The relative sensitivities of sensory and motor fusion to small binocular disparities.

Horizontal binocular disparity is the fundamental stimulus for both fusional vergence and stereopsis, but whether common disparity-sensitive mechanisms are involved in both responses is unknown. To determine whether the sensitivities of motor and sensory fusion are interdependent, we studied vergence eye movements and depth discrimination, using stimuli with haplopic binocular disparities, in subjects with normal stereopsis and in subjects with mild to severe stereoanomalies. Our results showed that the subjects' disparity discrimination functions varied from nearly perfect discrimination to chance performance for all of the experimental stimuli. Their sensory functions did not necessarily predict the shape of their motor fusion functions, but in most cases were correlated with the subject's fixation disparities. The results support the conclusion that the stereoanomalies and vergence anomalies that previously have been described for coarse binocular disparities also extend to the small, haplopic binocular disparities. The independence of the response properties of sensory and motor fusion suggests that neural pathways for sensory and motor fusion separate after the initial disparity-selective mechanisms in primary visual cortex.     

Interocular inhibition in strabismus

Summary Many patients with acquired strabismus do not suffer from diplopia and confusion after an individually and age-dependent interval. They inhibit the image of the deviated eye by binocular rilvary and particularly by the physiological ability to disregard visually disturbing stimuli. In strabismus with early onset, binocular rivalry is also demonstrable, even for stimuli that do not normally lead to suppression. On the basis of anomalous retinal correspondence, this rivalry occurs between retinal points onto which the same object projects. The retinal area with the lesser eccentricity receives the dominance. The fovea of the deviated eye is therefore not suppressed. In small-angle strabismus with smaller functional differences between anomalous corresponding retinal points anomalous fusion and even stereopsis can be possible as long as strong suprathreshold stimuli are presented. Strabismic amblyopia as a consequence of interfoveal suppression can only develop before anomalous retinal correspondence dominates in the strabismic child.      

A neurophysiological model for anomalous correspondence based on mechanisms of sensory fusion

Normal retinal correspondence is not stable. The arguments for the plasticity of correspondence in normal binocular vision have been given in two previous papers (Nelson, 1975, 1977). In this paper, both laboratory research and the clinical strabismus literature are reviewed to show similarities between normal and abnormal binocular vision. In particular, it is argued that sensory fusion (Panum's areas) and anomalous retinal correspondence (AC) obey similar principles, and so a sensory fusional model of AC may be developed.Recent advances in the neurophysiology of binocular vision are reviewed, but current laboratory knowledge cannot account for many phenomena known clinically unless certain postulates are made. Two hypothesized intracortical interactions among binocular disparity detectors, termed disparity domain inhibition and spatial domain facilitation, play key roles in extending the neurophysiology of binocular vision to an account of both normally - and clinically - observed plasticities of correspondence.The fusional model of retinal correspondence developed here from postulated domain interactions contrasts with the older concept of fixed corresponding points, an approach which has failed to provide a unified foundation for the treatment of normal and abnormal binocular vision.     

Short-latency disparity-vergence eye movements in humans: sensitivity to simulated orthogonal tropias

Small disparity stimuli applied to large random-dot patterns elicit machine-like vergence eye movements at short latency. We have examined the sensitivity of these eye movements to simulated orthogonal tropias in three normal subjects by recording (1) the effects of vertical disparities on the initial horizontal vergence responses elicited by 2 degrees crossed and uncrossed (horizontal) disparity stimuli, and (2) the effects of horizontal disparities on the initial vertical vergence responses elicited by 1.2 degrees left-hyper and 0.8 degrees right-hyper (vertical) disparity stimuli. Initial vergence responses were strongest when the orthogonal disparity was close to zero, and decreased to zero as the orthogonal disparity increased to 3 degrees -5 degrees, i.e., there was only a limited tolerance for orthogonal disparity. Tuning curves describing the dependence of the initial change in the vergence angle on the orthogonal disparity were well fit by a Gaussian function. An additional subject, who had an esotropia of approximately 10 degrees in our experimental setup, showed almost no horizontal vergence responses but did show vertical vergence responses to vertical disparity stimuli at short latency (albeit slightly longer than normal) despite the fact that her esotropia resulted in uncrossed disparities that would have totally disabled the vertical vergence mechanism of a normal subject, cf., anomalous retinal correspondence.     

