Implementation of a mental spelling system based on steady-state visual evoked potential (SSVEP)

In this study, we implemented a new mental spelling system based on steady-state visual evoked potential (SSVEP), adopting a QWERTY style layout keyboard with 30 LEDs flickering with different frequencies. During the offline experiments performed with five participants, we optimized various factors influencing the performance of the mental spelling system, such as distances between adjacent keys, light source arrangements, stimulating frequencies, recording electrodes, and visual angles. The online experiments were conducted with six participants to verify the feasibility of the optimized mental spelling system. The results of the online experiments were an average typing speed of 9.39 letters per minute (LPM) with an average success rate of 87.58 % corresponding to an average information transfer rate of 40.72 bits per minute, demonstrating the high performance of the developed mental spelling system. Indeed, the average typing speed of 9.39 LPM attained in this study was one of the best LPM results among those reported in previous BCI literatures. To further enhance the performance of our mental spelling system, we combined eye direction information (`left' or `right') extracted from a web camera with the SSVEP responses. As a result of the online experiments performed with 10 participants, the hybrid speller could reduce 16.6 typing errors on average.