are not contaminated by noise-induced cross-channel suppression because contrast gains affect signal and noise by the same proportion leaving the signal-to-noise ratio intact. We also review empirical findings showing that detecting a target in 0D noise involves a different processing strategy than detecting in absence of noise or in extended noise. Given that internal noise is extended as a function of all dimensions, we therefore recommend using external noise that is also extended as a function of all dimensions when assuming that the same processing strategy operates in low and high noise.

Two Eyes Are Better Than One-Binocular Summation of Dark Vision in Healthy Individuals and Patients with Chronic Respiratory Disease

We compared monocular and binocular absolute thresholds of dark adaptation in two separate study populations. Eighteen healthy individuals (Group A) and 13 patients with chronic respiratory insufficiency (Group B) were examined three times each by computerised dark adaptometry with simultaneous but separate recordings from each eye and binocularly. The respiratory patients received oxygen supplement at visits 1 and 3. In Group A, at all three visits, binocular dark adaptation was significantly more sensitive (40.5%) than monocular dark adaptation with either eye. In Group B, at visits 1 and 3, binocular dark adaptation was also significantly more sensitive than monocular dark adaptation (40.5% higher than the right and 47% higher than the left eye). However, in Group B, at visit 2 without oxygen treatment, no significant differences were observed between monocular and binocular sensitivities. Binocular dark vision was superior to monocular dark vision in healthy individuals and in patients with respiratory insufficiency that were provided oxygen supplementation. Furthermore, deficit in oxygen seems to affect binocular summation, perhaps by impaired enhancement in the central nervous system.

Quality assessment of stereoscopic 3D image compression by binocular integration behaviors

The objective approaches of 3D image quality assessment play a key role for the development of compression standards and various 3D multimedia applications. The quality assessment of 3D images faces more new challenges, such as asymmetric stereo compression, depth perception, and virtual view synthesis, than its 2D counterparts. In addition, the widely used 2D image quality metrics (e.g., PSNR and SSIM) cannot be directly applied to deal with these newly introduced challenges. This statement can be verified by the low correlation between the computed objective measures and the subjectively measured mean opinion scores (MOSs), when 3D images are the tested targets. In order to meet these newly introduced challenges, in this paper, besides traditional 2D image metrics, the binocular integration behaviors-the binocular combination and the binocular frequency integration, are utilized as the bases for measuring the quality of stereoscopic 3D images. The effectiveness of the proposed metrics is verified by conducting subjective evaluations on publicly available stereoscopic image databases. Experimental results show that significant consistency could be reached between the measured MOS and the proposed metrics, in which the correlation coefficient between them can go up to 0.88. Furthermore, we found that the proposed metrics can also address the quality assessment of the synthesized color-plus-depth 3D images well. Therefore, it is our belief that the binocular integration behaviors are important factors in the development of objective quality assessment for 3D images.

Effects of visual noise on binocular summation in patients with strabismus without amblyopia

PURPOSE

Patients with strabismus often complain of difficulty navigating through visually stimulating environments without clear explanation for this symptom. Binocular summation (BiS), defined as the superiority of binocular over monocular viewing on visual threshold tasks, is decreased in conditions that cause large interocular differences in visual acuity, but is not well studied in strabismic populations without amblyopia. The authors hypothesized that strabismus may lead to decreased BiS for tasks related to discrimination within increased background complexity. The goal of this study was to test the extent of BiS in patients with strabismus during discrimination of a luminance target disk embedded in visual noise.

METHODS

Participants included 10 exotropic, 10 esotropic, and 13 age-matched control patients. Performance of a task detecting a luminance-target was measured at 0, 10, and 20 μdeg(2) of visual noise for binocular and monocular conditions. BiS was calculated as the ratio of binocular contrast sensitivity to monocular contrast sensitivity for the target embedded in noise.

RESULTS

Patients with strabismus had lower BiS values than controls, with a significant decrease on linear regression in patients with strabismus at 20 μdeg(2) of noise (P = .05), with a trend toward significance at 10 μdeg(2) of noise (P = .07). Patients with strabismus showed a mean binocular inhibition (summation ratio < 1) at both noise levels.

CONCLUSIONS

These findings support the hypothesis that strabismus can lead to decreased BiS and even binocular inhibition. Despite literature showing enhanced BiS in visually demanding situations such as high levels of visual noise or low contrast, BiS was not significantly affected by visual noise in either group.

Functional burden of strabismus: decreased binocular summation and binocular inhibition

IMPORTANCE

Binocular summation (BiS) is defined as the superiority of visual function for binocular over monocular viewing. Binocular summation decreases with age and large interocular differences in visual acuity. To our knowledge, BiS has not heretofore been well studied as a functional measure of binocularity in strabismus.

OBJECTIVE

To evaluate the effect of strabismus on BiS using a battery of psychophysical tasks that are clinically relevant and easy to use and to determine whether strabismus is associated with binocular inhibition in extreme cases.

DESIGN

Case-control study.

SETTING

University-based eye institute.

PARTICIPANTS

Strabismic patients recruited during 2010 to 2012 from a preoperative clinic and control participants with no history of eye disease other than refractive error.

INTERVENTION

A battery of psychophysical and electrophysiological tests including Early Treatment Diabetic Retinopathy Study visual acuity, Sloan low-contrast acuity (LCA) (2.5% and 1.25%), Pelli-Robson contrast sensitivity, and sweep visual evoked potential contrast sensitivity.

MAIN OUTCOME AND MEASURE

Binocular summation was calculated as the ratio between binocular and better-eye individual scores.

RESULTS

Sixty strabismic and 80 control participants were prospectively examined (age range, 8-60 years). Mean BiS was significantly lower in the strabismic patients than controls for LCA (2.5% and 1.25%, P = .005 and <.001, respectively). For 1.25% LCA, strabismic patients had a mean BiS score less than 1, indicating binocular inhibition (ie, the binocular score was less than that of the better eye's monocular score). There was no significant difference in BiS for contrast thresholds on Early Treatment Diabetic Retinopathy Study visual acuity, Pelli-Robson contrast sensitivity, or sweep visual evoked potential contrast sensitivity. Regression analysis revealed a significant worsening of BiS with strabismus for 2.5% (P = .009) and 1.25% (P = .002) LCA, after accounting for age.

CONCLUSIONS AND RELEVANCE

Strabismic patients demonstrate subnormal BiS and even binocular inhibition for LCA, suggesting that strabismus impairs visual function more than previously appreciated. This may explain why strabismic patients who are not diplopic close 1 eye in visually demanding situations. This finding clarifies the visual deficits impacting quality of life in strabismic patients and may represent a novel measure by which to evaluate and monitor function in strabismus.