Shown here is the stereovision test book that our doctors use to test patient’s depth perception and binocularity.

There are many advantages to having two eyes.  Most people probably take for granted the benefits that having two functioning eyes affords them.  We are going to examine why people have two eyes, and how having two eyes enhances the sense of vision, specifically considering binocular vision as a benefit.

First and foremost, having two eyes gives us a back-up plan.  Should one eye be lost to injury or disease, there will still be another eye available to provide vision.  A world without vision is not a happy scenario, and many monocular patients are still able to live their lives with their sense of vision intact thanks to their having had two eyes in the first place.

Next up, a single eye provides a smaller field of view than do two eyes.  The eyes are located side by side to provide a wide field of view.  Consider that the eyes are mobile within their sockets, and now you have a visual system that can see more than 180 degrees side to side!  This gives more safety when driving or moving around an environment, and requires less turning of the head to view objects.

When both eyes are working together, they provide better vision.  Most of the time people have both their eyes open.  The brain takes the images from each individual eye and adds them together to provide an exceptionally clear view.  This enhanced visual acuity allows a person to see their very best.

The fourth, and maybe the most fascinating advantage of having two eyes, is stereovision.  The single sense of vision is actually made of many sub-sensory components.  Eyes do more than simply detect light, they detect ranges of brightness, color, motion, and depth.  Depth perception is the result of both eyes working together to generate a composite image.  Because the eyes are separated horizontally, each eye will detect a slightly different image.  The brain is wired to utilize this tiny difference in image placement to allow us to judge depth, or the spatial differences between objects and ourselves.   3D movies are an example of tricking our depth perception into thinking a 2-diminesional screen is a 3-dimensional image.  This is achieved by isolating the eyes optically (usually with polarized lenses) and presenting each eye with a slightly different image.

It is important to appreciate that not every patient’s binocularity has developed properly.  Even if a person has two eyes that can see (or can be corrected to see) a nice 20/20 (which is a standard letter size representing good visual acuity), the eye’s ability to work together to achieve stereovision may or may not be present.  Picture two cameras that can take great, quality pictures.  Even though each camera takes a nice, clear picture by itself, the two images need to be combined somehow in order to achieve a useful stereo effect (also known as ‘binocularity’).  This binocular fusion is the job of the brain, though sometimes a wrench gets thrown in the gears and prevents stereovision from developing.

Probably the most common reasons that binocular vision doesn’t develop properly are because of poor, or differing image quality.  Dramatic differences in the quality of an image affect not only that eye’s ability to developmentally achieve good vision (amblyopia), but can also hinder the brain’s ability to fuse images together.  Let’s look at the graphic representing different visual scenarios in the context of stereovision.  In Scenario A, each eye is well-focused and this theoretical patient’s brain can fuse the two images together.  The result is that this person can see a nice, clear image with good binocularity and depth perception.  The patient in this example is either wearing their contact lenses or glasses, or they have well-focused eye naturally or as a consequence of refractive surgery (such as LASIK or even cataract surgery).

In Scenario B, the example patient has two eyes that are well-focused, but a dramatic difference in image size is present.  This can be the result of having a nearsighted eye and farsighted eye, and correcting for that refractive error with glasses.  If the difference between the eyes is large enough, the actual image that lands on the patient’s retinas will be of differing sizes due to the dramatically different magnifying effects of the lenses.  This is important because even if each eye individually sees great, the resulting size discrepancy in the images means that the brain will learn to ignore one of the images (suppression).  This person does not see clear, comfortable, binocular vision, but ends up relying on the vision out of one of their eyes.

In Scenario C, the left eye is well-focused but the right eye image is blurry.  This is commonly the result of one eye having a different glasses prescription than the other.  If the right eye in this scenario is not corrected, it may never see as clear as the left, and would be considered an amblyopic, or lazy eye.  From the standpoint of binocular vision, the brain is unable to fuse a clear image, and an image that differs too much in clarity gets ignored by the visual system.  This person does not see clear, comfortable, binocular vision, but ends up relying on the vision out of one of their eyes.

In Scenario D, both images are heavily blurred, and the example patient’s brain is unable to achieve good vision at all, binocular or otherwise.  This scenario could be due to a patient having very high nearsightedness, moderate to high farsightedness, or high astigmatism, or even congenital cataracts or corneal disease.  This person does not see clear, comfortable, binocular vision.

Binocular vision is an important component of our visual system.  Without both eyes working together, you really aren’t seeing the full picture.  Our eye doctors recommend an annual check-up for all adults and children.  Along with the American Optometric Association, our optometrists recommend children receive their first eye exam at 6-12 months of age to allow for early diagnosis of visual conditions that can prevent optimal eye and vision development.  With a through eye exam that includes testing of stereovision, you can be sure that your children’s eyes are in a good position to develop to their fullest potential.  Please call our office at (303) 450-2020 to schedule yours and your family’s annual eye exams!