The spot where your optic nerve connects to your retina is called the optic disc. There are no photoreceptor cells on this disc, so when an image hits that part of your retina, you can't see it. This is your blind spot. You don't notice this blind spot in every-day life, because your two eyes work together to cover it up. To find it, draw a filled-in, 1/4"-sized square and a circle three or four inches apart 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! Try again to find the blind spot for your other eye. Close your right eye and focus on the circle with your left eye. Move the paper until the square disappears.
What happened when the circle disappeared? Did you see nothing where the circle had been? No, when the circle disappeared, you saw a plain white background that matched the rest of the sheet of paper. This is because your brain "filled in" for the blind spot - your eye didn't send any information about that part of the paper, so the brain just made the "hole" match the rest. Try the experiment again on a piece of colored paper. When the circle disappears, the brain will fill in whatever color matches the rest of the paper.
The brain doesn't just match colored backgrounds. It can also make other changes to what you see. Try drawing two filled-in rectangles side by side with a circle in between them. A few inches 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 that happen? The circle in between the bars fell on your blind spot. When it disappeared, the brain filled in for the missing information by connecting the two bars!
Here is one final experiment with your blind spot. In this instance the brain doesn't match the blind spot with its immediate white background, but instead with the pattern surrounding it. 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 blue:
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!