Migrating songbirds fly thousands of miles over terrain they have never seen before, guided by the stars and by the earth's magnetic field. However, to find our way through a cluttered environment a compass is not enough, we need a map. Rats behave as if they use cognitive maps, and neurones in the hippocampus behave as if they encode parts of the map. John O'Keefe, of University College London, was the first to propose that the hippocampus provides the rat with a cognitive map. The activity of a hundred or so of the half-million neurones in the hippocampus can tell the rat where it is with a very high degree of precision.
Recent experiments show that place neurones pick up signals from landmarks in the environment.
When the frame of reference provided by distant landmarks surrounding the rat is rotated, the place fields rotate too. When the walls are moved further apart, the place-fields can stretch in the same direction, or split into two, one associated with each wall. One of the best tests of a rat's cognitive map is the milk maze, designed in the early eighties by Richard Morris of the University of Edinburgh. The rat is placed in a large pool of opaque milky water, with a submerged platform. The rat soon learns to use visible landmarks surrounding the pool to find its way to the platform, but if the hippocampus is damaged, or if its function is impaired by drugs, the rat can no longer do the task.