The spot where your optic nerve connects to your retina is called the optic disc, also commonly referred to as the blind spot. There are no photoreceptor cells on this disc, so when an image falls on this part of the retina, it is not detected. You don't notice this blind spot in everyday life, because your two eyes work together to cover it up.

To find it, draw a coloured, square and a circle 6 or 7 cm apart, as shown on the right, on a piece of white paper.

Hold the paper at arm's length and close your left eye.   Focus on the square with your right eye, and slowly move the paper toward you. When the circle reaches your blind spot, it will disappear! Although the red circle disappeared your brain filled in that space with the background, which happened to be white. Since the brain received no image from the eye it improvised and filled in the gaps with what it assumed should be there. Try this again with a different coloured paper or one with patterns.

 

The brain doesn't just match colored backgrounds but can also make other changes to what you see. Try drawing two coloured rectangles side by side with a circle in between them, as shown on the right. About 6 cm to the right of this, draw a square.

Close your right eye and focus your left eye on the square.   Move the paper until the circle disappears and the two separated bars become one bar. How did the brain compensate when the square reached the blind spot?  

 

Draw a line down the center of your page.   On one side draw a small square and on the other draw rows of circles.  Color the center circle red and all the others green as shown on the right.

Close your left eye and look at the square with your right eye.   As you move the paper, the red circle should disappear and be replaced by a blue one!

In this instance the brain doesn't match the blind spot with its immediate white background, but instead with the pattern surrounding it.