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Table of Contents

Chapter 1: Two Eyes: Blessing or Curse? ..Page 1

o Decussation........................................................................ Page 9 o Free fusion Stereogram ............................................... Page12 o X-Ray Vision .................................................................... Page 16 o Triangulation .................................................................. Page 18 o Spare Eye Clinical Evidence ...................................... Page 19 o Peripheral Vision ........................................................... Page 21 o More than 2 eyes ........................................................... Page 23 o Psycho-social consequences of strabismus ....... Page 25

Chapter 2: To be written…

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Two Eyes: Blessing or Curse?

“Excellent! Your eyes are smoothly moving in all directions the way they should! Now follow my light in towards your nose.”

As I bring the test light closer and closer to my patient’s nose, I notice that his two eyes seem to point inward together toward the moving light. When my light is about 12 cm from the tip of his nose, however, I observe his left eye swing out a bit towards his left ear.

I pull back my light to about 20 cm and say, “Concentrate on the light.” My patient’s left eye swings back towards his nose so that, once again, both of his eyes appear to be pointed at my test light. I move the light towards him, and when the light is about 12 cm from him nose, his left eye swings out towards his left ear.

“How do your eyes feel when you read?” I ask my patient.

“Well, I don’t really like to read much, but sometimes they feel strained a bit when I’m working on the computer.”

“What about 3-D movies?”

“I don’t like those.”

Is my patient’s discomfort when doing computer work and watching 3D movies related to his inability to keep both eyes aligned on my light as I bring it closer to him? The truth is, we don’t have enough information to make that conclusion. However, having a “remote near point of convergence” (not being able to follow the light inward with both eyes to a distance of at least 10 cm from his nose) is one sign of a treatable visual dysfunction called “convergence insufficiency”.

Which brings us to the question posed in the title: is the possession of two eyes a “blessing” or a “curse”? A patient with “convergence insufficiency” may consider having two eyes a curse because he has difficulty coordinating them when reading. His wife, however, may be grateful for the “spare eye” she has, having lost one in a bicycle accident as a child. You, and each of your patients, can “take a side” in this debate after deliberating the pros and cons of “bi-ocularity”, but the bottom line for each person is that we have to function with what we are given. Ideally, we would like our patients’ visual systems to function WELL, giving them clear and effortlessly single binocular vision in the course of their work and leisure lives. The comprehensive visual examination incorporates several measures of binocular coordination to detect inefficiencies and dysfunction. Symptomatic relief and enhanced function often can be achieved with lenses, prisms, and vision therapy, thereby enabling patients to operate more efficiently and comfortably while performing their daily tasks.

Before we delve into the workings of normal binocular vision, let’s briefly reflect on the advantages and disadvantages of having two eyes. Appreciation of the complexities of two-eyed vision can provide a foundation for the information we give to patients when discussing the treatment options available to them. I am not suggesting that we discuss all of the advantages and disadvantages of binocularity with each patient, but

http://ww1.prweb.com/prfiles/2011/05/10/8407738/eyes%20not%20working%20together.jpg

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patients certainly need to know the benefits and risks of efforts to improve binocularity in order to make properly informed decisions about their vision care.

Two eyes: advantages

1. Redundancy:

a. If you lose one eye, you still have one! (Great reason for treating amblyopia.)

b. An amblyopic eye can experience improved vision when the non-amblyopic eye is severely injured or lost.

c. Safety glasses for ALL!

2. Larger span of peripheral vision

a. Lateral eye placement: large panoramic view, but little overlap of the visual fields of the two eyes.

b. Frontal eye placement: greater overlap of the visual fields, but smaller panoramic view.

3. Better visual sensitivity

a. Slightly lower thresholds (i.e., slightly higher sensitivity) for contrast detection are measured with both eyes simultaneously viewing (i.e., “binocular”) compared to measurements taken with one eye viewing (i.e., “monocular”).

Fig. 1-1a: The prey Fig. 1-1b: The hunter

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4. Potential for “stereopsis”: 3D vision that arises from the cue of “binocular disparity”.

Stereo: solidity

Opsis: vision, sight

When viewing a three-dimensional, or “solid” object, the left eye’s view of that object is slightly different from the right eye’s view of that same object because the eyes are horizontally separated in the head.

