Well, hell. I finally managed to sit down and get past the Preface in Liveing's On Megrim, and ho-lee mackerel, I think I know exactly why this got Sacks' attention.

The very first chapter is "Illustrative Cases" -- a few case studies, both Liveing's own patients and those of a few other neurological writers of his time. He covers, in brief, Du Bois Reymond's case of a man with classic hemicranial cephalgia accompanied by nausea, erythema of the face, and blood pressure changes; his own patient, a young woman with severe headache and general 'sea sickness', whose attacks are terminated by sleep; another of his patients, a man who suffered pure ocular migraine with various fun scintillating scotomata in his central vision; a man whose migraine attacks could be brought on by either a violent emotional or physical effort or gastric upset, and comes on with a burst of flickering scotomatous lights; and mentions of patients belonging to Parry and Wollaston, the latter of whom treated a man with a combination of aura, transient scotomata, and brief but severe headache.

For those of you playing along at home, none of those cases have the exact same list of symptoms. And in fact, case three shares exactly zero explicit symptoms with either of the first two. Yet Liveing points out that all of these things are known as 'megrim'*. It's not unheard of for folk medicine to lump a bunch of unconnected things together under one name, but Liveing says outright that he thinks he's demonstrated satisfactorily that all of these cases fall under one umbrella, and you know what? If you have that kind of terrible unstoppable chronic pattern-matching mind, he has. The explicit symptoms are kind of grab-bag, but the underlying structure of the maladies keeps plucking strings of recognition -- the build-up to the attack, the inexplicable sense of inevitability, the paroxysmal quality when it finally hits the tipping point and wallops the poor migraineur, the peculiar mood changes that go with it.

I don't know if Liveing suffered from migraines, but Sacks did and I do, and the case descriptions given in the book are all very accurate, not just about what migraines feel like and look like from the interior, but about the strange state it puts you in, and about the nagging sense that all of these things are manifestations of one central goddamn weird thing your brain is doing that is fucking with all your various systems. He explicitly describes them as seizures, which, if I understand my epileptic friends correctly, follow more or less the same pattern of the tendency towards having one slowly builds until it hits a tipping point, after which the world goes a wee bit wonky and an attack is -- and feels -- inevitable. The storm is relatively quick but incredibly violent, and since it disrupts the brain itself, it's liable to show up as weird reactions in anything that depends on the brain for monitoring and control, which is literally everything about you as a human being.

The symptoms are almost irrelevant. It's the underlying pattern of how and why the brain-storm hits that matches. Liveing had the ability to overlay case upon case upon case and rotate each one just so until he found the angle where the silhouettes lined up, even when the edges of the shadows were smeared and indistinct. That is not common. Most people have the capacity to understand what's going on when you explain it, but being able to find it unprompted is one in a million. It is the thing I zero in on when spotting the other weird genius kids, because it is a trick that can be pulled off independently of education or cultural tradition. All it takes is having a truckload of raw data spread out in front of you.

Sacks had it in spades, which is how he wound up doing chaos theory math in the back of a book about neurological case studies with a side of epidemiological history, in which he quoted a fair amount from Marcel Proust. I have it, and I have amassed an extensive collection of funny looks from other people to prove it. He fell in love with Liveing's writing for the same reason I fell in love with his: When you match patterns constantly, linking up with another chronic pattern-matcher doesn't make your collection of patterns go up arithmetically, it goes up exponentially. Your combined Zustandsräume become a much bigger place to think around in. People who compulsively network things to other things in their heads tend to find it an inherently rewarding activity, so picking up entirely new comparative frameworks from other smart people is a fun way to lose a weekend.

Sacks also, in his foreword, answers my question about how the hell he found this book, which is that it was in the rare books section of a library and he wheedled a librarian into letting it circulate home with him just this once. Upon having to give it back, and realizing that even if he went through a proper antiques dealer another copy would be damnably difficult to find, he arranged to photocopy the whole blessit thing. I would not find that quite so funny if I had not done exactly the same with several volumes of the collected works of A M Turing about a decade ago, when I pestered them out of the university library at my alma mater.

*: "Megrim" is an obsolete British form of migraine, which pops up a lot in your life if  you develop the sort of personal problems that lead you to go binge-reading Victorian neurology. The word comes originally from the Greek hemikrania, "(pain on) one side of the head", which refers to the peculiar tendency of the migraine gnomes to pick only one of your eyeballs to jab from behind with a flaming pitchfork. It's pronounced MEE-grim, much as British English still pronounces "migraine" as MEE-grain. The American pronunciation of MY-grain appears to have come from someone who reverse engineered the reading from the printed word without having heard it spoken.