title: Beyond maximum speed--a novel two-stimulus paradigm for brain-computer interfaces based on event-related potentials (P300-BCI).
abstract: OBJECTIVE
  The speed of brain-computer interfaces (BCI), based on event-related potentials (ERP), is inherently limited by the commonly used one-stimulus paradigm. In this paper, we introduce a novel paradigm that can increase the spelling speed by a factor of 2, thereby extending the one-stimulus paradigm to a two-stimulus paradigm. Two different stimuli (a face and a symbol) are presented at the same time, superimposed on different characters and ERPs are classified using a multi-class classifier. Here, we present the proof-of-principle that is achieved with healthy participants.
  
  
  METHODS
  Eight participants were confronted with the novel two-stimulus paradigm and, for comparison, with two one-stimulus paradigms that used either one of the stimuli. Classification accuracies (percentage of correctly predicted letters) and elicited ERPs from the three paradigms were compared in a comprehensive offline analysis.
  
  
  RESULTS
  The accuracies slightly decreased with the novel system compared to the established one-stimulus face paradigm. However, the use of two stimuli allowed for spelling at twice the maximum speed of the one-stimulus paradigms, and participants still achieved an average accuracy of 81.25%. This study introduced an alternative way of increasing the spelling speed in ERP-BCIs and illustrated that ERP-BCIs may not yet have reached their speed limit. Future research is needed in order to improve the reliability of the novel approach, as some participants displayed reduced accuracies. Furthermore, a comparison to the most recent BCI systems with individually adjusted, rapid stimulus timing is needed to draw conclusions about the practical relevance of the proposed paradigm.
  
  
  CONCLUSIONS
  We introduced a novel two-stimulus paradigm that might be of high value for users who have reached the speed limit with the current one-stimulus ERP-BCI systems.
authors: Department of Psychology I, University of Würzburg, Marcusstr. 9-11, 97070 Würzburg, Germany. Institute of Clinical Medicine, University of Oslo, Kirkeveien 166, 0450 Oslo, Norway.
authors: Tobias
authors: T
authors: Kaufmann
authors: Andrea
authors: A
authors: Kübler
mesh_terms: D000328:Adult; D062207:Brain-Computer Interfaces; D004562:Electrocardiography / Q000379:methods*; D018913:Event-Related Potentials, P300 / Q000502:physiology*; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D010775:Photic Stimulation / Q000379:methods*; D012107:Research Design; D013647:Task Performance and Analysis; D014796:Visual Perception / Q000502:physiology*; D014956:Writing; D055815:Young Adult