Touch a pencil with all five fingertips of one hand, close your eyes, and ponder: If each fingertip forms a distinct tactile "image" of the pencil, why do you perceive a single pencil instead of five unconnected pencil fragments?
The answer is that your brain has special circuits that help you build complete pictures from fragmented sensory information. In effect, these gap-filling circuits induce your brain to perceive what it expects to see, instead of what it actually sees. When these expectations accurately reflect the objective world around you, your perceptions will be on target. Sometimes, however, what your brain expects to see is far from an accurate representation of reality.
Cross the index and middle fingers of your dominant hand as far as you can, forcing them apart with your other hand so that you can place your nose between the outside surfaces of the crossed fingers. Then close ...
