Multisubject learning for common spatial patterns in motor-imagery BCI

Abstract

Motor-imagery-based brain-computer interfaces (BCIs) commonly use the common spatial pattern filter (CSP) as preprocessing step before feature extraction and classification. The CSP method is a supervised algorithm and therefore needs subject-specific training data for calibration, which is very time consuming to collect. In order to reduce the amount of calibration data that is needed for a new subject, one can apply multitask (from now on called multisubject) machine learning techniques to the preprocessing phase. Here, the goal of multisubject learning is to learn a spatial filter for a new subject based on its own data and that of other subjects. This paper outlines the details of the multitask CSP algorithm and shows results on two data sets. In certain subjects a clear improvement can be seen, especially when the number of training trials is relatively low.

Figure 1

(a) shows the training set that is used to compute both the bCSP and clmtCSP filters. The data points themselves are not plotted, instead we only draw the standard deviation contours of the data's estimated covariance matrix, together with its corresponding principal vectors (representing the ellipse's principal axis). Blue and black contours correspond to the first class or condition, while green and red contours represent the other class. The goal of the computed filters is to align the principal vectors to the axes. The results for both bCSP and clmtCSP are shown in (b) and (c) figures, respectively. Here, the contours denote the standard deviations according to the estimated covariance matrix of the “unmixed” sources. Concerning the clmtCSP method, if the contours are drawn in blue and green, it means that they have been estimated as being in the first cluster according to the algorithm. If it is red and black, the task is estimated as belonging to the second cluster. The true cluster number is given in the title of each subplot.

Figure 1

(a) shows the training set that is used to compute both the bCSP and clmtCSP filters. The data points themselves are not plotted, instead we only draw the standard deviation contours of the data's estimated covariance matrix, together with its corresponding principal vectors (representing the ellipse's principal axis). Blue and black contours correspond to the first class or condition, while green and red contours represent the other class. The goal of the computed filters is to align the principal vectors to the axes. The results for both bCSP and clmtCSP are shown in (b) and (c) figures, respectively. Here, the contours denote the standard deviations according to the estimated covariance matrix of the “unmixed” sources. Concerning the clmtCSP method, if the contours are drawn in blue and green, it means that they have been estimated as being in the first cluster according to the algorithm. If it is red and black, the task is estimated as belonging to the second cluster. The true cluster number is given in the title of each subplot.

Figure 2

(a) compares the variance ratios of the bCSP solution with the clmtCSP solution on the first cluster, while (b) makes the comparison for tasks of the second cluster. The number above or below each pair of bars is the P  value according to the paired Wilcoxon signed rank test. The numeric suffix on the tick labels of the x-axis denotes the source number.

Figure 3

Cross-validation accuracies per parameter combination of λ ₁ and λ ₂ on the BCIC3 data set. We performed 5-fold cross-validation per subject. Averaging the result over all folds and all subjects gives the final result as plotted in the figure.