**1.3. Steady-state visual evoked potential**

Regan experimented with long trains of stimuli that comprised of sinusoidally modulated monochromatic light [8]. Small amplitude stable VEP were generated which were entitled "steady-state" visually evoked potentials (SSVEPs) of the human visionary system. There hence, steady-state visual evoked potentials (SSVEPs) are defined as the potential elicited by the change in the visual field with the frequency higher than 6 Hz.

When a user is presented with some periodic stimuli, SSVEP is generated strongly at the occipital areas of the brain [10]. SSVEP is usually acquired from various electrode sites like Oz, O1, O2, Pz, P3, P4, and some surrounding locations to occipital region. While the most commonly used SSVEP frequency range is 4–60 Hz, the resonance phenomenon is generally observed around 10, 20, 40 and 80 Hz [11].
