The blind spot is a part of the retina where there are no photoreceptors. To demonstrate its existence to yourself, close your right eye, look at the + sign below with your left eye, then move your head toward or away from the screen slowly while continuing to watch the + sign. The big black dot will disappear as it passes through the blind spot of the retina of your left eye.

Thus there is a portion of your field of vision that you would expect to experience as missing. The reason this does not happen is that your brain fills in the blind spot with the colour and texture of the area surrounding it. In the above experiment, the black dot was replaced with the white background of this Web page. The following example works exactly the same way but is even more striking, because your brain fills in the break in the line.

If your visual cortex is capable of filling in the image in your blind spot in this way, then chances are good that it does the same thing throughout your field of vision. Consequently, what you are aware of seeing may not be exactly what is actually being imprinted on your retina, as if it were just a simple piece of film. Instead, what you are seeing may already have had several "special effects" added.