Modified pattern-reversal visual checkerboard stimuli with dual alternating frequencies for multi-class ssvep-based brain-computer interfaces

In this study, a new dual-frequency stimulation method that can produce more visual stimuli with limited number of stimulation frequencies was proposed for use in multiclass SSVEP-based BCI systems. The new stimulation was based on a conventional black-white checkerboard pattern; however, unlike the conventional approach, ten visual stimuli could be generated by combining four different stimulation frequencies. Through the offline experiments, we confirmed that all ten visual stimuli could evoke distinct and discriminable single SSVEP peaks, of which the SNRs were high enough to be used for practical SSVEP-based BCI systems. In order to demonstrate the feasibility of the proposed method, a mental keypad system was implemented and online experiments were conducted with additional ten participants, from which we achieved an average information transfer rate of 33.26 bits per minute and an average accuracy of 87.23 %.