Motion VEPs, stereopsis, and bifoveal fusion in children with strabismus

PURPOSE: The link between nasal-temporal motion asymmetries and anomalous binocular sensory function in infantile esotropia (ET) has led to the idea that visual evoked potential responses to horizontal motion (mVE) is an alternative measure of sensory binocularity to stereopsis. A second hypothesis is that the mVEP response is a marker for bifoveal fusion. The purpose of this study was to directly evaluate these two hypotheses by examining the correspondence between the mVEP response and both stereoacuity and bifoveal fusion in a cohort of strabismic patients with variable binocular sensory function. METHODS: Motion VEPs, random dot stereopsis, and bifoveal fusion were measured in 94 children: 20 with infantile ET, 16 with infantile accommodative ET, 22 with late-onset accommodative ET, 10 with intermittent infantile strabismus, and 26 normal control participants. RESULTS: Patients with infantile ET and infantile accommodative ET had high concordance between mVEP responses and stereoacuity and mVEP responses and bifoveal fusion. Asymmetric mVEP responses were highly concordant with both no measurable stereopsis and an absence of fusional vergence. Patients with late-onset accommodative ET and intermittent infantile strabismus revealed discordance between the mVEP response and stereoacuity and high concordance between the mVEP response and bifoveal fusion. Asymmetric mVEP responses were highly concordant with the absence of bifoveal fusion and the minimum-size prism to elicit fusional vergence. CONCLUSIONS: The qualitative and quantitative relationship between the mVEP response and fusional vergence suggests that the mVEP response is an objective measure of bifoveal fusion. The availability of such a test will facilitate studies of normal development of bifoveal fusion and development of monofixation syndrome in strabismus.     

Four diopters prism test recording in small angle esotropia: a quantitative study using a magnetic search coil

PURPOSE: To determine the vergence response, if any, in a homogenous group of patients suffering from strabismus and submitted to environmental induction of retinal disparity with respect to a control group of subjects. MATERIALS AND METHODS: A series of 13 subjects was included in the present study (5 normal subjects and 8 patients suffering from small angle esotropia). The patients were evaluated in an illuminated environment where they were told to stare at a target which subtended a visual angle of 20 degrees and, successively, in the dark using a luminous stimulus which subtended a visual angle of 3.5 minutes. The vergence movements were evaluated using a four diopter base-out prism; eye movements were recorded under conditions of both binocular and monocular vision. Ocular movements were recorded by means of a magnetic search coil; the amplitude and peak velocity of the vergence movements were evaluated by computer analysis. Statistical analysis was performed using the Multiple Anova test for repeated measures. RESULTS: The data obtained in the present study demonstrate that: 1) there is a significant difference between binocular and monocular vision in both groups of subjects; however, the ocular response in the patients affected by strabismus was significantly different from that in the control group of subjects; 2) during monocular vision in the dark, the response of the strabismic patients was highly variable; 3) for both groups of subjects no significant differences were observed in the vergence response when a large-angle stimulus was employed in light conditions nor when a small-angle stimulus was utilized in a dark environment.     

Four diopters prism test recording in small angle esotropia: a quantitative study using a magnetic search coil

PURPOSE: To determine the vergence response, if any, in a homogenous group of patients suffering from strabismus and submitted to environmental induction of retinal disparity with respect to a control group of subjects. MATERIALS AND METHODS: A series of 13 subjects was included in the present study (5 normal subjects and 8 patients suffering from small angle esotropia). The patients were evaluated in an illuminated environment where they were told to stare at a target which subtended a visual angle of 20 degrees and, successively, in the dark using a luminous stimulus which subtended a visual angle of 3.5 minutes. The vergence movements were evaluated using a four diopter base-out prism; eye movements were recorded under conditions of both binocular and monocular vision. Ocular movements were recorded by means of a magnetic search coil; the amplitude and peak velocity of the vergence movements were evaluated by computer analysis. Statistical analysis was performed using the Multiple Anova test for repeated measures. RESULTS: The data obtained in the present study demonstrate that: 1) there is a significant difference between binocular and monocular vision in both groups of subjects; however, the ocular response in the patients affected by strabismus was significantly different from that in the control group of subjects; 2) during monocular vision in the dark, the response of the strabismic patients was highly variable; 3) for both groups of subjects no significant differences were observed in the vergence response when a large-angle stimulus was employed in light conditions nor when a small-angle stimulus was utilized in a dark environment.     

