(A) Rats are placed in a circular pool about the size of a child’s swimming pool, filled with water that has been made opaque. Around the outside of the pool, objects such as doors, windows, and curtains are arranged as landmarks. A small platform is placed in the pool, just below the opaque water’s surface, which makes it invisible. While the rat swims around the pool trying to find this platform to rest on, a video camera records its path (represented by the lines in these figures). After the first few tries, normal rats are able to swim directly to the platform on each attempt.

(B) In contrast, when the experiment is conducted with rats whose spatial memory has been impaired by lesions in their hippocampus, their path indicates an inability to remember where the platform is, even after many trials.

(from Schenk and Morris, 1985.)