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.
Brain cannot tell the difference
Studies show that processing ASL (or another signed language) activates the same linguistic regions of the left brain as spoken languages. That is, the brain cannot tell the difference between hands and lips, as renowned neuroscientist Dr. Petitto famously noted. That is, speech is not central to language.
Brain controls the same of language development
Research studies show that the milestones of language acquisition (L1) in sign language 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).
Sign language is not universal
Like speech language, sign language is not universal nor international, contrary to common belief among hearing speakers. Signed languages around the world are as distinct as, for example, English and Japanese.
Sign language is not gestural nor pictorial.
Sign language is not made up of a standardized system of gestures. Despite some iconicity (no different from speech as in iconic sounds or onomatopoeia), ASL signs are as abstract as any spoken languages.
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. Signed languages have their complexities.
Sign language is not a substitute of speech
Nor it is a signed version of 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.
You may also be interested in American Sign Language
Dictionary: sign language dictionary.