Recording of disparity vergence in comitant esotropia

Vergence movements induced by base-out prisms were recorded, with an infrared eye movement recording system, in 5 patients with comitant esotropia and anomalous retinal correspondence and in 5 normal volunteers. The findings support the hypothesis that vergence movements in strabismus are induced by disparity, since accomodative as well as proximal vergence had been ruled out. Vergence movements are much slower and with different characteristics in strabismic patients with respect to normals. It may be argued that vergence movements represent the motor fusion component left over in strabismus. They may be considered the objective representation of the well-known phenomenon of prism compensation or adaptation, found in many strabismic patients.     

Stereoscopic cooperation between the fovea of one eye and the periphery of the other eye at large disparities Implications for anomalous retinal correspondence in strabismus

In normal human observers we searched for the largest amount of visual disparity that can still provide depth information; we compared (1) crossed and uncrossed disparities and (2) symmetrical and asymmetrical locations of disparate stimuli. A pair of 3° discs projected for 100 ms served as targets. Symmetrical stimuli were projected on temporal or nasal retinal loci in both eyes; asymmetrical stimuli were projected on the fovea of one eye and on the nasal or temporal periphery of the other eye. Thresholds were determined using a two-alternative forced choice procedure. Subjects had to distinguish binocular disparate images from monocular double images of identical angular separation. Among six subjects, crossed disparities were recognized by one up to 6°, by three up to 9°, by one up to 18°, and by one up to 21°. Uncrossed disparities were recognized by two at 3°, by two up to 6° and by two up to 9°. Hence, crossed disparities could be recognized up to higher angles than uncrossed. No consistent difference was found between symmetrical and asymmetrical stimuli. Stimuli with crossed disparity appeared smaller and with uncrossed disparity larger than monocular stimuli of the same objective size, suggesting that the size-constancy mechanism operates when disparity stimuli are presented as briefly as 100 ms, i.e., without simultaneous vergence eye movements. We speculate that the far-reaching interocular connections demonstrated in normal subjects might also be utilized in the case of strabismus: these interocular connections could form the basis for anomalous retinal correspondence.     

The accuracy of binocular vergence for peripheral stimuli

Two experiments were conducted to investigate the roles of dark vergence and retinal eccentricity as factors influencing binocular vergence responses. A nonius alignment technique was used to measure vergence responses in total darkness (dark vergence) and for dim binocular stimuli presented at retinal eccentricities ranging from 2° to 8° over distances ranging from 28.5 to 342cm. The results indicate that vergence was progressively less accurate with stimuli at increasing retinal eccentricities and that errors of fixation were biased toward the individual's dark vergence position. The hypothesis that dark vergence represents the functional resting state of the vergence system which influences oculomotor fusional responses under a variety of conditions is discussed.     

Eye-movement responses to disparity vergence stimuli with artificial monocular scotomas

The effects of artificial monocular scotomas on eye-movement responses to horizontal disparity vergence stimuli were studied in six subjects with normal binocular vision. Subjects viewed stereoscopic 1.5 degrees horizontal step disparity vergence stimuli through liquid crystal shutter glasses. The central portion of the stimulus presented to the right eye was removed to simulate monocular artificial scotomas of variable diameters (2 degrees to 10 degrees ). Eye movements were recorded with a binocular head-mounted eye tracker. Responses included pure vergence, vergence followed by saccades, and pure saccadic eye movements. The rate of responses with saccadic eye movements increased with the diameter of the artificial scotoma (p < 0.0001); there was an increase in the rate of responses starting with saccades (p < 0.0001), as well as an increase in the rate of saccades after initial vergence responses (p < 0.01). The probability of saccades after initial vergence responses was affected by the open-loop gain of the vergence response (p < 0.001). The open-loop gain decreased with increased diameters of the artificial scotomas (p < 0.0001). As the diameter of the artificial scotomas increased, the amplitude of the initial vergence eye-movement responses decreased, and the prevalence of saccadic eye movements and asymmetric vergence increased. The effects of the diameter of artificial monocular scotomas on eye-movement responses in subjects with normal binocular vision are consistent with the effects of diameter of suppression scotomas on eye-movement responses to disparity vergence stimuli in patients with infantile esotropia.     

Zero retinal image disparity: A stimulus for suppression in small angle strabismus

Suppression was examined with binocular perimetry in patients with small angle strabismus. Suppression stimuli were presented to the fovea of the fixating eye or 5 degrees nasal or temporal from the fovea. Suppression was found in the deviating eye of patients with normal or anomalous correspondence at retinal points corresponding to the retinal locus of the suppression stimulus. Suppression was demonstrated on both the nasal and temporal hemiretina of the deviating eye. Results indicate that zero retinal disparity is a strong stimulus for suppression in the central and peripheral visual field of patients with small angle strabismus.This study was supported by a grant, E4-20573, from the National Eye Institute.     

