Strabismus is a disease characterized by misalignment of the eyes that affects 1-2% of the population of the United States. Normal stereovision is impossible, because the eyes are unable to fuse together on a target. As a result, children with strabismus have poor depth perception, impairing their ability later in life to perform certain jobs, to compete in sports, and to enjoy a normal three-dimensional view of their environment. In addition, many children with strabismus eventually develop amblyopia, depriving them of normal sight in one eye. If anything happens to their good eye, they face a grave predicament. There are also intangible, but significant, psychological aspects to strabismus. Because direct eye contact is so important in social interactions, patients with strabismus are sometimes at a disadvantage, and may suffer discrimination when seeking employment.

      The crucial point is that strabismus occurs without any weakness of the eye muscles, abnormality of the cranial nerves, or intrinsic disorder of the eyes. The primary culprit is a failure of the brain mechanisms responsible for establishing or maintaining binocular fusion. Our goal is to elucidate these neural deficits. We are motivated by a conviction that understanding these deficits is essential to finding better methods of preventing and treating strabismus.

      To make progress against strabismus, we employ a two-pronged approach by conducting research in both humans and monkeys (Fig. 1). Laboratory studies in humans have allowed us to define more precisely the ocular motor and perceptual abnormalities that are present in strabismus. Observations made in patients with various types of strabismus are unarguably relevant – because the data require no extrapolation from animal to man. However, to unravel the neural abnormalities responsible for strabismus, one must employ invasive experimental techniques that cannot be used in humans. As a substitute, we are privileged to be able to use the macaque monkey as an experimental model. This species is extremely similar to the human in terms of its visual function. Extensive testing has shown that macaques, like humans, have 20/20 visual acuity, trichromatic color vision, and high level stereoscopic depth perception.

Fig. 1) Juvenile macaque raised with exotropia from age 1 month. The right eye is deviated outwards, causing nasal displacement of the corneal light reflex. The young girl displays a similar outwards deviation of her right eye. Her eyes were aligned following eye muscle surgery performed by Dr. Horton.

      Kiorpes and colleagues have reported that baby macaques develop strabismus spontaneously, just as do human infants. However, such cases are quite rare, so it is not easy to obtain monkeys with spontaneous strabismus for research. As a substitute, we induce strabismus artificially in monkeys by releasing the medial rectus muscle in each eye at age 1 month. Immediately after bilateral medial rectus tenotomy, infant macaques display a large angle exotropia, with virtually no adduction in either eye. Over a period of weeks, the deviation angle diminishes and adduction improves. However, the monkey is always left with an exotropia, which varies in magnitude from animal to animal. Usually one eye is favored for fixation in primary gaze, but monkeys with surgical exotropia freely alternate between the eyes, and generally do not develop amblyopia. Our surgical macaque model of alternating exotropia has been described in this publication: Economides JR, Adams DL, Jocson CM, Horton JC. J. Neurophysiology, 2007.

      We recognize that some people are opposed to research that involves animals. We respect their opinions and have listened carefully to them. Nonetheless, our view is that finding a cure for eye diseases that affect children, such as strabismus, requires conducting experiments in animals. As long as such experiments are performed humanely, we believe that they are ethically justified. In the final analysis, human life is more precious than animal life. At the same time, every animal’s life is valuable, and we strive to use as few animals as possible to answer each scientific question. We also work hard, along with the veterinary staff at UCSF, to make sure that each animal in our laboratory is healthy and comfortable. In fact, some are so contented that they enjoy naps in the middle of experiments.

      Although strabismus can take many forms, there exist 3 predominant pediatric phenotypes: accommodative esotropia, infantile esotropia, and exotropia. In accommodative esotropia, children experience blurred vision from uncorrected hyperopia. They attempt to overcome their blurred vision by accommodating. Unfortunately, accommodation is linked in the brainstem to convergence, inducing the eyes to cross. Prescription of eyeglasses ameliorates this problem by reducing the urge to accommodate. If detected in time, accommodative esotropia can often be treated effectively simply by prescribing eyeglasses. A major problem is that many children are not referred soon enough for evaluation and treatment of accommodative esotropia. Even a few months of ocular misalignment are sufficient to cause a permanent breakdown in binocular function, and hence, the ability to fuse.

