Guessing What's on Your Mind: Using the N400 in Brain Computer Interfaces

A method is explored for using a simple neurophysiological brain responses to determine a deeper underlying brain state. The response chosen for the research is the N400 potential, a linguistical dectection method for priming. The goal is to construct a BCI that can determine what the user is `thinking about', where `thinking about' is de ned as being primed on. Existing N400 research primes the subject by showing an explicit priming stimulus, such as a word or picture, before showing a probe stimulus. The amplitude of the N400 potential, which is an ERP component occurring roughly 400ms after the presentation of the probe, is modulated by the semantic/associative relatedness of the prime and probe stimuli. Four experiments were performed which explore the possibility for achieving the N400 e ect without showing priming stimuli. Two experiments repeat earlier work, which uses conventional means of eliciting the N400 e ect. One experiment uses trials consisting of 7 words sentences where the last word was either semantically congruent with the rst 6 words or not. The other experiments used word pairs, of which the rst word acted as prime and the latter as a probe. The other two experiments are variations of the previous experiments. Instead of each trial consisting of a prime and a probe, one of two physical objects was given to the subject before the experiment, which the subject was instructed to think about. During the experiment itself, no more priming stimuli are presented, but multiple probe words are shown corresponding to both possible objects. 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. This opens the door to the possibility of BCI that can be controlled by simply thinking about a word. The nature of the N400 allows support for a very large vocabulary, as the probing scheme can perform a binary search.