This difference in perspective between the eyes is called “binocular disparity” and is one of the cues the visual system can use to perceive depth within an object or scene.

Note: Stereopsis can only be appreciated in those areas of space where the visual fields of the left and right eyes overlap.

5. Potential for “x-ray vision” (also called, “breaking camouflage”)

Fig. 1-3 is a “free fusion” stereogram that illustrates the ability of a binocular viewer to “see through” clutter. Fuse the left two images with “uncrossed” viewing, or fuse the right two images with “crossed” viewing to see how simultaneous viewing with frontally located eyes allows the observer to see more of the environment lying beyond (or through, hence “xray”) a leafy environment.

Fig. 1-2

Fig. 1-3

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6. Gauging the distance of objects

When viewed from a single vantage point (as when viewing with one eye), the distance of an object from that vantage point is ambiguous because we have only the retinal image to give us information. To explain this concept, we will ignore prior knowledge that a human has about the object being viewed, such as the typical size of a car relative to the other objects around the car. (The context in which a known object is viewed can give clues to the distance of that object.) As you can see in Fig. 1-4, the same retinal image size is created by objects at different distances; thus, when no other clues are present, retinal image size alone cannot give information about the object’s distance:

If an object is viewed from TWO vantage points, then we can use Euclidian geometry to calculate the distance of that object from the observer. This technique, used in geographical surveying, is called “triangulation”, and a free video explanation of triangulation can be found at http://www.fizzics.org/Pages/Astronomyvideo.aspx. (Scroll down to the video entitled, “Measuring distance by triangulation and parallax”.)

Why not MORE than 2 eyes?

With 3 or more eyes, we could have better visual sensitivity! (Could we see 20/2?!)

• The probability of detection increases with the number of detectors.

• More detectors, however, require greater neural complexity to combine the images if more than one detector is stimulated at the same time.

More than 2 eyes could give us a panoramic, 360⁰ visual field!

• Again, more detectors means more wires and trying to keep them all arranged in the brain to make sense of what’s out there!

On a more practical note:

• Spectacles would certainly look funny.

• We would have to spend more money on contact lenses and make-up!

Fig. 1-4

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Two Eyes: Disadvantages

1. Two pictures are generated for a single object in space!

Diplopia : simultaneous perception of two images of a single object in space.

Although two images of some objects might be appealing, the consequences of diplopia could be injurious, even life- threatening. Which image represents the “true” location of the object in space?

• The eyes must be “aligned” to yield a single percept of an attended object in space. When the eyes are aligned, the image of the attended object falls on the FOVEA of each eye.

• Since the FOVEA is the part of the retina that has the best visual acuity, images falling on the fovea of the eye convey information about the fine detail of the object.

• When we visually attend to a particular object in space, even if only with one eye, the brain signals the ocular muscles to rotate the eye until the image of the object of attention falls on the fovea of the eye.

• Ocular alignment is achieved through the coordination of TWELVE (six per eye) “extraocular muscles”! (More neural machinery to coordinate.)

Motor fusion: coordinated movement of the two eyes in order to achieve “bifoveation”. In other words, the eyes move together (but not necessarily in the same direction) until the image of the attended object falls on the fovea of each eye.

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• Chronic ocular misalignment, particularly beginning at a young age, can result in reduced visual function (amblyopia).

• Failure to align extraocular muscles results in “cosmetic irregularity”. (Beauty is in the eye of the beholder, yes, but observable deviations from normal in physical appearance and the accompanying loss of visual function can have deleterious socio-economic consequences.)

Adequate motor fusion in normal binocular vision enables the observer to experience HAPLOPIA: the PERCEPTION of a single object in space generated from the neural processing of TWO retinal images of the attended object (one from each eye).

Inadequate motor fusion results in DIPLOPIA (see above) and CONFUSION. Confusion is the perception that two different objects are occupying the same place in space. (Confusing, indeed!)

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2. A large difference in refractive error between the two eyes may make it impossible for the brain to

“fuse” the two images, even if the eyes are physically aligned.

• Differences in image magnification between the two eyes (aniseikonia) occur if the refractive errors are different between the eyes (anisometropia).

