Cooper, R. M., Zubek, J. P. 1958. Effects of enriched and restricted early environments on the learning ability of bright and dull rats. Canadian Journal of Psychology 12, 159-164.

The bright animals reared in enriched environments [group-housing with numerous enrichment gadgets] showed no improvement in learning ability over bright controls reared under normal laboratory condition [group-housing in barren cage]. The dull animals, on the other hand, benefited greatly from the enriched experience and attained a level of performance equal to that of the bright animals. Rearing in restricted environments had converse effects. The dull animals suffered no impairment as compared with dull controls, while the bright animals were retarded to the levels of the dulls in learning performance.