infant "sign"

[Professor Annette Karmiloff-Smith]

I have been asked to find out what serious sceintific studies have
been done on the effects of teaching hearing infants to use "signs"
(I am not talking about the real language, ASL or BSL, but a list of
lexical signs taught to "advance infant communication skills" before
they are able to vocalise).

I'd appreciate refs, abstracts of refs, of scientific studies, as
well as any personal experiences.
Rather urgent please.
Many thanks, and HAPPY NEW YEAR TO ALL,
Annette K-S