ACTA OPHTHALMOLOGICA 63 (1 985) Suppl. 173,50-53

Single Binocular Vision Clinical Aspects

Eilif Gregersen

Department of Ophthalmology (Head: Eilif Gregersen), Rigshospitalet, Copenhagen, Denmark.

Vision in itself is an extremely complex function, and single binocular vision even more so. Its clinical features constitute such an enormous group of subjects that the framework here does not permit any discussion of its electrophysiology, histochemistry, or histology, although great advances in these fields have been made during recent years.

By way of introduction let me emphasize that the physiology and pathophysiology of single binocular vision still remain unelucidated in many ways. Monocular vision is the natural requisite for binocular vision. Fusion of the monocular sensory impressions from the right and left eye into a joint, unified, general sensory impression is what is known as single binocular vision.

Duration, frequency, intensity, and wavelength of the light stimulus

A number of visual qualities are the same for monocular and binocular visual function. The duration and frequency of a flash of light are the same for both. The same applies to the intensity of a light stimulus.

The wavelength of the light is perceived in the same way by monocular and binocular function. If, in an experimental design, or per- haps in unilateral cataract, light of different intensity or colour is received by the retina in the right and left eye, there may occur a rivalry (e.g. Worth 4 lights test), and exceptionally fusi- on of colours.

Visual field and gaze field

In dealing with differences and similarities between monocular and binocular vision it

must of course be borne in mind that the binocular visual field and the binocular gaze field are smaller than the monocular ones.

Simultaneous perception, correspondence, and horopter

Normal single binocular vision is conditioned primarily by a simultaneous perception from the right and left eye and secondarily by normal retinal correspondence. Fig. 1 presents

Fig. 1. Illustration indicates the concave horopter plane and that retinal points in one eye correspond to small retinal areas

in the other eye (f = fovea).

the well-known horopter. When the position of the eyes is normal and the gaze is directed at a site on the plane, all points on the plane will be perceived by receptors in the right and left retinas as being of the same direction. These are retinal localizations whose nerve paths

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from the right and left eye respectively perhaps/ presumably end in the same cortical cells in the striate cortex. As is well-known, the concavity of the horopter decreases with the remoteness of the object.

As a general rule, we talk of corresponding retinal points. The fact is most probably that one point in e.g. the right retina corresponds with a small area in the left retina, and vice versa. As a result, the two eyes may slide

and appurtenant convergence and divergence functions as well as versions and ductions have to be normal. Where the vergences are con- cerned, the motor functions of single binocular vision naturally depend upon the presence of normal sensory functions of single binocular vision.

Stereopsis (Panum’s fusion area) a bit in relation to each other, the so-called cortical or retinal ”slip”, a phenomenon which gives rise to the physiological fixation disparity.

Even though parts of the observed objects are ,,, little,, anterior or posterior to the horopter plane, but within the so-called Panurn area, fusion can take place, even to the extent of stereopsis, as there is a question of slightly disparate stimuli (Fig. 2).

Fusion (central and peripheral, sensory and motor)

Simultaneous perception and a normal retinal correspondence are the presuppositions for nor- mal fusion, i.e. a normal fusion of 4 half images to one joint, unified, general sensory perception. It must be pointed out that so- called central fusion must be differentiated from peripheral fusion. The term central fusion is at times replaced with the term foveal fusion. Both are somewhat ill-defined concepts. The term bifoveolar fusion seems preferable, as it defines and restricts this element of visual function on the retinal level to a pure cone function. It is estimated that bifoveolar fusion in the retinal plane is restricted to less than 1.5”, i.e. to less than about 0.5 mm of retinal extent.

Bifoveolar fusion and peripheral fusion are rather different in certain respects. Central or bifoveolar fusion is conditioned by the cones of the macular region which have a 1:l corti- cal representation, whereas the rods in the extramacular areas, which condition peripheral fusion, are many to each cortical cell. The difference between central and peripheral fusion consists, int. al., in the fact that central fusion conditions normal stereopsis, but cannot induce fusional movements. This is saying that central fusion cannot induce so-called motor fusion. Peripheral fusion, on the other hand, conditions normal fusional amplitude; in other words, peripheral fusion creates a normal motor fusion, but it can ”yield” only very gross stereopsis.

Normal single binocular vision of course also depends upon intact motor elements of single binocular vision. That is, fusional amplitude

image to left eye right eye

Panum’s area of >2 object fusion (6-7’centraL points for 6-7” periph.) stereo

I J 4 inverted half images ”in“

optic nerve, tract & radiation

2 inverted stereoscopic half images after intra-hemispheric fusion

complete reinverted stereo image after inter-hemispheric fusion

Fig. 2. Illustration indicates that both fusion and stereovision is located to Panum’s area and that fusion and stereo- vision of the four half images takes place on different

central levels.

L J

\f

Relation or independence of the elements of single binocular vision

Although simultaneous perception, fusion, and stereopsis are often described as different degrees of the function which is single binocular vision, it must be emphasized that various factors indicate that these 3 sensory compo- nents are nevertheless rather different. It may

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be presumed, therefore, that separate and special- ized cortical cells are ”reserved” for each com- ponents of single binocular vision. Perhaps the only thing they have in common is their simultaneous processing of visual stimuli from the right and left eyes. As regards the difference between the components of single binocular vision it may be mentioned, as an example, that stereopsis can be induced only by horizontally disparate stimuli, whereas the fusional amplitude manifests itself also in the vertical plane.

