How It Works

Structure of the human eye. [1]

The figure above shows the structure of a human eye and the retina inside it.  The retina, which is similar to a camera's image sensor, is made up of millions of photoreceptors (pixels) called rods and cones.  The visual information recorded by these photoreceptors are collected by the optic nerve [2].  Since there are no photoreceptors at the head of the optic nerve, it forms a spot of no visual input.  Any image formed at this spot is not sensed.  While the size of this spot varies from individual to individual, it is actually rather huge - about 4° to 6°.  That means that if you close one eye, there is a hole in your visual world that is an inch or two across at arms length!

As you saw for yourself, the brain automatically "fills in" the blind spot with a simple extrapolation (extension) of the image surrounding the blind spot.  The white disc in the pictures above was filled in with the surrounding black background.  The sun was seamlessly replaced by the sky.  This is why you do not notice the blind spot in your day-to-day observations of the world, but as you might have noticed from examples above, the brain only fills it in with plausible "patterns" and not plausible "information."

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• Learn: Viewfinder and Eye: Human eye

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