A further complication is what is known as 'inattential blindness': we don't see things because we are too busy concentraiting on something else. My favourite example is the 'invisible gorilla'. Here's a clip from a website describing one such experiment.

Subjects were asked to monitor one of the teams of players by counting the number of passes they made. After about 45 seconds of performing this task, an unexpected event occurred. In all versions, many observers failed to see the change. For more details about this inattentional blindness, visit the lab page and look under inattentional blindness.

The Video