7.10 Because it's there

(From twoday.everest.org)

Sir Edmund Hillary (1919-2008) and Mr Tenzing Norgay (तेन्जिङ नोर्गे शेर्पा, 1914-1986) climbed Mount Qomolangma, as Mt Everest is called in Tibetan, and reached its summit together on May 29, 1953. Many have tried, while some have since succeeded, more have succumbed to various high-altitude (HA) illnesses, from exposure to both the extreme cold (i.e., frostbites) and the low atmospheric pressure (i.e., low O2 level). The oxygen level near the top of Mt Everest is actually 66% less than that at sea level. Depending on the altitudes and how fast the climbers ascend, these illnesses can be mild, e.g., acute mountain sickness (from accumulation of some fluid in the brain) and HA edema (swelling of hands, feet, and face). The more severe forms, HA pulmonary edema and HA cerebral edema can both be fatal. These illnesses are largely a result of hypoxia-induced leakage of plasma from the capillaries.

Interestingly, there are now findings that pertain to the eye, e.g., those reported in“The Eye in the Wilderness” in Wilderness Medicine (Auerbach PS, editor, Mosby publisher, 2001). Even though none of the eye problems can kill you, each can still be a nuisance when you are 29,000 feet above the sea with no medical facilities in sight - unless a fellow climber happens to be an eye doctor.

1. Retinal hemorrhage may develop at altitudes of >8,000 feet. There are no symptoms except if it occurs within the macula, then a small scotoma may result. This maybe similar to retinal hemorrhage from polycythemia vera (see image below), i.e., too many red cells that thicken the blood, impede blood circulation, and occlude retinal veins. Indeed, humans adapt to long-term high-altitude hypoxia by producing more red cells that often cause chronic mountain sickness.

(An example of retina hemorrhage - from central retinal vein occlusion)

2. Snow blindness or UV-keratitis. UV radiation increases at ca 5% every 1,000 feet above sea level and UV rays are reflected by the surface of snow/ice that further increases the intensity. If unprotected, the cornea and iris are injured. The eyes become red, painful, and photophobic. The North American Inuits have long used slit sunshields made from wood or bones to protect their eyes.

3. Contacts maybe a problem as well. The lack of atmospheric oxygen further aggravated the inherent corneal hypoxia with the contact lens wear. The climber should carry a pair of back-up glacier glasses. High-O2 transmitting silicone hygrogel lenses that have been approved for extended-wear should be used. And 4th generation fluoroquinolone eyedrops qdx7 days also should be used at the first sign of bacterial keratitis.

4. Climbers with radial keratotomy (RK) have reported drastic changes in vision in the form of a hyperopic shift. PRK and LASIK appear less affected. In all cases, there is a thickening of the peripheral cornea possibly from edema. And in the case of RK, the incision-weakened peri-central cornea probably becomes further flattened from changes in the periphery.

High-altitude illnesses can be reversed by moving down to lower altitudes. Some mountaineers believe that by drinking enough water, these illnesses can be avoided, presumably the blood can thus be "thinned". Perhaps researchers in HA medicine should examine this claim and provide some definitive answers.

For us couch potatoes, the question is always: "Why do you want to climb Mt Everest?" And George Mallory (1886-1924; remains finally found on the North Side in 1999) still put it best, "Because it is there." The risks are, however, still beyond our imagination.

7.9 Wild crazy eyes

Every Halloween (celebrated on the night of Oct 31 wherever you are), someone will feel obliged to decorate their eyes for theatrical effects (in addition to wearing outlandish costumes) and show up at your door for "trick-or-treat"ing. This is done by using contact lenses with interesting patterns. A few examples are shown below:

( "Wild Eyes" by Ciba Vision)

Notice the most popular Cat's Eye (top left) usually rotate with each blink. For the onlookers, the appearance of asymmetrical vertical pupils is often quite disconcerting.

Often these lenses are used in the entertainment world. Master Po, in the Kung Fu TV series (1972-5), and Mr Richard B Riddick of the Chronicles of Riddick (2004) fame both wore white contacts with clear pupils (the latter further enhanced digitally). The Devil in Rosemary's Baby (1968), of course, was in Devil's Eye lenses. Judge Dredd in the movie of the same title (1995), on the other hand, appeared to have used cheap color contacts (more below).

