Online classification of imagined speech using functional near-infrared spectroscopy signals

Most brain-computer interfaces (BCIs) based on functional near-infrared spectroscopy (fNIRS) require that users perform mental tasks such as motor imagery, mental arithmetic, or music imagery to convey a message or to answer simple yes or no questions. These cognitive tasks usually have no direct association with the communicative intent, which makes them difficult for users to perform. In this paper, a 3-class intuitive BCI is presented which enables users to directly answer yes or no questions by covertly rehearsing the word 'yes' or 'no' for 15 s. The BCI also admits an equivalent duration of unconstrained rest which constitutes the third discernable task. Twelve participants each completed one offline block and six online blocks over the course of 2 sessions. The mean value of the change in oxygenated hemoglobin concentration during a trial was calculated for each channel and used to train a regularized linear discriminant analysis (RLDA) classifier. By the final online block, 9 out of 12 participants were performing above chance (p<0.001), with a 3-class accuracy of 83.8+9.4%. Even when considering all participants, the average online 3-class accuracy over the last 3 blocks was 64.1+20.6%, with only 3 participants scoring below chance (p<0.001). For most participants, channels in the left temporal and temporoparietal cortex provided the most discriminative information. To our knowledge, this is the first report of an online fNIRS 3-class imagined speech BCI. Our findings suggest that imagined speech can be used as a reliable activation task for selected users for the development of more intuitive BCIs for communication.