Friday, January 4, 2008

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])

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.

Wednesday, January 2, 2008

7.4 Tako sashimi

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