How the blind spot works

Every human eye has a small region on the retina called the optic disc - the point where the optic nerve exits the eye and where blood vessels enter. Because there are no photoreceptors (rods or cones) at this location, light that falls on the optic disc produces no visual signal. The result is a blind spot in your visual field at roughly 15° from the center of your vision in each eye.

The brain fills in the gap automatically using information from the surrounding retinal area (a process called perceptual interpolation), which is why you don't normally experience a black hole in your vision.

Discovery of the blind spot

The blind spot was first described by French physicist Edme Mariotte in 1660. He noticed that a small bright spot placed at a specific position in the visual field of his test subjects seemed to vanish. In France, the blind spot is still sometimes called la tache de Mariotte (Mariotte's spot) in his honor.

How to find your blind spot

Close your left eye and focus your right eye on the fixation cross (+).
Hold the screen about 25 cm (10 inches) from your face.
Slowly move the screen closer or farther until the dot on the right disappears.
At that distance, the dot is falling on your optic disc - your blind spot.

To test your left eye, close your right eye and focus on the dot; the cross should disappear at the correct distance.

Why don't we normally notice it?

Two reasons make the blind spot invisible in everyday life:

Binocular vision: With both eyes open, the blind spot of each eye falls in a region that the other eye can see. The two visual fields overlap, completely covering the gap.
Perceptual filling-in: Even with one eye closed, the visual cortex interpolates a seamless continuation of the surrounding texture or color into the blind spot region. This is the same mechanism that makes the dot disappear rather than leaving a black void.