Documenta Ophthalmologica 47.1 : 163-199, 1979 W H A T IS N O R M A L B I N O C U L A R V I S I O N ? R.A. C R O N E & S A N J O T O H A R D J O W I J O T O

(Amsterdam, The Netherlands /Bandung, Indonesia) Keywords: Oculomotor balance, Anomalous fusion, Fixation disparity, Random dot stereograms.

ABSTRACT The vergence position of the eyes is determined by the near fixation-accommodationmiosis synkinesis and the fusion mechanism. The contribution of both systems~was analysed in 30 normal subjects and 16 subjects with abnormal binocular vision. Prism fixation disparity curves were determined in three different experimental situations: the routine method according to Ogle, a method to stimulate the synkinetic convergence (Experiment I, with one fixation point as sole binocular stimulus) and a method to stimulate the fusion mechanism (Experiment II, with random dot stereograms). Experiment I produced fiat curves and Experiment II steep curves. The mean diameter of the horizontal Panum area was 5 minutes of arc in Experiment I and 2 ~ in Experiment II. On the basis of these findings, it was postulated that the synkinetic system operates in the absence of fixation disparity and the fusion system in the presence of fixation disparity. In Experiment II, esodisparities of 100 minutes of arc occur in a number of normal subjects. The dividing line between normal and abnormal binocular vision therefore is blurred. Normal persons can display disparities, the order of magnitude of which is equal to that of the angle of squint in micro-strabismus. INTRODUCTION T h e t h e o r e t i c a l insight i n t o s t r a b i s m u s has b e e n s u b s t a n t i a l l y d e e p e n e d b y w o r k carried o u t in t h e f u n d a m e n t a l sciences. C o n c e p t s s u c h as fusion and stereopsis have a s s u m e d a n e w d i m e n s i o n as a result of t h e w o r k o f n e u r o physiologists like H u b e l a n d Wiesel, Bishop and m a n y others, b u t also t h r o u g h t h e p s y c h o p h y s i c a l r e s e a r c h u n d e r t a k e n b y Ogle a n d Julesz. Clinicians, t o o , have c o n t r i b u t e d to a n e w view of strabismus. A characteristic f e a t u r e of this n e w c o n c e p t i o n is a lessening of t h e d i s t i n c t i o n bet w e e n n o r m a l b i n o c u l a r vision and squint. Lang p o i n t e d to the great imp o r t a n c e of m i c r o - s t r a b i s m u s , s t r a b i s m u s w i t h a small angle o f squint, of w h i c h t h e u p p e r limit has b e e n fixed at a b o u t 6 ~ , b u t o f w h i c h t h e l o w e r limit has n e v e r b e e n stated. Bagolini again drew a t t e n t i o n to t h e fact t h a t m o t o r f u s i o n - w h i c h in f o r m e r t i m e s was generally regarded as an a t t r i b u t e of n o r m a l b i n o c u l a r vision - occurs also in strabismus. It is t h e n ' a n o m a l o u s f u s i o n ' , because t h e objective is n o t bifoveal f i x a t i o n , b u t s t a b i l i z a t i o n of t h e eyes at t h e angle of squint.

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The work of Ogle in the area of fixation disparity has perhaps contributed the most to lessening the distinction between strabismus and normal binocular vision. Fixation disparity is strabismus which as a rule does not exceed about ten minutes of arc. Twenty years ago, Jampolsky, writing on the subject of fixation disparity, had this to say: 'Fixation disparity is most likely an ocular deviation occupying an intermediate status between that of orthophoria, with hi-fixation, and a manifest strabismus.' All this has imparted topicality to the question: What, in precise terms, is binocular vision? The answer appears simple, but, as the literature shows, there is a difference of opinion on the subject. In seeking to define normal binocular vision, one can direct one's attention to various characteristics of binocular vision, such as stereoscopic vision and amplitude of fusion. This, however, does not provide a precise demarcation between normal and abnormal binocular vision. It is also necessary to examine the binocular eye position. Then, 'orthophoria' appears to be a correct description of normal binocular vision. But is this really so? Is a small degree of esophoria perhaps more normal, as Jonkers has postulated? And is there really any point in formulating binocular vision on the basis of a measurement in dissociated vision? Pursuing this line, 'absence of fixation disparity' would seem to be a better equivalent of normal binocular vision. But then the question arises: Is the absence of fixation disparity normal, or do most normal subjects have a certain esodisparity, just as they have a slight esophoria? Perhaps the reader would have wished to pose a preliminary question, namely that if fixation disparity does indeed exist, can it be used as a criterion for normal binocular vision, or is it an artificial product of a laboratory test situation? This treatise seeks to provide an answer to the question: What is normal binocular vision? Put another way, how precisely can we formulate the boundaries between normal binocular vision and strabismus? Observing that fixation and anomalous motor fusion are the two phenomena which have contributed most to blurring the contrast between normal binocular vision and abnormal binocular vision, we shall direct our attention particularly to these phenomena. Prior discussion of the known facts and existing theories concerning normal oculomotor balance is a prerequisite for a sound understanding of the results of our investigations. These investigations will lead to a more precise definition of the mechanism which regulates normal binocular vision. OCULOMOTOR BALANCE Many factors contribute to the accurate maintenance of bifoveal fixation, both anatomical and innervational. No one doubts that an intact anatomy of the oculomotor apparatus is an important factor. In some forms of stra-

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bismus, a disturbance of the anatomical relationships seems to be the prime cause - this is so, for example, in strabismus divergens in patients suffering from Crouzon's disease. Yet the significance of anatomical factors in the subtle disturbances of the oculomotor balance is generally secondary, as will emerge later in this treatise. Electromyographic investigations have shown that a lasting innervation of the ocular muscles occurs. This lasting tonus is caused by various nervous mechanisms which are involved in the process of bifoveal fixation. The centres from which this tonus emanates cannot be precisely determined. The vestibular tonus and the conjugate gaze innervation will contribute to it, but here we shall confine ourselves to the disjunctive innervations. In these we distinguish three mechanisms, which we shall designate the 'F', 'S' and 'O' systems.

The F system (motor fusion) The motor fusion serves to correct various types of disturbance of the binocular fixation. The stimulus for this correction is the so-called disparity, i.e. the situation in which identical images fall on non-corresponding retinal points. Fusional movements may be horizontal, vertical of torsional. The range increases as a function of the amount of contour in the visual field. A great deal of evidence exists that the foveal stimulus is of secondary importance for the occurrence of motor fusion. The F system, as we, following Ogle's example, shall term motor fusion, operates as a 'psycho-optical reflex'; that is to say, an involuntary process which occurs compulsively provided that attention exists. The human visual cortex may well be organized to bring about fusion by the presence of 'disparity detectors' (Barlow et al., 1967). These nerve cells are believed to process the data which lead to stereoscopic vision and motor fusion. Julesz (1964), with his 'Random Dot Stereograms', demonstrated convincingly that the recognition of patterns (a higher level of visual-psychic activity) is not necessary in order to achieve stereoscopic vision. We may assume that this also holds true for motor fusion, which may depend upon the activity of these same disparity detectors. Contours in the visual field are stimuli for motor fusion; consciously recognized visual objects are not necessary f o r this process.

