Simultaneous

• Spoken languages use sounds more or less one after another to create words, with tone, volume and facial expression adding to the meaning. But sign languages are visual, with interpreters simultaneously using facial expression and posture as well as their hands to communicate meaning.
  
  

Natural

• Since the 1960s research on sign languages has revealed that they are natural languages in the same way as spoken languages and must be analyzed with the same theoretical tools. This has spurred interest in signing among linguists and psychologists, and a lot of research has been done on the grammar, processing and acquisition of various sign languages, plus work on related cognitive areas such as memory.
  
  It is now widely held that sign language research provides a unique opportunity to study the human mind and human language. Linguists and psycholinguists increasingly use sign language as an arbiter for or against hypotheses on the nature of language and the mind.
  
  

Understanding

• Recent research on signing deaf people and nonsigning speakers has led to more detailed understanding of how the brain processes different aspects of language in general and whether the visual nature of signing affects where language is processed.
  
  The evidence supports the theory that sign languages are largely processed in language processing areas rather than speech processing areas. There is also evidence that deaf people who suffer strokes experience the same types of language processing issues as users of spoken language.
  
  

Dialects

• Like spoken languages, sign languages have different dialects, depending on the location of schools for the deaf. In the United States, for instance, there are 115 universities and colleges teaching American Sign Language (ASL), and according to the dialect theory local dialects will influence the way students learn ASL.
  
  

Theories

• Although signs have been used for centuries for communication by and with deaf people, the development of sign language theory and practice has only taken place in the last two centuries. Much work has been done in recent decades to turn theories of interpretation into practical value.
  
  A theory currently being studied concerns the system of English "mouthing" by interpreters. This silent visual representation by a signer is done concurrently with hand signs and is found particularly among bilingual interpreters who use ASL and spoken English. It is distinct from other systems of grammatical mouthing.