      For infantile esotropia, the instigating factors are unclear and there is no effective preventive strategy. Infantile esotropia appears in babies soon after birth (in fact, it was formerly called “congenital esotropia”). Binocular function does not appear to ever become properly established. This form of strabismus is the most resistant to effective intervention. Strabismus surgery is rarely successful at restoring bifoveal fixation.

      Our research has concentrated on the third form of strabismus: exotropia. It generally manifests at a later age than either infantile or accommodative esotropia. Usually there is a prolonged “twilight” period when exotropia occurs intermittently, offering the potential to intervene therapeutically before the development of constant exotropia. For this reason, we consider it the most treatable form of strabismus, with the best prognosis for salvage of binocular function. There is also a practical reason we have chosen to study exotropia. One can reliably induce alternating exotropia in monkeys be performing a tenotomy of the medial rectus muscles during infancy. Surprisingly, attempts to create esotropia by tenotomy of the lateral rectus muscle are usually not successful.

      Historically, research on strabismus has been carried out predominantly by clinicians, who have concentrated, quite appropriately, upon empirical approaches. Important progress has come from devising better methods to screen pediatric populations for strabismus, using the corneal light reflex, random-dot stereograms, and photoscreening devices to detect eye misalignment. As a result, children with strabismus are being identified in greater numbers than ever before. As mentioned above, strabismus in some patients can be treated by prescription of eyeglasses because misalignment of the eyes is driven by an uncorrected refractive error. In other children, strabismus eventually requires surgery on the eye muscles. Surgery can improve a child’s appearance, but unfortunately, often fails to result in perfect eye re-alignment with restoration of binocular function. In such cases, the fundamental problem is that the neural mechanism for ocular fusion has been lost. Muscle surgery can re-position the eyes, but without a neural drive to fuse, the eyes never again lock precisely onto a common target.

      During the first few weeks of life, the eyes wander and cross quite freely, until an extraordinary event takes place. The brain learns to fuse the stream of images coming from each eye, to create a single view of the world. Each retina contains a specialized region called the fovea, capable of highest visual acuity (20/20). The challenge for each newborn child is to align and move the eyes together, so that visual targets are projected accurately onto each fovea. Once this goal is achieved, stereoscopic depth perception begins to emerge and maturation of visuomotor behavior accelerates rapidly.

      In a small number of children, the eyes drift out of alignment again, resulting in strabismus. In children with exotropia, we do not understand the instigating factors. It is clear, however, that once strabismus develops, few children complain of diplopia (“double vision”). In part, this reflects the fact that many are too young to speak. Another critical factor is that children learn rapidly to suppress the vision from the deviated eye, in order to get rid of double vision. Thus, suppression plays a key role in strabismus, because it eliminates the “error signal” that would normally induce children to re-adjust muscle tone to bring their eyes back into alignment. A major objective of our research is to explain how visual suppression occurs.

      Some children alternate fixation between the two eyes, looking for a few moments with one eye, and then switching abruptly to the other. In such cases, each eye maintains normal visual acuity, but the child loses binocular function. In other children, one eye becomes dominant, and the other remains constantly deviated. Because this eye is always suppressed, it frequently develops amblyopia. These children not only lose stereovision, but face the threat of blindness in one eye. Amblyopia is often referred to as “lazy eye”, an inaccurate term used to soften bad news for parents by implying that nothing too serious is wrong, except that the eye is mischievously not working hard enough. In fact, the problem in amblyopia lies not with the eye, but the visual cortex, where connections onto neurons serving the amblyopic eye are lost.

