Review of the BCI Competition IV

Michael Tangermann, Klaus-Robert Müller, Ad Aertsen, Niels Birbaumer, Christoph Braun, Clemens Brunner, Robert Leeb, Carsten Mehring, Kai J Miller, Gernot R Müller-Putz, Guido Nolte, Gert Pfurtscheller, Hubert Preissl, Gerwin Schalk, Alois Schlögl, Carmen Vidaurre, Stephan Waldert, Benjamin Blankertz, Michael Tangermann, Klaus-Robert Müller, Ad Aertsen, Niels Birbaumer, Christoph Braun, Clemens Brunner, Robert Leeb, Carsten Mehring, Kai J Miller, Gernot R Müller-Putz, Guido Nolte, Gert Pfurtscheller, Hubert Preissl, Gerwin Schalk, Alois Schlögl, Carmen Vidaurre, Stephan Waldert, Benjamin Blankertz

Abstract

The BCI competition IV stands in the tradition of prior BCI competitions that aim to provide high quality neuroscientific data for open access to the scientific community. As experienced already in prior competitions not only scientists from the narrow field of BCI compete, but scholars with a broad variety of backgrounds and nationalities. They include high specialists as well as students. The goals of all BCI competitions have always been to challenge with respect to novel paradigms and complex data. We report on the following challenges: (1) asynchronous data, (2) synthetic, (3) multi-class continuous data, (4) session-to-session transfer, (5) directionally modulated MEG, (6) finger movements recorded by ECoG. As after past competitions, our hope is that winning entries may enhance the analysis methods of future BCIs.

Keywords: BCI; brain-computer interface; competition.

