Sign Language: what is and what is not
Sign language is a language used in visual-spatial modality. It is a natural language as sophisticated and complex as any speech language. It has been primarily developed and used by culturally Deaf people.
Studies show that processing ASL (or another signed language) activates the same regions of the left brain as spoken languages. This suggests that speech is not central to language.
Research studies show that the milestones of language acquisition (L1) in signlan is on the similar timeline as speech from babbling to two-word stage and beyond. (Petitto) This suggests that language development is maturationally controlled by the brain regardless of the modality (signlan or speech).
Like speech language, sign language is >not universal nor international. Signed languages around the world are as distinct as, for example, English and Japanese.
Sign language is not made up of a standardized system of gestures. Numerous lingustic studies show that sign languages have their own grammatical rules, syntax, phonology, morphology, and other linguistic features similar to that spoken languages have.
Sign language is not a substitute of speech language nor it is a signed version of a speech language. It stands in its own. Ameslan (American Sign Language) and Auslan (Australian Sign Language), for example, are not signed versions of English nor are they based on English.