Interesting. You mean there comes a moment of epiphany for the babies later when something "clicks" and they go "AH-HAH!! So THAT'S what I've been saying!"? Are you sure? Just wondering.
The lipreading and the babbling are developmental steps toward the combined skills of the mechanics and the understanding for language. I just read an article about this in the last few months, and I am trying to find it again. I've found these instead:
Researchers have found that they can identify the language base of a baby's family before the baby learns to talk:
Here they can tell from the babbling what language a child has been
exposed to after only five hours.
there's a definite difference in the babbling sounds between babies who speak different languages once exposed to speech a very short while.
Most of the studies I've read on babbling are focused on hearing babies. It would be cool to see similar studies for deaf babies from signing families, and it looks like that work has been done and collected in the book [ame="http://www.amazon.com/gp/product/0743247132/ref=as_li_ss_tl?ie=UTF8&tag=cmasonideas-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=0743247132"]Talking Hands, What Sign Language Reveals About the Mind[/ame]. Here's a quote from about page 65 to 66:
“The babbling has a linguistic purpose. At first, babies produce a wide range of speech sounds, including those not found in the language of the home…. Over time, as the infant gains more exposure to her mother tongue, these ‘foreign’ sounds disappear, and her babbling falls in line with those sounds actually present in the adult language around her. Linguists regard the babbling stage as a kind of extended phonological rehearsal… in which, from the set of all possible speech sounds, the infant gradually homes in on the particularly subset she’ll actually need. As it turns out, exactly the same thing happens with deaf infants exposed to sign as a first language, only here the babbling is manual. Using their hands and fingers, these deaf babies rehearse a wide range of ‘nonsense’ gestures. Over time, they unconsciously zero in on the set of meaningful gestures in the particular sign language to which they are exposed. Like the verbal babbling of hearing babies, the manual babbling of deaf babies, seemingly random at first, gradually tailors itself fo the linguistic demands of the babbler’s native language.”
[in this case, sign].
Both hearing and deaf babies babble about the same way at first. Gradually deaf babies quit the oral babbling, but if signed to, they continue to develop language the same way and the same pace (except I think faster in the first stages in particular), nonsense babbling (or so it appears), more definite babbling (manual or oral), and then words- as all human infants in the entire world. It's really pretty cool.
I think several things happen- they understand what they want to say, and they lipread to figure out the mechanics of how to say what they want to say. it's obvious babies understand far more than they can actually say- My two year old son could not speak, but if we asked him which of his cars was the biggest, the smallest, the second largest, the yellow one, etc. he knew and would show us. Deaf babies sign their first word months before hearing babies speak their first word, because sign is easier for a baby to do than forming a word orally. How do they learn the mechanics of saying/signing the word if not by watching to see how it is done?
That's what I do as an adult- when my friend is teaching me a new sign I copy her. She laughs and says no, and signs it again, and I watch her closely, copy her again.
I did this in my Japanese class, too, lipread carefully to figure out how to make my mouth form the sounds.
They also have parents imitating what the baby says, and then often say the word the babbling most sounds like, "Oh, you said baba, do you mean you want the bottle?" or whatever. The baby watches the parent's face closely, and collects information- and words with understanding follow- signed or spoken.
I'm not really sure what your point of disagreement is? Can you explain your reasons for thinking lipreading has nothing to do with the development of speech? Babies watch the mouths of speaking people carefully, and they watch the hands of signing people carefully. Both are useful for developing communication skills.