The S system (synkinesis o f near fixation, accommodation and miosis) The S system is employed for near vision. Its neurological localization differs from that of the F system. According to Jampel (1959), there ir a parieto-occipital centre. Near fixation, unlike motor fusion, is not a psycho-

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optical reflex, but a conscious eye movement, just as are the movements of gaze. Near fixation is directed towards a bifoveally fixatable nearby object which is identified as such. Because of this property, the S system is an activity requiring higher visual-psychic integration than fusion. Fusional and synkinetic convergence As a convergent eye movement can be brought about by both the F and the S systems, it is of importance to asses the contribution made by each mechanism. The contribution made to convergence by the S sysCem can be investigated in a situation in which stimulation of the F system is minimal: during bifoveal fixation of a consciously identified, nearby visual object in the absence from the field of vision of contours which can lead to a psycho-optical fusional reflex. Certain aspects of the S sytem, voluntary, accommodative and proximal convergence, can be observed by completely eliminating binocular vision. The essential element of convergence - b i f o v e a l fixation - will then, however, be lost from sight. The contribution made to convergence by the F system can be investigated by offering a pattern in which peripheral contours greatly outweigh contours in the central part of the visual field. No consciously identifiable, nearby visual object must be present. The foregoing distinction between the F and S systems differs substantially from the well-known list of types of convergence. Maddox (1929), as is known, distinguished fusional, accommodative,proximal and tonic convergence. Maddox's summary (to which 'voluntary convergence' could have been added) has only the appearance of being complete. It ignores the fact that the main task of the system S lies in functioning ~turing binocular vision, and that this task may not be described as fusional convergence if one is seeking to avoid confusion between the F and S systems. There are, in fact, two main forms o f convergence: fusional convergence and synkinetic convergence. The 0 system {the slow mechanism o f orthophorization) The F system is a psycho-optical reflex. The fusional movements take place at a speed of several degrees per second, and if the stimulus for fusion is removed, the eyes return to the position of rest at approximately the same speed. Synkinetic near fixation occurs at an analogous speed and can be terminated at the same tempo. In addition to these two rapid mechanisms there exists a slow stabilizing system which operates in both the vertical and horizontal planes. Few researchers have devoted explicit study to this system, which I will call the 'O' system.

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The O system is triggered off when a subject wears prisms for a considerable period. When base-out prisms are interposed, a compulsive fusional movement in the convergent direction is produced. If a Maddox rod is then held in front of the glasses, the fusional tonus is interrupted and an 'exophoria', which corresponds to the strength of the prisms, can be measured. When the prisms have been worn for some time, this 'exophoria' gives way to a pseudo-orthophoria: measured through the prisms glasses, the red line of the Maddox rod again runs through the light! The results produced by Carter's first subject (Carter, 1965) are reproduced in Fig. 1. Had the O system not been operating, the measurement points would have lain along the dotted line. The slow compensation is a mechanism whose origin we do not know. It is a system which, by restoring the orthophoria, reduces the demand for a fusional vergence. The term 'orthophorization' (O system) would appear to be appropriate to describe this. The 0 mechanism is o f great significance: in clinical strabology. The O system affords lasting compensation for disturbances of the oculomotor balance, without the need for fusional effort (activity of the F system.) One example may be cited. Nystagmus operations according to Kestenbaum, performed on patients who were previously orthophoric, do not lead to heterophoria (Crone,1979). Yet extensive surgery is performed on four ocular muscles. It is unthinkable that these operations could have been performed with such precision that the intended movement of the eyes was achieved with an accuracy of one degree. The O system, which renders the F system superfluous and eliminates the heterophoria, is responsible for the orthophoria in operated patients with torticollis. ESOPHORIA

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The 0 system and heterophoria The O system is a characteristic of normal binocular vision. Within certain limits, its impedes the occurrence of heterophoria following anatomical or functional disturbance of the oculomotor balance caused, respectively, by surgery and prisms. For these reasons, the notion that common heterophoria in itself can be adequately explained by an anatomical or functional disturbance of the oculomotor balance must be rejected. The O system must be very closely allied to the phenomenon of heterophoria. There are in principle two possibilities: either heterophoria occurs as a result of inactivity of the O system, or the O system itself is abnormal irl heterophoria. The latter has proved to be the case. The O system does operate in heterophoria; however, it then exerts an unexpected effect: it stabilizes not the orthophoria, but the heterophoria. In heterophoria, the O system can be described as the 'principle of the maintenance of heterophoria'. This principle is illustrated in Fig. 2, which is derived from Schubert (1943). A subject with 30 minutes of arc of exophoria is given a base-in prism of 6 diopters. Measured through the prism, there is at first a pronounced esophoria, as might be expected, but after 11 minutes the original exophoria returns! The 'principle of the maintenance of heterophoria' is in fact the action of the O mechanism in anomalous fusion. We shall deal with anomalous fusion in the following section.

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FIXATION DISPARITY Fundamental characteristics A characteristic of binocular vision which has not yet been dear with in discussion of the oculomotor balance is fixation disparity. This is a minor inaccuracy in the eye position which can arise during binocular vision. The phenomenon has been investigated at length by Ogle and his associates, and was summarized in 'Oculomotor Imbalance' (Ogle, Martens and Dyer, 1967). Subsequent work in this area was done by, among others, Crone and associates and is summarized in 'Diplopia' (Crone, 1973). Where reference is made to pages in these two works, they will be prefaced by the letter 'O' or 'C'. Fixation disparity can easily be demonstrated and measured by the method devised by Ogle. The subject or patient looks at a fusion target (e.g. a chart containing letters). At the centre of the fusion target is a square field of, say, 2 x 2 ~ containing two polarized nonius lines which are mounted one above the other and are capable of being moved horizontally towards or away from each other. The subject views the field through polaroid plates and moves the nonius lines until, subjectively, they form a continous line. If in reality the lines are separated by X minutes of arc, there is a fixation disparity of X minutes of arc. Fixation disparity can be measured at reading distance or during distant vision. For reasons which will be explained in due course, we shall discuss only measurements during distant vision. Fig. 3 shows the situation of exodisparity which is caused by base-out prisms. The nonius lines are imaged on corresponding vertical meridians and are thus seen as forming a continuous line. The fusional objects are not imaged on corresponding points of the retina, and the eyes are slightly divergent. It is commonly assumed that spontaneous fixation disparity is characteristic of heterophoria and that it does not occur in orthophoria. In normal subjects, fixation disparity can be induced by imposing a strain on the binocular system. This can be done in two ways: 1. Prisms change the direction of the incoming rays of light. A corrective fusional movement occurs and is accompanied by fixation disparity. 2. Lenses change the tonus of accommodative convergence; this demands a corrective fusional movement with fixation disparity. Fig. 4 (O, 144) shows the essential points: A. With base-in prisms, the rays enter convergently. The compensatory divergent fusional tonus is incomplete, producing esodisparity. The disparity increases as the strength of the prisms increases. With base-out prisms there is exodisparity.

