Guessing What’s on Your Mind: Using the N400 in Brain Computer Interfaces Marijn van Vliet, Christian M¨ uhl, Boris Reuderink, and Mannes Poel University of Twente, Human Media Interaction, Enschede 7522 NB, NL, w.m.vanvliet@student.utwente.nl Abstract. In this paper, a method is proposed for using a simple neuro- physiological 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 pre- sented 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. 1 Introduction Brain Computer Interfaces (BCI) are devices that let a user control a computer program, without any physical movement. A BCI measures the activity within the brain directly, interprets it and sends a control signal to a computer. By actively or passively changing his brain activity, the user can send different control signals and by doing so, operate the system. The effectiveness of a BCI depends highly on the ability to measure relevant processes within the brain and the performance of the classification of the signal. 1.1 Low level brain responses versus high level cognitive processes Without significant progress in recording and signal processing technology, brain- computer interfaces (BCI) that rely on electroencephalography (EEG) record- ings only have access to basic, neurophysiological responses. Examples include the P300 response, event related (de)synchronization (ERD/ERS) and steady state visually evoked potentials (SSVEP). These directly measurable phenom- ena can be regarded as low level responses. These are manifestations of higher level cognitive processes that are more complex and cannot be directly measured, such as object recognition, intention of movement and visual processing. When measuring the low level responses, most information about specifics of the higher level processing is lost. However, the low level responses can give insight into the higher level brain processes by using probes in a search scheme. By using probes, different possibilities for the high level brain state can be tested until the correct one is found.