Copyright 1995 Larry Bickford. All Rights Reserved.


The EyeCare Reports


Binocular/Stereoscopic Vision (The 3-D View of Things)

Strabismus

Amblyopia



Binocular Vision

Definition: Binocular: adj.: the simultaneous use of both eyes, two-eyed or two-sights.

That¹s just the beginning of the story. When most people hear the word ³binocular,² they envision a compact, hand-held, two eyepiece telescope used to watch birds, or whales or whatever. Magnifying and viewing distant objects as if you 85;ve been transported there is the function of this optical device. The optical and vision related binocular has a more involved functional definition.


Stereoscopic imaging and depth perception

Binocular vision requires two views of an object, each seen from a slightly different angle (parallax) combined to form a three dimensional (stereoscopic) (3-D) presentation of that visible space. Our eye s are placed some distance apart, with a divider (the nose) in between, creating the ability to observe two separate images. Our brain combines these images to create a stereoscopic, three dimensional reference. When referring to the human vision system, we call this fused, simultaneous binocular vision. The perception is that of an object-oriented spatially real image.

But sometimes objects aren¹t necessarily real. Let¹s look at ³The Magic Eye², those 3-D composite images that you stare at, and then.......pufffff......as if magic, a picture with depth and space appears from a flat photograph!

It¹s not magic, of course, it¹s just stereoscopic vision. Two views are able to be deciphered from the single picture because you have artificially created double vision. Normally, when you view an object at a fixed distance in front of you, bot h eyes turn in slightly (converge) so that they¹re aimed squarely on the object. The focusing system also adjusts for that distance. By holding the picture close and turning your eyes so that they are aimed at an imaginary place in front of you, you& #185;ve de-focused and un-crossed (diverged) your eyes to create a double vision view of the picture. Your brain combines the two images and decodes the not-so-random patterns into a 3-D view! So why not 3-D television and movies?? Stay tuned, it¹ s coming soon!

One eye works well and the other doesn¹t. What then?

Let¹s examine the situation where you only have use of one eye. Do you see 3-D? That question has been bantered about for years and we do have good answers. Like, yes and no. True stereoscopic 3-D vision is not possible without the ability to process two, combined images. But within the human vision system, another factor comes into play: depth perception.

Depth perception is the process by which you understand the relative distances of objects in space and how they relate to each other. With binocular, stereoscopic vision, the task is simple. There is very clear spatial presentation. But along with this true 3-D view of things, our brain has learned to process visual images by comparing the relative sizes, horizontal and vertical orientations, object overlap and shadows projected within the field to further define our perceptions of the world around us. These are monocular functions: only one eye is needed.

Back to the original question: If you have the use of only one eye, you¹ve probably learned to understand spatial reality, but how you ³get it² is different than those of us with full binocular vision.

Here are some home and field experiments for you to try.
These will help you clearly define these differences in real-life terms

1. View one of those 3-D photographs or holographic film. If you can see the stereo image, close on eye. The 3-D image is gone. If you do not have the ability to see nearly equall y with both eyes, you can not perceive 3-D from these sources.

2 View a scene with great depth, for example an outdoor viewscape with trees or other large objects in the background and some smaller objects in the foreground. The far-point background should be a clear sky (some clouds floating by would be a nice touc h!) Stare out into the distant sky and view the scene with one eye covered. At first, it may appear noticeably flat but soon you¹ll begin to see some ³depth.² Now view the scene with both eyes. You will notice an immediate, significant &# 179;expansion of depth¹² as it may be called. This is the true binocular stereo effect kicking in.

What if you don¹t have binocular vision?


