Babbling found in infants exposed to signlan from birth
Canonical babbling is the stage (7-10 months) in which babies produce identical, repetitive syllables which later develop into language.
In speech, babies produce repetitive consonant-vowel patterns. E.g. "dadada", "bababa". On the other hand, babies who are exposed to signlan, also produce reduplicated patterns. E.g. opening and closing hands repeatedly, moving hands up and down, etc.
Studies show that manual babbling occurs in deaf and hearing infants who are exposed to sign language from birth.
It is language-centered, not speech-centered, since language in the brain is amodal. That is, babbling is cross-modal.
Longitudinal studies also show the similarities of developmental patterns of language and communication in deaf and hearing --
from manual/vocal babbling
to manual/vocal gesture (prelinguistic communication)
to early lexical development to full-fledged language.
These similar milestones show an evidence that the development of language in the brain occurs regardless of which language and which modality is used (visually speaking / vocally speaking).
References and suggested readings
Petitto, Laura Ann and Marentette, Paula. "Babbling in the Manual Mode: Evidence for the Ontogeny of Language." Science Reports. Vol 251, 22 March 1991. Page 1493-1496.
Petitto, Dr. Laura Ann. Untitled. Montreal: McGill University media. October 28, 1997.