Normal Binocularity Research Articles

The effects of interocular differences in retinal illuminance on vision and binocularity

It is known that binocular function is affected by interocular differences in retinal image size, shape, clarity, and illumination. The present study was performed to systematically examine the effects of interocular differences in retinal illuminance on monocular visual acuity, fusion, and stereopsis. Fifty adults with normal binocularity and a best-corrected visual acuity of 20/20 or better were enrolled. We examined best-corrected visual acuity, fusion, and stereoacuity as monocular retinal illuminance was gradually reduced by increasing neutral density filters from 0.2 neutral density (ND; 63% transmission) to 3.0 ND (0.1% transmission) in increments of 0.2 ND. We measured the smallest size of fusion slide by major amblyoscope, and evaluated stereoacuity with the Titmus test and the Lang test. The best-corrected visual acuity began to decrease significantly when the value of the neutral density filter reached 2.0 ND (1% transmission; p<0.01). The level of fusion decreased significantly when monocular retinal illuminance was reduced to 2.0 ND (p<0.01). Stereoacuity began to decline significantly when the value of the neutral density filter was 1.4 ND (4% transmission) with the Titmus test and 1.6 ND (2.5% transmission) with the Lang test. A binocularly conducted Titmus test was the most sensitive to the presence of monocularly induced retinal illuminance deterioration.

Fourier-analysed Steady-state VEPs in Pre-school Children with and without Normal Binocularity

Pre-school children aged 4-5 were examined with steady-state VEP in response to a sinusoidal grating pattern with a spatial frequency of 4 c/deg, reversing at rates 5, 10, and 15 Hz. Normal children (n = 10) were compared with subjects lacking stereo perception (n = 6) and with subjects showing significant unilateral amblyopia with visual acuity in the worse eye <0.5 (n = 7). Fast Fourier Transform was used for analysis of the crude steady-state VEP responses. Compared to normals, the subjects lacking stereo perception showed a significantly lower power of the second harmonic in the response evoked by binocular stimulation with gratings reversed at 15 Hz. The amblyopic group showed a significant difference between the dominant and the non-dominant eye regarding the first harmonic power in the responses evoked by gratings reversed at 5 and 10 Hz. These findings are discussed in relation to the magnocellular and parvocellular visual pathways and suggested models for linear and non-linear processing of visual signals.

Rapid extragranular plasticity in the absence of thalamocortical plasticity in the developing primary visual cortex.

Monocular deprivation during early postnatal development remodels the circuitry of the primary visual cortex so that most neurons respond poorly to stimuli presented to the deprived eye. This rapid physiological change is ultimately accompanied by a matching anatomical loss of input from the deprived eye. This remodeling is thought to be initiated at the thalamocortical synapse. Ocular dominance plasticity after brief (24 hours) monocular deprivation was analyzed by intrinsic signal optical imaging and by targeted extracellular unit recordings. Deprived-eye responsiveness was lost in the extragranular layers, whereas normal binocularity in layer IV was preserved. This finding supports the hypothesis that thalamocortical organization is guided by earlier changes at higher stages.

Concomitant strabismus: of strabismus in strabismus syndromes

To show how to progress from the deviation of the visual axis (provided the diagnosis of concomitant strabismus is certain) to the classification of the squint in one out of the different squint syndromes and consequently to adopt the appropriate therapeutic strategy. Every sign correlated with the deviation contributes to progress step by step to the diagnosis of a given squint syndrome. The age on onset of strabismus, either convergent or divergent, and its characteristic, intermittent or constant, allow in a first step to evaluate the potential binocularity, as well in early as late (acquired) strabismus. The first group of early strabismus includes manifest infantile strabismus and microstrabismus. Both have abnormal binocularity. The possibility of functional amblyopia, angle variability and additional incomitances have to be investigated. Early intermittent strabismus keeping a potential normal binocularity are seldom. In the second group of late onset strabismus, retinal correspondence has to be investigated by correspondence tests and prism or bifocus compensation to distinguish between the two possible types (including the accommodative forms of strabismus), i.e. decompensated microstrabismus with abnormal binocularity or normosensorial strabismus with potential normal binocularity. In some cases potential binocularity may be initially uncertain and/or remain later on subnormal. As the result of this systematic approach, every cases of squint can be classified in one out of the different squint syndromes. Based on the precise diagnosis, the appropriate treatment can be carried out. The goals of treatment which can be reached in every syndrome are indicated. For an overall view of the squint syndromes a classification with two entrances are necessary, on the one hand early or late onset, on the other hand normal or abnormal binocular conditions. This approach of concomitant strabismus should serve as guide lines for clinical practice.

