A U.S.-British scientist and a Norwegian husband-and-wife research team won the Nobel Prize in Medicine Monday for discovering the brain's navigation system — the inner GPS that helps us find our way in the world — a revelation that could lead to advances in diagnosing Alzheimer's.

The research by John O'Keefe, May-Britt Moser and Edvard Moser represents a "paradigm shift" in neuroscience that could help researchers understand the sometimes severe spatial memory loss associated with Alzheimer's disease, the Nobel Assembly said.

"This year's Nobel laureates have discovered a positioning system, an `inner GPS' in the brain, that makes it possible to orient ourselves in space," the assembly said.

O'Keefe, 74, a dual U.S. and British citizen at the University College London, discovered the first component of this system in 1971 when he found that a certain type of nerve cell was always activated when a rat was at a certain place in a room. He demonstrated that these place cells were building up a map of the environment, not just registering visual input.

Thirty-four years later, in 2005, May-Britt Moser and Edvard Moser, married neuroscientists at the Norwegian University of Science and Technology in Trondheim, identified another type of nerve cell — the grid cell — that generates a co-ordinate system for precise positioning and path-finding, the assembly said.

Monday's award was the fourth time that a married couple has shared a Nobel Prize and the second time in the medicine category.

"This is crazy," an excited May-Britt Moser, 51, told The Associated Press by telephone from Trondheim.

"This is such a great honour for all of us and all the people who have worked with us and supported us," she said. "We are going to continue and hopefully do even more groundbreaking work in the future."

Her 52-year-old husband didn't immediately find out about the prize because he was flying to the Max Planck Institute in Munich, Germany, to demonstrate their research. Edvard Moser told the Norwegian news agency NTB he only discovered he had won after he landed in Munich, turned on his cellphone and saw a flood of emails, text messages and missed calls.

"I didn't know anything. When I got off the plane there was a representative there with a bouquet of flowers who said `congratulations on the prize,"' he was quoted as saying.

The Nobel Assembly said the discoveries marked a shift in scientists' understanding of how specialized cells work together to perform complex cognitive tasks. They have also opened new avenues for understanding cognitive functions such as memory, thinking and planning.

"Thanks to our grid and place cells, we don't have to walk around with a map to find our way each time we visit a city, because we have that map in our head," said Juleen Zierath, chair of the medicine prize committee. "I think, without these cells, we would have a really hard time to survive."

O'Keefe told the AP he was working at home when his office called to say "there's a gentleman from Sweden who wants to have a word with you."

Professor John O'Keefe speaks in an interview Monday after he was named a co-winner of the 2014 Nobel Prize in Medicine. (Matt Dunham/Associated Press)

"Before I called him, I took a long, deep breath," O'Keefe said, speaking at his office at University College London.

O'Keefe was born in Harlem and raised in the South Bronx. "If you can survive the South Bronx, you can survive anything," he said.

He moved to England for postdoctoral training and found the place cells in a part of the brain called the hippocampus. The Mosers, meanwhile, identified the grid cells in a nearby section of the brain known as the entorhinal cortex.

O'Keefe said his work could be used as a basis for investigating Alzheimer's, for example, by designing tests to try to pick up the first signs of the disease.

"So we can not only use brain imaging to see the earliest signs of the disease in this part of the brain, but we can begin to see how it is affecting their memory, particularly their spatial memory," he said.

David Foster, a neuroscientist who studies place cells at Johns Hopkins University in Baltimore, said O'Keefe's discovery of place cells was controversial in 1971 but became widely accepted over the next few decades as more researchers began to study it.

"He founded the field," Foster said.

All three Nobel laureates won Columbia University's Louisa Gross Horwitz Prize last year for their discoveries. They will split the Nobel prize money of 8 million Swedish kronor (about $1.1 million Cdn), with half to O'Keefe and the other half to the Mosers.

The Nobel awards in physics, chemistry, literature and peace will be announced later this week and the economics prize will be announced next Monday. Created by Swedish industrialist Alfred Nobel, the Nobel Prizes were first awarded in 1901. The winners collect their awards on Dec. 10, the anniversary of Nobel's death in 1896.

Last year's medicine award went to researchers who discovered how substances are transported within cells, a process involved in such important activities as brain cell communication and the release of insulin.