That is a good question! I could not come up with anything using my limited intellectual ability. So I checked google.
http://www.straightdope.com/columns/031226.html
Quote:
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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."
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I doubt that the straightdope would work as a source, but that gives you an idea at least.