(Ecco)

“Hel-lo!”

It’s 9:07 a.m., Oct. 17, 1965, the eve of Margot Sharpe’s 24th birthday: “The single defining moment of Margot Sharpe’s life as it will be the single defining moment of Margo Sharpe’s career.”

So begins Joyce Carol Oates’s wonderfully illuminating novel “The Man Without a Shadow.” Margot, a fledgling PhD neuroscientist at a prestigious institute, has just been introduced to “E.H.”: Elihu Hoopes, an amnesiac famous within academic neurological circles.

Because the hippocampus of his brain was ravaged by encephalitis fever, Elihu can form no new short-term memories. Forever 37, a privileged son of a rich Philadelphia family, a Renaissance man, athletic, well read, artistic and witty, Elihu is nevertheless completely incapacitated by his disability. Living with an adoring aunt, Elihu is driven to the institute several times a week to undergo batteries of tests. Not to help or heal him — that is not possible — but for the neuroscience community to gain insight into the human brain. There is more than a whiff of exploitation.

From the beginning, Margot is convinced that she is special to Elihu. He leans close to her, smells her hair. Every time they meet anew, she feels his eager attention and even a sense of familiarity, although other scientists decry that as impossible. “Hel-lo!” he says, with interest, each time. Thus begins one of the most curious and moving love affairs in contemporary fiction.

Author Joyce Carol Oates (Dustin Cohen)

What starts out as a professional relationship between subject and scientist gradually becomes romanticized — at least on Margot’s part. Keen to make sense of his bewildering world, Elihu picks up on cues, both physical and verbal, that Margo begins to drop when he “meets” her. “Are you my wife?” he continually asks her, and if no one is around, she answers, “Yes.”

The book examines the nature of passion, affection and, above all, the loneliness that permeates even the longest and most intimate relationships.

In her amnesia logbook Margot speculates:

How do we know who we were, if we don’t know who we are?

How do we know who we are, if we don’t know who we were?”

The scientific community feeds off Elihu, running hundreds of experiments, publishing dozens of papers. Careers are made on him, Margot’s among them. When the department head, Margot’s mentor and onetime lover, wins a Nobel Prize, Oates probes some of the ethical issues that arise from the activities of the institute — and of academic research in general.

Decades pass, but Elihu remains 37 in his mind, perhaps somewhat less compliant, less gentlemanly with the institute staff, but essentially the same. And Margot, also, refuses to age. She dresses in the same clothes, wears her hair in the same eccentric side braid as it slowly turns white. Fourteen years younger than Elihu, she gradually ages beyond his ability to recognize her as a peer.

Margot muses on this at one of the isolated times she confronts the reality of her life: “Strange that she, who’d so often been the youngest person to give a paper at a scientific gathering, the youngest full professor in the Psychology Department, the youngest award recipient, is no longer young but middle-aged; no longer promising but accomplished; no longer envied but revered.”

One of the extraordinary aspects of this novel is watching the simultaneous ascendancy of Margot into the stratosphere of the neuroscience community — her groundbreaking work with Elihu honored and her scientific rigor emulated and admired — and her descent into what can only be called a kind of madness.

Because Margo does go slightly mad. At first able to keep her passion for Elihu under control, she eventually takes enormous risks to be with “my dear husband,” as she calls him, placing silver matching rings on their fingers when they are alone and guiding him into the bushes for assignations. Oates captures the exquisite satisfaction — physical and emotional — that Margot feels when she succeeds at consummating their “marriage,” despite the professional and personal ramifications she would face if her forbidden love were made public. She is convinced that Elihu knows who she is, even if he doesn’t know her name, and that the essence of their decades-long relationship is embedded in an undamaged part of his brain.

Oates dramatizes achingly well that when the two lovers are not together, they are utterly alone. Margot’s life outside the lab is a solitary one. She drinks — to excess. She has no friends. She spurns her family members when they reach out. When he’s not at the institute, Elihu retreats to his bedroom in his elderly aunt’s house, watching classic movies that he has long since memorized. He occasionally surprises his watchers with the clarity of his observations: “The annihilation is not the terror. The journey is the terror.”

There are mysteries within mysteries in this book, beautifully rendered. Who is the dead girl lying in a stream, her body sparkling, who appears in the sketches Elihu continually draws? How much does Elihu retain — if not consciously, then unconsciously — from his daily slate of mind-numbingly tedious and sometimes cruel memory tests? Oates provides no easy answers.

The book’s ending, also, is heart-rending. Not because Elihu, in his 70s, is dying, but because Margot, returning to his bed from a brief bathroom break, realizes that Elihu has never seen her in his life.

“Hel-lo!” he says.

LaPlante is the author of “Turn of Mind” and “A Circle of Wives.” Her most recent book, “Coming of Age at the End of Days,” is out in paperback.

The Man Without a Shadow

By Joyce Carol Oates

Ecco. 384 pp. $27.99