Each of your eyes has a blind spot on the retina. This blind spot is where the optic nerve connects to the retina. Watch this video to find your blind spot. The reason you cover one of your eyes is to prevent that eye from compensating for the blind spot in the other eye. Without that compensation, your mind then fills in the blind spot with surrounding visual stimulus. In other words, if you used a white 3 x 5 card then you noticed that the dot disappeared into the white of the 3 x 5 card. Video by Dayton Koons Voice by Emme Packer