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Mental workload is defined as the proportion of the information processing capability used to perform a task. High cognitive load requires additional resources to process information; this demand for additional resources may reduce the processing efficiency and performance. Therefore, the technique of workload estimation can ensure a proper working environment to promote the working efficiency of each person. In this paper, we propose a three-dimensional convolutional neural network (3D CNN) employing a multilevel feature fusion algorithm for mental workload estimation using electroencephalogram (EEG) signals. The 1D EEG signals are converted to 3D EEG images to enable the 3D CNN to learn the spectral and spatial information over the scalp. The multilevel feature fusion framework integrates local and global neuronal activities by workload tasks in the 3D CNN algorithm. Multilevel features are extracted in each layer of the 3D convolution operation and each multilevel feature is multiplied by a weighting factor, which determines the importance of the feature. The weighting factor is adaptively estimated for each EEG image by a backpropagation process. Furthermore, we generate subframes from each EEG image and propose a temporal attention technique based on the long short-term memory model (LSTM) to extract a significant subframe at each multilevel feature that is strongly correlated with task difficulty. To verify the performance of our network, we performed the Sternberg task to measure the mental workload of the participant, which was classified according to its difficulty as low or high workload condition. We showed that the difficulty of the workload was well designed, which was reflected in the behavior of the participant. Our network is trained on this dataset and the accuracy of our network is 90.8 %, which is better than that of conventional algorithms. We also evaluated our method using the public EEG dataset and achieved 93.9 % accuracy. INDEX TERMS Convolutional neural network, electroencephalogram (EEG), feature fusion, mental workload, working memory.

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In this paper, we propose dissimilarity regularization with a multistage fusion stream for a synthetic aperture radar (SAR) and infrared (IR) sensor fusion using deep learning. The multistage fusion structures are composed of multiple layers for fusing all the feature maps generated by the convolutional neural networks. The proposed structure combines feature maps of equivalent levels, ensuring that the spatial information of the corresponding levels can be utilized for fusion. Dissimilarity regularization is the sum of the normalized cross-correlation between the features generated in two different single-sensor streams. The proposed regularization is added to the conventional learning problem of a single-sensor stream, and each single-sensor stream is promoted to learn the disparate types of features for fusion. To evaluate the proposed algorithm, we compare the recognition rate of the proposed algorithm with that of the conventional fusion approaches using the SAR and IR image databases. Finally, the effects of the proposed architecture and regularization on the fusion result are analyzed.