Reliable visual stimuli on LCD screens for SSVEP based BCI

A Brain-Computer interface (BCI) enables a direct communication between human and computers by analyzing brain activity. When a visual stimulus with a constant frequency is presented to the user, it is possible to observe a continuous brain response at the visual cortical area. This response is called the Steady-State Visual Evoked Potential (SSVEP) and it can be used for BCI. This paper deals with the methods for creating reliable visual stimuli on LCD screens to evoke SSVEP responses and how to compare them. Three techniques are proposed and compared for the production of stimuli: LEDs, a LCD screen using timers, and a LCD screen using the vertical refresh rate for synchronizing the visual stimuli. The comparison is based on the offline classification of five SSVEP responses. The different visual stimuli were tested with ten subjects. The visual stimuli on the LCD screen based on its vertical refresh rate offer the best recognition rate for the classification of SSVEP responses. The mean accuracy was improved of about 5% thanks to this strategy.