Curiouser and Curiouser : THE ISLAND OF THE COLORBLIND.<i> By Oliver Sacks</i> .<i> Alfred A. Knopf: 280 pp, $24</i>

The islands of the Pacific have a habit of unsettling the intellectual assumptions of Western civilization. The encounter between Louis Antoine de Bougainville and the “noble savages” of Tahiti gave Jean Jacques Rousseau ideas that led indirectly to the rights of man. Ruminations on Galapagos animals gave Charles Darwin ideas that led to the theory of evolution by natural selection. And Margaret Mead’s (now largely discredited) studies of adolescent sexuality on Samoa set in train the 20th century’s long obsession with the mutability of human nature. Pacific islands have held a miniature mirror up to civilization.

Oliver Sacks, now a famous New York neurologist and writer, grew up in London in World War II fascinated by this Pacific history and dreaming of islands. In the early 1990s, he took the chance to fulfill his dream in the old tradition. He used his island travels to hold a mirror up to neurology and returned much the wiser. This is the story of those travels.

For the reader, there is something almost delicious about gently absorbing some really quite sophisticated lessons about the structure and workings of the human brain while reading about beaches, breadfruit trees and coral reefs. So skillfully does Sacks weave together his travelogue with his science that the result is a book of beguiling beauty.

It comes in two parts, corresponding to two journeys, and is told in reverse chronological order. The first tale (and second journey) is about Pingelap, a remote atoll in the Caroline Islands where nearly 10% of the residents are afflicted with a peculiar visual defect called achromatopsia; the second story (and first journey) is about Guam, where an unusually high proportion of the local people suffer from a baffling array of degenerative neural diseases known collectively as lytico-bodig.

If, like me, you have never heard of lytico-bodig, Pingelap or achromatopsia, you almost get the feeling that the whole book might be fictional, a feeling not dispelled by the way Sacks’ story unfolds. He contacted a Norwegian scientist, Knut Nordby, to find out about an island of colorblind people called (implausibly) Fuur, in Denmark. Fuur turned out to have lost its neurological distinction, the colorblind inhabitants having long since dispersed. So instead Nordby and Sacks decided to set out for the far more remote destination of Pingelap together with an American scientist named Robert Wasserman.

Nordby is himself an achromatope. His retinas lack the cone cells that give most of us color vision and that make up the central, visually acute region of our vision. As a result, he sees in black and white only, is blinded by ordinary daylight and can read and see detail only with great difficulty with the help of a magnifying glass or monocular.

The reason so many Pingelapese are achromatopes is a typical story of island biogeography. In about 1775, a typhoon destroyed virtually everything on the low-lying atoll. Those people who survived the typhoon itself soon starved as the palms and fruit trees lay in ruins and the taro crop failed to thrive in salt-saturated soil. Of the thousand inhabitants on the island, about 20 survived, including somebody who must have possessed the unlucky genetic mutation of achromatopsia, or “maskun,” as it is known locally. His or her descendants carry and sometimes express the gene.

Achromatopes are happiest at night, and Sacks’ companion, Nordby, enjoys night fishing with the Pingelapese because he can see the fish unusually clearly. It is tempting to conclude that immediately after the typhoon, when the survivors ate only fish, the achromatopes were actually at an advantage, but this is speculation.

The arrival on Pingelap of Sacks, Nordby and Wasserman is occasion enough for excitement. Simply by distributing sunglasses, they greatly relieve the difficulties of those afflicted with the gene. Rarely can such a simple medical cure have been so effective. But it is Nordby’s presence that is most reassuring, because most Pingelapese achromatopes harbor misconceptions that the disease is progressive, disabling (they find it hard to read blackboards or books) or is caused by something they or their parents did or ate. Nordby stands as proof that a determined person can rise above the disability and explains the fundamentals of heredity to them, disabusing them of the notion that it began when their mothers walked on a beach in bright sunlight.

