Dynamic time warping-based transfer learning for improving common spatial patterns in brain-computer interface

Objective. Common spatial patterns (CSP) is a prominent feature extraction algorithm in motor imagery (MI)-based brain-computer interfaces (BCIs). However, CSP is computed using sample-based covariance-matrix estimation. Hence, its performance deteriorates if the number of training trials is small. To address this problem, this paper proposes a novel regularized covariance matrix estimation framework for CSP (i.e. DTW-RCSP) based on dynamic time warping (DTW) and transfer learning. Approach. The proposed framework combines the subject-specific covariance matrix (Σss) estimated using the few available trials from the new subject, with a novelDTW-based transferred covariance matrix (ΣDTW) estimated using previous subjects' trials. In the proposedΣDTW, the available labelled trials from the previous subjects are temporally aligned to the average of the few available trials of the new subject from the same class using DTW. This alignment aims to reduce temporal variations and non-stationarities between previous subjects trials and the available few trials from the new subjects. Moreover, to tackle the problem of regularization parameter selection when only few trials are available for training, an online method is proposed, where the best regularization parameter is selected based on the confidence scores of the trained classifier on upcoming first few labelled testing trials. Main results. The proposed framework is evaluated on two datasets against two baseline algorithms. The obtained results reveal that DTW-RCSP significantly outperformed the baseline algorithms at various testing scenarios, particularly, when only a few trials are available for training. Significance. Impressively, our results show that successful BCI interactions could be achieved with a calibration session as small as only one trial per class.