These lenses are an outgrowth of artificial pupils, i.e., custom-lenses that can mask the traumatized eyes with torn irises hence distorted pupils, or from extensive scarring of the cornea known as band keratopathy. An example of the latter is shown below (the opaque cornea in the right eye, top; and with an artificial pupil, bottom):

These lenses are created by using an opaque lens upon which the iris pattern matching that of the fellow normal eye is superimposed - often a painstaking job for the artists. The pupillary area can be either clear of black (black in the above example). Each lens can cost hundreds of US dollars, however. And soft contacts do have limited useful lifespans. In some cases, corneal tattooing is an option and which is performed by an eye doctor who knows what he or she is doing. Interestingly, there appears a new fad now, or at least the beginning of it, i.e., tattooing of healthy corneas for whatever reason the recipient convinces him-/her-self of. This is a potentially blinding practice that probably should be banned outright.

Inexpensive off-the-shelf color contacts also can be used for damaged eyes; however, because of the transparency, the result is not as satisfactory. These lenses are usually manufactured with the combination of different patterns:

(A Freshlook lens made by Ciba Vision; the clear center allow vision)

The above, for example, is a cosmetic color contact lens based on three patterns (top), suitable for patients with light-colored irises. For dark irises, only deep blue or light hazel, sometimes gray, is appreciable. Color preference is naturally a very personal choice. These color lenses, although not designed on purpose, do reflect lights that makes the eyes "shine" - certainly an added attraction.

Most people, however, do not realize that these cosmetic lenses, even the ones with no refractive power, are still medical devices subject to FDA regulation. And they are by prescription only, not to be shared, and are to be worn under doctor's care.

There is a good reason to be careful: Usually these cosmetic contacts are "tight" lenses - to avoid excessive movement and off-center positioning from blinking - or the patient's vision is blurred from seeing the "pupillary" rims of the lenses. This is especially disturbing at night when the patient's pupils dilate to larger than the diameter of the clear zone of the lens. And a tight low-oxygen-transmitting lens often causes corneal hypoxia and its painful sequelae, inflammation and infection.

A trip to the emergency room in the middle of the night can certainly put a quick end to the fun.

7.8 Now you see it, now you don't

3D-illusion is a fun subject. It is enjoyed by people with binocular vision, i.e., the two eyes must work together and each with a good central vision. The foundation is stereopsis which can be exploited for 3D illusion. And, based on preliminary functional MRI, stereopsis seems to be coded in the cortex adjacent to the intraparietal sulcus. Not surprisingly, in patients with amblyopia, there is a decreased activation in V1, the primary visual cortex.The above image shows a stereopsis testing kit, known as the Stereo Fly - found in all eye doctors' offices. Children are asked to put on the polarizer spectacles, look at the huge scary fly on the right, and pinch its wings. On the left panel are 9 sets of buttons and 3 rows of cartoons, both of which with decreasing depths. The children are asked to push the buttons/cartoons from the most to the least obvious. Stereopsis can thus be quantified. This test is based on retinal disparity. In real life, the two eyes look at a 3D object, each from a different angle (as part of convergence), and the images are then reconstructed in the brain and perceived as 3D. The Stereo Fly is in 2D, the 3D effect is really a simulation of the retinal disparity.

There are many other ways of appreciating pseudo-3D images, with (e.g., the red-green 3D glasses at IMAX theaters, or the polarizers shown above), and without equipment (by converging or diverging the eyes to fuse the images). The basic principle is the same: one eye sees (or is allowed to see) one image of an object taken from one angle and the fellow eye, another image taken from a slightly different angle. The two images then fuse together to produce 3D. This angle is determined by the distance between the two eyes, known as the interpupillary distance or PD - typically around 65mm in adults.

In addition to superimposing two images as in the Stereo Fly above, there are different ways of generating stereograms. The example below is a computer-generated single-image random-dot stereogram, in which, a semi-sphere can be seen by simply diverging your eyes (just stare at it, the eyes/brain will take care of the rest):

(From wwwchem.uwimona.edu.jm:1104/gifs/caff3d.gif)

A more decorative, copy-righted version is known as the Magic Eye. Actually, a very simple way of making stereograms is to photograph the same object twice each at a slightly different angle to simulate the viewing position of the two eyes. Then lined up the two pictures horizontally, and by converging the eyes or using a stereoscope, the 3D effect becomes apparent. 3D cameras also have been in existence since the 1960s.

