Via James, I have just come across this absolutely fascinating 10-minute talk.
It demonstrates that there is a vital and very early window of learning in which babies acquire the typical sounds of their native language - has to happen well within the first year of life.
But partway through the talk, there is a piece of research showing that audio / TV make virtually NO difference to learning new sounds, it HAS to be through direct human contact.
To me, a possible implication is that this connection of learning with the social part of the brain is that it may also be true for later learning.
Perhaps teachers cannot be replaced by computers, after all - despite this new project in the USA by a private company looking to get into educational provision.