ORIGINAL RESEARCH ARTICLE published: 14 October 2011 doi: 10.3389/fnins.2011.00112 Listen, you are writing! Speeding up online spelling with a dynamic auditory BCI Martijn Schreuder 1*, Thomas Rost 1,2 and Michael Tangermann1 1 Machine Learning Laboratory, Berlin Institute of Technology, Berlin, Germany 2 Bernstein Center for Computational Neuroscience, Berlin, Germany Edited by: Leonardo Cohen, National Institutes of Health, USA Reviewed by: Kenji Kansaku, Research Institute of National Rehabilitation Center for Persons with Disabilities, Japan Michal Lavidor, Bar Ilan University, Israel Surjo R. Soekadar, National Institutes of Health, USA *Correspondence: Martijn Schreuder , Machine Learning Laboratory, Berlin Institute of Technology, FR6-9, Franklinstraße 28/29, 10587 Berlin, Germany. e-mail: schreuder@tu-berlin.de Representing an intuitive spelling interface for brain–computer interfaces (BCI) in the audi- tory domain is not straight-forward. In consequence, all existing approaches based on event-related potentials (ERP) rely at least partially on a visual representation of the inter- face. This online study introduces an auditory spelling interface that eliminates the neces- sity for such a visualization. In up to two sessions, a group of healthy subjects (N = 21) was asked to use a text entry application, utilizing the spatial cues of the AMUSE paradigm (Audi- tory Multi-class Spatial ERP). The speller relies on the auditory sense both for stimulation and the core feedback. Without prior BCI experience, 76% of the participants were able to write a full sentence during the ﬁrst session. By exploiting the advantages of a newly intro- duced dynamic stopping method, a maximum writing speed of 1.41 char/min (7.55 bits/min) could be reached during the second session (average: 0.94 char/min, 5.26 bits/min). For the ﬁrst time, the presented work shows that an auditory BCI can reach performances sim- ilar to state-of-the-art visual BCIs based on covert attention. These results represent an important step toward a purely auditory BCI. Keywords: brain–computer interface, directional hearing, auditory event-related potentials, P300, N200, dynamic subtrials INTRODUCTION Recent successes have been booked in the application of brain– computer interface (BCI) technology at the end-user’s home, both for communication (Nijboer et al., 2008b; Sellers et al., 2010) and other purposes (Münssinger et al., 2010). BCIs allow for a direct connection between the brain and the external world. Interpretation of the brain signals is generally performed in real- time and users can thus interact with a device by changing their brain state. Apart from applications for healthy users (Nijholt et al., 2009; Tangermann et al., 2009; Blankertz et al., 2010), BCI technology was ultimately meant to allow otherwise para- lyzed people to communicate and interact with their environment again. Most of the aforementioned studies are based on visual event- related potentials (ERP). Their use in BCI was introduced by Farwell and Donchin (1988) and they have been a major focus in BCI research. This study describes a BCI that is based on audi- tory ERPs, which may extend the work on visual ERPs in several important aspects. In an exploratory study (Huggins et al., 2011), 61 people suf- fering from mid-stage amyotrophic lateral sclerosis (ALS) were included with ALSFRS-R ratings ranging from 18 to 33 (Cedar- baum et al., 1999). The authors found that a larger portion of their population suffered from auditory (42%) than visual (25%) deﬁcits. On the other hand, a recent study which followed a patient with ALS through the late stages of the disease, reported that a disease-resistant BCI may only be possible through the auditory or proprioceptive pathways (Murguialday et al., 2010). Indeed, sev- eral studies show that the traditional visual BCI paradigms may not function well when eye-gaze control is limited (Brunner et al., 2010; Treder and Blankertz, 2010). A user group with such het- erogeneous abilities and needs clearly stipulates the necessity for a range of diverse BCI systems. For these reasons, the interest in additional BCI paradigms has grown in recent years. Some studies report that by using a different interface, a visual ERP BCI can be realized that does not depend on eye-gaze con- trol (Acqualagna et al., 2010; Liu et al., 2011; Treder et al., 2011). Another approach could be to switch to a different sensory domain altogether, by using auditory- (Hinterberger et al., 2004; Kanoh et al., 2008; Nijboer et al., 2008a; Kim et al., 2011; Vlek et al., 2011) or vibrotactile cues (Cincotti et al., 2007; Brouwer and van Erp, 2010) for feedback or stimulation. Auditory stimulation usu- ally involves an oddball paradigm where the user is exposed to stimuli that differ from each other on some property. The user is required to focus attention to one of them. Such paradigms gener- ally involve tones with differences in pitch, resulting in a binary BCI (Hill et al., 2005; Halder et al., 2010). Multi-class approaches have been described using different environmental sounds (Klobassa et al., 2009) or spoken words or numbers (Sellers and Donchin, 2006; Furdea et al., 2009; Kübler et al., 2009; Guo et al., 2010). Though this increases the number of options per selection, pre- sentation of such stimuli is inherently slower. Furthermore, the mapping is often not intuitive, so that the matrix is still shown (albeit not ﬂashed). Of special interest is a recent study (Kübler et al., 2009), which describes a patient trial with the paradigm previously reported in (Furdea et al., 2009). Four subjects suffering from ALS received extensive training with the auditory BCI. Three of them were close www.frontiersin.org October 2011 | Volume 5 | Article 112 | 1