Change blindness

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Change blindness is a phenomenon in visual perception where apparently large changes within a visual scene are undetected by the viewer. Typically for change blindness to occur, the change in the scene has to coincide with some visual disruption such as an eye movement or a brief obscuration of the observed scene or image. When looking at still images, change blindness can be achieved by changing a part of the image in 13 seconds or longer.

When change blindness was first explored systematically by George McConkie and his colleagues in the late 1970s, the phenomenon was largely limited to the study of changes introduced to words and text during eye movements. A student of McConkie's, John Grimes, was the first to extend this phenomenon to the domain of scene perception (in a conference presentation in 1992, not published until a book chapter in 1996). Grimes showed that people miss large changes to scenes when the changes are introduced during an eye movement. For example, many people failed to notice when two people in a scene exchanged heads! In these saccade-contingent change blindness studies, changes to the scene were synchronized with measured movements of the observer's eyes, so that the changes occurred only when the eyes were moving. Under these conditions, changes are often hard to detect. A number of studies since then have explored saccade-contingent change blindness (e.g., Henderson & Hollingworth, 1999; McConkie & Currie, 1996).

Later experiments showed that change blindness was not specifically related to eye movements – other forms of visual disruption could also induce change blindness. Rensink et al, popularized the "flicker" technique in which two images alternate repeatedly with a brief (80ms) blank screen after each image (giving the display a flickering appearance). With the blank screen in place, surprisingly large changes could be made to the scene without the observer noticing. Rensink et al (1997) also introduced the term "change blindness."

Other studies showed that change blindness occurs when the change is introduced during a cut or pan in a motion picture, even when the change is to the central actor in a scene (Levin & Simons, 1997). People also regularly fail to notice editing errors in commercial movies, despite the intense scrutiny of movies during the production process.

Change blindness can be particularly dramatic when changes occur unexpectedly, with many observers even failing to notice when a person they were talking to was surreptitiously replaced by a different actor (Simons & Levin, 1998). Change blindness has now been shown to occur with a wide variety of visual disruptions (e.g., blinks, transient noise flashed on a display, etc).

Change blindness is related to other induced failures of awareness, such as inattentional blindness. A crucial difference is that successful change detection in the presence of a visual disruption requires a comparison of one image to another one held in memory. Consequently, change blindness can occur due to a failure to store the information in the first place or to a failure to compare the relevant information from the current scene to the representation (hence models of visual short term memory may be important for understanding the phenomenon). In contrast, inattentional blindness reflects the failure to detect an unexpected stimulus that is fully visible in a single display – it does not require a comparison to memory.

In the real world, change blindness may be responsible for traffic accidents. If drivers fail to notice significant changes around them (e.g. the presence of a pedestrian in their path).

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