• Failure of sensory fusion (i.e., the brain cannot put the two pictures together) results in suppression, rivalry, diplopia, and/or amblyopia.

Bottom line: Under circumstances of normal development (i.e.,when everything goes right), the advantages of having two functioning eyes appear to outweigh the disadvantages.

Clinical assignment: Practice describing these advantages and disadvantages of having two eyes to your classmates, and then to your family and friends, using layperson’s terms.

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References

Jones, RK, Lee, DN. (1981). Why two eyes are better than one: the two views of binocular vision. J Exp Psychol, 7(1): 30-40.

Boothe, RG, Brown, RJ. (1996). What happens to binocularity in primate strabismus? Eye, 10: 199-208.

Rose, D. (1978). Monocular versus binocular contrast thresholds for movement and pattern. Perception, 7: 195-200.

Visual Pathways and Decussation

Your patient, a precious 5 year old, asks you, “How come I have two eyes, but only see ONE of YOU?”

You respond, “You have two eyes, but you only have one brain. We use our brain to see.”

She finds it unnecessary to ask HOW the brain puts the two pictures together, and you are relieved because your neuro-anatomy notes were recycled long ago.

To appeal to your inner nerd (because we know you’re going to look it up when your patient leaves), we will outline the gross manner in which the visual signals from the two eyes are combined in the brain to generate a single percept of an object in the environment.

The diagrams in this section are adapted from Ramon y Cajal (1911).

In humans, who have forward-facing eyes, the primary visual pathway is diagrammed in Fig. 1. To convey a true representation of the location of objects in space, the signals generated by the detection of those objects need to leave the eye and arrive at the brain in a reasonably organized fashion. Consider the signal detected by a single eye:

• Images of points in space nasal to fixation are formed on the retina temporal to the fovea.

o Axons from ganglion cells in temporal retina exit the eye via the optic nerve, form the temporal aspect of the optic chiasm, and synapse in the ipsilateral lateral geniculate nucleus.

• Images of points in space temporal to fixation are formed on the retina nasal to the fovea.

o Axons from ganglion cells in nasal retina exit the eye via the optic nerve, cross the midline (decussate) via the optic chiasm, and synapse onto cells in the contralateral lateral geniculate nucleus.

This “hemidecussation” of ganglion cell axons after they leave the eye allows the signal from each eye that represents a single location in space to go to the same side of the brain. Fig. 1

Let’s look at what would happen to space representation in the brain if ganglion cell axons did not decussate, as shown in Fig. 2.

The object, an arrow, is being viewed by an observer having two laterally-facing eyes with visual fields that do not overlap. Thus, the left half of the arrow is imaged on the left eye’s retina, and the right half of the arrow is imaged on the right eye’s retina.

• With undecussated ganglion cell axons, the image of the left half of the arrow is represented in the left half of the brain, and the image of the right half of the arrow is represented in the right half of the brain.

• The image inversion that occurs due to the convex optical system of the eye combined with the undecussated signal transmission results in a disjointed image representation in the brain.

While animals could learn to interpret the disjointed representation of space in order to operate in their environment, with respect to survival, the speed of response can be crucial! Is there a more efficient way that space could be neurologically represented? Ramon y Cajal (1911) proposed that full decussation of optic nerve fibers, as found in submammalian vertebrates, evolved to create a “central neural map” in the brain that retains the positional relationships between object points in space, as shown in Fig. 3.

As for the previous example, the observer here has two laterally-facing eyes such that the visual fields of the two eyes do not overlap. The left half of the arrow is imaged on the left eye’s retina, and the right half of the arrow is imaged on the right eye’s retina.

• With full decussation of the optic nerves, the left eye’s image is represented in the right half of the brain, and the right eye’s image is represented in the left half of the brain.

• The image inversion that occurs due to the convex optical system of the eye combined with the fully decussated signal transmission results in a “central” image representation (“central” refers to the brain) that preserves the spatial relationships of object points.

