A novel motor imagery hybrid brain computer interface using EEG and functional transcranial Doppler ultrasound

•We introduce a novel hybrid BCI that uses EEG and fTCD as brain sensing modalities.•Flickering mental rotation and word generation mental tasks were employed.•Features derived from EEG and fTCD power spectrum were calculated.•Mutual information and SVM were used for feature selection and classification.•The proposed hybrid BCI outperforms EEG-fNIRS BCIs in literature in terms of speed. Hybrid brain computer interfaces (BCIs) combining multiple brain imaging modalities have been proposed recently to boost the performance of single modality BCIs. In this paper, we propose a novel motor imagery (MI) hybrid BCI that uses electrical brain activity recorded using Electroencephalography (EEG) as well as cerebral blood flow velocity measured using functional transcranial Doppler ultrasound (fTCD). Features derived from the power spectrum for both EEG and fTCD signals were calculated. Mutual information and linear support vector machines (SVM) were employed for feature selection and classification. Using the EEG-fTCD combination, average accuracies of 88.33%, 89.48%, and 82.38% were achieved for right arm MI versus baseline, left arm MI versus baseline, and right arm MI versus left arm MI respectively. Compared to performance measures obtained using EEG only, the hybrid system provided significant improvement in terms of accuracy by 4.48%, 5.36%, and 4.76% respectively. In addition, average transmission rates of 4.17, 5.45, and 10.57 bits/min were achieved for right arm MI versus baseline, left arm MI versus baseline, and right arm MI versus left arm MI respectively. Compared to EEG-fNIRS hybrid BCIs in literature, we achieved similar or higher accuracies with shorter task duration. The proposed hybrid system is a promising candidate for real-time BCI applications.