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Most brain-computer interfaces (BCIs) based on functional near-infraredspectroscopy (fNIRS) require that users perform mental tasks such as motorimagery, mental arithmetic, or music imagery to convey a message or to answersimple yes or no questions. These cognitive tasks usually have no directassociation with the communicative intent, which makes them difficult for usersto perform. In this paper, a 3-class intuitive BCI is presented which enablesusers 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 ofunconstrained rest which constitutes the third discernable task. Twelveparticipants each completed one offline block and six online blocks over thecourse of 2 sessions. The mean value of the change in oxygenated hemoglobinconcentration during a trial was calculated for each channel and used to traina regularized linear discriminant analysis (RLDA) classifier. By the finalonline 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%, withonly 3 participants scoring below chance (p<0.001). For most participants,channels in the left temporal and temporoparietal cortex provided the mostdiscriminative information. To our knowledge, this is the first report of anonline fNIRS 3-class imagined speech BCI. Our findings suggest that imaginedspeech can be used as a reliable activation task for selected users for thedevelopment of more intuitive BCIs for communication.

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