Towards Zero Training Figure 4. The feedback results for each of the six subjects. The feedback accuracy is denoted for the 100 trials of each run. The initial three runs, here marked as block ‘‘I’’, were done with the Zero-Training classifier, and in the following the order of the classifiers was randomly permuted in each block of two runs, here denoted as ‘‘II–V’’. The shift of the blue curve relative to the green curve within the shaded areas indicates the order of the classifiers within the block. doi:10.1371/journal.pone.0002967.g004 successful (‘‘hit’’) trials from each run. After the third run, the subjects could not know in advance, which one of the two classifiers (Zero-Training or ordinary CSP) was used for the generation of the feedback. For subjects zq, al and zk, the CSP feedback performed better than the Zero-Training feedback. In ay and aw, the feedback performance on the four blocks is very similar with both classifiers, whereas in subject zp, the Zero-Training feedback even outper- formed the CSP feedback. The performance over all subjects is shown in Fig. 5, where the feedback performance in each run of the four blocks is collected in a single boxplot for each classifier. The CSP performance is slightly higher on average, although this difference is not significant: a Wilcoxon ranking test yields a significance level of p = 0.05. B. Adaptation of the Classifier Bias The bias was updated at the beginning of every run. We can now check if this update was necessary for the accuracy of the classifiers. For run i and classifier j and movement class k, let mijk be the mean of the classifier output of the corresponding 50 trials. Then the value b^ij : ~ mij2{bijmij1 relates the optimal bias bij for run i and classifier j with the actual distance between the class means. A value of 1 would correspond to shifting the decision boundary by the entire inter-means distance. The results of this calculation are shown in Fig. 6. For most subjects, the required shift is moderate Figure 5. This figure shows the feedback performance of the CSP and the Zero-Training classifier over all subjects and runs. The median of the CSP feedback accuracy is slightly higher. This difference is not significant (Wilcoxon ranking test, p,0.05). doi:10.1371/journal.pone.0002967.g005 PLoS ONE | www.plosone.org 6 August 2008 | Volume 3 | Issue 8 | e2967