Anisometropia and Binocularity

To determine the effects of experimentally induced anisometropia on binocular function in healthy adults as a means of assessing the potentially detrimental effects of uncorrected anisometropia on binocular development in childhood. Nineteen adults with normal binocularity, ranging in age from 26 to 59 years, were studied. Unilateral myopia, hyperopia, or astigmatism (at 90 degrees or 45 degrees) was induced in each subject using trial lenses. Sensory status then was assessed by measuring stereoacuity, Worth four-dot fusion, and Bagolini lens response. All subjects showed a decline in binocular function with increasing levels of anisometropia. Foveal suppression was evident on the Worth four-dot test, and increased in proportion to the anisometropia. Stereoacuity was similarly degraded by the induced anisometropia, with some subjects showing significant loss of stereoacuity with as little as 1 diopter of spherical anisometropia. Bagolini lens responses were binocular in almost all patients, although occasional abnormalities were found. Relatively low degrees of anisometropia may cause significant abnormalities in high-grade binocular visual functions in adults. The potential effects of uncorrected anisometropia on binocularity in children require further investigation, but should be considered in developing guidelines for the empiric correction of refractive errors.

How useful is anomalous correspondence?

In anomalous retinal correspondence (ARC), retinal points in the right and left eyes which receive stimuli from one object in space have the same visual direction despite a manifest motor deviation. The mode of cooperation of these anomalously corresponding retinal points depends on their relative eccentricity or their relative functional level. If there is a great difference, as for the most part in large angle strabismus, the retinal point with less eccentricity and better resolution will prevail, while the other will be suppressed. This leads to a second area of high resolution in the binocular visual field with a possibly irritating effect. In microtropia, anomalous corresponding retinal points have functionally different levels only in the very centre of the visual field. ARC provides in this case a binocular cooperation very similar to normal fusion. To an individually different degree, ARC can adapt to new squint angles, which makes squint surgery possible without postoperative diplopia, but a change to normal binocularity with stable interocular connections can not be achieved. ARC enables the heterotropic patient to see with both eyes simultaneously without diplopia by a regionally changing use of inhibition and anomalous fusion in the interest of the best possible perception.

Dichoptic luminance beat visual evoked potentials in the assessment of binocularity in children.

Direct evidence of a distinct cortical binocular pathway has been provided by the production of nonlinear (difference) beats from dichoptic luminance stimulation in stereonormal adults and the absence or diminution of these beats in stereoblind subjects. We have investigated a clinically useful application of this technique in a pediatric population with potentially abnormal binocular vision. We recorded dichoptic luminance beat visual evoked potentials (VEPs) from 20 children (ages 7 months to 8 years) with abnormal binocular ability secondary to strabismus and/or amblyopia and compared this to a control group of 20 children with normal binocularity. Stereoblind children generated significantly lower dichoptic signal-to-noise ratios than stereonormal children (P < .001). Responses to monoptic multifrequency flicker were not significantly different between the two groups (P = .936). This dichoptic VEP can be performed quickly and easily on young children and gives a quantitative assessment of cortical binocularity that may not be determinable by standard clinical methods. This technique may also prove useful for the preoperative gradation of binocular potential and prediction of postoperative binocular fusion.

Brown's syndrome. A longitudinal long-term study of spontaneous course.

The prognosis for spontaneous improvement of Brown's syndrome has not yet been clarified. The present longitudinal long-term study comprised 10 patients with idiopathic Brown's syndrome arising or noticed during the first or second year of life. They were followed for an average of 13 years during which each had a mean of 9 examinations. Spontaneous improvement occurred in 9 of the patients, 3 of whom were cured, obtaining normal motility. All three cases had initially been permanent and monolateral, with initially mild, moderate, and severe restriction, respectively. In the remaining 6 patients who changed for the better, the spontaneous improvement consisted of a reduction of the initial hypotropia and the initial depression of the adducted eye. When the patients were last seen, the sensorial state of binocular vision demonstrated in 4 of the 10 normal binocularity, in 2 binocularity corresponding to microstrabismus, and in 4 alternating suppression.

Monocular Components of the Fixation Disparity Curve

Experiments were conducted to characterize the distribution of the monocular components of fixation disparity, i.e., ascertain whether or not the fixation disparity measured by nonius alignments is equally divided between the two eyes or has an unequal distribution. Computer generated nonius lines were used to measure the monocular components of the forced vergence fixation disparity curve (FDC) for subjects with normal binocularity. Thirty-three percent of all subjects tested showed unequal distribution of their monocular components. The composite FDC's for subjects with unequal distribution of the monocular components tended to have steeper slopes than those with equally distributed components. It was concluded that equally divided monocular components are not a prerequisite for good binocularity.