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Fig. 3. Ogle's method of measuring fixation disparity. The polarized nonius lines are haploscopically imaged on corresponding vertical meridians. Tile eyes are forced to convergence by the interposition of base-out prisms. The eyes lag slightly (exodisparity) and the fusion targets are fused, although they are imaged on non-corresponding retinal points. Above: target. Below: eyes, prisms and polaroid plates.

B. Minus lenses produce an increased accommodative convergence tonus which is incompletely compensated by a divergent fusion tonus, producing esodisparity. The disparity increases as the lenses become stronger. Positive lenses produce a reverse effect. C. From curves A and B, by eliminating the fixation disparity, a relationship can be found between the accommodative stimulus and the prismatic fusion stimulus. The data thus obtained generally lie along a straight line. The slope of the line is a measure of the so-called AC/A ratio.

170

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Clinical types of fixation disparity curves and their interpretation Crone (C, passim) described five types of prism fixation disparity curves (Fig. 5): 1. The normal curve is sigmoid and has a fiat centre section. Between the primary eye position and a certain degree of convergence there is no fixation disparity. The curve is not so characteristic in all cases, but in normal subjects the exodisparity over a large area of convergence is of small magnitude. 2. A sigmoid curve is characteristic of convergence insufficiency and convergence palsy. Apart from the disturbance of the mechanism of convergence, the binocular vision in such patients is intact, as is shown, inter alia, by an intact vertical vergence. 3. A flat curve is indicative of disturbance of binocular vision: intermittent diplopia, tendency towards suppression, etc. In such patients, the amplitude of convergence can be very great, especially following orthoptic exercises. 4. A curve with spontaneous esodisparity which is maintained during prismatic convergence is characteristic of esophoria. 5. A curve which reveals spontaneous exodisparity and which is entirely below the abscissa is characteristic of exophoria.

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Fig. 5. Five idealized types of fixation disparity prism curves. 1. Normal. 2. Convergence palsy. 3. Fusional insufficiency. 4. Esophoria. 5. Exophoria.

These five curves are ideal types. All the possible stages between normal and abnormal curves are encountered. In terpre ta tion o f clinical types

Comparison of curves 1, 2 and 3 leads to the following conclusions. Only the operation of the F system is accompanied by fixation disparity (curve 2); insufficiency of the F system (curve 3) leads to diminution of the fixation disparity. Put another way, the S system works without error, but a lag is an inherent feature o f the F system. It is plausible that the fixation disparity performs a certain function in the fusion mechanism. Ogle (O, 329) expressed the view that fixation disparity 'actually was the stimulus for the direction and magnitude of the innervations that provide binocular fixation and for the maintenance of the tensions of the convergence-divergence synergies to prevent diplopia'. He assumed that an approximately proportional relationship existed between the fixation disparity and the disjunctive innervations referred to. If indeed fixation disparity is a characteristic feature of the F system, and is unconnected with the S system, a significant conclusion can be drawn from curves 4 and 5, viz. Heterophoria is a disturbance o f the mechanism or fusion and one in which

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the fusion is anomalous. The fixation disparity curve is abnormal in general terms and points to disturbed fusion. Since the curve does not intersect the zero line, the fusion, in the narrower sense, is 'anomalous'. This statement is true both for horizontal and vertical (C, 265) heterophoria. Heterophoria is binocular vision with the aid o f disparate retinal points, as is the case in micro-strabismus. The abovementioned characteristic of heterophoria is based on the fixation disparity prism curves. The behaviour of the eye position during dissociation is not at issue. This is an epiphenomenon which is also observed in micro-strabismus and which demands separate interpretation (C, 132). We shall not deal with its interpretation here.

Fixation disparity and the 0 system The O system was described in the preceding section as a slow process which originally occurred as a result of disturbance of the binocular eye position. This system also operated in patients with heterophoria; the effect was then that, following disturbance (e.g. by prisms) and even prismatic compensation of the heterophoria, a certain degree of heterophoria returned after a time. If orthophoria is identical to 'absence of spontaneous fixation disparity', and spontaneous fixation disparity is precisely the nuclear symptom of heterophoria, it immediately follows that: The function o f the 0 system is to cancel out fixation disparity where this is artificially induced, or to restore pre-existent fixation disparity where this is artificially modified or eliminated. This indeed proves to be so. Fig. 6 (O, 317) shows the (less than complete) elimination of fixation disparity in a normal subject following the interposition of a base-out prism of 28 diopters. That the O principle not only cancels out artificially induced fixation disparity, but also serves to maintain pre-existent fixation disparity, can be seen in Fig. 7 (O, 283). This relates to a patient with a hyperphoria of 1.75 prism diopters who, before and after compensation with prisms of 2 diopters, and even after overcompensation with prisms of 4 diopters, retained the original fixation disparity (and hyperphoria!). Ogle observed: 'Although this result is rather startling, it is only a more pronounced example of the data found for subjects having near-normal eyes who compensate completely for vertical prisms.' (O, 284). Ogle thus established a connexion between orthophorization and the principle of the retention of heterophoria. The result was indeed 'startling' to him, for at the time he was not familiar with the concept of 'anomalous fusion'.