      More than a century ago, von Gräfe proposed that part of the visual field is suppressed in each eye in strabismus. These regions are called “suppression scotomas”. To map suppression scotomas, the fields must be tested dichoptically by stimulating each eye separately during binocular viewing (Fig. 2). ). We have developed a reliable method, using colored filters (red = right eye; blue = left eye) that match the dichroic filters in the color wheel of a digital light projector. The subject fixates a cross that is rear-projected on the center of a translucent tangent screen. The color of the cross – red or blue – changes randomly on each trial. After eye trackers detect fixation on the cross, a 1.0° purple spot is presented for 200 msec at a peripheral location. The purple spot is composed of isoluminant blue and red. The subject’s task is to report the color of the spot. If the right eye is suppressed locally in the visual field where the spot was presented, the subject will answer “blue”, and vice-versa. Catch trials consisting of red, blue, or no stimulus are interleaved randomly to assess reliability on unambiguous trials.

Fig. 2) Suppression scotomas in a 9-year-old girl with a 16° alternating exotropia. Visual field maps were compiled from interleaved trials with either the (a) left or (b) right eye fixating on a cross at the center of the screen. The center of gaze for the fixating eye has been set at the origin for all trials. Top row shows purple stimulus trials. Most points were tested 4 times; jitter in the location of each stimulus trial (white circles) reflects a correction corresponding to the difference in position between the fixation cross and the fixating eye, as measured by the eye tracker. The position of the deviated eye for each trial is plotted as a small black dot, forming a cluster underneath the letter for that eye. The fill color of the white circles indicates the subject’s verbal response to a spot at that location: “blue” = left eye perceiving; “red” = right eye perceiving. The color background is a smoothed Kriging interpolation of the data points. Middle row shows control trials with a red stimulus, with 99/99 correct responses. Bottom row shows control trials with a blue stimulus, with 106/111 correct responses. 48/49 blank trials were correctly ignored.

      The main result is that the temporal retina is suppressed in each eye, up to a demarcation line located between the fixating eye’s fovea and the projection of the deviated eye’s fovea. Surprisingly, the fovea of the deviated eye remains perceptually active. Visual confusion is avoided by shifting its perceived location, a phenomenon called anomalous retinal correspondence. The deviated fovea is subsidiary, because visual attention is directed to the fovea being used to saccade to targets. For a full account of our studies of suppression scotomas, see: Economides JR, Adams DL, Horton JC. J. Neuroscience, 2012.

      The division of the visual scene into regions perceived by only one eye – rather than both eyes – leads to altered neuronal activity in the primary visual cortex. Figure 3 depicts the impact of suppression of visual information from the peripheral temporal retina in each eye:

Fig. 3) Schematic diagram showing perception of the visual scene by subjects with alternating exotropia. Here, either eye is shown looking at a central cross, with the other eye deviated by 16°. The cross lands on the temporal retina of the deviated eye. Dashed lines represent the nasal extent of each eye’s visual field. Blue and red shading of eyes indicates portions of each retina engaged in perception; gray shading denotes suppressed temporal retina. The smaller the exotropic deviation, the closer temporal suppression extends towards the vertical meridians in each eye. In striate cortex, dark and light (depicted by blue/gray or red/gray columns) CO columns occur in the peripheral visual field representation (from 8° - 64°), reflecting suppression of neuronal activity in ocular dominance columns supplied by the peripheral temporal (ipsilateral gray) retinas. In the central visual field representation (from 0° - 8°) both retinas are active perceptually. Consequently, CO staining density is equal in the cores of the ocular dominance columns, indicated by alternating red and blue shading. It is reduced, however, in binocular border strips (thin white lines) along the edges of ocular dominance columns because fusion is absent.

