Binocular contrast summation and inhibition in amblyopia

The monocular contrast sensitivity loss in amblyopia is well documented. We investigated the influence of interocular sensitivity difference on binocular contrast sensitivity in amblyopia. Monocular and binocular contrast sensitivity functions of six amblyopes (three strabismic and three anisometropic) were measured. The monocular contrast sensitivity loss depended on the type of amblyope. Anisometropic amblyopes generally showed high frequency losses. Strabismic amblyopes showed losses at both low and high spatial frequencies. Binocular performance was assessed in terms of binocular ratios (binocular/non-amblyopic). A binocular ratio greater than 1 indicates binocular summation (binocular > monocular) while a ratio less than 1 shows binocular inhibition (binocular < monocular). In all subjects, the binocular ratio depended on the difference between the amblyopic and the non-amblyopic eye. Minimal interocular difference produced binocular summation, the magnitude of which decreased as the difference between the two eyes increased. Further increases in the monocular difference produced binocular inhibition. Anisometropic amblyopes showed a greater degree of binocular summation at low spatial frequencies compared to strabismic amblyopes. Both types of amblyopes showed binocular inhibition at high spatial frequencies. Clinical implications of binocular summation and inhibition in amblyopia are discussed.     

Binocular interaction in the peripheral visual field of humans with strabismic and anisometropic amblyopia

Residual binocular interaction was tested across the visual field of 9 strabismic and 2 anisometropic amblyopes in threshold and suprathreshold conditions.Binocular summation and interocular transfer of adaptation after-effects, both absent or very much reduced in the central region of the visual field of squint amblyopes, were highly significant in the periphery. The regions of preserved binocularity were well correlated with the areas for which there was no acuity loss and no interocular suppression. In anisometropic amblyopes, both binocular summation and interocular transfer of adaptation were lost at all tested eccentricities.Dynamic local stereopsis was also present in the periphery, but not in the central field of both squint and anisometropic amblyopes.     

Binocular summation to gratings in the peripheral field in older subjects is spatial frequency dependent

PURPOSE: Binocular summation for contrast detection in the fovea is lower in older normal subjects. At present, no binocular summation data exist for contrast detection in older subjects in the peripheral field. METHODS: Monocular and binocular contrast thresholds were measured for sine-wave gratings of 1 c/deg (F1) and 4 c/deg (F4) in the fovea in young and older visually normal subjects. Measurements were also obtained at 8 degrees in the superior field for gratings of 1 c/deg (P1) and 4 c/deg (P4). RESULTS: Binocular summation ratios in young subjects at the fovea and periphery were not statistically different at either spatial frequency. In older subjects, analysis of the relevant conditions showed significantly lower summation ratios for: P4/F4 combination (p = 0.05), and P4/P1 combination (p = 0.03). No significant differences existed for F1/P1 and F1/F4 combinations (p > 0.05). CONCLUSIONS: Lowest binocular summation ratio is shown with older subjects for gratings of 4 c/deg in the periphery (P4). Further analysis suggests the possibility that this is due to a larger relative loss in binocular sensitivity. Data are discussed in terms of selective loss of binocular neurons mediating peripheral sensitivity in the ageing eye.     

Binocular summation in the peripheral visual field

Purpose: To investigate binocular summation for a light detection task in the central and peripheral retina along 4 meridians. Methods: 10 young subjects aged 22-30 years (mean 26 years) with healthy eyes and normal visual fields participated in the study. A custom-designed program on a Humphrey Visual Field Analyser measured thresholds at the fovea and at 5°, 10°, 15°, 25°, 40° and 55° along the horizontal meridian, and at 5°, 10°, 15°, 25°, 40° along the vertical, 45° oblique and 135° oblique meridians. Right and left monocular fields and binocular fields were measured on each subject using white spot stimuli (Goldmann sizes I and III). Binocular summation ratios were calculated for each eccentricity by dividing binocular sensitivity be the better monocular sensitivity. Results and conclusion: As expected, binocular and monocular sensitivities decreased with increasing eccentricity. The mean binocular summation ratio at the fovea was 1.38 for size I stimulus and 1.42 for size III. Binocular summation was demonstrated in the peripheral retina along all meridians and is of similar magnitude to foveal binocular summation. ANOVA showed that binocular summation was significantly higher in the vertical meridian than horizontal meridian for both size I and III stimuli. There was no significant variation in binocular summation ratios with eccentricity along any of the 4 meridians examined.     

