Because this inability to perceive, this sighted blindness, seemed to be caused by the fact that subjects were not attending to the stimulus but instead were attending to something else ... we labeled this phenomenon inattentional blindness (IB).*
Neisser, Simons, and Chabris have replicated and extended the work of Mack and Rock with experiments that have subjects attending to a specific task while watching a film, such as counting how many times a basketball is passed from one team member to another, while someone walks through the scene carrying an umbrella or wearing a gorilla suit. A surprisingly large percentage of subjects do not perceive something as obvious as a person in a gorilla suit moving through the scene they are observing, if they are attending to something else in their visual field. (Several examples of these experiments can be viewed on the Simons Lab page of the University of Illinois.)
The possibility of inattentional blindness should always be considered when dealing with conflicting eyewitness testimony where one of the parties claims that he did not see something that the other party saw and that might seem like something that anybody who was looking should have seen. The party who says he did not see what might seem too salient to miss may have been paying attention to something else at the time. Of course, one could be lying about not seeing something that one saw, but even the most honest person in the world might not see something that you think anybody should have seen.
Inattentional blindness may explain, for example, how a pilot with an interest in crop circles could fly right over one without even noticing it. The pilot had flown to see a recently discovered crop circle near Stonehenge. After visiting the site, he flew back to the airport to refuel before setting off on a trip that took him back over the site he had just visited. On the return flight he noticed another crop circle near the one he had visited earlier in the day and swears that the new circle was not there just forty-five minutes earlier. The new circle is very elaborate and could not have been produced by human hoaxers in such a short time. He concludes that some mysterious force must have been at work. Perhaps, but it seems more likely that the pilot experienced inattentional blindness when he was flying to the airport. He was focused on other tasks when he flew over the site and didn’t notice what was right beneath him all the time. (See "Crop Circles - Quest for Truth.")
Research by Chabris and Simons indicates that inattentional blindness is a "necessary, if unfortunate, by-product of the normal operation of attention and perception" (The Invisible Gorilla: And Other Ways Our Intuitions Deceive Us. 2010, p. 38). They point out that even radiologists, who are highly trained experts at detecting visual signs of medical problems, "can still miss subtle problems when they 'read' medical images." This may explain why my dentist didn't see a crack in one of my teeth on an x-ray until I started to complain about the pain in a particular area. To eliminate inattentional blindness, we'd have to eliminate focused attention. That would not be a good idea. Even worse would be the condition of being able to attend to everything in our sensory field at once. It would drive us mad.
Research also shows that training people to improve their attention abilities may do nothing to help them detect unexpected objects. "If an object is truly unexpected, people are unlikely to notice it no matter how good (or bad) they are at focusing attention" (Chabris and Simons: p. 32). Remember this the next time you're at the airport watching the transportation security screener do his or her job. It should not be surprising to find that these folks miss a lot of contraband, some of which has been planted by their bosses to test them.
It should go without saying--but I'll say it anyway--that magicians take advantage of inattentional blindness when doing sleight of hand tricks. The process of misdirection involves getting you to pay attention to one thing while the magician does something right before your eyes that you don't see or pay attention to.