How does Stephen Hawking speak and write?

The Professor has managed to write books and give lectures despite his disease

The Professor who is world renowned, has a progressive disease called ALS (amyotrophic lateral sclerosis) that has gradually paralysed him over the years, yet despite this he is still able to give lectures on a global stage and write dozens of books.

75 year old Hawking is able to do this using a single cheek muscle which is attached to a computer to communicate simply by tensing it. At the start of his disease he had a hand held clicker that spelled out words, he now uses the cheek based device that moves a cursor to find the first letter of the word, after typing a few letters sophisticated predictive text technology which suggests the best-fit words.

The professor has previously tested eye-tracking technology and a system controlled by moving his eyebrows but he found the cheek movement sensor the most effective for him.