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Analysis of the work of Ogle Ogle's writings on the subject of fixation disparity contain a wealth of valuable data. Yet his book failed to achieve the status of a masterpiece in the annals of strabology. In one very important area, namely the relationship between fixation disparity and clinical symptoms, the reader is left less than satisfied. Ogle was the first to admit this. In his preface he wrote: 'The promise of this study was not fully realised', and on page 93: 'Any attempt to find relationships between the particular type of fixation disparity prism curve and the patient's oculomotor imbalance (phoria), oculomotor coordination (prism vergences), refractive errors, or symptoms has not been rewarding.' Ogle described four types of prism fixation disparity curve (Fig. 8). Type 1 (O, 76) is the normal type. The normal (sigmoid) curve should be the sum of two exponential curves (O, 113); the upper half was termed the 'conver-

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gence synergy', and the lower half the 'divergence synergy' (see Fig. 9, derived from O, 114). The horizontal part of the curves of type II and III need not correspond exactly to the abscissa. These curves can occur if the exponential coefficients of the divergence, or convergence, synergy are small. 'Only a few suitable, but consistent variations in the basic assumptions about the characteristics of the S system are necessary' (O, 343). Type IV is a flat curve which is seldom encountered and then only in cases of disturbed binocular vision. The type IV curve was difficult to interpret (O, 351). Ogle's inability to assemble his research data into a clear system lies in shortcomings in his theoretical assumptions, notably in two respects:

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Fig. 9. Diagram showing that a sigmoid fixation disparity prism curve can be described as the sum of two separate component curves. (Ogle).

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1. No clear distinction is made between S and F systems Ogle adhered to Maddox's division of the S system into tonic, proximal and accommodative components. He does, however, say on page 125 : 'Perhaps bifoveal fixation reflexes might be included also under this category. However, we do not do so, because of the fact that fusional movements to direct the convergence of the eyes can be and often are initiated solely by images falling on the peripheral areas of the retinas of the two eyes ( . . . . . ). Perhaps some individuals can modify convergence by volition, such as the overconverging of the eyes in order to fuse images of two separated X-ray films; this kind of convergence change belongs in this second category. However, if the separation of diplopic half-images of the whole binocular visual field is small, the average person cannot voluntarily prevent fusional movements.' Here, Ogle referred to two important aspects of synergic near fixation, only to brush them aside again. The first is foveal bifixation, and the second the voluntary nature of this. On page 125, when discussing the specific character of fusional convergence, he comes even closer to a better separation of the F and S systems when he writes: 'As a reflex it belongs in an entirely different category (in the opinion of the authors) than do the three components just described previously and may possibly be subject to a different neurologic control.' It would have been easy to make a mental jump from this 'different neurological control' to the more or less proportional regulating mechanism of fusion (O, 329) and then to add the errorless-operating control mechanism of the S system as the missing element in the formation of the theory.

2. The second problem which remained unrecognized is that o f anomalous fusion. Ogle adhered strictly to the classic image of heterophoria - an unsatisfactory eye position corrected by sound power of fusion. The fact that fusion, admittedly, can be weaker or stronger, but is fundamentally normal, runs like a thin blue line through his study. Only in an older publication (Ogle, 1950) is the possibility of anomalous fusion touched upon, but then promptly rejected. On the subject of the curves which lie above the zero line throughout the whole of their length (and which, in the context of the study, are classified as the 'second group'), the author says: 'It might be argued that the constant deviation in the second group could result from an adopted change in the sensory subjective visual directions, in the same sense as that found in an anomalous correspondence, conditioned by a constant fixation disparity under the influence of a constant neuro-muscular tension. However, the increase in the fixation disparity that occurs as the fusion details of the

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target are confined more and more peripherally, shows this not to be true.' His argument against anomalous retinal correspondence is weak when viewed in the light of the present knowledge of strabology, for the angle of anomaly at which eyes co-operate binocularly depends upon many factors and can vary widely in some patients. Another example serves to show that Ogle rejects the concept of anomalous fusion. In discussing the F and S systems (O, 320), he refers firstly to the 'central convergence reflexes: accommodation-convergence synkinesis, fixation reflexes and anomalies of these'; and secondly to 'the compulsionfor-fusion reflex'. Of anomalies of the latter there is no mention! Therefore, he attributes the deviation of the fixation disparity curves observed in heterophoria to an anomaly of the S system (O, 434) and not one of the F system. The two theoretical starting points referred to lead to numerous internal contradictions. Two are mentioned, namely: 1. The difference between step and fiat fixation disparity curves. Ogle does not succeed in interpreting the steep and flat curves, and the reason for this is that he identifies the S system too closely with the accommodation-convergence relationship. Of the steep sigm'qd curve, he says on page 79: 'If one interprets the fixation disparity as being a measure of the oculomotor imbalance ( . . . ) we would say that the oculomotor imbalance is greatly affected by changes in the accommodation-convergence relafionship.' In clinical terms, this cannot be reconciled with the fact that it is precisely when the curves are steep that near vision (the S system) is weak (O, 46). Dealing with the variant of the normal curve with a flat centre section, he says (page 80): 'We might assume that the fusional ability of the oculomotor processes is highly developed, that is: strong, in the sense that this ability is such as to overcome any embarrassment of the accommodationconvergence relationship.' Clinically, this, too, cannot be reconciled with the fact that it is the flat curves (Ogle's type IV) which have weak 'fusional ability'. 2. The Iinearity o f the derived curve. It can be seen in Fig. 4 in this article that the derived data for the AC/A ratio lie along a straight line. This appears to be the case in 92% of the patients examined, many of whom had atypical fixation disparity curves (O, 185). This fact is easily explained if the S mechanism is normal and abnormal fixation disparity curves are allied to an abnormal F system: after all, the abnormal F system is eliminated in calculating the derived data. It is, on the other hand, difficult to reconcile with Ogle's notion that the S system itself is the cause of the abnormal curves. 'This result in so many patients is somewhat remarkable', he wrote, indicating that he continued to be critical towards his own work. 178

INVESTIGATION OF NORMAL SUBJECTS

Closer definition of this investigation in terms of fixation disparity A description of the S and F mechanisms, the clinical correlation between prism fixation disparity curves and disturbances of the oculomotor balance, and, finally, a critical appraisal of the work of Ogle lead to a relatively simple view, namely that fixation disparity is a characteristic of the F mechanism, and that the S system operates in the absence of fixation disparity. This assertion demands further proof, and this will be furnished in the following sections of our study. To obtain it, it was necessary to investigate the fixation disparity curves of normal persons under two different circumstances, i.e. maximum stimulation of the S system, and maximum stimulation of the F system with suppression of the S system. If our predictions are correct, the former investigation may be expected to produce a flat curve, and the latter a steep curve. Normal binocular vision is brought about by the action of the F, S and O systems. The O system will be ignored for the purposes of this study. If the O system has only a stabilizing effect in normal persons and heterophorics (and there are strong clinical arguments to support this view), it is not necessary to include this system in the present study.

