Abstract. In this paper, a method is proposed for using a simple neurophysiological brain response, the N400 potential, to determine a deeper underlying brain state. The goal is to construct a BCI that can determine what the user is `thinking about', where `thinking about' is defined as being primed on. The results indicate that a subject can prime himself on a physical object by actively thinking about it during the experiment, as opposed to being shown explicit priming stimuli. Probe words are presented that elicit an N400 response which amplitude is modulated by the associative relatedness of the probe word to the object the user has primed himself on.