Thanks to a pointer from Kristy Cooper, I've followed links to this interesting page on brain plasticity and language acquisition for deaf children.

http://gazeshift.blogspot.com.au/p/blog-page_6.html?spref=fb

"Traditionally, the term neurotoxicity refers to damage to the brain or nervous system caused by exposure to natural or man made substances.  However, I think we need not limit causes of damage to tangible substances.  We can think of language deprivation as a type of poison.  We now have evidence that brain restructuring, altered processing, and decreased cognitive performance are measurable effects of lack of access and the resultant deprivation.  Therefore the behaviors that lead to this end are toxic, and the lack of access is the toxin."