For a fascinating look at the relationship between deafness, sign,
speech and
language, see
Oliver Sacks' book
Seeing Voices. He studies the evolution of
ASL, contrasting it with the much less evocative and flexible
Signed English. The latter was devised by hearing people, and there was oppression of ASL as 'ethnic' almost, where deaf children were forced to use Signed English.
Perhaps the most powerful passage in the book is one where Sacks is on Martha's Vineyard to study indigenous sign, due to the genetic traits that resulted in a high percentage of deaf residents of the island. He comes across an old woman sleeping in her chair on her porch. Her hands are twitching, and after a moment he realizes she is signing in her sleep - she's dreaming in MVSL.