Schreuder et al. Listen, you are writing! FIGURE 5 | Early stopping performance. For (A), data is collapsed over all subjects that participated in Session 2. The number of correct and incorrect trials are plotted as a function of the iteration in which a trial was stopped. Accuracy is high in general, and particularly for those trials that were stopped before the 15th iteration. The same data is plotted in (B), but separately for each subject and for sentences 2a and 2b. The black markers indicate the minimum number of selections necessary for writing a sentence. Subject VPfch did not perform sentence 2b. Table 3 | Grand-average confusion matrix. Selected class Target class PPV 1 2 3 4 5 6 Sensitivity N 1 765 2 18 3 24 4 24 5 12 6 44 86.2 9 881 18 26 8 84 85.9 72 26 543 15 10 66 74.2 16 29 12 629 12 71 81.8 27 37 41 54 539 59 71.2 23 72 9 54 7 773 82.4 83.9 82.9 83.9 78.4 91.7 70.5 912 1063 647 802 588 1097 Confusion matrix of classiﬁcation results over all subjects. Sensitivity, positive predictive value (PPV), and the occurrence (N) of each target are given for convenience. and bilateral auditory cortical areas. The later positive compo- nent is focused over the central mid-line electrodes and toward the temporal areas. This is in line with previous ofﬂine results (Schreuder et al., 2010). It can be noted that in the manual feature extraction procedure, different intervals can be assessed based on this physiological plausibility before being included for online use. DIRECTIONAL PREFERENCE The grand-average CM can be found in Table 3. Figure 7 dis- plays matrix F of the pairwise F -scores, where high values indicate a good separability between the indicated classes. Diagonal val- ues are irrelevant and marked X. Other elements are marked as location neighbors (l), sound neighbors with 1.5 tone pitch differ- ence (s) and locations that are symmetric in the front–back plane (fb); all are expected to hamper separability. Tone locations were optimized such that no entry has two such labels. Directions without any degrading factor had an average pair- wise F -score of 0.89 (SD: 0.15). This was only slightly degraded for spatial neighbors (0.87, SD: 0.16), but both pitch neigh- bors (0.82, SD: 0.19) and especially the front–back confusion (0.76, SD: 0.22) had a larger effect on pairwise discriminabil- ity. These effects were investigated by performing pairwise t -tests FIGURE 6 | Grand-average physiology. Calibration data of the ﬁrst session is collapsed over subjects and trials. Average time series for channel Cz are plotted together with the positions of ﬁve consecutive stimuli (A). Scalp potential distributions for target and non-target trials are shown in (B) and distributions of class-discriminative information in (C). Two intervals were hand-picked based on the signed AUC values, and are represented by the shaded gray areas. Early negative components over frontal and temporal regions are followed by a positive component over centro-parietal areas. Frontiers in Neuroscience | Neuroprosthetics October 2011 | Volume 5 | Article 112 | 8