Change blindness is a perceptual phenomenon where observers fail to notice visual changes, sometimes very extreme ones, illustrating that the brain does not always process scenes in precise detail, but rather provides a general overview. A famous and often-repeated experiment to demonstrate change blindness is the basketball game scenario, where observers are asked to watch a short clip of a basketball game. Often, the experimenter provides a task, such as counting passes or falls. Most observers fail to notice that a person in a gorilla suit walks into the middle of the court, gesticulates, and then leaves again.
Despite the name, change blindness is not a visual impairment, and it's also not a disorder. All people demonstrate it to some extent, although some appear to be less prone than others. Autism, for example, tends to reduce the amount of change blindness an observer experiences. Research on autism and visual processing suggests that autistic people are less capable of ignoring extraneous details, and thus tend to notice when something in their environment changes.
Visual images go through a number of forms of processing when they hit the brain so the observer can make sense of the image. The brain can assign a sense of direction, name objects in the scene, and provide context to help the observer interpret it. Change blindness appears to be the result of the brain's desire to provide a quick overview of a scene for the benefit of the observer; for example, the brain might decide that a person is looking at a crowded airport terminal or a woodland scene on the basis of general information, but would not provide specific details like the precise location of every tree, or the color of the baggage at the corner of the eye.
Change blindness becomes even more extreme when a visual distraction is present. In the woodland scene example, if a deer bounds across the observer's view, he may fail to notice that a hunter has appeared. In a crowded airport terminal, the eye will be drawn toward a moving baggage cart or a plane taking off, and may miss a change at a departures or arrivals board. Asking people to complete tasks can also exacerbate change blindness, as they focus on the task instead of what they are seeing. Thus, the busy flier scanning the departures board for a connecting flight may not notice someone stepping into her way.
Researchers who study this phenomenon conduct experiments like randomly switching the person a subject is talking to or editing films to insert scenes or make the characters change heads halfway through a scene. In both examples, many observers fail to notice the change, although it becomes apparent once pointed out and will always be easy to spot in the future.