An evoked potentials (EPs) is an electrical manifestation of the brain's reception of and response to an external stimulus. The classification of visual evoked potentials (VEPs) depend primarily on stimulus characteristics. The visual content of a stimulus can be divided into two types: patterned and diffuse. VEPs to patterned stimuli are due mainly to the density of light and dark contrast borders; VEPs to diffuse light flashes are due to change in luminance only. Transitions between light and dark pattern elements have sharp borders in checkerboard patterns and in bar gratings. Those transitions can be represented by a spatial square wave. In sine wave gratings the change of luminance between light and dark stripes is gradual. The visual angle describes the size of the image of one light or dark element. The spatial frequency equals the number of repetitions of one light plus one dark element.