Our attention is usually drawn to changes in the environment, so noticing an abrupt alteration in a photograph should be easy. However, some conditions can make this task difficult. Try the first 10 examples in , showing photographs. You probably found some of them hard and some of them easy. Once you noticed the alteration, even with the flicker, it probably became impossible not to see the changes. They seem to pop out of the image at you. Even in the hardest ones, removing the flicker made the task trivial. Now try the last 10 examples in , showing abstract scenes. Were those more difficult?
Research starting in the 1970s has shown that someone paying close attention to one event or object may not notice odd happenings outside the area of attention. This has been termed inattentional blindness, failure to see unattended items. In contrast, change blindness is failure to see changes to an image when the change occurs along with a brief eye movement or flash in the image. Unlike inattentional blindness, change blindness persists when one is actively looking for a change. The relationship between these two phenomena is still not clear and both are subjects of current research.
As you saw in the demonstrations, it is easy to see changes when they occur without a flash and much more difficult when the flash is present. In fact, the flash need not even obscure the area of change. It needs only to distract. Normally, the visual system automatically directs attention to change, using local motion cues in the visual field. However, when a blank field is inserted between images, that flicker may overwhelm the mechanisms that respond to local motion. Lacking that automatic control, you must instead direct your attention by slower high-level mechanisms. You search the scene systematically until you happen to hit the area that is changing.