Microtropia

Microtropia is an unilateral strabismus of less than 5 degrees, usually with harmonious anomalous correspondence. Three forms may be distinguished: Primary constant, primary decompensating and consecutive microtropia. In three instances microtropia is important for the ophthalmologist: In assessment of amblyopia apparently without strabismus, in evaluation of strabismus treatment results and in evaluation of hereditary factors in strabismus. Amblyopia is more pronounced in cases with anisometropia and eccentric fixation, but usually responds well to occlusion treatment. Because of a typical 'reading amblyopia', treatment with alternating partial occlusion should be carried out until a child can read fluently with each eye. It is estimated that about 1% of general population has a microstrabismus. Primary microtropia is probably due to a primary sensorial defect, which predisposes to anomalous retinal correspondence. Primary microtropia may decompensate into a larger angle. After therapy, not a complete parallelism but a consecutive microtropia results.     

Fast disconjugate adaptations of saccades in microstrabismic subjects

In normal subjects, saccade amplitude inequality can be induced almost immediately when the image is made larger for one eye. This disconjugacy allows binocular fusion at the point of regard despite the image size inequality. It persists under subsequent monocular viewing which suggests a fast adaptive mechanism. This study tests whether such disconjugacy can be induced in subjects with microstrabismus who do not have foveal fusion. Three microstrabismic subjects viewed a random dot pattern the size of which was 10% larger in one eye. Within 40 sec horizontal saccades became larger in the eye viewing the larger image by 4–10%; the induced disconjugacy persisted under subsequent monocular viewing. Thus, fast disconjugate adaptation is possible in microstrabismus demonstrating that foveal fusion is not necessary to achieve it.     

Binocular visual perception in strabismics studied by means of visual evoked responses

Ten normal subjects and 14 patients with comitant esotropia were examined by means of pattern visual evoked responses (VER) under monocular and binocular viewing conditions. When both eyes were stimulated together a VER summation was noted both in normals and in strabismics with small-angle deviation and anomalous retinal correspondence (ARC). This is considered as an objective proof of binocularity. Patients with large-angle strabismus and/or suppression of the image of the deviated eye did not show summation. The significance of summation and its relationship with binocular vision was analyzed by recording binocular VER in normals in which diplopia was artificially induced and in strabismics who spontaneously exhibited double vision. A simple way for differentiating normals from strabismics by means of VERs is presented, considering that the presence or absence of summation per se does not achieve this result. This method is based on the anteposition in front of the fixing eye of neutral filters of increasing density. Summation disappears in strabismics with much weaker filters than in normals (0.5 versus 1.6 log. unit).     

Horizontal fixation disparity measures with nonius lines.

PURPOSE: It has been reported that changes in visual direction and retinal correspondence may limit the validity of nonius lines as a subjective measure of vergence, at least in particular viewing conditions, e.g., dynamic or forced vergence. Nonius lines may be valid at larger spatial separation between fusion contour and nonius lines. Therefore, we measured fixation disparity varying the amount of a vertical gap between nonius lines. METHODS: A static central fusion stimulus was given at a 100-cm viewing distance. The nonius lines were presented with various vertical gaps either continuously (and adjusted to subjective alignment) or flashed in a series of trials (in adaptive testing). Twelve subjects with normal binocular vision were tested repeatedly to identify individual vergence characteristics. RESULTS: When the vertical gap between nonius lines was increased up to 7.9 deg, the amount of subjective fixation disparity tended to increase in part of the subjects, predominantly in those with an exo fixation disparity. Most subjective fixation disparity measures were correlated with each other and with tonic vergence (i.e., dark vergence tested subjectively without fusion stimulus); however, flashed nonius lines at larger nonius gaps were an exception. CONCLUSIONS: We found physiologically plausible measures of idiosyncratic fixation disparity with continuous nonius lines at any amount of nonius gap or with flashed nonius lines at small gaps. In these conditions, the intersubject variability of fixation disparity was much larger than effects of the spatial separation between fusion stimulus and nonius lines.     

