Schreuder et al. Listen, you are writing! caveats itself, as up–down localization may suffer from similar In a recent study it was shown that auditory distraction has no problems as front–back localization. effect on BCI systems based on mental tasks Friedrich et al. (2011). It is not clear if this holds true for auditory ERP BCI systems, as DYNAMIC STOPPING METHOD interaction in the same modality occurs. The level of distraction Increase in performance due to any dynamic stopping method could depend on the relative loudness and the user’s ability to is promising but largely neglected in literature. Though several concentrate. That this is not necessarily a problem became obvi- methods have been introduced before (Serby et al., 2005; Lenhardt ous when subject VPkw helped us out in a demonstration for et al., 2008; Zhang et al., 2008; Liu et al., 2010), they seldom ﬁnd the local television. She performed a near-perfect spelling session, their way into online studies other than for the purpose of val- whilst TV cameras were being set up in the room and the crew was idating the methods themselves. With an early decision in 41% talking. of all trials and a false positive rate of 5%, we succeeded in con- Though not explicitly tested here, the current setup with added structing a conservative yet effective method for dynamic stopping. pitch cues may be particularly suited for patients that are practiced The method is loosely based on Lenhardt et al. (2008). However, musicians, as their improved tone hearing can increase the beneﬁt instead of one global threshold, subject- and iteration-speciﬁc from these pitch cues (Micheyl et al., 2006). thresholds are used. By taking a relative conﬁdence measure (p), the method is robust against biases. The results show again that FUTURE RECOMMENDATIONS dynamic stopping can signiﬁcantly increase performance. Although the AMUSE paradigm was now embedded in a speller Performance in traditional visual ERP application is generally system, it may proof useful in other domains. For instance, the good even at a low number of iterations. However, things get more speaker distribution on the azimuth plane could provide a user difﬁcult when overt attention is impossible (Brunner et al., 2010; with an intuitive set of commands for spatial navigation. Also, by Treder and Blankertz, 2010), where performance breaks down and including secondary ERP signals such as the ErrP (Schalk et al., a quality check as provided by a dynamic stopping method may 2000; Schmidt et al., 2011) for post hoc analysis of a decision, the be desirable. The same can be said for auditory paradigms, where system could be expected to increase in usability. covert attention is the standard and the resulting ERPs are less The SOA of 175 ms used in this study was not optimized. Any strong. Dynamic stopping methods should thus ﬁnd their way SOA represents a trade-off between single-subtrial SNR and the into BCI research and applications. number of repetitions needed. On the one hand, each subtrial Here a decision is enforced after 15 iterations, but an additional is expected to contribute more class-discriminant information beneﬁt of dynamic stopping methods could be to refrain from with a longer SOA. This can be explained by changed average any decision if no threshold is surpassed. It might prove useful amplitudes of target components, e.g., increased target P3 and to allow for trials were no decisions is made, to implement a no- N2, due to larger target-to-target distances (Gonsalvez and Polich, control state or to simply refrain from taking uncertain decisions. 2002; Allison and Pineda, 2006) and longer inter-stimulus inter- This could be particularly useful in long-term use of a system, vals (Polich et al., 1991). But even other non-target ERPs can have where at times the user may be in a no-control state (Huggins an inﬂuence on the classiﬁcation accuracy, e.g., the sensitivity of et al., 2011). N1 and P2 to SOA and target-to-target distances (Budd et al., 1998; Allison and Pineda, 2006). On the other hand, the BCI sys- TOWARD A PURELY AUDITORY MODALITY tem agglomerates evidence over time by repeated presentation Most auditory BCI spellers described in literature depend to a of the stimuli. With a fast SOA, more repetitions can be per- certain extend on the visual modality for presenting a helping formed within the same time, which compensates for the lower matrix (Furdea et al., 2009; Klobassa et al., 2009; Kübler et al., 2009; information content in each individual subtrial. Due to increased Höhne et al., 2010; Belitski et al., 2011) to the user. By designing overlap in fast paradigms, this interaction has to be considered an intuitive interface, presentations of such a (static) matrix was carefully. As the best SOA value is probably subject dependent, not necessary in the current study. Users could easily learn the an individualization for patients or other long-term users seems required steps for writing a letter. Sentence 1 and 2b were writ- advisable. ten without any visual feedback other than the writing progress. Investigations of the basic ERP characteristics of ALS patients Although this requires some additional time to present to the user report a decrease of amplitude not only for the P3(a) (Silvoni et al., his current position in the spelling tree, it represents a new step 2009) component, but also for the N1 and MMN (N2) components toward a purely auditory BCI. Though not presented here, it is (Raggi et al., 2008) in auditory and visual oddball paradigms. An worth noting that the system has been used successfully by a blind increase of P3 latency has also been described (Hanagasi et al., collaborator to spell a sentence, using exclusively the auditory cues. 2002; Paulus et al., 2002), even in direct correlation with ALS sever- Of course, a completely auditory BCI is not always required. ity and months from disease onset (Raggi et al., 2008). Still, reports Residual eye control may not sufﬁce for a visual BCI, but could on ERP based BCI systems for patients are encouraging. Silvoni be good enough for presenting a static matrix. Furthermore, there et al. (2009) found no correlation between clinical data and BCI are cases imaginable where auditory stimulation may be prefer- performance for 21 early and mid-stage ALS patients in an audi- able over visual ones even when the visual modality is in tact. For tory ERP BCI, even though the P3 amplitude was reduced when instance, auditory stimuli may be easier to ignore and less intru- compared to healthy controls. Furthermore, several applications sive when the user is engaged in a (primary) visual task such as for ALS patients have been published that show the feasibility of reading. visual and auditory ERP paradigms (Sellers and Donchin, 2006; Frontiers in Neuroscience | Neuroprosthetics October 2011 | Volume 5 | Article 112 | 10