Previously researchers have proposed that the nature of speech itself is what drives the language acquisition process and milestones. If this is true, hearing children exclusively exposed to a signed language should have a language acquisition process quite different from that of children exposed to speech. However, in Petitto (1997) we see that hearing monolingual children exposed only to a signed language exhibit identical milestones to hearing children acquiring a spoken language.
Additionally, the stages and structure of babbling are the same in signed and spoken languages. Manual babbling is separate from general communicative gesturing exhibited by both deaf and hearing babies. Just like hearing, English-exposed babies produce vocal babbling of phonetic units (e.g., pa pa, ba ba, ma ma) that are distinct from other vocalizations, sign-exposed babies produce manual babbling of signed phonetic units that is distinct from general communicative gestures.This similarity raises the question, how do they have such strikingly similar syllabic structure despite the different modalities?
Evidence from the similarity in linguistic milestones and timing of language acquisition across modalities supports the idea that there is a genetic component underlying language acquisition that are not modality specific. Instead the genetic foundation of language is at the level of language patterning. The findings related to language acquisition milestones and babbling across modalities indicates that the brain cannot be genetically hardwired for spoken language only; rather, it detects the patterns of language, regardless of modality. This ability to recognize and produce language patterns, spoken or signed, suggests that there is a common mechanism operating in the language acquisition process.
In The Beginning: On the Genetic and Environmental Factors that Make Early Language Acquisition Possible. (1997) Petitto; Please see Petitto’s published papers and abstracts here.
Keywords: Babbling, Language Acquisition, Milestones, Bimodal Bilinguals, Genetic foundation of language