Objective vs subjective measures of fixation disparity for short and long fixation periods

Purpose: Fixation disparity, i.e. the vergence error for stationary fusion stimuli, can be measured objectively with eye trackers and subjectively with nonius lines. Between these two measures, previous studies found differences that tended to be proportional to the amount of forced vergence, i.e. the discrepancy between vergence and accommodative stimulus. We investigate whether objective and subjective fixation disparity might be similar without forced vergence. Method: We simultaneously measured fixation disparity with the EyeLink II system and with flashed dichoptic nonius lines in 17 subjects who observed targets at a vergence stimulus of 6 deg in an haploscope with 60 cm viewing distance. Results: We found individual differences in objective fixation disparity in a range of about 20 (eso) to ?60 (exo) min arc which was considerably larger than the range of subjective fixation disparity. Results were similar for long fixation periods (about 15 s) and short fixation periods (about 1.5 s) between 5 deg saccadic gaze shifts. Further, objective fixation disparity was correlated with objective heterophoria, i.e. the vergence state without a fusion stimulus (r = 0.8, p < 0.001). Conclusion: Subjective fixation disparity explained only about 25% of the inter‐individual variability in objective fixation disparity. The discrepancy between these two measures might be explained by sensory shifts in retinal correspondence, also in the present condition without forced vergence.     

Management of strabismus with hemianopic visual field defects

Background

Hemianopia and strabismus leads to severe disturbance of visual orientation and diplopia under binocular conditions if the deviated eye has a normal retinal localization.

Subjects and methods

Four cases with homonymous (two) and bitemporal (two) hemianopia and strabismus will be described with respect to binocular visual field and diplopia/confusion. All of them were recommended for strabismus surgery. Preoperatively, prism adaptation test was carried out to analyze functional results and fusional competence. In three of the four cases, strabismus surgery was successfully performed to avoid diplopia/confusion. All three patients revealed normal retinal correspondence. Furthermore, in two cases surgery led to an extension of binocular visual field; in one case with a bitemporal hemianopia and hemifield-slide phenomenon, blurred central vision and reading problems reduced significantly postoperatively. In one patient with anomalous retinal correspondence due to early childhood trauma exotropia led to an extension of the binocular visual field. In this case, strabismus surgery would have been unfavourable.

Results

Case 1 showed a homonymous hemianopia to the left and acquired exotropia of the right eye, leading to binocular diplopia. Case 2 with homonymous hemianopia to the right and exotropia of right eye revealed anomalous retinal correspondence after history of perinatal brain injury, resulting in absence of diplopia and enlargement of visual field to the right. Cases 3 and 4 with bitemporal hemianopia suffered from sensory disturbances caused by additional acquired strabismus. The exodeviation of the right eye in case 3 led to a restriction of binocular visual field with overlap of the nasal parts causing diplopia, whereas the esodeviation of case 4 resulted in a “gap” between the nasal parts (blind area).

Conclusion

Depending on the extent of visual field defects and on retinal correspondence, functional consequences for binocular vision and binocular visual field should be considered prior to surgery. In normal retinal correspondence, strabismus surgery will be indicated in most cases because of diplopia. However, surgery might result in a reduction of binocular visual field. Preoperatively, it is important to map monocular and binocular visual fields, to examine retinal correspondence, and to undertake prism adaptation test to imitate the postoperative functional result and risk of double vision.     

Fusional responses of strabismics to foveal and extrafoveal stimulation

Horizontal fusional responses of 11 strabismics, with normal or anomalous retinal correspondence, were studied. These included four small-angle esotropes with amblyopia, one intermittent esotrope (overcorrected intermittent exotrope), two intermittent exotropes (preoperative esotropes), one accommodative esotrope, and three microtropes with amblyopia. Fusional stimuli (with a constant accommodative demand) ranging from very small, central stimuli to those that cover the central, the peripheral, and both the central and the peripheral visual fields were used. All 11 strabismics produced fusional vergence responses. Many of the fusional responses included large nonmotor components. Small central stimuli were not effective in producing fusional vergence, whereas stimuli that covered the visual periphery produced fusional responses that were similar to those by subjects with normal stereoscopic vision.     

Fixation disparity

Fixation disparity may be a symptom of binocular stress or a purposeful error signal to drive vergence eye movements. Two similar units for detecting fixation disparity (Mallett and Sheedy units) are compared and evaluated. The Mallett unit was found to be more reliable, with similar readings in symptom-free subjects, whereas the Sheedy unit gave a larger spread of fixation disparity measurements and appears to be less useful than the Mallett unit for routine clinical use. The exact role of fixation disparity remains unresolved. Fixation disparity has two components in symptomatic subjects; the vergence signal component and an oculomotor imbalance component.