Module 28 

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Module 28:

Binocular Vision

Section 1

The miracle of binocular vision

Have you ever wondered why we look the way we do?  Humans, that is.  After all, we could have been made to look like this, with  compound eyes giving us hundreds of tiny pictures in a mosaic:

But God in His infinite wisdom gave us two eyes, mounted forward on our heads, with 6 pair of extra-ocular muscles that normally work in perfect unison to track objects in such a way that the images from the two eyes are perceived by the brain as one image oriented in 3 dimensional space. The complexity of the neuro-mechanics involved is mind boggling.  This is the visual system, when working perfectly, that allows the major league baseball player to hit a slider coming at him at 90 miles per hour. Pretty amazing. 

Of course, it is that mind boggling complexity that fills the ophthalmic assistant with dread when he or she thinks about dealing with extra-ocular muscles and ocular motility problems.  But, like all things complex, if you break the system down into smaller parts, it becomes much more simple, and it becomes easier to understand.

Binocular Vision

What is binocular vision?  Your handy-dandy Quick Reference Glossary * defines "binocular" as "visual properties or processes that involve both eyes working together."  Each eye tracks the same object and each eye sees an image of the same object.

Fusion

Fusion is the term given to the process by which the brain takes the image from each eye and "fuses" the images into a single image in the brain (perception).  Be aware that you can have binocular vision without having fusion.  After all, a person experiencing double vision (no fusion) has binocular vision.

Fusion has two components: sensory and motor.  Sensory fusion is the perception of a single image that occurs in the brain.  Motor fusion is the binocular combination of eye movements (tracking) required for each eye to maintain the image.  For example, as an object moves toward you, the eyes have to converge to maintain simultaneous focus on the object.

Stereopsis

Because the eyes are separated by a significant distance in our forehead, each eye sees the same object at a slightly different angle.  So, the two images are very similar, but not exactly the same.  The brain uses the differences in the two images to compute the object's position in space.  This is stereopsis, the so called "three dimensional" effect (3-D). 

Stereopsis is the highest form of binocular vision, and there are even various grades of stereopsis.  Most folks can perceive that Aunt Minnie is standing 2 feet in front of Uncle Clyde, but not everyone can perceive that the image of the Lincoln Memorial is raised slightly from the surface of a penny.  

One of the functions of the Titmus Stereo Test is to measure the patient's degree of stereopsis.  Some people have only "gross" stereopsis. They can pick the wings of the fly, but they can't identify the animal that sticks out from the rest.  Other people have finely developed stereo perception and they can identify the circle that is elevated in the ninth group.  A detailed discussion of stereo testing is found in Section 2.

The degree of stereo perception is a function of the "angle of parallax", "P" in the drawings below. The greater angle, the more pronounced the 3-D effect.  Since the side of the triangle represented by the distance between our pupils does not change, then the angle is changed by the distance from our eyes of the viewed object.  The farther the object is from us, the less the stereo effect will be for a given object size.  The lesson here is to hold that needle and thread close to you for a greater stereo effect.

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Depth Perception

Depth perception is not synonymous with stereopsis.  Stereopsis is a form of depth perception.  However, we do have depth perception without having stereopsis.  Our visual system uses clues other than stereopsis to give us a sense of the relative position of objects in space.  These are some of the other visual clues we use to perceive depth:

• Converging lines:  Parallel lines appear to converge as they recede into the distance, and they diverge as they come closer to us, e.g. railroad tracks.

• Relative size: An object of a known size will appear to be smaller in the distance.  A car next to you looks much larger than a car a mile away.

• Relative motion:  As you move, objects that are closer to you appear to pass by more quickly than objects in the distance.  As you look out the car window, those telephone poles are whizzing by, but that mountain in the distance barely seems to move.

• Shadowing and highlights:  The way in which light is reflected from an object, or the way that light is blocked by an object, gives us a sense of the object's position in space.  This phenomenon is used by artists and graphic designers to simulate a 3-D effect.

So, even the one-eyed person has depth perception, but she does not have stereopis, which is the most advanced form of depth perception.

Stereo Viewing and Stereo Simulation

Stereopsis can be simulated photographically by taking two different photos from slightly different perspectives.  The two photos side by side below are of the same scene, but the camera was moved about 3 inches to the right before the photo on the right was taken.  The photo will be in stereo if viewed with a stereo viewer.  

The stereo effect can also be simulated using a "flicker photo", as demonstrated below.  This is a gif animation that will not be appreciated unless you are viewing it on a computer screen.  The animation "flickers" between the two different photos taken from slightly different perspectives, simulating what is seen with each eye.  You will notice that the stereo effect can be appreciated with only one eye.  Concentrate on the foreground of the photo.  This photo was taken from the 6th floor of the Eye Foundation of Kansas City.   Yes, it was a windy day.

