How come flashing lights cause seizures?

Photoparoxysmal resp: Photoparoxysmal responses (ppr) is an abnormal visual sensitivity of the brain in reaction to intermittent photic stimulation. Photosensitivity depends on the intensity, the contrast of the visual stimulus, and the specific frequency of flashing. The flashing patterns of certain games and television shows trigger epileptic seizures in only 5% of epileptics. Photosensitive epileptics peak at 10-14.