  The N400 is an Event Related Potential that is evoked in response to
conceptually meaningful stimuli. It is for instance more negative in response
to incongruent than congruent words in a sentence, and more negative for
unrelated than related words following a prime word. This sensitivity to
semantic content of a stimulus in relation to the mental context of an
individual makes it a signal of interest for Brain Computer Interfaces. Given
this potential it is notable that the BCI literature exploiting the N400 is
limited. We identify three existing application areas: (1) exploiting the
semantic processing of faces to enhance matrix speller performance, (2)
detecting language processing in patients with Disorders of Consciousness, and
(3) using semantic stimuli to probe what is on a user's mind. Drawing on
studies from these application areas, we illustrate that the N400 can
successfully be exploited for BCI purposes, but that the signal-to-noise ratio
is a limiting factor, with signal strength also varying strongly across
subjects. Furthermore, we put findings in context of the general N400
literature, noting open questions and identifying opportunities for further
research.