      Relative levels of neuronal activity in the brain can be visualized anatomically by processing tissue for cytochrome oxidase (CO), a metabolic enzyme. Regions that are more active physiologically contain higher amounts of CO. In the primary visual cortex, input from the retina is segregated into separate, interlacing zones called “ocular dominance columns”. In normal subjects, with good binocular vision, no pattern of cytochrome oxidase stripes is seen in layer 4 of the cortex. But in subjects with exotropia, the suppression of signals from the temporal retina of each eye produces two distinct patterns of CO activity (Fig. 4):

      1) Pale Border Strips: In primary visual cortex where the central visual field is represented, signals from both retinae are perceived, so there is strong CO activity in the core of each ocular dominance column. Along the borders between columns, CO activity is reduced because these regions are rich in binocular cells, which require ocular fusion to maintain normal levels of metabolic activity.

      2) Thin Dark/Wide Pale Columns: In primary visual cortex where the peripheral visual field is represented, signals from the ipsilateral temporal retina are suppressed. There is also loss of activity in binocular cells, concentrated in border strips. As a result, CO reveals alternating dark and light columns, with the dark columns corresponding to cortex driven by signals from the contralateral nasal retina.

Fig. 4) (A) Pattern of pale border stripes, present in representation of peripheral visual field, where input from the ipsilateral temporal retina is suppressed. (B) Pattern of thin, dark columns alternating with wide, pale columns, seen in representation of central visual field, where both input from both eyes in perceived.

      The abnormal patterns of CO activity present in the cortex of macaques with alternating exotropia are described further in: Adams DL, Economides JR, Sincich LC, Horton JC. J. Neuroscience, 2013. This study demonstrates for the first time anatomical changes in the visual cortex that might explain suppression in strabismus. Like a switch, inhibitory mechanisms functioning between ocular dominance columns may allow one eye to turn off neuronal activity in the other eye’s columns, leading to suppression of perception. However, this evidence is still preliminary. Our laboratory is currently engaged in further psychophysical and electrophysiological experiments in awake, trained macaques raised with strabismus. Our goal is to record the activity of single cells in the cortex, to learn if their firing rates correlate with the monkey’s report of which eye is actively perceiving.

      In normal subjects, the visual environment is explored through constant execution of saccades. When an object of interest appears, its image falls on corresponding points in the peripheral retina of each eye. A conjugate saccade moves the eyes so that the target is projected onto each fovea. In strabismus, the situation is more complicated because the foveae are directed at two different locations.

      It is common for a person with alternating exotropia to view a target with one eye and make an accurate saccade to fixate a new target with the other eye. It is unknown how this feat is accomplished. It is possible that the new target is perceived via the peripheral retina in the initially fixating eye. In that case, to program an appropriate movement for the other eye, the brain would have to take into account the exotropia to calculate the correct vector of the intended saccade. Alternatively, the new target might be detected via the peripheral retina in the deviated eye. The brain could then derive saccade parameters directly from information provided by the eye destined to acquire the new target. The third possibility is that either eye could supply the information required to make an appropriate saccade for a fixation swap. To resolve this issue, we have presented targets dichoptically to subjects with alternating exotropia that were visible to the fixating eye, the deviated eye, or to both eyes. We then compared the subjects’ choice of eye for target acquisition with the organization of their suppression scotomas. Data from a representative patient is shown below (Fig. 5).

Fig. 5) Saccades to targets presented to both eyes. a, b) Blue cross/purple target and red cross/purple target trials. A purple target elicits a different pattern of saccadic behavior than a blue (Fig. 2a) or a red (Fig. 2b) target. There are separate zones, where either the left eye or the right eye makes a saccade to the target. c, d) Perceptual data for blue cross/purple target and red cross/purple target trials acquired in a separate experiment, showing the patient’s verbal identification of target color. In regions shaded blue the right eye is suppressed, and vice-versa. Correlation between saccade data and perceptual data implies that saccades are made to each target with the eye that perceives the target. For more details, see: Economides JR, Adams DL, Horton JC. J. Neuroscience, 2014