THE MAN WHO MISTOOK HIS WIFE FOR A HAT; And Other Clinical Tales. By Oliver Sacks. Summit. 233 pp. $15.95.

IN THE TITLE essay of his new book, Oliver Sacks describes one of his patients, the genial teacher Dr. P., as a man who greeted fire-hydrants and parking meters as if they were children, talked to the knobs on furniture, and puzzled over a glove: "A continuous surface . . . infolded on itself. . . five outpouchings. . . . A container of some sort?" He even once reached for his wife's head, mistaking her for a hat.

P. could "see," but his brain could not process what his eyes took in. He recognized his brother Paul by facial characteristics that were familiar -- "That square jaw, those big teeth. I would know Paul anywhere!" -- but he could not recognize faces or visualize an entire scene. "He construed the world as a computer construes it, by means of key features and schematic relationships . . . without the reality being grasped at all."

Sacks, a neurologist, compares P.'s peculiar view of the world to neurologists' classical interpretation of the mind. Like P., he argues, doctors identify the brain's chambers and describe their technical functions, but fail to grasp their relationship to the whole. Historically, doctors focused on the left hemisphere of the brain, the "unique flower of human evolution," while Sacks concentrates on the more primitive right, which controls "the crucial powers of recognizing reality" -- faces, for example -- "which every living creature must have in order to survive."

Sacks says, "Neurology and psychology, curiously, though they talk of everything else, almost never talk of 'judgment.'"

Many of the patients Sacks describes have been consigned to the back wards of hospitals, nursing homes, and to wandering the streets. Ignored by doctors and forgotten by society, they cannot make sense of their senses.

Mr. G., 49, has no memory of anything between 1945 and the last few seconds. He can play a quick game of checkers, but not chess, for the moves are too slow and he forgets what he is doing. He is "isolated in a single moment of being . . . a constantly changing, meaningless moment," Sacks says. Mrs. S., a stroke victim in her sixties, has lost her understanding of the concept of "left." She wears her lipstick only on the right side of her mouth; eats only the food on the right side of her plate. The partially deaf Mrs. O'M., in her eighties, continually hears songs of her childhood, such as "Easter Parade," "Glory, Glory, Hallelujah," and "Good Night, Sweet Jesus." Sacks says many patients hear these tunes, a sort of "Top Ten of the Cortex."

Clearly, Sacks' patients are lost in different worlds, but he insists that they are not necessarily worlds in which all must be lost. Sacks finds that G., though lacking memory, comes alive in church, absorbed by the ritual. The woman who could not fathom "left" consumes an entire meal by swiveling in a wheelchair to her right until she comes full circle, thus bringing the left portion on her plate into view.

The retarded twins, John and Michael, institutionalized since they were 7, find serenity in the invocation of numbers. They can instantly recall 300-digit numbers, and they spend hours communicating with each other through a language of six-figure primes. Mr. A., also retarded, memorized the entire nine-volume, 6,000-page 1954 edition of Grove's Dictionary of Music and Musicians after it was dictated to him by his devoted father, an opera singer.

SACKS PORTRAYS an eerie, higher intelligence in the elderly, deranged patients on the aphasia ward. The vocabulary of words is lost to them, but they nevertheless laugh heartily as they watch a televised speech by President Reagan. These patients, Sacks says, have a stunning appreciation of utterance, of facial expression and intended meaning that accompany speech and that can never be "faked, as words alone can." Unable to grasp words, they cannot be deceived by them, not even by the Great Communicator. "Here then, was the paradox of the President's speech. We normals -- aided, doubtless, by our wish to be fooled, were indeed well and truly fooled. . . . And so cunningly was deceptive word-use combined with deceptive tone, that only the brain-damaged remained intact, undeceived."

All this is not to say that Sacks romanticizes mental illness. His precise, wry, and often wrenching accounts of his patients' afflictions simply offer a series of unsettling perceptions about damaged minds and our own faulty reactions. In one of his most disturbing essays, "The Possessed," he describes a woman on the street suffering from a severe form of Tourette's syndrome, a personality disorder manifesting itself in "tics, jerks, mannerisms, grimaces, noises, curses, involuntary imitations and compulsions of all sorts."

Sacks says, "In the course of a short city- block this frantic old woman frenetically caricatured the features of forty or fifty passers- by, in a quick-fire sequence of kaleidoscopic imitations, each lasting a second or two, sometimes less, and the whole dizzying sequence scarcely more than two minutes.

"And there were ludicrous imitations of the second and third order; for the people in the street, startled, outraged, bewildered by her imitations, took on these expressions in reaction to her; and those expressions in turn were re-reflected, re-directed, re-distorted, by the (woman), causing a still greater degree of outrage and shock. This grotesque, involuntary resonance, or mutuality, by which everyone was drawn into an absurdly amplifying interaction, was the source of the disturbance I had seen from a distance. This woman who, becoming everybody, lost her own self, became nobody. This woman with a thousand faces, masks, personae -- how must it be for her in this whirlwind of identities?"

How must it be for all of his patients? Sacks asks, with fascination and concern. And, implicitly, how must it be for us? Rather than concentrate only on what is lost in the minds of his patients, Sacks searches for what is there, "the qualities of mind which are preserved, even enhanced." The favorite word of neurology is "deficits," he says, but he argues eloquently that the study of how the mind restores itself, strives to "preserve its identity," is at least as critical as the losses themselves.