Rat in maze. White laboratory rat (Rattus norvegicus) in an eight-arm radial maze. This apparatus is used to test spatial memory. A common experiment is to train the rat to retrieve a small amount of food from each arm, and test its ability to remember which arms it has already visited. This experiment is part of a study on the effect of cannabinoids (the active compounds in cannabis) on behaviour. The white rat is one of the most commonly used experimental mammals. It is an albino strain bred from the original brown or Norway rat. Photographed in a laboratory at the University of Milan, Italy.