[a-dub]

the existing state of the art is called the "p300 speller" and i believe it's been around for decades.

the user has a screen in front of them where the alphabet is cycled through at about 0.5 ch/s. scalp electrodes detect a signal associated with novelty or surprise allowing the user to spell out communications, very slowly.

edit: the cycling is in random order, thus exploiting the p300 surprise signal when the right character is highlighted. there's also some manipulation of spatial location to enhance the evocation of the signal.