Change blindness

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In visual perception, change blindness is the phenomenon that occurs when a person viewing a visual scene apparently fails to detect large changes in the scene. For change blindness to occur, the change in the scene typically has to coincide with some visual disruption such as a saccade (eye movement) or a brief obscuration of the observed scene or image. When looking at still images, a viewer can experience change blindness if part of the image changes.


[edit] Background

The first explorations of change blindness appear to have been conducted by George McConkie and his colleagues in the late 1970s, focusing on changes made to words and text during saccadic eye movements. A student of McConkie's, John Grimes, extended this phenomenon to the domain of scene perception (in a conference presentation in 1992, later published in 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. (For more recent studies of saccade-contingent change blindness, see Henderson & Hollingworth, 1999, and McConkie & Currie, 1996.)

Beginning in the late 1980s, research began to reveal that other forms of visual disruption besides eye movements could also induce relatively poor change detection. Pashler (1988) showed that observers were quite poor at detecting changes introduced into arrays of letters while the display was flickered off and on, even if the offset was as brief as 67 milliseconds (although offsets briefer than that produced better change detection). He concluded by noting that people report having a "clear sense of apprehending the identities and locations of large numbers of objects in a scene" (p. 377), and that given these introspections, it seemed surprising that people's ability to detect changes proved to be so poor.

Later, Rensink et al., popularized the "flicker" technique in which two images of scenes alternate repeatedly with a brief (80 millisecond) 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 reliably noticing them. Rensink et al. (1997) also introduced the term "change blindness."

Other studies showed that change detection is also poor 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).

[edit] Causes and relationship to other phenomena

Change blindness may be 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.

It has been shown that change blindness can even occur immediately after an observer has identified all of the objects in a display. Becker and Pashler (2002) had observers name the highest digit in an array of digits exposed for 2 seconds, at which time the display flickered off and on (with one of the digits changed). Even though observers were almost perfect at naming the highest digit in the initial display, they were still performing at the usual, relatively low, level in spotting the change. When the highest digit itself changed, though, this change was almost always noticed.

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