Time-varying and simultaneous frequency stimulation for multi-class SSVEP-based brain-computer interface

In recent years, there has been increasing interest in using steady-state visual evoked potentials (SSVEP) in brain-computer interface (BCI) systems for their high signal to noise ratio. However, due to the limitations of brain physiology and the refresh rate of the display devices, the available stimulation frequencies that evoke strong SSVEPs are limited. The goal of this paper is to investigate time-varying and simultaneous frequency stimulation in order to increase the number of visual stimuli with a fixed number of stimulation frequencies in multiclass SSVEP-based BCI systems. This study analyzes the SSVEPs induced by groups of light-emitting diodes (LEDs). The proposed method produces more selections than the number of stimulation frequencies through an efficient combination of time-varying and simultaneous frequencies for stimulation. The feasibility and effectiveness of our proposed method was confirmed by a set of experiments conducted on six subjects. The results confirmed that our proposed stimulation is a promising method to increase the number of stimuli using a fixed number of frequencies for multi-class SSVEP-based BCI tasks.