Selective losses in binocular vision in anisometropic amblyopes

Human anisometropic amblyopes typically exhibit reduced contrast sensitivity in the amblyopic eye, especially at higher spatial frequencies. We determined whether this spatial frequency selective loss in contrast sensitivity is accompanied by selective losses in binocular function. Binocular summation (the improvement in one eye's detection performance produced by a subthreshold pattern presented to the fellow eye) was measured at several spatial frequencies. Normal observers exhibited equivalent binouclar summation at all spatial frequencies, whereas all anisometropic amblyopes exhibited normal summation at low spatial frequencies but none at high spatial frequencies. Stereoacuity (minimum resolvable disparity) was also measured as a function of spatial frequency. For normal observers, stereoacuity was best at the highest spatial frequency; for anisometropes stereoacuity was normal at low spatial frequencies, subnormal at intermediate spatial frequencies, and unmeasurable at higher spatial frequencies. Anisometropia may represent a form of selective binocular deprivation that affects neural mechanisms underlying binocular summation and stereopsis.     

The development of the Quality of Life Impact of Refractive Correction (QIRC) questionnaire

Purpose:  Binocular summation (binocular sensitivity > monocular sensitivity) and binocular inhibition (binocular sensitivity < monocular sensitivity) have been shown in various laboratory and clinical situations. The occurrence of habitual binocular summation/inhibition is unknown. The aim of this study was to investigate binocular performance in 100 'ocularly normal' subjects over the age of 45 years attending an optometric practice. In addition, very little data exist on habitual quality of life (QOL) scores in normal subjects. We aimed to investigate whether binocular performance for different visual tasks affected QOL scores.
Methods:  Right eye, left eye and binocular contrast sensitivity (CS), near and distance logMAR acuity (VA), and face recognition (FR) were measured. Vision-related quality of life (QOL) scores were measured using the standard NEI–VFQ 25 questionnaire.
Results:  Habitual binocular summation ratios were similar to those reported under laboratory conditions. Binocular summation ratios for visual acuity were 26%, 47% for CS and 11% for FR. Binocular inhibition was shown by 12 subjects with VA, seven people in CS and 31 subjects with FR. No significant associations were shown between the summation ratios of distance VA, near VA, CS & FR and the 13 different QOL categories.
Conclusions:  Face recognition showed the poorest summation as it falls under the complex pattern recognition known to produce lower summation under laboratory conditions. Binocular summation ratios were not significantly associated with either the age of the subject or to the differences between the two eyes. The lack of correlation between binocular summation ratios and QOL was because of the high scores shown by the normal subjects.     

Effect of stimulus size on binocular summation within the binocular visual field

PURPOSE: To study the influence of stimulus sizes on binocular summation using the modified Octopus 201 combined with a space synoptophore. METHODS: Four normal subjects, aged 21 to 26 were tested. Using the SARGON program, we designed a new program to test 37 points in the central 6 degrees visual field. Sensitivity of the central 6 degrees visual field under monocular and binocular conditions was measured while the fusion patterns were displayed on the space synoptophore. The visual fields were measured at stimulus sizes 1, 3, and 5. RESULTS: The visual sensitivity under binocular conditions was higher than under monocular conditions for all the stimulus sizes. Binocular summation for stimulus size 1 was present in a flat form, for stimulus size 3 in a convex form, and for stimulus size 5 in a concave form in the central 6 degrees visual field. CONCLUSION: Binocular summation differed in stimulus size and retinal eccentricity. Binocular summation for stimulus size 3 increased in the fovea and it increased for stimulus size 5 in the peripheral area in the central 6 degrees visual field.     

A psychophysical study of human binocular interactions in normal and amblyopic visual systems

During infancy and childhood, spatial contrast sensitivity and alignment sensitivity undergo maturation, and during this period the visual system has considerable plasticity. The purpose of this study was to compare the nature of interocular interactions of these spatial functions in normally sighted children and adults, and to study the extent to which interocular interactions are impaired in anisometropic amblyopia. Spatial functions were measured under three viewing conditions: monocular (fellow eye occluded), dichoptic (uniform stimulus presented to the fellow eye but with a peripheral fusion lock), and binocular. Measurements were made in each eye during monocular and dichoptic viewing. In the contrast sensitivity task, Gabor stimuli were presented in one of two temporal intervals. For the alignment task, a three-element Gabor stimulus was used. The task of the subject was to indicate the direction of displacement of the middle patch with respect to the outer patches. The findings indicate that in children, binocular contrast sensitivity was better than monocular (binocular summation) but so too was dichoptic sensitivity (dichoptic summation). The magnitude of binocular/dichoptic summation was significantly greater in children than in normally sighted adults for contrast sensitivity, but not for alignment sensitivity. In anisometropic amblyopes, however, we find that for the group as a whole the amblyopic eye does not benefit when the fellow eye views a dichoptic stimulus, compared to dark occlusion of that eye. In addition, we found considerable inter-individual variation within the amblyopic group. Implications of these findings for techniques used in vision therapy are discussed.     

