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Figure 1.

The experimental setup.

For the physiological experiments, all eight speakers were used. For the BCI experiments, only the front semi-circle was used (speakers 1,2,3,7, and 8).

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Table 1.

Cue properties in BCI experimental round.

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Table 2.

Rejection rates for all conditions.

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Figure 2.

Averaged positive waveforms (condition C1000).

Only the channel with the highest positive ROC value between 300 and 650 ms is presented here. The shaded interval indicates the area where this highest ROC value was found. Intervals were handpicked. Scalp topographies in Figure 3 are taken from this interval. Horizontal black bars mark the time of stimulus presentation.

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Figure 3.

Scalp topographies for the P300 interval (condition C1000).

Scalp topographies indicate the average potential over the interval marked in Figure 2. ROC plots do not necessarily indicate the magnitude of the difference between the two curves, but rather the significance of that difference. For most subjects this is concentrated over the parietal area. Each row corresponds to a different subject. Note that not all subjects have the same number of electrodes available.

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Table 3.

P300 waveform characteristics.

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Figure 4.

Scalp topographies for the negative deflections (condition C1000).

Scalp topographies indicate the average potential over the interval marked in Figure 5. ROC plots do not necessarily indicate the magnitude of the difference between the two curves, but rather the significance of that difference. For most subjects this is concentrated over the frontal and temporal area. Each row corresponds to a different subject. Note that not all subjects have the same number of electrodes available.

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Table 4.

Subject performances for the key response task.

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Figure 5.

Averaged negative waveforms (condition C1000).

Only the channel with the largest negative ROC value between 100 and 300 ms is presented here. The shaded interval indicates the area where this largest negative ROC value was found. Intervals were handpicked. Scalp topographies in Figure 4 are taken from this interval. Horizontal black bars mark the time of stimulus presentation.

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Figure 6.

Averaged waveforms of all subjects and conditions from the second experimental round.

Only the channel with the highest ROC value between classes is presented here. All ERPs in the left column come from condition C300. The middle column represents condition C175 and images in the right column are taken from condition C300s. Every row represents a subject. The shaded area in condition C175 marks the high ROC interval that is used for scalp topographies in Figure 7. Horizontal black bars mark the time of stimulus presentation.

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Figure 7.

Scalp topographies for the P300 interval (condition C175).

Scalp topographies indicate the average potential over the interval marked in the second column of Figure 6. ROC plots do not necessarily indicate the magnitude of the difference between the two curves, but rather the significance of that difference. The area where the high ROC values are concentrated has shifted to the frontal area as compared to Figure 3. Each row corresponds to a different subject. Note that not all subjects have the same number of electrodes available.

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Figure 8.

Distribution of channels selected for classification in various conditions over all subjects.

Scalp topographies indicate the prevalence of selection of different channels during the cross validation steps averaged over all subjects of a particular condition. Negative values indicate channels selected for negative ROC values, positive values indicate channels selected for positive ROC values. Values have been normalized to the maximum possible occurrences (nr subjects x nr of crossvalidation folds). The frontal cross indicates the Fz channel, the posterior cross indicates the Pz channel. Channels with negative ROC values are consistently selected from the frontal regions. For condition C1000, the channels with positive ROC values are concentrated over the parietal- and occipital areas, whereas for the faster conditions these are more diffuse.

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Table 5.

Speaker loudness.

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Figure 9.

Polar sensitivity plot for condition Cr.

The confusion matrices for condition Cr of all subjects are summed and represented as sensitivity plot. The black line indicates the sensitivity at each speaker location. Direction confusion is represented by the green (neighboring direction) and red (other) arrows. The length of the arrow indicates the amount of error in that direction. Speaker 1 (front) and 5 (back) are difficult to distinguish, as can be seen from their exclusive confusion. Direction labels correspond to those in Figure 1.

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Table 6.

Averaged performance for different directions (condition C1000).

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Figure 10.

Subject performances for all conditions.

The left column shows the selection scores and the right column shows the corresponding ITR. Both are plotted as a function of the number of iterations. Each rows indicates a different condition. The horizontal line in the left column indicates the 70% threshold. Although we do not report on the classification on condition C1000, we have included the figures here for comparison. They have only 10 iterations, as no more stimuli were presented in the physiological experimental round. Accuracy scores for condition C1000 increase faster for than for the other conditions. However, because of the long ISI (1000 ms) the ITR is relatively low.

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Table 7.

Classification performance for all BCI conditions.

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