Healthy rats are separated by chance to live in one of three different environments, either standard, enriched, or deprived. After several months, the rats were killed, and researchers measured the size of their brains and the number of receptors for the neurotransmitter acetylcholine (ACh), which is important for memory and learning. The standard environment was a typical laboratory colony cage where several rats are held with an adequate amount of space, food, and water. The enriched environment the rats had adequate food and water as well as mazes and wheels for stimulation and were placed with other rats with which to socialize. In the deprived environment, they were alone in a cage that had adequate food and water, but that had no other rats and no opportunities for stimulation in the form of mazes or wheels. (Rosenzweig and Bennett, 1972)