Binocular summation of contrast remains intact in strabismic amblyopia

PURPOSE: Strabismic amblyopia is typically associated with several visual deficits, including loss of contrast sensitivity in the amblyopic eye and abnormal binocular vision. Binocular summation ratios (BSRs) are usually assessed by comparing contrast sensitivity for binocular stimuli (sens(BIN)) with that measured in the good eye alone (sens(GOOD)), giving BSR = sens(BIN)/sens(GOOD). This calculation provides an operational index of clinical binocular function, but does not assess whether neuronal mechanisms for binocular summation of contrast remain intact. This study was conducted to investigate this question. METHODS: Horizontal sine-wave gratings were used as stimuli (3 or 9 cyc/deg; 200 ms), and the conventional method of assessment (above) was compared with one in which the contrast in the amblyopic eye was adjusted (normalized) to equate monocular sensitivities. RESULTS: In nine strabismic amblyopes (mean age, 32 years), the results confirmed that the BSR was close to unity when the conventional method was used (little or no binocular advantage), but increased to approximately radical2 or higher when the normalization method was used. The results were similar to those for normal control subjects (n = 3; mean age, 38 years) and were consistent with the physiological summation of contrast between the eyes. When the normal observers performed the experiments with a neutral-density (ND) filter in front of one eye, their performance was similar to that of the amblyopes in both methods of assessment. CONCLUSIONS: The results indicate that strabismic amblyopes have mechanisms for binocular summation of contrast and that the amblyopic deficits of binocularity can be simulated with an ND filter. The implications of these results for best clinical practice are discussed.     

Regional variations in binocular summation across the visual field

Binocular summation for a contrast detection task was measured as a function of eccentricity and target size along the horizontal and vertical meridians for ten young normal subjects. Binocular summation at the fovea was of the order of 1.4 for all target sizes, although there was some intersubject variation. Binocular summation was highest along the vertical meridian. With increasing eccentricity from the fovea, binocular summation for target size I (0.108 degrees projected diameter) decreased, remained relatively constant for target size III (0.431 degrees projected diameter) and increased with increasing eccentricity from the fovea for target size V (1.724 degrees projected diameter). For target sizes I and III, binocular summation was present only when interocular differences in sensitivity were under 5 dB, for target size V this relationship did not hold. Influences such as stimulation of corresponding retinal points and cortical representation are considered.     

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

PurposeBinocular 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.MethodsThis 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.ResultsAll 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 = (Left^m + Right^m)^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.ConclusionsCortical 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.     

The effect of pupil size on binocular summation at suprathreshold conditions

PURPOSE: To determine the influence of retinal illuminance on monocular or binocular visual reaction time (VRT). METHODS: On two normal subjects, uniform circular stimuli were presented with respect to a reference stimulus at the fovea under suprathreshold conditions, and the detection of positive and negative luminance variations was recorded. Binocular and monocular reaction times were measured (by the index finger pressing on a mouse key) for viewing with both natural and artificial pupils. RESULTS: Binocular reaction times were shorter than monocular reaction times; nevertheless, this binocular-summation effect was less marked in trials with the artificial pupil. Analyses of binocular-summation ratios for contrast changes for both pupil types indicated maximum and minimum binocular-saturation values depending on contrast variations in both positive and negative luminance changes. CONCLUSIONS: Binocular summation can be influenced by pupil size under suprathreshold conditions. Results are discussed in terms of retinal illuminance and cortical pupil response mechanisms.     

