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."