One of the other indigenous myths to explain maskun is that it first occurred after white men took away some of the Pingelapese to work in the phosphate mines of the island Nauru. Indeed, a local tradition attributes the source of the “infection” to Scandinavians and is therefore bizarrely supported by the appearance of Nordby. The myth is later explained as a coincidence when the author visits a community of Pingelapese on nearby Pohnpei and finds a man who has visited the west and heard the story of Fuur.

Nonetheless, in Sacks’ gentle prose, it feels at this point as if we are drifting into a novel of Latin American magical realism. He quotes his diary written when he was high on sakau, the local plant hallucinogen:

“Floating over coral-heads. Lips of giant clams, perseverating, filling whole visual field. Suddenly a blue blaze. Luminous blobs fall from it. I hear the falling blobs distinctly; amplifying, they fill my auditory sensorium. I realize that it is my heartbeats, transformed, that I am hearing.”

On to Guam, where the story to be told is at once less neat and less cheerful, even though it concerns a neurological condition that is slowly dying out. Lytico-bodig consists of two overlapping and highly variable diseases, both of which seem to strike only native Chamorros born before 1952. One--lytico--causes progressive paralysis; the other--bodig--is more like Parkinson’s disease, a loss of mental initiative that leaves its victims unable to move, speak or act without prompting. It is sometimes accompanied by dementia.

The mystery is what causes them, and Sacks joins the hunt after many great minds have been baffled by Guam’s curious disease over 40 years. The fact that lytico-bodig is concentrated among the Chamorros of Guam, who suffered a demographic bottleneck similar to that of the Pingelapese (caused by Spanish genocide, not typhoons), seems at first to suggest a genetic cause, as on Pingelap. But the sudden disappearance of the disease in those born after 1952 and the fact that the disease often affects husbands and wives in a family as well as brothers and sisters seem to overturn the genetic hypothesis.

Instead, for a long time, suspicion fell on cycad trees--heavy, primitive palm-like plants that flourished all over the world in the age of the dinosaurs and now live only on Guam and similar islands. Cycad seeds have toxic side effects on cattle. To eat them, people have to grind them and then wash the flour carefully to remove the toxins. During World War II, the inhabitants of Guam took to the hills and forests to escape the occupying Japanese, and many of them lived on a diet of almost nothing but “fadang,” or cycad, flour.

However, lytico-bodig has none of the characteristics of poisoning. In some people it has only now begun to emerge, 50 years after the war. Symptoms like lytico-bodig can be induced in animals only by the use of highly concentrated doses of some of the chemicals in the cycad seeds, and even then the symptoms are temporary, not progressive, and appear fairly quickly.

Briefly, a few years ago, there was a flurry of support for the idea that lytico-bodig is caused not by genes or food but by water. The water is, in some similarly affected parts of New Guinea and Japan, low in calcium and magnesium and high in aluminum. Aluminum has been implicated in Alzheimer’s disease. But the aluminum hypothesis, too, has fallen into disfavor: Guam’s water is not high in aluminum.

In other words, nobody yet knows the answer. It is a brave author who writes a detective story with no resolution, but I hope more science writers follow Sacks’ lead. The very uncertainty flatters the reader. It invites him to join the hunt, to test his own wits against the problem; it does not condescend to him by laying down the facts and accepting no challenge to them. People often prefer mysteries to answers, which is why they like the paranormal.

Sacks writes with his familiar ease and grace. But every now and then he shows a small factual lapse. Several times, for instance, he asserts that prion diseases are caused by viruses. This is not only out of date, it risks missing an important insight. We now know that some gradual and slow forms of dementia can be caused by a rogue protein molecule called a prion, progressively catalyzing its own creation from non-rogue versions, and can therefore be both infectious and hereditary. This is at least an intriguing parallel to lytico-bodig, and it would have been good to see the parallel explored.

But this is not just a book about neurology. Sacks comes to Guam equipped with a surprising and relevant qualification. One of his obsessions is the study and cultivation of cycads. So the link between lytico-bodig and cycads, however dubious, gives him the perfect excuse to indulge himself with a whole chapter about a visit to Rota, Guam’s neighbor, where forests of these ponderous trees still flourish. The book, which has been in turn a travelogue, a study of visual handicap and a scientific mystery story, ends in pure botany.