Single binocular vision in the normal population (bifixation, possibly monofixation)

It is reasonable to ask about the function of single binocular vision in the normal population, if a normal population is defined as being persons without manifest squint, without uni- lateral visual impairment, and without major anisometropia. Among the central single binocular visual functions the acuity of stereo- vision is the most demanding and most per- fected component. A study of this component, therefore, is the most useful tool in evaluating central single binocular vision in normal per- sons. The normal values for stereovisual acuity is generally taken to be = or below a disparity of 40” in the Titmus and 60” in the TNO test. The best stereovision is the least disparity which can induce stereopsis. If the above- mentioned stereothresholds are present, they indicate normal central single binocular visual functions, i.e. bifoveolar fixation. (This does not rule out reduced fusional amplitude).

The acuity of stereovision in the normal population differs. In materials of normal populations after sorting off persons with a manifest squint or impaired vision in one eye, 2/3 - 3/4 of the subjects can manage the named, normal stereolevels. In a material in which also the relatives of strabismic persons are sorted off nearly all persons may manage a normal stereoacuity. Of course, the presence of anisometropia, its severity and correction, does play a role. The stereo performance also depends upon the method of testing and upon the time allowed. Moreover, the testhetest conformity is limited, since training, int. al., is a factor too. In conclusion, it may be said concerning the stereoacuity in the normal

population that not uncommonly subnormal or very deficient values are found in persons who are otherwise completely normal and with- out symptoms. Some of these persons presumably have alternating foveal suppression, i.e. they are monofixators. Others may have major phorias or varying degrees of anisometropia which has been corrected insufficiently and/or late in life.

Manifest strabismus and defective single binocular vision

In order not to exceed the given frame- work, the classical and pronounced anomalies of single binocular vision in manifest strabismus will be illustrated only by Fig. 3.. In the top left-hand corner it presents the usual situation

diplopia

\ ‘---4 extramacular suppression

harm. a.cc,

confusion

maculqr suppression

0 0 f a k f

inharm. a.r.c.

Fig. 3. Illustration of the classical sensorial disturbances of

binocular vision in manifest strabismus.

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in strabismus with diplopia due to macular and paramacular image formation and in the upper right-hand corner confusion caused by two different macular images. In the centre there are the usual squintconditioned suppression phenomena, on the left of a paramacular site of diplopia and on the right of the macula in the confusion situation. The two drawings at the bottom indicate the retinal areas in the right and left eye to which a squint patient attaches an identical directional value and which he therefore selects in the synoptophore test. At the bottom on the left there is a presentation of harmonious abnormal retinal correspondence with coincidence between the everyday image site and the correspondence site. Bottom right shows unharmonious abnormal retinal corres- pondence, meaning that the everyday retinal image site and the retinal area preferred by the patient in the synoptophore test are not identical.

Treatment of defective single binocular vision (gains and limitations)

Amblyopia. It is well-known that strabismic amblyopia

and anisometropic amblyopia can be cured in about 90% of the cases if treatment is started in time, consistently carried through, and followed up by possible maintenance measures.

Cosmetic, but not functional cure. It is well-known that by a squint operation

it is nearly always possible to obtain a position of the eyes which per se permits normal single binocular vision in a major or minor area of the gaze field. On the other hand, years of trials and experience have shown that in cases with early-onset strabismus, i.e. onset within the first 6 months of life, it is nearly always impossible to create a normal retinal corres- pondence. Patients with early-onset strabismus hardly ever (in less than 5 % ) attain bifoveolar fixation in spite of optimal surgery and pos- sibly many years' orthoptic treatment. In these patients with early-onset strabismus (including microstrabismus), defects of sensory functions

presumably constitute a predominant element in the impaired single binocular vision. Thus, orthoptic and surgical treatment can, as a main rule, not train single binocular visual functions which have never existed.

On the other hand, when the squint angle has been reduced to around 5" or less by surgery, these patients with early-onset strabis- mus develop in the course of everyday life an abnormal correspondence which may result in the development of a certain abnormal fusional amplitude and abnormal gross stereopsis. This gives the patients a certain evaluation of dif- ferences in distance and a convergence function as well as some stability of the position of the eyes, so that an aggravation with an increased squint angle can be avoided to some extent. Such a final situation, thus, represents a con- dition which is not at all bad.

In the above-mentioned categories of patients, it must be pointed out that orthoptic treatment on a "loose" indication is generally not only unnecessary, but may entail irreversible and very disagreeable diplopia. Late operation, i.e. in the teens or later, may also entail bothersome diplopic complaints.

Cosmetic and functional cure. If the squint has not arisen until after the

child has enjoyed normal single binocular vision during the first few years of life, it is possible to obtain bifoveal fixation in the majority of cases. In such patients there are often good functional results of surgery and orthoptic treatment. Thus, treatment with spec- tacles, operation, antisuppression exercises, con- vergence training, fusional exercises, prisms, etc. often lead to a cure. Some patients need only surgery or the non-surgical therapeutic measures, while in other patients both have to be combined.

Author's address: E. Gregersen. Bjenafdelingen. Rigshospitalet. Blegdamsvej. 2100 Ksbenhavn 0. Denmark.

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