Saturday, May 23, 2015

The specialness of islands

kw: book reviews, nonfiction, medicine, islands, cycads

Perception may not be reality, but it certainly feels that way. We go through life contentedly thinking other people are a lot like us. The growth from naivete to sophistication is largely involved in learning the ways we differ from others, and how to effectively cope with that. When we say someone "sees things differently", we usually mean understanding, not a different kind of physical vision. But among people who are not entirely blind, one man in twelve, and one woman in about 250, sees the world around as having different colors than those seen by most of us.

This illustration from 1895 is an attempt to show people with "normal" color vision the effects of a few kinds of color blindness. A truer depiction would have both flags II and III with brown stripes, just of differing contrast, and flags IV and V would be blurrier, because the blue-blind and total monochromats have much lower acuity of vision. Note that in flag IV the star field is black rather than blue.

I once studied color vision for 50 of my colleagues using printed color wheels, asking them to name the colors at certain angles, marked around the margin of the wheel. I tracked down a few folks who were color blind also, because the aim was to produce color maps for geologists that everyone could use with equal ease, and many geologists are color blind. As it happened, the results were somewhat confounded by the printing process, which used three colored inks. These are easier to distinguish from one another even by people with the usual, red-green, kinds of color blindness. Thus, we had  more flexibility using map colors than we'd originally expected.

What do we mean by red-green color blindness? It is actually of two varieties. The formal terms are protanopia and deuteranopia, based on the numbers 1 and 2. "1" refers to the red-sensitive cone cell (R cone), and "2" refers to the green-sensitive cone cell (G cone). However, the sensitivity curves of these two kinds of cells overlap quite a lot.

As seen in this diagram, the R cone and G cone have very similar sensitivity curves, just shifted from one another. It is somewhat surprising that the B cone has such a low sensitivity. In normal daylight, there is a lot of blue light, so it doesn't need a lot of sensitivity. In low light, such as the light of a full moon, for humans at least, the rod cells begin to work as the cones lose effectiveness. Rods are also blue-sensitive, though sort of between G cones and B cones. This is why moonlit scenes appear bluish. The most sensitive vision cells active under moonlight are blue-sensitive.

If either the R cones or G cones are missing or inactive, red, yellow and green shades can be distinguished from blues, but not from each other. The main difference is that the deepest reds appear totally dark to a protanope, while there is a blue-green region of the spectrum that is hard for a deuteranope to see. But because both conditions make the person unable to distinguish red from green, both are called red-green color blindness.

If none of the cones are present, or are not active, a person has only "night vision", with only the rods working. This is the primary type of achromatopsia, or total color blindness. The word is composed of "achromat", meaning "no color" and "opsia", referring to the eye. According to an old estimate, about one person in 30,000 has this condition, and it occurs in more men much more than women. Apparently, all kinds of color anomaly and color blindness are X-linked, so it is quite rare for a woman to have the same anomalous gene on both X chromosomes.

Because the "day vision" system isn't working, and rod cells typically bleach out entirely in bright light and stop working, achromatopes are day-blind or very sensitive to bright light, and cannot function in daylight without strong filters over their eyes. There are other unfortunate characteristics of the syndrome, such as nystagmus (rapid and unusual movement of the eyes), which may be side effects of the day-blindness.

Oliver Sacks, a polymath who almost incidentally is a neurologist, became fascinated upon learning of a pair of islands in the south Pacific, where more than 5% of the people are achromatopes. In the local language the condition is called maskun, meaning "not see", because of their day-blindness. They can function well enough in lower light, so they can do certain kinds of work. They are well accepted in their communities. Dr. Sacks and two others, one a scientist named Knut Nordby who is an achromatope, visited Pingelap and Pohnpei to study the phenomenon, and to bring dark glasses and other vision aids to people that have been coping without them. Their travels and work are described in a delightful way in Sacks's book The Island of the Colorblind, bound together with Cycad Island in the volume I read.

Both books explore the way island populations tend to concentrate certain characteristics. When you have a small population that has little or no contact with others, the statistics of gene-shuffling through the generations can exaggerate some conditions. In the case of Pingelap, a disastrous typhoon and following famine killed all but 20 people residing there, and fear of disease kept people from neighboring islands from visiting for more than a generation. One of those 20 carried the gene for achromatopsia, and inbreeding brought it into expression, as mentioned, to an incidence of about 5%. The numbers of those with maskun would be greater if it were not hard for them to find marriage partners.

The presence of Dr. Nordby was crucial to getting the people to talk willingly about their condition, and to help the research team gather useful genealogical data. The largest nearby island, Pohnpei, has a valley populated by immigrants from Pingelap, and a similar incidence of maskun. The team also went there to help those they could, and study the condition further.

Color blindness of this kind is trouble enough, but it doesn't kill you. A different medical condition is found on Guam, as Dr. Sacks tells us in Cycad Island. A slow, progressive disease called lytico-bodig in the local language has been endemic there for several generations. In the lytico form it resembles ALS, most famously afflicting Dr. Stephen Hawking, and earlier called Lou Gehrig's Disease, for that ballplayer died from it. The bodig form is more like Parkinson's Disease, leading to rigidity and paralysis, and is often accompanied by dementia. One sufferer cheerfully told Dr. Sacks, "Come back soon. I won't remember you, so I'll have the pleasure of getting to know you all over again."

Much of the latter book details a series of frustrated efforts over the years to determine what causes lytico-bodig. No final conclusion is offered, but the language strongly hints that the best hypothesis is poisoning of people who are genetically susceptible, in two different ways, to certain chemicals in the resin of cycads. The residents of Guam enjoy certain foods they prepare from cycad seed pods and other parts of the plant. They go to great lengths to detoxify them, because untreated, a small amount can kill. It seems to be like Japanese fugu, the puffer-fish, which an expert chef can prepare so it is safe to eat, if a bit "tingly", but several people are badly injured or killed each year from eating fugu. On Guam, cycad preparation is carried out with various levels of diligence. Also, a local fruit bat eats cycad fruits, and something that concentrates in its flesh can be damaging to the people, who enjoy bat meals.

It is stated that nobody born after 1952 suffers from lytico-bodig. This is probably because of changes in diet. Cycads are eaten much less now, and the fruit bat is seldom eaten because it is getting very scarce. Thus, a resident scientist named John Steele, who has studied the disease and befriended its sufferers for half his life, may find that the disease vanishes before he is able to prove its source. This is scientifically frustrating, but socially, it is a great relief.

Dr. Sacks's interests are wide-ranging, and I find he has five other books in print. Guess I'll track them down.

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