Eye, Brain, and Vision
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                                             STRABISMUS
The commonest cause of amblyopia in humans is strabismus, or squint, terms that signify nonparallel eyes—cross-eye or wall-eye. (The term squint as technically used is synonymous with strabismus and has nothing to do with squinching up the eyes in bright light.) The cause of strabismus is unknown, and indeed it probably has more than one cause. In some cases, strabismus comes on shortly after birth, during the first few months when in humans the eyes would just be starting to fixate and follow objects. The lack of straightness could be the result of an abnormality in the eye muscles, or it could be caused by a derangement in the circuits in the brainstem that subserve eye movements.
In some children, strabismus seems to be the result of long-sightedness. To focus properly at a distance, the lens in a long-sighted eye has to become as globular as the lens of a normal eye becomes when it focuses on a near object.
To round up the lens for close work means contracting the ciliary muscle inside the eye, which is called accommodation. When a normal person accommodates to focus on something close, the eyes automatically also turn in, or converge. The figure on the facing page shows the two processes. The circuits in the brainstem for accommodation and convergence are probably related and may overlap; in any case, it is hard to do one without doing the other. When a long-sighted person accommodates, as he must to focus even on a distant object, one or both eyes may turn in, even though the convergence in this case is counterproductive. If a long-sighted child is not fitted with glasses, turning in an eye may become habitual and eventually permanent. This explanation for strabismus must surely be valid for some cases, but not for all, since strabismus is not necessarily accompanied by long-sightedness and since in some people with strabismus, one or other eye turns out rather than in.

 

 

 

 

 

 

 

 




Strabismus can be treated surgically by detaching and reattaching the extraocular muscles. The operation is usually successful in straightening the eyes, but until the last decade or so it was not generally done until a child had reached the age of four to ten, for the same reason that cataract removal was delayed—the slight increase in risk.
Strabismus that arises in an adult, say from an injury to a nerve or eye muscle, is of course accompanied by double vision. To see what that is like, you need only press (gently) on one eye from below and one side. Double vision can be most annoying and incapacitating, and if no better solution is available, a patch may have to be put over one eye, as in the Hathaway shirt man. The double vision otherwise persists as long as the strabismus is uncorrected. In a child with strabismus, however, the double vision rarely persists;
instead, either alternation or suppression of vision in one eye occurs.
When a child alternates, he fixes (directs his gaze) first with one eye, while the nonfixating eye turns in or out, and then fixes with the other while the first eye is diverted. (Alternating strabismus is very common, and once you know about the condition, you can easily recognize it.) The eyes take turns fixating, perhaps every second or so, and while one eye is looking, the other seems not to see. At any instant, with one eye straight and the other deviating, vision in the deviated eye is said to be suppressed. Suppression is familiar to anyone who has trained himself to look through a monocular microscope, sight a gun, or do any other strictly one-eye task, with the other eye open. The scene simply disappears for the suppressed eye. A child who alternates is always suppressing one or other eye, but if we test vision separately in each eye, we generally find both eyes to be normal.
Some children with strabismus do not alternate but use one eye all the time, suppressing the other eye. When one eye is habitually suppressed, vision tends to deteriorate in the suppressed eye. Acuity falls, especially in or near the central, or foveal part of the visual field, and if the situation continues, the eye may become for practical purposes blind. This kind of blindness is what the ophthalmologists call amblyopia ex anopsia. It is by far the commonest kind of amblyopia, indeed of blindness in general.
It was natural for us to think of trying to induce strabismus, and hence amblyopia, in a kitten or monkey by surgically cutting an eye muscle at birth, since we could then look at the physiology and see what part of the path had failed. We did this in half a dozen kittens and were discouraged to find that the kittens, like many children, developed alternating strabismus; they looked first with one eye and then the other. By testing each eye separately, we soon verified that they had normal vision in both eyes. Evidently we had failed to induce an amblyopia, and we debated what to do next. We decided to record from one of the kittens, even though we had no idea what we could possibly learn. (Research often consists of groping.) The results were completely unexpected. As we recorded from cell after cell, we soon realized that something strange had happened to the brain: each cell responded completely normally, but only through one eye. As the electrode advanced through the cortex, cell after cell would respond from the left eye, then suddenly the sequence would be broken and the other eye would take over. Unlike what we had seen after eye closure, neither eye seemed to have suffered relative to the other eye in terms of its overall hegemony. Binocular cells occasionally appeared near the points of transition, but in the kittens, the proportion of binocular cells in the population was about 20 percent instead of the normal 85 percent, as shown in the graph on this page.
We wondered whether most of the originally binocular cells had simply died or become unresponsive, leaving behind only monocular cells. This seemed very unlikely because as the electrode advanced, the cortex of these animals yielded the usual richness of responding cells: it did not seem at all like a cortex depleted of four-fifths of its cells. In a normal cat, in a typical penetration parallel to the surface in the upper layers, we see about ten to fifteen cells in a row—all dominated by the same eye, all obviously belonging to the same ocular-dominance column—of which two or three may be monocular. In the strabismic animals we likewise saw ten to fifteen cells all dominated by one eye, but now all but two to three were monocular. Each cell had apparently come to be dominated completely or almost completely by the eye it had originally merely preferred.
To appreciate the surprising quality of this result you have to remember that we had not really interfered with the total amount of visual stimulus reaching either retina. Because we had no reason to think that we had injured either eye,

   
 




After we cut one eye muscle in a kitten at birth and then recorded after three months, the great majority of cells were monocular, falling into groups 1 and 7.


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When we look at a near object two things happen: the lens rounds up because ciliary muscles contract, and the eyes turn in.