This seemed important:

Environmental enrichment prevents effects of dark-rearing in the rat visual cortex
Nature Neuroscience, March 2004 Volume 7 Number 3 pp 215 – 216
Alessandro Bartoletti, Paolo Medini, Nicoletta Berardi & Lamberto Maffei
Abstract: Environmental enrichment potentiates neural plasticity, enhancing acquisition and consolidation of memory traces. In the sensory cortices, after cortical circuit maturation and sensory function acquisition are completed, neural plasticity declines and the critical period ‘closes’. In the visual cortex, this process can be prevented by dark-rearing, and here we show that environmental enrichment can promote physiological maturation and consolidation of visual cortical connections in dark-reared rats, leading to critical period closure.

Presumably because environmental enrichment encourages cannibalisation of the proto-visual areas by other functions. Another tale of activity dependent neural development…