Figures

Figure 1
Figure 1
Citations of the overview articles on previous competitions. The histogram shows how many times the editorial articles on BCI competitions I (Sajda et al., 2003), II (Blankertz et al., 2004), and III (Blankertz et al., 2006) have been cited in ISI-indexed journals. Data were retrieved from the ISI Web of Knowledge on December 1st 2011.
Figure 2
Figure 2
Citations of the articles by the competition winners. The histogram shows how many times the articles of the winning teams of BCI competition II (describing the winning algorithms) have been cited in ISI-indexed journals. Data were retrieved from the ISI Web of Knowledge on December 1st 2011.
Figure 3
Figure 3
(Data set 1 – trial structure). Training data was collected in the calibration runs. Arrows pointing left, right, or down have been presented as cues for imagining left hand, right hand, or foot movements. After a fixation cross was presented for 2 s, the directional cue was overlaid for 4 s. Then the screen was blank for 2 s. In the test runs used for evaluation, spoken words have been presented as cues.
Figure 4
Figure 4
(Data set 1 – glance at the neurophysiology). The first row displays the averaged spectra of the two chosen motor imagery tasks (red: left hand, green: right hand; blue: foot) in the training data. A selected subject-specific frequency band is shaded in gray. The second row shows the average amplitude envelope of that frequency band with 0 being the time point of cue presentation. The time interval which was used to calculate the spectra shown above is shaded. The bottommost row displays the (signed) r2-difference in log band-power between the individually chosen motor imagery tasks as scalp maps. Band-power was calculated in the frequency band that is shown in the topmost row and averaged across the time interval that is indicated in the middle row. Three of those seven data sets have been artificially generated, see main text.
Figure 5
Figure 5
(Data set 1 – histogram of results). Performance of the first five ranked submissions is shown in terms of their mean squared error (MSE) wrt. the true labels. Only results for the real (i.e., not artificially generated) data sets are shown. The mean across the four data sets is plotted as a horizontal red line. The MSE for constant prediction output of 0 are 0.507, 0.515, 0.491, 0.524 for data sets a, b, f, g, respectively.
Figure 6
Figure 6
(Data set 1 – distribution of classifier outputs). These (normalized) histograms display the distribution of the classifier outputs of the winning algorithm. Each row corresponds to one data set (a, b, f, g). The left column is a histogram for those time points in which the true label is −1, for the middle column it is 0 (no control), and for the right column it is 1. The true label is indicated by the blue triangle.
Figure 7
Figure 7
(Data set 1 – trace of classifier outputs). The labels of the true mental state are displayed in blue. The red line shows the classifier outputs of the winning algorithm. This example is a selected segment of 100 s taken from data set g in which the classification is quite successful. The MSE in the shown segment is 0.171.
Figure 8
Figure 8
(Data Set 1 – artificial). Left: spectra of the signal at all channel locations for the two conditions, eyes open and eyes closed. Right: scalp plot of the signal power at 10 Hz for the two conditions (eyes open and closed). The actual noise of the artificial data varied linearly in time between both conditions, depending of the task of the BCI user.
Figure 9
Figure 9
(Data Set 1 – artificial). Example of fast and slow baseline drifts that are observable in unfiltered BCI competition IV data. The figure depicts the time course of the amplitude of the EEG in one channel.
Figure 10
Figure 10
(Data Set 1 – artificial). Location and direction of selected dipoles in the head.
Figure 11
Figure 11
(Data Set 1 – artificial). Power topographies at the μ rhythm frequency generated by the dipoles.
Figure 12
Figure 12
(Data Set 1 – artificial). Spectra of the EEG signal in two discriminative channels. The discriminability between the classes is shown at the bottom of each spectrum. This Figure illustrates an example of asymmetry of the μ rhythm peak in each hemisphere. Also the harmonic of the μ rhythm is observable in the beta band.
Figure 13
Figure 13
(Data Set 1 – artificial). Top row: scalp plots of one eye movement and one eye blink generated for the synthetic EEG data sets of the BCI competition IV. Bottom row: corresponding time course of eye movements and blinks.
Figure 14
Figure 14
(Data Set 1 – artificial). Linear regression of the rank position (left) and performance of the method (right). The x-axis corresponds to the results submitted for the real EEG data sets, whereas y-axis corresponds to those of the synthetic EEG.
Figure 15
Figure 15
(Data Set 2a). Timing scheme of one session.
Figure 16
Figure 16
(Data Set 2a). Timing scheme of the paradigm.
Figure 17
Figure 17
(Data Set 2a). Left: electrode montage corresponding to the international 10–20 system. Right: electrode montage of the three monopolar EOG channels.
Figure 18
Figure 18
(Data Set 2b). Timing scheme of one session (for screening and feedback sessions).
Figure 19
Figure 19
(Data Set 2b). Electrode montage of the three monopolar EOG channels.
Figure 20
Figure 20
(Data Set 2b). Timing scheme of the paradigm. (A) The first two sessions (01T, 02T) contain training data without feedback, and (B) the last three sessions (03T, 04E, 05E) with smiley feedback.
Figure 21
Figure 21
(Data Set 3). Schematic overview of different recording techniques for BMIs (from Waldert et al., with permission).
Figure 22
Figure 22
(Data Set 3). Time course of a trial with time constraints (from Waldert et al., with permission).
Figure 23
Figure 23
(Data Set 3). Results of the BCI competition IV and, for comparison, the average result of applying a RLDA and linear SVM to the low-pass filtered and resampled activity of the data.
Figure 24
Figure 24
(Data Set 4). The ECoG signals in train_data (time, channel) and test_data (time, channel) were acquired from each electrode with respect to a scalp reference and ground before re-referencing with respect to the common average.
Figure 25
Figure 25
(Data Set 4 – event-related potential). Illustration that the characteristic changes in the power spectral density changes with activity are not due to an reproducible event-related potential shift (ERP). Two adjacent electrodes are shown in (A). One has an ERP, and one does not, but both have the characteristic peri-movement spectral changes. (B) Individual (gray) and averaged thumb movement (black, left) or index finger movement (black, right), locked to the first movement from the appropriate movement cue. (C) The normalized power spectral density (“PSD”) as a function of time. It demonstrates the classic spectral changes just prior to movement onset for both thumb and index finger. Note that the decrease in power at lower frequencies (α/β/μ range), and the increase in power at higher frequencies (above about 40 Hz) both begin before movement onset. (D) Individual and averaged raw potential traces around each of the first movements from appropriate thumb or index finger movement epochs. There is no significant event-related potential (ERP) effect for thumb, but there is for the index finger.
Figure 26
Figure 26
(Data Set 4). Time courses of finger flexion, broadband, LMP, and the raw electric potential. The LMP (Schalk et al., ; Kubanek et al., 2009) has been shown to hold information about different motor behaviors. Spectrally broadband change, corresponding to 1/f type change in the electric potential power spectrum (Miller et al., 2009a,b), can be captured as another powerful correlate of motor behavior. By synthesizing different features, more powerful brain-computer interfacing algorithms may be obtained.
Figure 27
Figure 27
(Data Set 4). Examples of the normalized power spectral density (PSD) of the potential time series around finger flexion. The PSD was calculated from 1 s windows centered at times of maximum flexion and also during rest. (A) Mean PSD of index finger movement samples (light trace) and rest samples (black trace). (B) Average time-varying PSD (scaled as percentage of mean power at each frequency) with respect to first index finger movement from each movement cue.
Figure 28
Figure 28
(Data Set 4). Cortical activation maps for movement of different fingers in one subject. The changes in power between 126 and 150 Hz are focused in the classic hand area of the brain. The spatial distribution for 76–100 Hz are nearly identical, as might be expected since both are reflections of the broadband feature highlighted in recent literature (Miller et al., 2009b). Low frequency changes are spatially much more broad, corresponding to fluctuations in the classic motor rhythms. Figure 29 shows that the spatial representations for high frequencies are very different for different finger movement types, within a general hand region. Electrode positions are shown with white dots, and power change with light and dark gray patches on the brain surface.
Figure 29
Figure 29
(Data Set 4). A blow-up of the sensorimotor region for high frequencies from Figure 28. Note that this variability across electrodes allows for robust segregation of different finger movements during classification.
Figure 30
Figure 30
(Data Set 4). Time course of ECoG in adjacent electrodes reveals individual digit representation. (A) X-ray of the ECoG array in situ, with three electrodes labeled, corresponding to the numbers in (C). (B) Flexion time course of each finger. (C) Projections of the time-frequency representation to broadband spectral change (Miller et al., 2009b). Each electrode is specifically and strongly correlated with one movement type (r = 0.46 for broadband from electrode 1 with thumb position; r = 0.47 for electrode 2 with index finger; r = 0.29 for electrode 3 with little finger; cross-combinations had a mean correlation of −0.09, indicating light hyperextension of other fingers while flexing the appropriate finger in this subject), over 10 min of continuous data (3.6 × 106 samples).

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