Bennett, E. L., Rosenzweig, M. R., Diamond, M. C. 1969. Rat brain: Effects of environmental enrichment on wet and dry weights. Science 163, 825-826.

Wet weight of rat cerebral cortex was increased by exposure to an enriched environment [10 or 12 animals in large cage with toys], as compared with standard colony [2 or 3 animals in small cage] or impoverished conditions [single-housing].