Moms and Dads are the number one source of brain growth for the infant if, only they will pick their babies up and talk to them a lot.
Happily, most families can afford a human face. So all that is left to do is to pick the child up frequently during the day (and night as needed) and talk to him/her a lot. If you are not sure about it, error on the high side of frequently and a lot.
Some parents (baby sitters and/or day care professionals) are overjoyed when a baby spends long hours in a crib without complaining.
That frees time to watch TV, do house cleaning, or attend to other children who are complaining.
But for the "quiet" child, irretrievable windows (minutes) of opportunity for neural growth may be slipping away for ever. We have seen how in the first six months brain growth can be measured in milligrams per minute.
We have seen how children deprived of stimulation (touch) alone have less brain tissue than those that are touched.