In humans, both stereopsis and motion parallax co-exist. The advantage of stereopsis is depth perception especially at close range if monocular cues are ambiguous. At a distance, parallax dominates, e.g., a house behind a tree moves in the same direction of your motion and the tree in the opposite direction. To the brain, this clue indicates the house is farther away than the tree from you. So by relying on parallax, the distance judgment of an amblyope or a one-eyed person is not impaired at all.

Optical illusion, on the other hand, can be appreciated monocularly. And based on fMRI, Area MT (medial temporal) in the brain seems responsible. An example of optical illusion is shown in the image below, squares A and B actually have the same shade. You can print it out, cut out A and B and compare them side by side.

(From: http://wapedia.mobi/en/Image:Grey_square_optical_illusion.PNG)

If you don't want to bother printing anything, look at the above diagram, the cords all have the same length, yet the one on the left appears shorter. There are simply too many varieties of optical illusions to mention here. Luckily, most are now available on the Net.

Optical illusion is based on the brain's interpretation drawing upon past visual experiences. Indeed Area MT is associated with recognition of the spatial localization of one's self. You might say that MT imposes its will on the eyes - the true origin of optical illusion.

The above are two more unique aspects of higher-order vision that make our visual world vibrant and exciting. Of course, you should also remember: "Seeing is believing" is not without limits.

7.7 Sharp shooters

(Qin Dynasty, 221-206BC, standing archer)

Sharp shooters are most likely endowed with unique physiological characteristics. Ideally, they should have 20/15 or better vision with miotic pupils (<2.5mm, even at night), no dry eyes or glare sensitivity (i.e., ocular media remain clear), supplemented with better than average eye-hand coordination and a well-regulated adrenal gland a la Mr John Glenn.

Before the advent of guns and rifles, there were famous sharp-shooting archers. The best known are the horseback riding Mongols led by Genghis Khan - now a popular competitive sports in modern day Mongolia. A stylized form is the Japanese ritual archery, known as Yabusame (流鏑馬). Then there were such accomplished individual archers as Hou-Yi (后羿), Yue Fei (岳飛), Nasu no Yoichi (那須与一), Wilhelm Tell, Robin Hood, et al, all of whom were noted for their supernatural aims and spectacular deeds.

There are parallels in training an archer and a sharp shooter - assuming he/she has passed the ocular physical. We will now go historical and discuss the archer training. It is a combination of whether the person is on foot or on a horseback, and whether the targets are stationary or moving. Training is based on relative motion. The fundamentals, however, are still visual acuity enhancing and eye-tracking. The handling of the bow and arrow is largely mechanical (although it has been decreed that the two hands must be posed such that the left hand is like upholding the Tai Mountain [泰山] straight and taut, and the right, cradling a baby).

Let's start by looking at some legends popular in China.

For visual acuity enhancement, a famed archery instructor tied a flea with a hair from the ox tail, dangled the contraption from under the top window sill and the student was to stare at the flea from across the room, without blinking, until the morphological details could be discerned. For three years, the student practiced this visual training until the flea appeared as big as a cartwheel. By then, he could easily shoot an arrow through its heart. This story was recorded in a Shang Dynasty (1600-1046BC) document (列子.湯問). (Modern day shooters instead use a telescope.)

And Lesson No 2 - eye tracking: a skilled archer was to be able to hit, at 100 paces away, the branches of a willow tree swaying in the gentle breeze. Again, practice makes perfect. General Li Guang (李廣, ? - 119BC) of the Han Dynasty was a huge success at this game. In fact. legend has it that General Li, with his long and powerful arms, once shot a tiger at a long distance. Which turned out to be a tiger-like rock formation, yet the arrow was so deeply embedded as if it had penetrated the flesh.

Hmm..., if you really think about it, the training methods described above are still in use today; albeit in different formats and for entirely different purposes.