Fig. 2

Fig. 3

When overlap of the visual fields of the two eyes is negligible, as in animals with laterally-facing eyes, the two eyes send the brain information about entirely different locations in space, and the brain has no need to merge those images, except where the nasal regions of the visual fields meet (see top of Fig. 3). For animals with frontally-facing eyes (like humans), however, the left eye conveys information about a large region of visual space that the right eye also represents, as shown in Fig. 4.

• If the visual fields overlapped as shown, but the observer had full decussation of the optic nerves, then a complete image of the arrow would be represented in the right half of the brain (from the left eye) AND a complete image of the arrow would be represented in the left half of the brain (from the right eye). While I cannot be certain of what the observer would see, two separate image representations in the brain from a single object in space would likely result in diplopia, or “double vision”.

• With hemidecussation, the signals from each eye that represent the same half of object space are sent to one side of the brain. The result shown in Fig. 4: each half of the brain receives TWO images, one from each eye. When binocular vision is normal, the two half images that one side of the brain receives are so similar that the brain “fuses” the two images, generating a single percept of the object in space.

Because the two frontally-facing eyes are horizontally separated in the head, the two images of an object that one half of the brain receives are not EXACTLY the same. These slight differences in space representation, referred to as “binocular disparity”, do not prohibit fusion of the images. When the binocular disparity between the images is large enough to be detected, the fused image is perceived to have depth. Thus, binocular disparity is the stimulus for stereopsis.

Fig. 4

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“Free fusion” stereogram: a device-free way of presenting separate targets of the same picture to the two eyes (at the same time).

• The diagram below shows how a “free fusion” stereogram works. Review the diagram to understand the concept of how to view these types of stereograms, then try out the stereograms that follow.

• Some people have difficulty “fusing” these stereograms. In other words, they have trouble controlling their MOTOR vergence well enough to center each eye’s target to allow sensory fusion to occur.

• Remember, MOTOR vergence allows you to change the point in space where your visual axes intersect. Not only do you need to make your visual axes intersect off the physical plane in which the targets are located, but you also have to MAINTAIN that vergence posture while keeping your focus (accommodation) on the physixal plane where the targets are located.

• It’s ‘unnatural’ to focus in one plane and converge in another, but it can be done! If you have difficulty, consider using a prism bar to help you fuse these stereograms, and get thee to a practitioner who specializes in diagnosing and treating your vergence disorder!

Try this simple free fusion stereogram. You can either converge your eyes CLOSER to you than the physical plane of the computer’s screen, or FARTHER from you than the plane of the computer screen. The stereo percept that you experience will differ depending on where you place your vergence. (We will discuss why the stereo percept changes with vergence in these stereograms in the chapter on stereopsis.)

A: Normal viewing. When you look at the physical card on which the targets are located, your visual axes intersect at that plane and your focus is also at that plane. The card should look clear and you should just see one card with 2 targets on it.

B: Crossed viewing: your visual axes converge in front of the card, but your focus is farther away, at the plane of the physical card.

C: Uncrossed viewing: your visual axes converge behind the card, but your focus is closer.

A B C

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• You will know that you have achieved the correct vergence position of your eyes when you see THREE magenta rings and THREE blue rings. The CENTER rings should appear the clearest.

• If your convergence is IN FRONT of the plane of the computer screen (crossed viewing), then with respect to the CENTRAL image (which is the fused image), the blue ring will appear CLOSER to you than the magenta ring.

• If your convergence is BEHIND the plane of the computer screen (uncrossed viewing), then with respect to the fused central image, the blue ring will appear FARTHER from you than the magenta ring.

• Where you place your eyes does not constrain the relative depth relationship between the different parts of the stereogram. Below is the same stereogram with one slight difference. Instead of the two inner circles being shifted slightly outward from center, the blue rings are shifted slightly inward from center.

• If your convergence is IN FRONT of the plane of the computer screen (crossed viewing), then with respect to the CENTRAL image (which is the fused image), the blue ring will appear FARTHER from you than the magenta ring.

• If your convergence is BEHIND the plane of the computer screen (uncrossed viewing), then with respect to the fused central image, the blue ring will appear CLOSER to you than the magenta ring.

