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In most scientific studies involving animals such as rats, the animals are kept in individual houses, to minimize variation in what each animal experiences during the course of the experiment, and also to reduce the risk of disease passed from animal to animal. Such a sterile environment is, of course, very different to what the animal would experience in nature.

Some studies provide the test animals with "enriched environments". In these cases, animals may be housed together in a colony, and may be provided with toys and opportunities for exercise and social interaction. They may also receive handling by humans and other forms of unusual or stimulating experience. Scientific studies have shown that animals placed in enriched environments show increased brain growth and better capacity for learning than animals housed in sterile environments. These results have been taken as evidence for the "use-it-or-lose-it" theories of brain function: that is, that people (and animals) with active, stimulating lifestyles maintain brain function better than those experiencing less stimulation.

Further reading: H. van Praag and others, "Neural consequences of environmental enrichment," in Nature Reviews Neuroscience, December 2000, vol. 1, pages 191-198.

Further reading: "Use it or Lose it"