If a person is born deaf, what language do they think in?

Sign language. They really picture themselves signing in their head, much like when non-deaf people think in their own voice.

I took ASL and asked my teacher about this. She says that actually, they don't think words like we do. They think things as images, smells, feelings, etc., because they most likely weren't introduced to words. They know concepts, not words.

I guess what ever language they read in?