NEW YORK — Ted James and Lysa Grant hit it off immediately when they met at a study group for a psychology class. The two students at New York University knew something was special, and four years later they are now engaged.

James, 24, vividly remembers the first thing he noticed about Grant. “I just loved her smile,” he said.

For Grant, the connection was more cerebral. “I could see he was really smart,” she said. “That was a turn-on.”

But says R. Douglas Fields, neither intelligence nor charm had much to do with their mutual attraction. Rather, a little-known cranial nerve brought them together, he believes. Few neuroscientists are even aware that this “nerve zero” exists, but Fields, an adjunct professor of neuroscience at the University of Maryland, believes it may be the key to lust.

His theory is that nerve zero transmits sex pheromones to the brain. Pheromones are chemicals that one member of a species emits that trigger an innate behavioral response in another member of the same species. They are generally detected by the sense of smell. The notion that smell is important to the sexual drive of animals is well-established, but nerve zero may be the “missing link” that confirms human beings rely on pheromones, Fields said.

Data exists

“Human behaviors are much more complex than other animals’, but there are several studies showing that the sense of smell does affect sexual behavior in people,” Fields said. “And here is a nerve that connects the nose to the part of the brain involved in sexual reproduction, which helps prove it.”

Pheromones were discovered in insects in 1959, and later studies suggested that they also induced sexual reactions in people. In 1995, for example, Claus Wedekind, a researcher at the University of Bern in Switzerland, asked a group of women to smell T-shirts worn by men they did not know. The women preferred the smell of men who had different immune systems from their own, which would enhance the likelihood that they would have healthy children.

Such theories are controversial in part because they lack hard scientific evidence.

Besides the debate over whether sex pheromones even exist, few people in the field know about nerve zero, which was discovered in the human brain in 1913, well after the other 12 cranial nerves. Researchers called it nerve zero rather than rename the others.

Being so thin, this obscure nerve usually is overlooked in medical research as it is often stripped away when the brain is exposed for dissection. For this reason, nerve zero doesn’t appear in most neuroscience textbooks or medical brain maps.

For those who know about nerve zero, its function has long been debated. Some scientists believe that it is a branch of the olfactory nerves or that it has lost its purpose over time, much like the appendix.

For that reason, the jury is still out on Fields’ theory, said Michael Meredith, co-director of the neuroscience program at Florida State University.

“I don’t know that there is good evidence of that,” he said. “There are a lot of if’s, and’s and but’s. It’s prevalent in all vertebrates, which suggests that it does have a function, but we don’t know that it has an adult function.”

A conduit

But Fields insists evidence is stacking up that the sense of smell affects one’s choice of sexual partner and that nerve zero is the conduit. He points to a 1987 study on hamsters by Celeste Wirsig, then a postdoctoral fellow at Baylor University. The rodents failed to mate after their nerve zeros were severed. Because the nerve systems of hamsters and humans are similar, it stands to reason, Fields said, that nerve zero has the same purpose.

Fields has supporters, among them James Kohl, co-author of “The Scent of Eros,” a book on pheromones.

“He’s right on,” Kohl said. “We have known there is some physical link, but [nerve zero] really helps to define it. Maybe a lot of neuroscientists don’t know about it, but people who study the olfactory system and pheromones see that and say that’s really important.”

The thought that they might be together because they “smell right” makes James and Grant a little uncomfortable, although they accept that it’s possible. Even if scent is what got them started, James believes love is much more complicated than a pleasing aroma.

“She’s my soul mate,” he said about his fiance. “No whiff of sweat is going to make me feel the way I feel about her.”