Filling in, and the moments you are blind

When the line you are looking at runs straight through the blind spot, you do not see a broken line. You see one continuous line. When the background is a texture, the texture appears to continue. This is called filling-in (or perceptual completion): the brain creates the missing part from the surroundings, so the hole stays invisible.

That means what you "see" at your blind spot is made up, not recorded. It is a very good guess most of the time, which is why nobody notices. It would be a bad guess if something small and different were hiding exactly there.

Your eyes also make about three fast jumps every second. These jumps are called saccades. During a big saccade the image sweeps across the retina far too fast to read, and visual sensitivity is turned down (saccadic suppression). The brain does not show you the smear and does not leave a dark gap. It skips the interval, which is why the world seems steady while your eyes jump.

Try it: look in a mirror at your left eye, then your right eye, then back. You will never see your own eyes move, because the jump itself is edited out.

There is a third thing hiding in plain sight: only the centre of your gaze is sharp. The sharpest region, the fovea, covers only about 2 degrees of vision, roughly the width of your thumbnail at arm's length. Outside it, detail drops quickly, and colour and fine print fade. You feel as if the whole scene is crisp because your eyes jump to whatever you ask about, so the sharp spot is always there when needed.

Common myth: "Our whole field of view is equally sharp." It is not. The sharp area is tiny, and your eyes move so quickly that you get a sense of a detailed whole.