The subjects The investigation covered 30 normal subjects, the majority of whom were between 20 and 30 years of age. All had normal visual acuity, with or without the aid of spectacles, and a stereoacuity of at least 60 seconds of arc, as measured by the TNO random dot test. In no case did the heterophoria exceed 1~ A fixation disparity curve of each subject was made in accordance with the method indicated by Ogle (O, 28) and described on page 169 of this article. A new version of the Crone apparatus (C, 82) was used, in which the subject was able to remotely move both nonius lines horizontally towards or away from each other. The subject looked through a double rotating prism and polaroid plates. The fusion target, which contained Snellen optotype characters, had an outer border of 32 x 32 cm, and an inner square border of 15 x 15 cm. The observation distance was 3.5 m. The apparatus was placed in a well-lit room. Fixation disparity prism curves always provide a wealth of numerical data. The horizontal width of the curve indicates the total amplitude of fusion. The vertical amplitude of the curve is the amplitude of the sensory fusion and thus indicates the horizontal diameter of the Panum area. In addition, the curve reveals the spontaneous (non-prism) fixation disparity and the

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degree of exodisparity (or esodisparity) produced by subjecting the fusion to the effect of prisms. Although there are fairly wide differences between the individual results, the pattern of curve 1 (Fig. 5) is generally recognizable in all 30 subjects. The curves of the normal subjects are reproduced in Fig. 10. ESO DISR 10I 5

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Stimulating the S system (Experiment I) The principal role played by the S system in binocular vision is to produce deliberate bifoveal fixation during convergence and accommodation. The isolated influence of the S system on the eye position can thus best be investigated by presenting a bifoveal fixation point upon which the subject can convergence and accommodate. Apart from the foveal stimulus, no contours which could stimulate the F system may be present in the visual field. Our test arrangement failed to meet one of the conditions referred to, in as much as the foveal stimulus was at a distance of 3.5 m, with the result

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that the accommodation was not stimulated. The reasons for this were not only of a technical nature, but also reflected the fact that all our fixation disparity investigations, and also the routine examinations according to Ogle and the investigations of the F system, were conducted with fusional objects at an apparent distance of 3.5 m. The apparatus was the same as was used for the routine fixation disparity examinations according to Ogle, but with a number of modifications. The nonius lines were shortened to 1 cm and the horizontal line between them was widened to 1 cm. A 2 mm diameter red Light Emitting Diode was introduced at the centre of this horizontal line. The room was completely darkened, with the result that the subject saw only the red light source and the two short polarized nonius lines. The fixation disparity was determined with the variable prism in various positions. The experiment was difficult for the majority of the normal subjects, for they had a tendency to fuse the nonius lines by means of a vertical fusion movement. The subject then saw two red lights, one above the other, showing once again that even weak, non-foveal stimuli can dominate the foveal stimulus. The test was carried out on 20 of the 30 normal subjects. In one case (subject number 3) it failed owing to diplopia. The results are shown in Fig. 11. They can provisionally be summarized as follows: the amplitude of prism fusion was small in the majority of subjects and in many cases was absent on the divergent side. The majority of those subjects who had a strikingly large amplitude of fusion were capable of squinting at will. In almost all cases the fixation disparity was small. The vertical range of the majority of the curves did not exceed 2 - 3 minutes of arc. This implies that the horizontal diameter o f the Panum areas o f the subjects who took part in this experiment was between two and three minutes o f arc. There were, however, a few exceptional subjects who achieved a considerable fixation disparity. The most striking example was subject number 1; but subject number 13 achieved a fixation disparity of 15 minutes of arc, albeit at the end of a very strong convergence. Stimulating the F system (Experiment II) The F system controls the binocular eye position by detecting similar contour elements in the fields of vision of both eyes. When a certain disparity is detected in a number of contour elements, and another disparity in another group, stereoscopic vision occurs. A fusional movement can then be performed which eliminates one of the disparities. In principle, this process takes place subconsciously, though a certain degree of visual attention is indispensable.

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The stereoscopic random dot patterns according to Julesz were chosen as a suitable stimulus for the F system. The fusion of two patterns produces a single pattern containing a square which, depending on the test situation, may be in front of or behind the surrounding pattern (see Fig. 12B). A fusional object of this nature has many advantages: 1. The random dot patterns can be presented at a large image angle, producing an optimum peripheral stimulus. Yet the central part of the pattern contains sufficiently fine details, so that in this respect the test conditions do not differ from those of experiment I. 2. The subject is instructed to scan the inner square of the stereogram, so that the foveal fixation of a deliberately chosen structural element can be omitted. Accordingly, the S system is not stimdated to any appreciable degree. 3. The stereoscopic disparity information may be considered to be an optimal stimulus for the disparity detector system which lies at the root of the F system. As it happens, the stereo effect is indispansable for the experiment on practical grounds, since the depth effect is the criterion for fusion. 4. The portion of the target to which the gaze is directed is the central square b e h i n d the surround. This serves to inhibit as far as possible the near reflex, and thus the S system.

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The apparatus is shown in Fig. 12 A. The subject viewed the random dot patterns - which were illuminated by a 300 lux source - haploscopically via beam splitters with 50% reflection. The random dot patterns measured 40 x 40 cm and were placed 80 cm in front of the eyes, making a total visual angle of 33 ~ There was a disparity of 90 minutes of arc between the innermost square and the apparently nearer surround. Nonius lines, which were also presented haploscopically by polarization, were at a distance of 3.5 m. The subject was able to move the lines with respect to each other. Viewed through lenses, random dot patterns and nonius lines were presented at optical infinity. The outer edge of the random dot patterns had been rendered indistinct in order to suppress conscious identification of the outer square.

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Fig. 12B. Experiment II. Subjective aspect of random dot stereograms and nonius lines. When the stereograms are united by a convergent eye movement an inner (distant) square will be seen. The nonius lines will not be seen aligned in case of fixation disparity. In the actual experiment the thickness of the lines is 1 min. arc, and there is no rivalry. The subject saw only the random dot patterns and the nonius lines; the room was darkened. He was instructed to visually explore the inner square and to align the nonius lines. Each time the position of the variable prism was changed, he was instructed to re-align the lines. All normal subjects found the task to be easy. The square at the rear was easely visible to all, and the amplitude of fusion could be determined simply, since the transition from stereopsis to loss of stereopsis was clearly defined. The results are shown in Fig. 13, from which the following can be deduced: 1. In most cases a small amplitude of horizontal prism fusion was observed. 2. The vertical range of the curves was large - in extreme cases, such as subject 8, more than 4~ This implies that the horizontal diameter o f the Panum area in this situation was more than 4 ~ 3. In spite of the fact that the test was based on normal subjects, remarkably wide individual disparities were observed. 4. Many of the subjects displayed a very large spontaneous esodisparity which could not be converted into exodisparity by the interposition of prisms. In a number of these subjects the curve contained two plateaux separated by about 90 minutes of arc.

