I never knew, but Helen Keller had meningitis when she was 19 months old. Why is this so important? Because,
… she probably had 19 months of normal exposure to language … (by 18 months, some children start to express structured sentences, showing that they had been understanding them even earlier). So she probably soft-wired her brain for structured stuff like syntax before losing sight and sound.
This came up in a NY Times Book Review of a new book on Consciousness by the neurologist, Adam Zeman. The reviewer points out that Zeman overlooks Keller’s early exposure to language.
One of the fascinating things about language learning is that it shows how silly the usual “nature vs. nurture” debates are. By nature, children are predisposed to learn language, but the input must come from their environment. As the reviewer puts it,
Certainly, one of life’s major tragedies occurs when a child is not recognized as being deaf until well after the major windows of opportunity for soft-wiring the brain in early childhood have closed (much of structuring is not hard-wired instinct). We know how essential this tune-up period is for normal adult consciousness from the short-sentence, present-tense-only adult abilities of deaf children of hearing parents who failed to provide an environment during preschool years that was adequately rich in sign language. (Deaf children of signing deaf parents do fine.)
That, in my mind, is what is so unique about humans — we are instinctively programmed to be able to learn from our environment. But that ability to learn is not the same throughout the course of our life. Of course, we can still learn second languages later in life, but there are important differences between those who become bilingual as young children and those who learn their second language later in life. However, second language learning is quite different, since we already learned to speak one language.
UPDATE: It seems that Helen Keller makes this clear in her autobiography:
I am told that while I was still in long dresses I showed many signs of an eager, self-asserting disposition. Everything that I saw other people do I insisted upon imitating. At six months I could pipe out “How d’ye,” and one day I attracted every one’s attention by saying “Tea, tea, tea” quite plainly. Even after my illness I remembered one of the words I had learned in these early months. It was the word “water,” and I continued to make some sound for that word after all other speech was lost. I ceased making the sound “wah-wah” only when I learned to spell the word.