Locked-in-syndrome (...) The patients retain consciousness, but are not able to communicate (de-efferent state), which can be verified via electrophysiological methods. In some cases residual functions of the optico-motoricity are still present, e.g. vertical movements of the eyeballs, which can be enlisted in elaborating a code toward communicating with the outside world.

(Handbuch der Intensivmedizin, Georg Thieme Verlag)