Interestingly, the brain doesn’t need to see “contour lines” to see depth. The above two stereograms have “contour lines” that let you know before you have fused the two half views what form the fused image will have – two circles, with one inside of the other. The next stereogram does not have discernible “contour lines” in each half view, yet when you fuse the stereogram, you get a distinct

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impression of a form or shape that is in a different depth plane than its surround. Try it! Again, use either crossed or uncrossed viewing to obtain motor fusion of the two half views of the following stereogram.

• Crossed viewing generates the perception of a smaller square in the center popping out into space CLOSER to you than the surrounding dots.

• Uncrossed viewing generates the perception of a smaller square in the center receding away from you with respect to the surrounding dots.

The brain doesn’t even need two entirely separate targets to fuse, as demonstrated with the “autostereograms” below. How these autostereograms work will be explained in greater detail in the chapter on stereopsis. However, to fuse these, the same basic principle of changing your vergence posture relative to the plane of the stereogram applies. The vergence movement you will need to make, however, is a little smaller than required by the stereograms above. Again, if you have difficulty fusing the autostereograms, try a little base in or base out prism to help you. You won’t need much—probably in the ballpark of 6-10 prism diopters. Just concentrate on keeping the plane of the target in focus and let the prism change the posture of your visual axes.

(Percept with crossed viewing, but of course the actual percept retains all of the dots.)

Once you have this autostereogram fused with UNCROSSED viewing, you will be able to see a cylinder in the upper left, a sphere in the lower left, and a cube to the right. (Crossed viewing will make the shapes appear to recede rather than protrude, so the forms may be more difficult to identify.)

• Until I learned how to do these, I thought it was a hoax!

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• Although not obvious in many autostereograms, they actually have strips of repetitive patterns. To best identify the stereo shape in the stereogram, you need to converge your eyes just enough to allow two adjacent strips to overlap. On the stereogram below, these strips of repetitive patterns are more easily seen (and are marked by the arrows). If you converge your eyes so that non-adjacent strips overlap, you may see depth, but the form designed into the stereogram may appear distorted.

Once you have this autostereogram fused with UNCROSSED viewing, you will be able to see a 11 circular groups of the candies popping out towards you relative to the surround. CROSSED viewing will make those areas appear to recede away from you relative to the surround.

Hypothesis: the forward-facing position of the eyes allows the observer to “see through” (hence, “x-ray vision”) a leafy environment.

When asked about the main benefit of binocular vision, most clinicians will say, “stereopsis”. The advantage of stereopsis is also promoted in both clinical and scientific textbooks. Being able to see the depth in a 3D movie is entertaining, and excellent depth perception certainly enhances the performance of certain tasks, such as microsurgery (Sachdeva R, Traboulsi EI, 2011).

As clinicians who will be considered “experts” in binocular vision, we must be careful about how much importance we give stereopsis with respect to our patients’ ability to perform daily tasks. For example, an optometrist once commented on his experience working with an ophthalmic surgeon who was amblyopic in one eye from childhood (personal communication with the author) as follows. The optometrist noted that the surgeon, who specialized in anterior segment surgery (ex. cataracts), had “the best hands” of anyone in the surgical group. In other words, the surgeon demonstrated excellent skill in microsurgical techniques despite poor stereoacuity. However, this same surgeon had difficulty identifying “macular edema”—accumulation of fluid in the macula of the eye that can be observed clinically (when of sufficient severity) as an elevation of the macula relative to the rest of the retina. In other words, this surgeon had difficulty using depth as a cue to a change within the eye when he did not have “feedback” from the motor interaction that he enjoyed during surgical manipulation.

The evolution of forward-facing eyes occurred long before the development of microsurgery, and long before the development of most other tasks whose performance is improved when stereo cues are utilized. So, if improved stereoacuity is not the biological incentive for the development of forward-facing eyes, what advantage of forward-facing eyes would initiate the evolutionary shift from laterally-facing eyes?

In 2008, Changizi and Shimojo proposed that forward-facing eyes facilitate a mammal’s ability to “see through” a cluttered environment. Conversely, mammals in non-cluttered environments can best see what’s around them with laterally placed eyes. Thus, the position of the eyes (lateral versus frontal) should have evolutionarily evolved differently for animals who live in cluttered versus non-cluttered environment. They categorized 319 species across 17 mammalian orders as dwellers of either cluttered, semi-cluttered, or non-cluttered environments and then compared the eye position and separation for each species to the size of the leaves in that species’ typical environment. Their data showed that eye position and separation tended to be related to leaf size in those animals living in cluttered (leafy) environments, but there was no such relationship for animals living in non-cluttered environments.

