title: A study of some enriching variables in a free-environment for rats.
mesh_terms: D000818:Animals; D004193:Discrimination Learning; D004777:Environment; D006799:Housing, Animal; D008124:Locomotion; D008297:Male; D009043:Motor Activity; D010988:Play and Playthings; D011340:Problem Solving; D051381:Rats; D014796:Visual Perception