Optimal visual stimuli on LCD screens for SSVEP based brain-computer interfaces

In this work, different stimulation frequencies for steady-state visual evoked potentials (SSVEP) based brain-computer interface (BCI) were evaluated in a spelling task with the Bremen-BCI system. The classical two dimensional BCI control requires five classes: four classes are dedicated to the directions (up, down, left and right) and one class for action (select). The number of producible frequencies on the standard liquid crystal display (LCD) is limited due to the vertical refresh rate of 60 Hz and the number of simultaneously used stimuli. In order to find optimal stimulation frequencies, the Bremen-BCI was evaluated in the case study with 37 naive subjects (without any SSVEP-BCI experience), including eight handicapped users, at the international rehabilitation fair RehaCare2008. During online spelling task, subjects spelled few words with the Bremen-BCI system and the timings for classifying different flickering frequencies have been investigated. The fastest SSVEP response was achieved for the stimulus frequency of 6.67 Hz.