Vergenee control by the S and F systems. On clinical grounds it was postulated that fixation disparity is a characteristic of the F system, and that the S system operates without an inherent lag

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in the eye position. This clinical hypothesis was confirmed by our investigation of 30 normal subjects: During an attempt to isolate the F mechanism, very large magnitudes of fixation disparity were observed, while the fixation disparity which accompanied the isolation of the S system was minimal. As we need not assume that complete isolation of either mechanism was in fact achieved, we may fairly safely extrapolate that fixation disparity is part of the fusion mechanism, but is absent from the 'near reflex'. This suggests that fusion and near reflex have differing regulating systems: fusion probably has a type of proportional system in which the fixation disparity is the 'steady state error'; the S system - in contrast, but in common with other voluntary eye movements - has no residual error. In the field of control technology, such cases are referred to as integral control systems. The difference between the two systems is discussed in Crone et al. (1978)*). PERSONS WITH OCULOMOTOR IMBALANCE During the investigation of normal persons, the question arose: How would patients with oculomotor imbalance behave in the various test situations? This cannot be entirely predicted. It is certain that the anomalous nature of the F system, which is characteristic of heterophoria, would be bound to emerge in the test situation in which the F system is stimulated. If the horizontal portion of the anomalous curves of the heterophorics is attributed to the S system, one might anticipate steep and short curves during heterophoria in the last-named test also. With this in view, a small number of esophorics and exophorics were investigated. One of the patients suffered from convergence palsy. In addition to the routine fixation disparity curve test, Experiment II was carried out on the patients described below. Experiment I was in general unsuccessful and was completed in only a few cases.

Convergence palsy One patient suffering from this condition was investigated. The curves according to Ogle and according to Experiment II are reproduced in Fig. 14. A 26-year old man, suffered a severe cranial trauma two years ago, after which he experienced diplopia as a result of a bilateral trochlear palsy. A

* In this publication, all fixation disparity measurements were made after the fixation disparity at zero prism strength had been zeroed by adjustment of the apparatus. Only later did it become clear that, in normal subjects without heterophoria, large spontaneous esodisparities could occur during Experiment II. This discovery, however, was not of fundamental significance in relation to the subject then discussed.

186

year later there was virtual remission, but the patient continued to see double when reading. A convergence and accommodation palsy was diagnosed. The accommodation was still 3 diopters in each eye. Vision was ODS 6/4 (+ID, Sph.). The patient read monocularly with Sph. +3.

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Esophoria

Eight cases of esophoria were investigated. The routine fixation disparity curves are reproduced in Fig. 15. Patients A and B were subjects in whom an esophoria was found by chance. The remainder were patients of the orthoptic department. Experiment II was performed on all the esophorics (Fig. 16) and Experiment I on patients A, B and C, though it was successful only in the last-named case. A., 30 years of age, vision ODS 5/5 (E). Maddox rod at 2.5 m: +2 ~ Normal stereoscopic vision, no complaints. B., 26 years of age, vision 5/5 (E). Maddox rod at 2.5 m: +5 ~ Normal stereoscopic vision. No complaints except diplopia after consuming alcohol. C., 18 years of age. At the age of 11, developed diplopia when reading. 1971: vision ODS 6/5 (Sph. + 0.75D). Maddox rod: +5 ~ esophoria at 2.5 m and 30 cm. After a week of trial occlusion, homonymous diplopia at 12 ~ Advised to undergo an operation, but declined. The patient was given prism spectacles with 5 PD base-out. Situation still: Maddox at 2.5 m: +5 ~ (without spectacles). Stereoscopic vision: 40 seconds of arc (Titmus test). The fixation disparity curves made at the ages of 11 and 18 years are virtually identical. D., 51 years of age. Suffered a mild contusio bulbi OS with nerve fibre bundle scotoma caused by a trauma of the optic nerve in 1964. Vision now OD 5/5 (Sph. +ID), OS 5/10 (Sph. +ID). Intermittent diplopia and asthenopia caused by esophoria 7 ~ Weak stereoscopic vision (circles 200"). A recession of the internal rectus muscle OS was performed. Maddox rod at 2.5 m one month after the operation: +3 ~ esophoria. T h e patient no longer suffered from diplopia, but had asthenopic complaints. E., 17 years of age. Had developed an intermittent convergent squint with esophoria of 13 ~ at the age of 6. Vision 5 / 5 0 D S with Sph. +2. With spectacles, 6 ~ of esophoria and no diplopia. The findings ten years later were virtually identical. Stereoscopic vision: 60" (TNO). Complained only of diplopia after removal of spectacles. F., 30 years of age. Recession performed at the age of 20 on account of 5 ~ esophoria with asthenopia and intermittent diplopia. Vision at present ODS 5/5. Spectacles OD Sph. +1.75D, prism 2D, base-out; OS Sph. 0.50D = Cyl. 0.75D, axis 90 ~ Maddox rod at 2.5 m: +2 ~ Stereoscopic vision: 40" (TNO). This patient is described in C, 175, case 9.7. She has since given birth to a daughter, who at the age of four suddenly developed strabismus, having earlier been diagnosed as having an esophoria of 3 ~ After an operation, the child now has straight eyes and normal binocular vision.

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G., 28 years of age. Convergent squint at the age of two; operated on at the age of four. Present situation: Vision ODS 5/5. Spectacles Sph. +3D = Cyl. - 1 D , 115 ~ and 30 ~ Maddox rod at 2.5 m: +6 ~ esophoria. Weak stereoscopic vision: the patient cannot see the TNO test but can see the square used in Experiment II. H., 35 years of age. Developed intermittent diplopia as a result of 15 ~ esophoria at the age of 22. Therapy: recession of the internal rectus muscle OS. Present situation: Vision ODS 5/5. Spectacles: OD Sph. - 3 . 2 5 D , 3 PD, base-out; OS Sph. - 5 . 2 5 D , prism 3 PD, base-out. Maddox rod at 2.5 m 6~ esophoria. Weak stereoscopic vision (fly positive).' Adequate amplitude of fusion. Asthenopic complaints while reading. I., 16 years of age. Developed intermittent diplopia as a result of 11 ~ esophoria at the age of 10. Therapy: recession of the internal rectus muscle OS. Post-operative: 9 ~ esophoria. Titmus test: 60". Present situation: Vision ODS 5/5 Sph. - 1 D = Cyl. - 4 D , axes 53 ~ and 150 ~. Maddox rod at 2.5 m: 10 ~ esophoria. Stereoscopic vision: 240" (TNO). The patient will be operated on for intermittent diplopia.

