Schreuder et al. Listen, you are writing! to, or in the locked-in state (LIS, only residual eye-movement), whereas the forth was entering the totally LIS (TLIS, no residual eye-movement). Though all four subjects had control over a visual BCI, they performed poorly on the auditory BCI. The authors acknowledge however the importance of the auditory modality for BCI. One of their suggestions for improvement is a spatial distribution of the stimuli. Several studies indeed show that using stimuli with spatial fea- tures can improve performance (Schreuder et al., 2009; Höhne et al., 2010). In ofﬂine studies, adding spatial information to an auditory stimulus was beneﬁcial to the recognition, both expressed in reaction times or accuracy (Belitski et al., 2011) and classiﬁca- tion performance (Schreuder et al., 2010). Furthermore, it allows for an intuitive multi-class auditory BCI without sacriﬁcing speed. Such paradigms could even be used to extend traditional (visual) BCIs to improve performance or facilitate training during the late stages of ALS (Belitski et al., 2011). The current study is an extension of Schreuder et al. (2010), where the principle of a paradigm using spatial auditory stimuli was ﬁrst shown (it was later called Auditory Multi-class Spatial ERP, or AMUSE). Here, AMUSE drives a spelling interface in online mode. Using six spatial locations and a two-step, hex-o- spell like speller interface (Blankertz et al., 2006), subjects were enabled to write by attending tones in space. It is shown here that AMUSE, combined with the speller inter- face, allows for reliable online spelling for a majority of healthy subjects. The proposed speller interface is intuitive and easy to learn. Its core components (stimulation, spelling tree navigation, and result feedback) are realized in the auditory domain. Average spelling performance was high, out-performing current auditory BCIs. Furthermore, a new method for dynamically changing the number of stimuli was introduced. Based on the data gathered, a trial could thus either be stopped if a threshold was reached, or continued. This method further increased the performance signiﬁcantly by reducing trial length. MATERIALS AND METHODS PARTICIPANTS Participants were 21 subjects that had never worked with a BCI before (BCI naïve). They reported no current or prior neuro- logical disorder and normal hearing. The latter was not formally tested. Subjects were ﬁnancially compensated for their participa- tion. Age ranged from 20 to 57 (m = 34.1, SD = 11.4). Procedures were approved by the Ethics Committee of the Charité Univer- sity Hospital. All subjects provided verbal and written informed consent and subsequent analysis and presentation of data was anonymized. DATA ACQUISITION EEG was recorded using a ﬁxed set of 56 Ag/AgCl electrodes and BrainAmp ampliﬁers (Brain Products, Munich, Germany). Chan- nels were referenced to the nose. Electrooculogram (EOG) was co-recorded with two bipolar channels. All impedances were kept below 15 kΩ. Only for subject VPfce did the impedance exceeded this threshold at the end of the session. The signals were sam- pled at 1 kHz and ﬁltered by a hardware analog band-pass ﬁlter between 0.1 and 250 Hz before being digitized and stored for ofﬂine analyses. For online use, the signal was low-pass ﬁltered below 40 Hz, down sampled to 100 Hz and streamed to the online Berlin BCI system. The stimulus presentation, the online Berlin BCI system and the ofﬂine analyses were implemented in Matlab (MathWorks), mak- ing use of the Psychophysics Toolbox (Brainard, 1997) for mul- tichannel audio presentation and the open-source text-to-speech system Mary (Schröder and Trouvain, 2003). A multichannel, low- latency ﬁrewire soundcard from M-Audio (M-Audio FireWire 410) was used to individually control the active, off-the-shelf computer speakers (type Sony SRS-A201). PARAMETER SCREENING A dependence of ERP amplitude and latency on stimulus ampli- tude and duration, particularly in the early components, has previ- ously been reported (Gonsalvez et al., 2007). To investigate this for the AMUSE paradigm, two parameter screenings were conducted, testing for the inﬂuence of stimulus loudness and duration on classiﬁcation performance. The studies were performed with six and seven healthy subjects, respectively. Results of both the parameter screening did not expose a systematic inﬂuence on the classiﬁcation performance. Also, the subjectively reported favorite condition could not be matched with the condition giving the highest score. Parameters were therefore ﬁxed to those used in Schreuder et al. (2010; ∼58 dB and 40 ms). STIMULI The localization of a stimulus in the azimuth plane depends mainly on two principles, interaural timing differences (ITD) and inter- aural level differences (ILD), which both have their own optimal frequency range (Middlebrooks and Green, 1991). Where the ITD effect is most efﬁcient for frequencies up to 1–1.3 KHz, the ILD effect provides the localization information for higher frequencies, roughly >3 KHz. Ofﬂine analyses showed that spatial location as a discriminative feature is enough to reliably elicit a P3 response (Schreuder et al., 2009, 2010). However, to optimally exploit both effects, stimuli consisted of a low frequent tone with harmonics and a high frequent, band-pass ﬁltered noise overlay (see Table 1). To further increase resolution, each of the six directions was asso- ciated with a unique combination of tone and noise to add an additional cue property. Stimuli can be found in Data Sheet 1 of the supplementary data. Table 1 | Stimulus properties. Direction Base tone (Hz) Noise range (KHz) 1 2 3 4 5 6 762 528 1099 635 915 440 3.6–9.2 3.2–8.0 4.0–10.5 3.4–8.6 3.8–9.8 3.0–7.5 The stimulus of each direction (see Figure 1) was a complex of a tone (base frequency and harmonics) and noise. Frontiers in Neuroscience | Neuroprosthetics October 2011 | Volume 5 | Article 112 | 2