Neue Methode zur Operation des Nystagmusblockierungs-Syndroms

Structural surgery is based on a model in which eye movements follow the ball-joint principle, i.e., there is absolute freedom of movement in all possible directions. Nystagmus blockage or convergence excess is regarded as a surplus of the medial rectus muscle. The relation between the weight of the globe and the muscle mass is taken into account. The medial rectus muscle is weakened by splitting it into 3 parts. The central part remains in its physiologic anatomic position in order to preserve normal convergence and binocularity, while the upper and lower portions prevent divergence. Additional prior weakening is accomplished by performing a tenonectomy.

Chromatic imbalance due to commonly used red-green filters reduces accuracy of stereoscopic depth perception.

This study was designed to determine the effect on stereopsis of interocular retinal chromatic and illuminance imbalance in 30 subjects with normal binocularity. A Randot 3 Circle Stereotest, viewed through polarizing glasses only, was the control condition. In each of three additional conditions, illuminance and chromatic imbalances were created by commonly used red and green filters and neutral density filters combined with the polarizing filters. The effect of each of these experimental conditions on stereopsis was determined by comparing total stereo judgement errors on the Randot Circle Stereotest for each condition to the control condition. Total average flux through the filter combinations was held constant by adjusting the luminance level of light reflected from the target. The chromatic imbalance created by the red-green filters significantly increased the number of stereo judgment errors (37%, p less than 0.05). However, illuminance imbalance (0.2 log units) of the magnitude created by the red-green filters did not significantly increase the number of errors (2.9%, p less than 0.65). Finally, the combined illuminance and chromatic imbalance created by the red-green filters significantly increased the error frequency (46%, p less than 0.05). The chromatic imbalance caused by red-green glasses significantly degrades stereopsis, whereas the illuminance imbalance caused by these filters has no significant effect on stereopsis.

Functional and neuronal binocularity in kittens raised with rapidly alternating monocular occlusion.

1. In order to determine the degree of synchrony of binocular activation required for the development of binocularity we reared 11 kittens with rapidly alternating monocular occlusion. Alternating occlusion was achieved with microprocessor-controlled electrooptic solid-state shutters, which were fitted to individually moulded goggles. The intervals of alternating occlusion were varied from 50 to 1,000 ms. Two normally reared kittens and three kittens that were reared with the shutters operating synchronously with open/close intervals of 50/50 ms, 200/200 ms, and 400/100 ms, respectively, were used as controls. Toward the end of the critical period we examined the kittens' ability for binocular depth discrimination and tested binocular luminance summation of the pupillary light reflex. Single-cell recordings were made from the visual cortex in order to determine the percentages of binocularly excitable neurons. 2. There was a good correlation between the degree of asynchrony of binocular experience, the impairment of depth discrimination, and the percentage of binocular neurons. Kittens reared with alternation rates of 200, 330, and 400 ms, respectively, had developed normal binocularity and were indistinguishable from the controls. Alternation rates of 500 ms or longer prevented the development of normal depth discrimination and luminance summation and resulted in reduced cortical binocularity. 3. A linear relationship between depth discrimination, binocular luminance summation, and percentages of binocular neurons was found. 4. Our findings indicate that an asynchrony of binocular activation of several hundred milliseconds is compatible with the development of normal binocularity in the kitten visual system.

Binocular vs. monocular task performance.

Functional advantages of binocularity were investigated by having 13 subjects perform a group of occupational-type tasks under monocular and binocular conditions. Significant binocular advantages ranging from 29.5% (pointers in straws) to 3.7% (reading speed) were measured. Tasks with many disparity cues showed the greatest binocular advantage. This shows that patients with normal binocular vision use binocular cues, most likely stereopsis, to enhance performance. In a second experiment, three subjects with normal binocular vision underwent monocular occlusion for 5 days to investigate whether monocular skills improved to compensate for the loss of binocular vision. During that period binocular performance was consistently better than monocular performance, and both monocular and binocular performance improved, even though the subjects were only gaining monocular experience. Although the 5 day occlusion period does not simulate the long-term denial of normal binocularity that strabismics or monocular patients experience, it shows that binocular superiority remains after short-term loss of binocularity.

Fusion lock diameter and the forced vergence fixation disparity curve.

The effect of the diameter of the fusion lock on the forced vergence fixation disparity (FD) curve was investigated because of ambiguity in the results of previous investigations. Data gathered from 20 asymptomatic subjects with normal binocularity showed that the fusion lock diameter had no noticeable effect on the shape of the curve. Further investigations using symptomatic subjects showing large FDs or steep forced vergence FD curves showed that fusion lock diameter did have an effect on the curves of these subjects. The results are interpreted for their clinical implications through a control systems model of the accommodation-vergence interaction.

