Greenough showed the effects of enriched and deprived environments on cortical connectivity and thickness in a series of studies. This is one of his early studies:
Science. 1972 Jun 30;176(4042):1445-7. Rearing complexity affects branching of dendrites in the visual cortex of the rat. Volkmar FR, Greenough WT.
"Higher-order dendritic branching is considerably greater in Golgistained neurons from the occipital cortex of rats reared in groups in a complex environment than in similar neurons of littermates reared individually in laboratory cages have intermediate amounts of branching, while lower-order branching did not appear to be affected by any rearing environment."