One is the well-known Bates method and its modern variations. The claim is that by relaxing extraocular muscles, the degree of myopia can be decreased to a point when glasses are no longer needed. However, this unique theory of accommodation is unsupported by physiological evidence; in which, the ciliary muscle alone is involved. In other words, you can exercise the EOMs all you want, it still won't change an iota of your accommodation.

The other is vision training/eye tracking, very popular among pro baseball players, or more accurately training of visual skills that involve dynamic vision, eye tracking/focusing, depth perception, fusion, and peripheral vision. There is, however, little consensus on the efficacy of this type of vision training. As usual, there are both believers and skeptics.

Amazing, isn't it? After 2-3 thousand years and people are still confused especially when legends, pseudo-science, and real science still collide.

7.6 T. rex vision

Have you ever wondered how the dinosaurs saw when you look at their immense fossil heads with huge empty eye sockets?

Of course, all the soft ocular tissues are long gone, so it is impossible to know the gross anatomy of the eye, let alone how many kinds or the density of photoreceptors in the retina. Without the information, all studies are essentially best guesses.

T. rex and its relatives, the theropod dinosaurs, all have a huge head, small fore-limbs with sharp claws, and walked or trotted on powerful hind legs. Based on the position of the orbits, it is possible to estimate the overlapping visual fields of the two eyes. It turns out that two possibilities exist, one with a 20° overlap (similar to that of the crocodiles) and the other 45–60° (similar to that of the birds). T. rex belongs in the latter group with a 55° overlap (see image at top).

So what does this overlap mean? Most likely for stereopsis at close range. According to some paleontologists: if you are a predator as the T. rex, it is a good idea to see what you are biting at. T. rex is known to leave its well-placed tooth marks on its victims. On the other hand, a scavenger needs only to know where the meal lies; and a wider peripheral visual field is advantageous for scanning the horizon, in case some big dangerous looking T. rex is lurking nearby.

The above seem reasonable. The next assumption is big eyes must have excellent vision. In fact, there has been some exercise fitting an enlarged version of reptile or bird eye into the eye socket of a T. rex and project what its vision could be. Some claimed T. rex had 13 times better acuity than humans. From the retinal point of view, this appears unlikely. The eyes of a T. rex maybe several times larger than that of the humans, it is not the number but the density of the of photoreceptors that determines visual acuity. That is, if T. rex did have a fovea as in the humans. Perhaps it had multiple foveas each for a different visual function for all we know (the eagles have two, for example). Without an actual sample of the retina, it is simply not possible to draw any definitive conclusions.

So how did a T. rex see? Very well, thank you very much.

7.5 Flowers, where?

Do animals have color vision or even how they see - no one really knows. We can, however, make some guesses. Dogs have two types of cones, humans three. So dogs don't have the same color vision as humans. Both dogs and humans have two types of photoreceptor cells, i.e., rods, and cones. Birds have three: cones, rods, and a mixed type, so some birds can see more colors (e.g., humming birds), while others can see more light at night (e.g., owls) than humans/dogs. Vision is not created equal at all .

Humans normally perceive color within the visible light spectrum of 380-750 nm. Not everyone does, of course. It is safe to say that one in twenty guys (5%) is color blind or more precisely, red-green deficient. This is caused by an X-linked recessive gene. Females can be R-G deficient, too, though much less common - it is hard to receive two copies of the rare gene. An even less prevalent form is blue-yellow deficiency and the most rare is total color blindness. Most eye doctors have never seen the latter two types in their entire professional careers.

Red-green deficiency is just that: the patients cannot readily differentiate, e.g., pink flowers from green leaves, or red from green apples (see images below). They will have trouble with traffic lights if the lights are arranged in an unfamiliar fashion (e.g., horizontal rather than the usual vertical with red at top). Not to mention the inability to tell whether a severe case of sunburn is coming on, a woman is wearing lipsticks, or a piece of steak is too well-done. And forget about reading litmus test results or color coordinating clothing. Otherwise the visual functions are quite normal.

The remedy, although not ideal, is to wear red-tinted contact lens (known as the X-Chrome lens) in one eye only. The patient can actually identify Ishihara color vision test plates just like any normal-sighted person. If you are R-G deficient, a DIY way is to cut out a piece of red mylar file cover, laminate it onto one of the lenses in your spectacles. Put your glasses on, look around, and be surprised at what you have been missing all your life.

