Mind Hacks has an interesting post about locked-in-syndrome, "where people are completely paralyzed despite having intact minds."
Some people with locked-in-syndrome can communicate by blinking or moving their eyes. Jean-Dominique Bauby wrote a book called The Diving Bell and the Butterfly by having someone sit next to him and slowly recite the alphabet. Bauby would blink his left eye when the person said the letter he needed.
But others are so locked-in they can't even blink. A recent article in New Scientist describes a 46-year-old woman who was totally locked-in. Her doctor, Dr. Birbaumer, wanted to ask her if she'd like to have electrodes implanted in her brain to help her communicate, but they couldn't figure out a way for the woman to answer.
[W]alking past an electronics store one day, Birbaumer's colleague Barbara Wilhelm spotted a medical device for measuring the pH of saliva, and had an idea. They trained the woman to change the acidity of her spit by imagining either the taste of lemon, or the taste of milk. She learned to push the pH one way to say "yes", the other to say "no".
Let me recommend to you I Raise My Eyes to Say Yes by Ruth Sienkiewicz-Mercer. She wrote a whole book with binary responses to large cardboard word boards.
Ruth died in 1998. She was discussing writing a murder mystery, her favorite flavor, right before her death. The sleuth was going to be paraplegic, like herself, with the hopped-up observational skills paraplegics tend to have. It would have been a great book.