Over the years his labs and others have shown that stimulating the brain makes it grow in almost every conceivable way. Animals raised in enriched environments—surrounded by other animals, objects to explore, toys to roll, ladders to climb, and running wheels—learn better than genetically identical animals that have been reared in impoverished environments. Acetylcholine, a brain chemical essential for learning, is higher in rats trained on difficult spatial problems than in rats trained on simpler problems. Mental training or life in enriched environments increases brain weight by 5 percent
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