(Top: red and green apples; bottom: as seen by a R-G deficient person [from answers.com])

There have been attempts at gene therapy repairing color blindness; although it is still unclear if any success (or even necessary).

If a normal-sighted person begins to lose color differentiation and who is afflicted with one or more of the diseases listed below. Then that is not so good because it indicates damages to the optic nerve and/or the retina. And the list is relatively long:

Eye diseases include POAG, AMD, and retinitis pigmentosa. Systemic diseases include Alzheimer's disease, diabetes, liver disease, chronic alcoholism, multiple sclerosis, Parkinson's disease, leukemia, and sickle cell anemia.

Many drugs also can have side-effects that involve disturbance to color vision. Usually your prescribing doctor will have informed you so.

And chemical poisoning by carbon monoxide, carbon disulfide, styrene, and lead can cause disturbance to color vision as well. The most famous example is Vincent van Gogh (1853-1890) who loved to paint in intense yellow (you all recall his "14 Sunflowers in a Vase"). Mr van Gogh was probably a walking medicine cabinet. In his time, he was treated for various ailments which have been theorized to be, take your pick, epilepsy, bipolar disorder, chronic sunstroke, acute intermittent porphyria, lead poisoning, and Ménière's disease. From his paintings, poisoning by absinthe liquor, digitalis, or even lead is indeed quite possible.

7.4 Tako sashimi

If you are a Jerry Lettvin fan, please skip to paragraph 2. Mere mortals, please read on:

RECIPE

Remove octopus guts, eyes, and beak and freeze for at least 72 hours.
Thaw and steam for 30 minutes or until done.
Cut off the tentacles and with a very sharp knife cut them into 1/8 inch slices at a 45 degree angle. Slice the belly into 1/4 inch strips.
Mix wasabi with soy sauce.
Dip octopus and enjoy.

(For more, visit http://www.freediver.net/freedivelist/recipes.html)

Save the eyes. Or visit your local Japanese restaurant, have some sashimi, and ask for octopus eyes while there. Why? A comparative anatomical study.

It is often mentioned that the octopus eye is structurally similar to the human eye, complete with cornea, iris, lens, vitreous, and retina. Presumably, it has sharp, color, and 3D vision, and is able to differentiate shapes.

However, a close examination shows that its crystalline lens has a fixed focal length like that of a camera, so the octopus focuses by moving the lens close to or away from the retina (some say by changing the shape of the eye globe). Also, the coordination of the octopus eyes is via the statocyst so the slit pupil of each eye is always in the horizontal position. Probably the most interesting feature is the retina: the photoreceptor cells are directed towards the light source.

If you go back to Topic 2.3.1, you'll see the human rods and cones are oriented against the light source, in other words, the photoreceptors are pointing towards the choroid/sclera, while that of the octopus, towards the vitreous. In terms of the efficiency in photon capture, the octopus obviously has the upper hand. Strange, huh? In fact, this has been used as the evidence of a design flaw of the human eye.

Actually, the octopus lives in (sea)water which has a refractive index of 1.33 and the sunlight is refracted and polarized when it enters water (especially at dusk and dawn). The octopus hunts at dusk. To see a prey such as a jellyfish, it will need a crystalline lens with even higher refractive index (than 1.33) in order to focus properly - because the corneal refractive power is neutralized by the water outside and the aqueous humor inside the eye. And equally important, it will need polarization vision (which actually works well with horizontal slit pupils). Polarized light is less intense and spectrally shifted, yet with more contrast, so it allows the octopus to better appreciate the pattern that leads to the capture of, e.g., a well camouflaged crab. Polarization vision will require the photoreceptors to directly analyze the incident light or the visual information is lost. The photoreceptors therefore should be oriented towards the light.

Humans can always wear Polaroid sunglasses, e.g., to see comfortably through light reflected and polarized by the surface of water/ocean. If you are a diver, try diving with your polarizers on at dusk and see if you can spot a Portuguese Man-of-War floating by. Report back after you have recovered from the pain. An octopus on land will need heavy prescription (for high myopia) just to navigate.

The take-home message: All eyes are developed to serve the one and only purpose: to see well to survive, wherever the rest of the body may reside.