Exophoria All subjects were patients of the orthoptic department. The curves are reproduced in Figs. 17 and 18. U., 50 years of age. Asthenopic complaints. Vision ODS 5/5 (E). Exophoria

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192

pia; also alternating hyperphoria. Vision ODS 5/5 Sph. - 0 . 5 0 D , E. Operation: recession of external rectus muscle OS. Post-operative: 5~ exophoria to intermittent exotropia. Weak binocular vision (TNO test 240"). X., 25 years of age. Intermittent exotropia of 19~ with purely cosmetic complaints. Recovery achieved by blinking. Vision OD 5 / 5 Sph. - 3 D . OS 5/5 Sph. - 1 . 5 D = Cyl. - 1 D , axis 750. . Stereoscopic vision: 40". Y., 26 years of age. Intermittent exotropia of 14 ~ with rapid recovery. Vision ODS 5/4 (E). Operation: recession and resection OD. Post-operative: 5 ~ exophoria to intermittent exotropia. Stereoscopic vision 100" (circles). Z., 15 years of age. Exophoria of 13 ~ with intermittent diplopia at the age of 10. Recession and resection OD performed at the age of 14. Present situation: vision OD 5/10 Sph. - 5 . 5 0 D = Cyl. - 2 . 5 0 , axis 50~ OS 5/10 Sph. - 5 D = Cyl. - 2 , axis 77 ~ Exophoria of 3 ~ at 2.5 m with quick recovery. Stereoscopic vision 120" (TNO test).

WHAT IS NORMAL BINOCULAR VISION? The dividing line between normal and abnormal is often difficult to define, and a person who is normal according to one criterion can be abnormal when judged by another. The criteria for normality which we employed were: Vision ODS at least 5/5; stereoscopic vision at least 60 seconds of arc; Maddox rod at 2.5 m at most 1~ We shall now pose the question whether our other methods of investigation lead to other criteria. We shall deal only with the curves according to Ogle and those found during Experiment II. Experiment I was virtually limited to normal subjects. There were practically no correlations with other data relating to the subjects. The experiment does not appear to be a suitable means for arriving at new criteria for normality and abnormality.

The curves according to Ogle Using the routine method, widely varying fixation disparity curves were found among the normal subjects (Fig. 10). The total amplitude of fusion varied between 12 PD and 50 PD. Spontaneous fixation disparity of more than 5 minutes of arc was found only in curves 3 and 14. Many curves lacked the downward portion (e.g. numbers 19. 27, 29 and 30). In spite of many variations, all curves met the definition of normality to the extent that curves of types 2 to 5 (Fig. 5) did not occur. In other words: I. There were no pure sigmoid curves (type 2), as are described for convergence palsy (although curve 28 is very similar).

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2. None of the subjects displayed an exceptionally small amplitude of sensory fusion (type 3). 3. There were no curves which pointed to obligate-anomalous fusion (types 4 and 5). If we now survey the patients with oculomotor imbalances, we find that: The patient with convergence palsy displayed a curve of type 2 in its pure form. The patients with esophoria could all be distinguished from normal persons by Ogle's method. The curves were of type 4 (Fig. 5), or they intersected the abscissa only when strong temporal prisms were interposed ('facurative micro-anomalous esophoria', C, 131). Only subject A, whose esophoria was limited to 2~ had a normal curve. The other accidentally discovered esophoric, patient B, had obligate-anomalous fusion. All the patients with exophoria or intermittent exotropia had anomalous fusion. They were not explicitly chosen for this reason. In our experience, a type 5 curve is usually encountered in patients of this type (see O, 243, however). Summarizing, we can say that routine fixation disparity curves afford a clear insight into the disturbance of the oculomotor balance. Admittedly, there is no strict correlation between the Maddox data and the spontaneous fixation disparity; but the patients with disturbed binocular vision can, almost without exception, be recognized by a deviant fixation disparity curve. Patients with a type 3 curve are poorly represented in this study. They are in any case rare, and the binocular vision does not meet the conditions imposed by Experiment II, i.e. they cannot perceive the depth in the random dot stereograms. Patients E and Y have a type 3 curve, which in the case of Y is also obligate-anomalous.

Experim en t II The normal subjects who took part in this test produced widely differing curves. Some were completely in the area of esodisparity, others were more symmetric with respect to the abscissa. There were no curves which lay completely in the area of exodisparity. A remarkable feature was the esocurves with two plateaux separated by approximately 90 minutes of arc. This is exactly the disparity between tlae inner square and the surround. The eye position of these subjects is evidently determined first by the surround (although they only explore the inner square). When the strenght of the base-out prisms is increased, the position is suddenly determined by the inner square. If, in a diagram, one shows the cortical representation of the corresponding retinal points according to Roelofs and R~Snne (C, 15) as

194

two lines, one above the other (corresponding elements being directly above one another), then in the case of subject 30, links will first be established in accordance with diagram A and afterwards in accordance with diagram B (Fig. 19). This subject would appear to have a tendency to see at least part of the pattern with normally corresponding retinal elements for a brief portion of the prismatically-induced vergence. Whether or n ~ the S system contributes to this process, it is difficult to judge. To the extent that disparities of more than 90' are avoided throughout the whole of the random dot test, the method of binocular vision of the subject 30 appears to be rational in physiological terms. This idea, while attractive, cannot wholly explain the tendency towards esodisparity. One would then expect to find an analogous curve with two plateaux in the area of exodisparity when the Julesz patterns were reversed: a spontaneous exodisparity of approximately 90', which gave way to zero disparity upon the interposition of base-in prisms. This supposition was not borne out. The situation in which the central square appeared in front of the surround was not systematically investigated, but in a small series of tests on ten normal persons it was found that the majority of the curves were not far below those pertaining to Experiment II. There was, however, a somewhat reduced tendency towards esodisparity, or a relatively greater tendency towards exodisparity, and the curves were considerably flatter (as a result of the stimulation of the S system). Only one subject (number 4) displayed a totally different type of curve when the patterns were reversed (see dotted curves in Fig. 13). Besides the curves with strong esodisparity, a second type of curve closely resembled the routine curves (numbers 1, 10, 11 and 22). Comparison of these with the corresponding curves according to Ogle and Experiment I, affords grounds for presuming that the efforts to separate the S and F mechanisms were unsuccessful in the case of these four subjects. The third type of curve which can be distinguished is the sigmoid curve, which is very steep and lies partly in the area of esodisparity and partly in that of exodisparity. This type of curve, which in fact was expected when the S system was eliminated, was found in ten subjects. There was a clearly positive correlation with early exodisparity in the curves according to Ogle (numbers 1, 2, 3, 7, 8, 9, 13, 14, 21, 25). The results of Experiment II as these relate to the patients give rise to the following observations: Convergence palsy: as expected, the curve is steep, sigmoid and narrow. Esophoria: the esodisparity is in most cases extreme and the curves are short. With patients E to I, the esodisparity cannot be influenced by prisms. The obligate esodisparity is of the same order of magnitude as the angle of squint in micro-strabismus.