Interocular interactions during acuity measurement in children and adults, and in adults with amblyopia

The binocular interactions that occur during dichoptic and binocular viewing were investigated using a letter acuity task in normally sighted children (age range 6-14 years) and adults, and in adults with anisometropic amblyopia. Our aims were to investigate the nature of binocular interactions that occur in each group, and the extent to which the characteristics of binocular interactions differ across the groups. The non-tested eye was occluded during monocular (baseline) viewing, and was allowed to view a uniform stimulus with fusion lock in dichoptic viewing. In adults and children with normal vision, acuity under dichoptic viewing was unchanged relative to monocular baseline in the dominant eyes, while acuity of the non-dominant eye improved under dichoptic viewing relative to baseline. The magnitude of dichoptic change in the non-dominant eyes was similar in the two normally sighted groups, but the dichoptic advantage was found to decrease with increasing age within the children tested. Binocular acuity was better than monocular acuity in normal subjects, and a decrease in binocular summation with age was noted within the age range of the children tested. In contrast, the amblyopic observers showed no change in acuity with viewing conditions. The results demonstrate development of interocular interactions during childhood, and wide inter-individual variation in pattern of interocular interactions among anisometropic amblyopic adults.     

Binocular contrast detection with unequal monocular illuminance

Binocular summation was measured in eight normal subjects by means of psychometric functions for contrast detection. An average 47% increase in binocular over monocular performance was obtained. Our data agreed with the simple summation model of Signal Detection Theory (Legge, 1984). Binocular psychometric functions were also measured when the sensitivity of one eye was decreased by means of a 1.0 neutral density (ND) filter. We found that binocular detectability in this case was reduced to below that of the better eye. This binocular inhibition was seen in all subjects. The slope of a contrast detection function gives a measure of the rate of change in detectability with contrast. If the slopes of two functions are equal, then the difference in detectability between these functions remains constant for all the contrast values used. When the slopes of the measured functions were analysed, no significant differences were found under any of the testing conditions. This indicates that the magnitude of summation (with equal monocular sensitivities), and of inhibition (with unequal monocular sensitivities), remains constant across the range of stimulus contrasts. The clinical implications of binocular inhibition are discussed.     

Detection and discrimination of the direction of motion in central and peripheral vision of normal and amblyopic observers

This paper describes the "motion" properties of the amblyopic fovea and compares them to the normal periphery. Specifically, thresholds for detection of the displacement of a grating pattern, and for discrimination of displacement direction were measured. The main findings of these experiments were: in the central vision of both normal and amblyopic observers, unreferenced displacement are detected with an accuracy equal to the observer's grating acuity; in the normal periphery, unreferenced motion thresholds fall off at a slower rate than does grating acuity; in amblyopic eyes, displacement thresholds are most elevated centrally; the addition of an abutting reference improves detection of motion for the normal fovea and in anisometropic amblyopes, but elevates motion thresholds in both the normal periphery and in the fovea of amblyopes with strabismus. The adequacy of the normal periphery as a model for the central vision of amblyopes is discussed.     

Contrast masking in strabismic amblyopia: attenuation, noise, interocular suppression and binocular summation

To investigate amblyopic contrast vision at threshold and above we performed pedestal-masking (contrast discrimination) experiments with a group of eight strabismic amblyopes using horizontal sinusoidal gratings (mainly 3c/deg) in monocular, binocular and dichoptic configurations balanced across eye (i.e. five conditions). With some exceptions in some observers, the four main results were as follows. (1) For the monocular and dichoptic conditions, sensitivity was less in the amblyopic eye than in the good eye at all mask contrasts. (2) Binocular and monocular dipper functions superimposed in the good eye. (3) Monocular masking functions had a normal dipper shape in the good eye, but facilitation was diminished in the amblyopic eye. (4) A less consistent result was normal facilitation in dichoptic masking when testing the good eye, but a loss of this when testing the amblyopic eye. This pattern of amblyopic results was replicated in a normal observer by placing a neutral density filter in front of one eye. The two-stage model of binocular contrast gain control [Meese, T.S., Georgeson, M.A. & Baker, D.H. (2006). Binocular contrast vision at and above threshold. Journal of Vision 6, 1224-1243.] was 'lesioned' in several ways to assess the form of the amblyopic deficit. The most successful model involves attenuation of signal and an increase in noise in the amblyopic eye, and intact stages of interocular suppression and binocular summation. This implies a behavioural influence from monocular noise in the amblyopic visual system as well as in normal observers with an ND filter over one eye.     

