Subnormal binocular vision with special reference to peripheral fusion

Cell-type-specific binocular vision guides predation in mice

Predators use vision to hunt, and hunting success is one of evolution's main selection pressures. However, how viewing strategies and visual systems are adapted to predation is unclear. Tracking predator-prey interactions of mice and crickets in 3D, we find that mice trace crickets with their binocular visual fields and that monocular mice are poor hunters. Mammalian binocular vision requires ipsi- and contralateral projections of retinal ganglion cells (RGCs) to the brain. Large-scale single-cell recordings and morphological reconstructions reveal that only a small subset (9 of 40+) of RGC types in the ventrotemporal mouse retina innervate ipsilateral brain areas (ipsi-RGCs). Selective ablation of ipsi-RGCs (<2% of RGCs) in the adult retina drastically reduces the hunting success of mice. Stimuli based on ethological observations indicate that five ipsi-RGC types reliably signal prey. Thus, viewing strategies align with a spatially restricted and cell-type-specific set of ipsi-RGCs that supports binocular vision to guide predation.

Separating fusion from rivalry

Visual fusion is the process in which differing but compatible binocular information is transformed into a unified percept. Even though this is at the basis of binocular vision, the underlying neural processes are, as yet, poorly understood. In our study we therefore aimed to investigate neural correlates of visual fusion. To this end, we presented binocularly compatible, fusible (BF), and incompatible, rivaling (BR) stimuli, as well as an intermediate stimulus type containing both binocularly fusible and monocular, incompatible elements (BFR). Comparing BFR stimuli with BF and BR stimuli, respectively, we were able to disentangle brain responses associated with either visual fusion or rivalry. By means of functional magnetic resonance imaging, we measured brain responses to these stimulus classes in the visual cortex, and investigated them in detail at various retinal eccentricities. Compared with BF stimuli, the response to BFR stimuli was elevated in visual cortical areas V1 and V2, but not in V3 and V4 - implying that the response to monocular stimulus features decreased from V1 to V4. Compared to BR stimuli, the response to BFR stimuli decreased with increasing eccentricity, specifically within V3 and V4. Taken together, it seems that although the processing of exclusively monocular information decreases from V1 to V4, the processing of binocularly fused information increases from earlier to later visual areas. Our findings suggest the presence of an inhibitory neural mechanism which, depending on the presence of fusion, acts differently on the processing of monocular information.

The decompensated monofixation syndrome (an American Ophthalmological Society thesis)

PURPOSE

To describe the clinical features and response to treatment of patients with decompensated monofixation syndrome (MFS) and to propose a hypothesis for a decompensation mechanism in such patients.

METHODS

Fourteen adults with MFS who had been symptomatically stable for a mean duration of 25 years developed diplopia in the absence of neurologic or orbital disease. After retrospective chart review, they underwent detailed orthoptic testing. Results from this cross-sectional analysis were compared with similar data from 16 control subjects with stable MFS.

RESULTS

Compared to stable MFS patients, decompensated subjects had significantly poorer horizontal fusional amplitudes but greater torsional fusional amplitudes; they were also more likely to have a small vertical strabismus and to have received initial treatment later. Stable subjects, however, also had subnormal horizontal as well as torsional fusional amplitudes. There was no difference between groups with respect to refractive error, amblyopia, type or prior treatment of strabismus, stereoacuity, or angle of deviation. After treatment, all patients regained monofixational alignment, but up to one-third had continued diplopia. Symptoms recurred in two patients whose treatment was initially successful.

CONCLUSIONS

Patients with MFS lose fusional amplitudes over time. In some cases this results in development of sensory torsion with secondary decompensation and diplopia. The rate of decompensation averages 7% per year from ages 20 to 70. Treatment for decompensation offers excellent motor results, but sensory symptoms may persist and recurrent symptoms may develop. Monitoring and maintenance of fusional vergence amplitudes should be part of the routine care for patients with MFS.

Intermittent esotropia

Not even half the number of the patients included in this study (N = 46) had been noticed as intermittent. Diplopia (N = 5) is not a reliable criterium. They need surgery with comparatively large amounts, on which behalf we prefer bi-medial rectus recessions. The prevailing result is "subnormal binocular vision," not to be confused with microtropia. Only four patients reached a cure, which means that they could not be differentiated from unaffected individuals any more.

Suppression scotomas in primary microstrabismus--a perimetric artefact

Static perimetry with stereoscopic targets, "stereo-perimetry", was performed on eight patients with primary microstrabismus to find out how strabismic subjects see under natural conditions, i.e. how they ordinarily make use of their squinting eye. In all cases, suppression scotomas were detectable using dissociating perimetric techniques. By means of stereoperimetry, however, the scotomas were not detectable. On the contrary, stereo-acuity was always best in the center of the suppression scotomas. This result indicates that suppression scotomas of the microstrabismic subjects examined here represented perimetric artefacts. In microstrabismic patients, the cooperation of the deviated eye in the central visual field is probably much better than has previously been thought.

Bowman lecture. Current concepts of infantile esotropia

Several forms of esotropia with a different pathophysiology that meet the criterion of an onset early in life must be distinguished from essential infantile esotropia. A hypothesis is presented, according to which a delayed development or a congenital defect of retinal disparity sensitivity (motor fusion) in an otherwise normal infant with immature sensory functions causes esotropia under the influence of various strabismogenic factors. Some of these factors are genetically determined, hence the familial occurrence of essential infantile esotropia. The absence or marked decrease of stereopsis and the asymmetry of optokinetic nystagmus are interpreted as the consequence of ocular misalignment early in life rather than of structural anomalies in the afferent visual pathways of esotropic patients. The therapeutic results after surgery are classified into four groups: subnormal binocular vision, microtropia, small angle eso- or exotropia and large angle residual or consecutive eso- or exodeviations. Analysis of data from 358 operated patients with a documented onset of esotropia prior to the sixth month of life has shown that the probability of obtaining an optimal functional result is increased when surgical alignment is completed before completion of the second year of life. However, surgery after the age of two or even four years of life does not preclude the development of binocular vision on a subnormal or anomalous basis.

Fusional movements by peripheral retinal stimulation ('peripheral motor fusion')

Vertical fusional movements elicited by stimulation of the non-foveal retina cannot be distinguished from movements elicited by foveal stimulation, except for a difference in the ratio between the amplitude of the response and the amplitude of the stimulus. This ratio is dependent on target configuration and on the retinal area stimulated. In some circumstances, if the size and richness of contour balances against the object fixated at the fovea, bifoveal fixation can be disrupted. It can be assumed that the stimulus for the motor response is made up of foveal as well as peripheral retinal stimulation. it is plausible that there is a gradient over the retina: the influence of the stimulated retinal area decreases form the fovea towards the periphery.

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 into 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.

[Fixation disparity in peripheral and central binocular vision under conditions of prismatic heterophoria (author's transl)]

A critical review of the literature shows that there are controversal interpretations of thephenomenon fixation disparity. The former findings of Ogle and coworkers are ascertained byinvertigator to close the central gap in the fusional patter. Formerly the gap was essentialdue to polarisation techiques. 1. Without central gap fixation disparity decreases, but remainssignificantly different from zero. 2. Diminution or closing of the gap leads to more preciseresults, since the position of the eyes is better controlled. 3. Straining the binocular vision with high prism power cortical tolerances increase when fusion was aided by central stimuli. Fixation disparity could be extended up to 20' (minutes of are) which is three times the well accepted size of the Panum area.

Th monofixation syndrome

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