The purpose of a language is for the instantiation of meaning in communication. Verbal languages commmunicate meaning through phonics, and over time, one's sub conscious mind will associate these sounds and meanings without one knowing. This is why people hear their natural language inside their heads. Sign language is no different to other languages, it just uses gestures to convey meaning rather than sounds. The brain would conjure up these gestures in ones head as thoughts pass, just the same as spoken words. It all depends on how long someone has been using sign language. The brain would need a certain amount of exposure before it begins to make these sub-conscious associations.

Not only do they think in sign language but they dream in sign too. This is only true if signing was learned in infancy and it became the only language they knew. ASL (American Sign Language) isn't like English at all and in fact it's more like Chinese. One sign/gesture can mean a whole phrase. There is PSL for Pakistan, NSL for Nicaragua and so on. Sadly, it’s thought that one in every thousand babies is born with a hearing loss and about one in two thousand become members of the Deaf community and use sign language. The World Federation of the Deaf was formed in 1951 and is active in 127 countries. www.wfdeaf.org There's also a European Union of the Deaf, which represents Deaf interests in the EU and has membership in all of the countries of the EU. www.eudnet.org

sign language!

The language of the deaf is a vast topic that has filled lots of books—one of the best is Seeing Voices: A Journey Into the World of the Deaf by Oliver Sacks (1989). All I can do in this venue is sketch out a few basic propositions: The folks at issue here are both (a) profoundly and (b) prelingually deaf. If you don't become totally deaf until after you've acquired language, your problems are . . . well, not minor, but manageable. You think in whatever spoken language you've learned. Given some commonsense accommodation during schooling, you'll progress normally intellectually. Depending on circumstances you may be able to speak and lip-read. About one child in a thousand, however, is born with no ability to hear whatsoever... The profoundly, prelingually deaf can and do acquire language; it's just gestural rather than verbal. The sign language most commonly used in the U.S. is American Sign Language, sometimes called Ameslan or just Sign. Those not conversant in Sign may suppose that it's an invented form of communication like Esperanto or Morse code. It's not. It's an independent natural language, evolved by ordinary people and transmitted culturally from one generation to the next. It bears no relationship to English and in some ways is more similar to Chinese—a single highly inflected gesture can convey an entire word or phrase. (Signed English, in which you'll sometimes see words spelled out one letter at a time, is a completely different animal.) Sign can be acquired effortlessly in early childhood—and by anyone, not just the deaf (e.g., hearing children of deaf parents). Those who do so use it as fluently as most Americans speak English. Sign equips native users with the ability to manipulate symbols, grasp abstractions, and actively acquire and process knowledge—in short, to think, in the full human sense of the term... The answer to your question is now obvious. In what language do the profoundly deaf think? Why, in Sign (or the local equivalent), assuming they were fortunate enough to have learned it in infancy. The hearing can have only a general idea what this is like—the gulf between spoken and visual language is far greater than that between, say, English and Russian. Research suggests that the brain of a native deaf signer is organized differently from that of a hearing person. Still, sometimes we can get a glimpse. Sacks writes of a visit to the island of Martha's Vineyard, where hereditary deafness was endemic for more than 250 years and a community of signers, most of whom hear normally, still flourishes. He met a woman in her 90s who would sometimes slip into a reverie, her hands moving constantly. According to her daughter, she was thinking in Sign. "Even in sleep, I was further informed, the old lady might sketch fragmentary signs on the counterpane," Sacks writes. "She was dreaming in Sign." http://www.languagehat.com/archives/001054.php

Helen Keller, ( according to some biographies ) did not think, just reacted, until the concept of language was introduced, from that point on she understood the world around her in the language of her peers. Our inborn ability to conceptionalize functions with the experience of our life and our ability to communicate with others give expression to "language" verbal, manual, or body language.

I think it's a combination of things. For example, as a native English speaker, I do not always think in English, or even in words. Neither do you, I'd be willing to bet. Have you ever said something that didn't come out the way you meant it? What about not being able to think of the right words to say what you're thinking? If you were thinking in words, that could never happen -- you'd just use the same words that were in your brain. We don't think in language all the time -- language is something we developed in order to help us communicate with one another.

I am deaf myself, yes I do think in sign language, and in words. sometimes hear myself.

I think people think in ideas and emotions which are translated into whatever language that person speaks for communication purposes.