Positional uncertainty in peripheral and amblyopic vision

Three experiments were performed to examine positional acuity and the role of spatial sampling in central, peripheral and amblyopic vision. In the first experiment, 3-line bisection acuity was compared to grating acuity. In normal foveal vision bisection acuity represents a hyperacuity. In anisometropic amblyopes, bisection acuity is reduced in rough proportion to their grating acuity. In strabismic amblyopes, and in the normal periphery, bisection acuity is reduced to a greater extent than grating acuity. This result implies that reduced contrast sensitivity of the spatial filters is not sufficient to account for the increased positional uncertainty found in peripheral vision and in strabismic amblyopia. In order to test the hypothesis that the high degree of positional uncertainty evident in these visual systems is a consequence of sparse spatial sampling, bisection thresholds and width discrimination thresholds were measured with stimuli comprised of discrete samples. The results showed that normal foveal vision and the vision of anisometropic amblyopes show little benefit from adding discrete samples to the stimulus. In contrast, the normal periphery, and the central field of strabismic amblyopes demonstrate marked positional uncertainty which can be efficiently reduced in proportion to the square root of the number of samples (up to about 10) comprising the stimulus in the direction orthogonal to the discrimination cue. In aggregate the results suggest that anisometropic and strabismic amblyopia are fundamentally different. The positional uncertainty in anisometropic amblyopia is consistent with the reduced sensitivity of the spatial filters. The data of the normal periphery and of the central field of strabismic amblyopes suggest that the cortical sampling grain imposes a fundamental limit upon their positional acuity.     

Binocular contrast sensitivity with monocular glare disability

Glare disability plays an important part in the investigation and assessment of cataract. We investigated the effect of monocular glare disability on binocular contrast sensitivity. The magnitude of binocular summation was measured at different degrees of monocular glare disability. In the absence of glare, maximum binocular summation was shown. With increasing glare disability, the degree of binocular summation decreased steadily until binocular inhibition was produced. Binocular inhibition is defined as a reduction in binocular sensitivity to reach a level below the monocular. The clinical implications of binocular inhibition with monocular glare disability are discussed.     

Binocular recognition summation in the peripheral visual field: contrast and orientation dependence

Spatial frequency thresholds for recognition were measured for binocular and monocular viewing conditions at two contrast levels (95% and 7%). Measurements were obtained at the fovea and at four different eccentric retinal locations. Each eccentric retinal location was 8 degrees from the fovea, one on the horizontal axis (180 degrees ), and the other three in the superior field on retinal axes of 90 degrees, 45 degrees and 135 degrees. For the superior and horizontal retinal locations, the orientations of the gratings tested were horizontal (180 degrees ) and vertical (90 degrees ). For the retinal points on the oblique axes, the orientations of the gratings were 45 degrees and 135 degrees. Measurements were also obtained at the fovea for all four different grating orientations at both levels of contrast. Recognition threshold was defined as the highest spatial frequency at which luminance gratings were perceived vertically. At the fovea, binocular summation ratios (binocular spatial frequency/monocular spatial frequency) showed no significant differences for gratings of either contrast levels or for any orientation (p>0.05). In the superior periphery, significantly higher summation ratios were shown by low contrast vertical gratings (p<0.05), and in the horizontal periphery by low contrast horizontal gratings (p<0.05). On the oblique axis, low contrast gratings that were parallel to the oblique meridian showed higher summation ratios compared to those at right angles to it. High contrast gratings, at least at 8 degrees eccentricity, did not show this effect. Data suggest that meridional organisation of the retina (e.g. vertical gratings seen maximally in the superior field) occurs for resolution and that it is evidenced closer to the fovea than previously shown.     

Suprathreshold spatial frequency detection and binocular interaction in strabismic and anisometropic amblyopia.

We have investigated suprathreshold contrast sensitivity and binocular interactions in strabismic and anisometropic amblyopes using a reaction time paradigm. For every spatial frequency, reaction time increased as the grating contrast decreased. At all spatial frequencies and contrast values the reaction times using the amblyopic eye were prolonged compared to the nonamblyopic eye, but most markedly at high spatial frequencies. In the middle range of spatial frequencies, the contrast vs. reaction time function for the nonamblyopic eyes was biphasic, suggesting that two separate mechanisms detect gratings at high and low contrast levels. These functions in deep amblyopia were monotonic, and in shallow amblyopia the break in the functions was present but shifted to lower contrast levels. Binocular interaction experiments showed that binocular summation was absent at all contrast levels, but binocular occlusion was evident at high contrast levels for amblyopic observers.