US-British scientist John O'Keefe and a 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 particular place in a room. He demonstrated that these "place cells" were building up a map of the environment, not just registering visual input.