References

Changizi, MA, Shimojo, S. (2008). “X-ray vision” and the evolution of forward-facing eyes. Journal of Theoretical Biology, 254: 756-767.

Sachdeva R, Traboulsi EI. (2011). Performance of patients with deficient stereoacuity on the EYESi microsurgical simulator. American Journal of Ophthalmology, 151(3): 427-33.

If you want to snatch this bag of money from the outlaw, you stand the best chance of getting away with it if you can accurately gauge its distance from you before you reach out your hand.

Having two eyes allows you to use “triangulation” to gauge the distance between you and the bag. Fortunately for animals, we learn to do this pretty early in life (imagine babies reaching for the swinging toys on a mobile) so that the distance judgment becomes automatic.

Essentially, you have a known distance between your two eyes. To point each eye at the target requires that each eye turn in the orbit towards the nose, creating a target angle of a known (from extraocular muscles) magnitude between the visual axes and the object. The information about the interpupillary distance and the magnitude of rotation of the eyes (from contraction of the extraocular muscles) can be used by the brain to “calculate” the angle 2θ.

Using trigonometry, we can drop the perpendicular to bisect the target angle and the interpupillary distance. Since θ is known and the interpupillary distance is known, the unknown target distance can be calculated from the tangent of θ:

Improved Vision in Amblyopic Eyes After Vision Loss in the “Good” Eyes

Several cases have been reported in the literature of patients who experienced unexpected improvement of vision in an amblyopic eye after losing vision in the non-amblyopic eye due to disease or injury. Some of the references are listed below for your review, one of which I will quote for you here.

Klaeger-Manzanell, C, Hoyt, CS and Good, WV. (1994). Two step recovery of vision in the amblyopic eye after vision loss and enucleation of the fixing eye. Br J Ophthalmol, 78: 506-507.

“A 44 yr old man with strabismic amblyopia of the right eye suffered a penetrating injury to the fixating left eye which resulted in cataract extraction and aphakic retinal detachment. The amblyopia in the right eye was due to a small angle esotropia that had never been treated. His best corrected visual acuity before the accident was 6/60 in the right eye (20/200) and 6/6 (20/20) in the left eye. Fixation in the right eye was central. The refractive error was +1.00 + 0.50 x 90 right eye and +0.25 +0.25 x 90 left eye. After the retinal detachment surgery the vision in the fixing eye dropped to counting fingers at 2 metres and remained there. Six weeks after retinal detachment surgery the visual acuity of the amblyopic eye had improved spontaneously to 6/24 (20/80).”

“During the following 13 months the visual acuity in the amblyopic eye remained stable at 6/24 while the injured eye deteriorated to light perception only and developed constant pain and phthisis that made enucleation necessary. Three weeks after the enucleation the visual acuity of the amblyopic eye had improved to 6/12 (20/40) and 4 months after the enucleation to 6/9 (20/30). It has remained at this level for 6 years on follow-up examination. The visual acuity was always measured in the same room at 6 metres with line Snellen optotype.”

In addition to case reports of humans with amblyopia experiencing improved vision in that eye when the vision in the “good” eye was compromised, research using a monkey model of amblyopia created by induced strabismus has shown improvement in the vision of the amblyopic eye when the “good” eye (also called the “fixing” or “fixating” eye) was enucleated (Harwerth, et al, 1986). Additional animal studies on the recovery of vision in amblyopia are listed in the references of these studies.

Other references citing improved vision in amblyopia with loss of the fixing eye:

Fronius, M, Cirina, L, Kuhli, C, Cordey, A, Ohrloff, C. (2006). Training the adult amblyopic eye with "perceptual learning" after vision loss in the non-amblyopic eye. Strabismus, 14(2): 75-9. Hamed, LM, Glaser, JS, Schatz, NJ. (1991). Improvement of vision in the amblyopic eye following visual loss in the contralateral normal eye: a report of three cases. Binocular Vision Quarterly, 6(2): 97-100.

