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
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
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
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
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
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
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
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