Right-and-left visual field stimulation：A frequency and space mixed coding method for SSVEP based brain-computer interface

On the condition of using limited frequencies, fewer targets could be presented in brain-computer interface （BCI） based on steady-state visual evoked potentials （SSVEP）. This paper proposes a novel coding method for SSVEP that, through a combination of frequency and spatial information, could increase the number of targets. Each target was composed of two flickers placed in the right-and-left visual fields. Given the role of the optic chiasm in the vision pathway, the two frequency components could be projected to contralateral occipital regions. Canonical correlation analysis （CCA） was utilized to detect the frequency components from the right or left visual cortex. Asymmetric index as a supplementary feature was also computed. Linear discriminant analysis （LDA） was used for target recognition. The attractive feature of this method is that it would substantially increase the number of targets. If the number of frequency was N, the method could present N times more targets than conventional SSVEP-based BCIs.