Lack of binocularity in cells of area 19 of cat visual cortex following monocular deprivation

We have studied the visual receptive field properties of neurons in cortical area 19 of monocularly deprived cats. Almost all visually responsive units responded to stimulation of the non-deprived eye only. Receptive field properties assessed through the non-deprived eye were found to be normal. Monocular deprivation appears thus to have sharply reduced the normal binocularity of neurons in area 19. Since the W-cell component of the visual pathways provide the predominant input to area 19, our results suggest that W-cells are vulnerable to environmental manipulation.

The course and effect of visual training on the vergence system.

The effect of a variety of vergence training procedures on the visual system of 35 asymptomatic young adults with normal binocularity was evaluated. The visual training took place on the weekdays of three consecutive weeks. Vergence ranges were measured before the study began and at the end of the 3-week period. Subpopulations were evaluated at one week and at 6 months to document further the course of the effects. Positive fusional vergence training significantly increased the vergence ranges after 1 week of training; a greater effect was measured after 3 weeks. The vergence capability was found to have decreased 6 months later, but the effects of the training were still apparent. Negative fusional vergence training was less effective; however, significant increases were demonstrable after 3 weeks of training. The phorias the accommodative amplitude were not affected by the orthoptics. We conclude that relatively short periods of training can provide long-lasting increases in the vergence ability of a group of binocular normals. The implications of such findings relative to possible neural control mechanisms and clinical visual training situations are discussed.

Analysis of the effect of temporal frequency on the dichoptic visual-evoked response.

When observers with normal binocularity view reversing black and white checkerboard patterns, the amplitude of the resulting binocular visual-evoked response (VER) is significantly greater than the amplitude of either corresponding monocular response. When the same checkerboard pattern is dichoptically presented with an interocular luminance difference (1.3 to 2.0 log units), the amplitude of the dichoptic VER is significantly less than either monocular response. In a previous study, we attributed this effect to interference between the monocular components of the binocular response resulting from a phase shift induced by the interocular luminance difference. To test this hypothesis, dichoptic VER's were recorded for three temporal frequencies (1.88, 3.75, and 7.5 Hz). The results obtained support the phase shift interpretation.

Studies on visual evoked cortical response by liquid crystal phase difference haploscope: amblyopia and/or strabismus.

Visual evoked cortical potentials induced by alternate stimulation of checkerboard pattern with the use of liquid crystal phase difference haploscope were investigated. The synchronization ratio of brain response was determined both in the group of normal binocularity and in the group with binocularity defect. The synchronization ratio of the group of normal binocularity (N = 30) is 100% at 1 Hz x 2, 98.5% +/- 3.9% at 5 Hz x 2, and 83.7% +/- 15.8% at 10 Hz x 2. On the other hand, in the group with binocularity defect, strabismus without amblyopia shows a reduction of the ratio only at the frequency of 10 Hz x 2. Anisometropic amblyopia shows a significant reduction of the ratio at 10 Hz x 2 and strabismus with amblyopia shows a remarkable reduction at 5 Hz x 2 and 10 Hz x 2. As for strabismus with or without latent binocularity, the latter cases show a significant reduction of the synchronization ratio. The reduction of synchronization ratio of VECP seems to have some relationship to the binocularity.

Patterns of interocular transfer of visuomotor coordination reveal differences in the representation of visual space

Acquisition and interocular transfer of adaptation to optical transformations of input were examined in strabismic and orthotropic subjects. Distinct patterns of behavior were displayed by three groups: strabismic alternating suppressors, strabismic and orthotropic constant suppressors, and orthotropes with normal binocularity of vision. It is suggested that these behaviors result from the way in which visual space is represented in each group, an outcome of the pattern of use of the eyes during early development. The conditions for modification of the representation of visual space appear to parallel those for initial acquisition.

The role of binocular neurons in the cat striate cortex in combining information from the two eyes.

Cats were raised under conditions of daily alternating monocular exposure, so that each eye received normal input, but the animals were never allowed to use both eyes simultaneously. With single cell recording techniques it could be shown that this led to a severe disturbance of the normal binocularity of cortical neurons. The cats were trained by operant techniques in a two-choice box to discriminate with each eye non-retinotopic stimulus pairs. To test the ability to combine binocular signals it was tested whether the learned discrimination would transfer to red/green colored stimuli which were viewed dichoptically through appropriately colored contract lenses. The arrangement was such that the positive stimulus could be distinguished from the negative stimuli only when the signals from both eyes were combined and used simultaneously. All cats showed immediate transfer, from the monocularly learned discrimination task to the dichoptic paradigm. This indicates that the presence of a normal population of binocular cortical cells in area 17 is not a prerequisite for the ability to use binocular cues for the solution of a pattern discrimination task.