Have you ever seen a movie in stereo (3-D)?  Disney Land and Disney World have at least one 3-D movie running all the time, and the "Spy Kids" movies would be recent Hollywood examples.  If 3-D movies are so life-like, why aren't all movies made in 3-D?  (Aside from the fact that the glasses are goofy.)  Simply because an effective "3-D" effect can be achieved by moving the camera to change the angle of view.  This is the same effect as demonstrated in the above "flicker photos".  

A good example is The Lord of the Rings movies.  The next time you view these, or any other action films, pay attention to how the camera angle changes.  Compare the action scenes to the character scenes.  In many of the action scenes, the camera moves, giving you the viewer a changing perspective and a "3-D" effect.

Abnormal binocularity

The are various processes by which the eyes are not in binocular alignment and a tropia exists.  The result is that the object of regard falls on the fovea of one eye (the "fixing" eye), and the image falls on a non-foveal area of the retina of the tropic (deviating) eye.  The patient experiences double vision (diplopia), at least in the short term.  Long term, the patient may compensate by either suppressing the image from the deviating eye, or by developing a crude form of fusion called "abnormal retinal correspondence" (ARC).

With ARC, the patient reports perceiving fusion in the presence of an obvious tropia.  The brain has fused the normal foveal image of the fixing eye with the blurred non-foveal image of the deviating eye.  There is no longer any double vision, but there is no significant stereopsis either.  Be aware that it is possible to have fusion without having stereopsis.

With suppression, the brain chooses to compensate for double vision by ignoring the image from the deviating eye.  If a young child suppresses because of strabismus, the suppression can lead to amblyopia.

Amblyopia

Amblyopia is decreased vision that cannot be improved with refraction, that is not caused by a specific organic problem.  In other words, suppression can lead to a permanent loss of vision.  There are several types of amblyopia.

Strabismic amblyopia results when the vision in the deviating eye of a non-alternating childhood tropia is suppressed and the vision does not develop normally.  A common example is an accommodative ET.  Do you remember the synkinetic near response?  If not, see Module 27.  These children have an ET from a non-treated hyperopia.  The child "prefers" one eye and the vision in the other eye is suppressed.  The child is treated with the full hyperopic glasses correction and patching of the preferred eye, which forces the child to use the eye that would otherwise be suppressed.

Anisometropic amblyopia results when each eye has a different uncorrected refractive error.  This can take several different forms.  One eye may be hyperopic and the other myopic.  If the hyperopic eye sees distance and the myopic eye sees at near, normal visual acuity may develop, but fusion and stereopsis will not.  If both eyes are hyperopic, but by significantly different amounts, the child will use the eye requiring less accommodation (the least hyperopic eye) and suppress the fellow eye.  The obvious remedy would be full correction of the refractive error in both eyes and patching if necessary. Patching must be started before age 9, and is most successful when begun by age 31/2.

Refractive amblyopia results in decreased visual acuity in both eyes.  This is caused by un-corrected high refractive errors.  This type includes meridional amblyopia which results from an uncorrected significant astigmatic error.  An axis aligned segment of vision develops normally while the segment 90 degrees away does not develop normally, resulting in decreased visual acuity.

Amblyopia ex anopsia (from disuse) results from disuse of the eye from a physical cause such as a congenital lid ptosis, or a congenital cataract.  Once the cause is removed, decreased vision may persist due to the length of time that the eye was not used.

Testing for Amblyopia

Amblyopia may be suspected when there is a one line best-corrected acuity difference between the two otherwise normal eyes.  The diagnosis is usually not official unless there is at least a two line difference.

Typically, an amblyopic eye has difficulty separating images that are close together.  This is sometimes termed "separation difficulty" or "crowding phenomenon".  When performing acuity testing, this means that the amblyopic eye will score much better when the vision is tested with single letters or symbols as opposed to a line of letters or symbols.  For this reason, children should always be tested with a full one of letters, numbers, or symbols ("E" game, or Allen pictures).

Testing for fusion and stereo-acuity

When testing binocular vision, it makes sense to start with stereo-acuity testing (unless the patient has an obvious tropia).  If the patient has good stereopis, you are finished with your testing.  Since stereopsis is the highest form of binocular vision, a person with stereopsis has normal sensory fusion and normal binocular vision.  Every child under the age of 10 should have stereo-acuity testing as part of the normal exam procedure, if the child is old enough to understand the test.

Common tests for stereo-acuity are the Titmus, Randot, and the Random dot E tests.  Common tests for sensory fusion are the Worth four dot, and the red filter test.

See Modules 29 and 30 for more information on other EOM related testing.

Reference:

* Quick Reference Glossary of Eyecare Terminology, Hoffman, Slack, Inc.