Saturday, January 19, 2008

7.10 Because it's there

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.

Friday, January 18, 2008

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.

Wednesday, January 16, 2008

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):


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.

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.

Sunday, January 13, 2008

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.