Various convolutional neural network (CNN)-based approaches have been recently proposed to improve the performance of motor imagery based-brain-computer interfaces (BCIs). However, the classification accuracy of CNNs is compromised when target data are distorted. Specifically for motor imagery electroencephalogram (EEG), the measured signals, even from the same person, are not consistent and can be significantly distorted. To overcome these limitations, we propose to apply a capsule network (CapsNet) for learning various properties of EEG signals, thereby achieving better and more robust performance than previous CNN methods. The proposed CapsNet-based framework classifies the two-class motor imagery, namely right-hand and left-hand movements. The motor imagery EEG signals are first transformed into 2D images using the short-time Fourier transform (STFT) algorithm and then used for training and testing the capsule network. The performance of the proposed framework was evaluated on the BCI competition IV 2b dataset. The proposed framework outperformed state-of-the-art CNN-based methods and various conventional machine learning approaches. The experimental results demonstrate the feasibility of the proposed approach for classification of motor imagery EEG signals.
Detecting a user's intentions is critical in human-computer interactions. Recently, braincomputer interfaces (BCIs) have been extensively studied to facilitate more accurate detection and prediction of the user's intentions. Specifically, various deep learning approaches have been applied to the BCIs for decoding the user's intent from motor-imagery electroencephalography (EEG) signals. However, their ability to capture the important features of an EEG signal remains limited, resulting in the deterioration of performance. In this paper, we propose a multi-layer temporal pyramid pooling approach to improve the performance of motor imagery-based BCIs. The proposed scheme introduces the application of multilayer multiscale pooling and fusion methods to capture various features of an EEG signal, which can be easily integrated into modern convolutional neural networks (CNNs). The experimental results based on the BCI competition IV dataset indicate that the CNN architectures with the proposed multilayer pyramid pooling method enhance classification performance compared to the original networks.
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