The following paper has been published in an SCI-journal (Brain Research).
Han-Jeong Hwang, Dong Hwan Kim, Chang-Hee Han, and Chang-Hwan Im, "A new dual-frequency stimulation method to increase the number of visual stimuli for multi-class SSVEP-based brain-computer interface (BCI),"
Brain Research, vol. 1515, pp. 66-77, 2013.