Harwerth, RS, Smith, EL III, Duncan, GC, Crawford, MLJ, von Noorden, GK. (1986). Effects of enucleation of the fixating eye on strabismic amblyopia in monkeys. Investigative Ophthalmology and Vision Science, 27: 246-254.

Karatza, EC, Shields, CL, Shields, JA. (2004). Visual improvement in an adult amblyopic eye following radiation-induced visual loss in the contralateral eye. Archives of Ophthalmology, 122(1): 126-8.

Rabin, J. (1984). Visual improvement in amblyopia after visual loss in the dominant eye. American Journal of Optometry and Physiological Optics, 61: 334.

An advantage of two eyes: increased peripheral vision

Compare the two photos below. The top photo shows you what a scene may look like when viewed with just the right eye open. The bottom photo shows you one advantage of having two functioning eyes: increased peripheral vision (which may increase your chances of survival!)

The outermost borders of the field of vision are typically measured while the eye is fixating a target (i.e., with the eyes stationary). Movement of the eyes and the head, of course, allow us to see more of the world around us. Without allowing eye and head movements, the normal dimensions of the visual field for humans are diagrammed below. Some important points to note are:

• The MONOCULAR visual field (i.e., with one eye open) spans about

• 160 degrees across the horizontal meridian.

From straight ahead, the normal visual field extends to about 100 degrees temporally;

From straight ahead, the normal visual field extends to about 60 degrees nasally (the nose gets in the way, so this can vary with nose size).

• 135 degrees across the vertical meridian.

From straight ahead, the normal visual field extends to about 60 degrees superiorly (the forehead/brow can affect this measurement);

From straight ahead, the normal visual field extends to about 75 degrees inferiorly.

Most clinical instruments can measure only the monocular visual field.

The diagram to the left is a polar grid onto which the outer extent of a normal visual field (black line) of the RIGHT EYE has been plotted.

• The red dot placed in the center of the grid represents the spatial projection of the FOVEA. During visual field testing, the observer is asked to look at a fixation target (generally placed “straight ahead” so that the eye is in the primary position of gaze). The red dot on the grid represents where the patient is (or should be!) looking during the visual field test.

• The small black oval located about 15 degrees to the right of the red dot represents the projection into space of the area of the retina where the optic nerve is located. Since that area of the retina has no photoreceptors, no light detection can take place there. Thus, an “absolute” blind spot is located in the visual field in the area to which the optic nerve projects.

Many members of the animal kingdom have more than two eyes, as demonstrated by this jumping spider, which has 8 eyes.

Four-eyed fish, found in the Amazon River delta of South America, actually only have two eyes, but both are divided into an upper aerial part and a lower aquatic part. The two retinal regions of each eye coincide with two different curvatures of the cornea above and below water. The corneal curvature differences are necessary to compensate for the different index of refraction for air versus water. This type of eye allows this fish to enjoy simultaneously clear vision above and below the water surface.

References

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References: Psychosocial consequences of strabismus and strabismus surgery

Satterfield D, Keltner JL, Morrison TL. (1993). Psychosocial aspects of strabismus study. Arch Ophthalmol, 111(8): 1100-5.

Olitsky SE, Sudesh S, Graziano A, Hamblen J, Brooks SE, Shaha SH. (1999). The negative psychosocial impact of strabismus in adults. JAAPOS,3(4):209-11.

Paysse EA, Steele EA, McCreery KM, Wilhelmus KR, Coats DK. (2001). Age of the emergence of negative attitudes toward strabismus. JAAPOS, 5(6): 361-6.

Coats DK, Paysse EA, Towler AJ, Dipboye RL. (2000). Impact of large angle horizontal strabismus on ability to obtain employment. Ophthalmology, 107(2): 402-5.

Uretmen O, Egrilmez S, Kose S, Pamukçu K, Akkin C, Palamar M. (2003). Negative social bias against children with strabismus. Acta Ophthalmol Scand, 81(2): 138-42.

Mojon-Azzi SM, Mojon DS. (2009). Strabismus and employment: the opinion of headhunters. Acta Ophthalmol, 87(7):784-8.

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