U.S.-British scientist John O'Keefe and Norwegian married couple May-Britt Moser and Edvard Moser won the Nobel Prize in medicine on Monday for discovering the "inner GPS" that helps the brain navigate through the world.

With experiments on rats, they discovered two different types of nerve cells that "constitute a positioning system in the brain," the Nobel Assembly said.

O'Keefe, of 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 May-Britt and Edvard Moser, of the Norwegian University of Science and Technology in Trondheim, identified another type of nerve cell — the "grid cell" — that generates a coordinate system for precise positioning and path-finding, the assembly said.

Hege Tunstad, a spokeswoman at the university in Trondheim, said May-Britt Moser was at the university when she found out she had won.

"She needed a minute to cry and speak with her team," Tunstad said. "Her husband was on a plane so he doesn't know yet."

The Nobel Assembly said that knowledge about the brain's positioning system may "help us understand the mechanism underpinning the devastating spatial memory loss" that affects people with Alzheimer's disease.

The discoveries have also opened new avenues for understanding cognitive functions such as memory, thinking and planning, it said.

The Nobel awards in physics, chemistry, literature and peace will be announced later this week. The economics prize will be announced next Monday.

The winners of the each category split the prize money of 8 million Swedish kronor (about $1.1 million). Each winner also receives a diploma and a gold medal.

Created by Swedish industrialist Alfred Nobel, the Nobel Prizes were first awarded in 1901. The winners always 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.

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Mark Lewis in Stavanger, Norway, contributed to this report.