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Exophoria: the curves differ little from the routine curves, but in the case of patients W and Z display a shift towards the area of esodisparity. The results obtained with exophorics in Experiment II imply that this experiment is unsuitable for clinical differentiation between normality and abnormality, and confirm the tendency towards esodisparity which was also found in normal persons during Experiment II. Summarizing, it can be said that Experiment II produces significant esodisparities in normal subjects, esophorics and even exophorics. The test is therefore not a suitable instrument for differentiating clinically between normality and abnormality. The dividing line between normal and abnormal binocular vision At the commencement of this investigation, the dividing line between normal and abnormal binocular vision was chosen somewhat arbitrarily. The criterion was very strict: a maximum heterophoria of 1~ at 2.5 metres. There was a reason for this, namely that, as experience has shown, even small degrees of heterophoria can be accompanied by obligate micro-anomalous correspondence (C, 171, case 9.3). The possibility that anomalous fixation disparity curves of type 4 and type 5 would be found in cases of a latent angle of less than 1~ was not excluded a priori. In the event, this situation did not arise. The curves according to Ogle admittedly displayed substantial differences from one subject to another, but in general they did not exceed the limit of normality as determined by our original criteria. The curves obtained during Experiment II, however, did so - and to a significant degree. With a number of normal persons, large spontaneous esodisparities occurred and the curve was of type 4, i.e. characteristic of esophoria. In other cases, the curve was of a sigmoid pattern with equal eso- and exodisparities. The question arises whether the different types of curves which were found during Experiment II point to different types of normal binocular vision? One may in particular wonder whether the situation of stereoscopic vision leads to pronounced esodisparity in one type of normal person but not in another type? In itself this is not inconceivable. For example, Richards (1969) described subjects in whom stereoscopic vision was possible only with crossed or only with uncrossed disparities. He described this situation as 'stereoblindness', thereby classifying it as abnormal. Our Experiment II cannot lead to so far-reaching a conclusion, but we can state that stereoscopic vision in subject number 30 in Experiment II appears to be highly preferred within the area of esodisparity, and that the subject behaves as an esophoric with anomalous fusion. Yet judged by other criteria, the subject concerned is normal.

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One could take the view that the test situation in Experiment II is itself abnormal - an artificial laboratory setup. This is very debatable. The visual target does not differ in principle from a daily scene such as one sees when looking out of the window. It is by no means inconceivable that, in the situation of daily vision, fixation disparities occur which until now would have been held to be highly abnormal.The unexpectedly large fixation disparities which were observed during Experiment II imply that the horizontal diameter of the (foveal) Panum area can be very large, even reaching 4 ~. This conflicts with the measurements made by other researchers, who in the majority of cases found a diameter of 1 0 - 2 0 minutes of arc. Fender and Julesz (1967) observed a horizontal diameter of 2 ~ when presenting random dot patterns in a situation of bilateral retinal image stabilization, and they only measured the temporalward extension of the fusional areas. The data of Fender and Julesz were treated with scepticism, and in the realms of strabology no attention was paid to them on the ground that measurements made in so artificial a test situation could not possess any practical value. PresumabIy the reverse is true: the random dot patterns are just as rich in contours as the scenes from daily life. The dimension of Panum areas quoted earlier were the result of laboratory tests in which the amplitude of sensory fusion was deliberately minimized. The test situation in investigations of that sort w a s highly artificial. The discovery that the horizontal range of the extension of the foveal Panum areas can amount to several degrees had important consequences for strabology. In the majority of textbooks, the concept of correspondence as laid down by Panum is defined as implying that one retinal element in one eye corresponds with a small Panum area in the other eye. Now that it has been shown that the Panum area can be measured in degrees instead of in minutes of arc, a new and much closer definition will have to be gi'r to the term 'normal correspondence'. In the introduction to this article, we asked ourselves how normal and abnormal binocular vision could be distinguished from each other now that the divisions between them have been blurred by the phenomenon of fixation disparity and anomalous fusion. Our investigation provided us with a reasonably good criterion, i.e. the shape of the fixation disparity curves according to Ogle. The experiment with the random dot patterns deprived us of that criterion, so that our final conclusion must be that the dividing line between normal and abnormal binocular vision remains obscure.

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REFERENCES Barlow, H.B., Blakemore, C. & Pettigrew, J.D. The neural mechanism of binocular depth discrimination. J. Physiol. 193:327-342 (1967). Carter, D.B. Fixation disparity and heterophoria following prolonged wearing of prisms.Amer. J. Optorn. 42:141-152 (1965). Crone, R.A. Diplopia. Excerpta Medica, Amsterdam & American Elsevier Publishing Cy. Inc., New York (1973). Crone, R.A. Vrooland, J.L. & Sanjoto Hardjowijoto. Proportionakegelung der Fusion, Integralregelung der willkfirlichen Konvergenz. Augenbewegungsst6rungen, Ed. G. Kommerell: 323-323. J.F. Bergmann Verlag, Miinchen (1978). Crone, R.A. Orthophorisation. Die binokulare Augenstellung nach Kestenbaumscher Operation. Klin. Mbl. Augenheilk. (to be published in 1979). Fender, D.H. & Julesz, B. Extension of Panum's fusional area in binocularly stabilized vision. J. Opt. Soc. Amer. 57:819-830 (1967). Jampel, R.S. Representation of the near response on the cerebral cortex of the macaque. Amer. J. Ophthal. 48:573-581 (1959). Julesz, B. Binocular depth perception without familiarity cues. Science 145:356-362 (1964). Maddox, E.E. Discussion on heterophoria. Trans. Ophthal. Soc. U.K. 4 9 : 3 1 - 4 4 (1929). Ogle, K.N. Researches in binocular vision. W.B. Saunders Cy. Philadelphia (1950). Ogle, K.N., Martens, T.G. & Dyer, J.A. Oculomotor imbalance in binocular vision and fixation disparity. Lea & Febiger, Philadelphia (1967). Richards, W. Stereopsis and stereoblindness. Exp. Brain Res. 10:380-388 (1970). Schubert, G. Grundlagen der beid/iugigen motorischen Koordination. Pfliigers Arch. ges. Physiol. 247:279-291 (1943). Authors' addresses: R.A. Crone Eye Dept. Wilhelmina Gasthuis 1 ste Helmersstraat 104 1054 EG Amsterdam, The Netherlands Sanjoto Hardjowijoto Pajajaran University Bandung, Indonesia

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