Most people get along just fine without true binocular vision. Some do have some difficulty with certain tasks under certain situations. Driving a motor vehicle, especially if the left eye is blurred or otherwise unused, can sometimes be troublesome. Th reading a needle is chore. Some sports need good binocular vision as does viewing holographs.! A young child who is delayed in learning to walk or, later, bumps into things (more than ³normal²) should be examined by an eye doctor, preferably an optometrist or someone who understands and can test binocular function. Sometimes there¹s a fairly straightforward diagnosis and management plan. There are a number of vision system causes for loss of binocular function. It is possible, although muc h more rare, for ³higher level neurological dysfunction² to be the culprit. These would be problems within the brain or the connections between the eyes and the visual processing center in the brain.

What are the causes and correction for loss of binocular vision

There are number of causes for the lose of binocular vision. The two primary issues are amblyopia and strabismus.

Amblyopia (lazy eye)

definition: a condition of reduced visual acuity, usually unilateral and infrequently bilateral, which is not correctable by refractive means and is not attributable to any obvious structural anomalies or ocular disease.

There are a number of causes for amblyopia. These include: visual deprivation in early childhood (for example, a child born with a cataract or corneal scar), strabismus (see below), uncorrected refractive error, and neurological disorders within the eye o r in the connections to the brain. Most of these causes result from early childhood deficits. Amblyopia can develop in an adult as a result of neurological trauma, nutritional deprivation or chemical insult. The primary chemical insults are nicotine ( 9;tobacco amblyopia²) alcohol, and salicylate poisoning. There are also cases of hysterical or psychogenic vision defects.

The vision system needs to see equally and simultaneously from both eyes. If only one eye has an optical focus error, or their degree of error is significantly different, the eyes don¹t work together. The fix: eyeglasses or contact lenses. When clea r vision develops in one eye but not the other, the condition called amblyopia occurs. In amblyopia, even when the brain receives a clearly focused image, it can not process that information. In certain circumstances, amblyopia can be helped. The treatme nt consists of hyper-stimulation of the weaker eye and de-stimulation of the stronger eye. This is accomplished by patching the good eye and/or through intense training of the weaker eye and the encouragement of neurological integration. The success of th e treatment of amblyopia varies (see comments below) and is considered very rarely functionally successful if it is a long standing deficit. But then again some people do walk after being paralyzed and recover from severe neurological impairments caused b y strokes. In modern medicine, many things are possible.


Strabismus (cross eye, wall eye, squint)

definition: misalignment of the eyes, failure of the eyes to look simultaneously at the same point in space.

Sometimes, the focus is correct but the eye muscles don¹t maintain ³ocular visual axis alignment.² One eye is turned out or turned in. This is called strabismus. The causes include: early childhood disruption of vision in one eye, congenit al defect in the length or function of one of the muscles that turn the eyes, neurological damage in the connections to the muscles or in the brain, or uncorrected large refractive errors, especially hyperopia. The last cause is called ³accommodative esotropia² and is a common and easily correctable form of strabismus. The fix for the various causes of strabismus is relative to the cause. If there is a farsighted correction, spectacle or contact lenses are indicated. If the muscle is too long or short, surgery is sometimes indicated. Exercises to strengthen weak muscles and/or facilitate neurological integration, or prism lenses to adjust the angle of the incoming image are other remedies. Sometimes these remedies are combined in a treatment pl an. Ophthalmologists are the practitioners who generally perform ³strabismus surgery.² Optometrists who specialize in developmental issues often treat the functional deficits. The success rates vary, and, as adults, some people seek a remedy sol ely for the cosmetic effect. A diagnosis and treatment plan will disclose outcome in a case by case situation, often with reasonably good certainty. As a basic statistical guideline, the earlier the problem is detected and mitigated, the better the progno sis for ³single, clear, simultaneous binocular vision.² As a ³rule of thumb,² children between age two and five are the best candidates for correction resulting in usable binocular vision. Adults desiring a cosmetic improvement are lik ely to be very pleased with the surgery.


additional information (as available):

EyeCare Reports: Contact Lenses > Bifocal contact lenses vs. ³Monovision²
Abstracts:Children¹s Vision
Abstracts: Color Vision > Enhancing perception: A discussion of the use of single monochromatic red lens to enhance color perception in people with red/green saturation deficiency.