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Demonstrations & Experiments
Change blindness is the phenomenon where a person viewing a visual scene apparently fails to detect large changes in the scene. Inattentional blindness is the phenomenon of not being able to see things that are actually there. Choice blindness is a phenomenon in which subjects fail to remember conspicuous mismatches between a chosen object and some attributed trait.
In visual perception, change blindness is the phenomenon where 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, change blindness can be achieved by changing a part of the image in 13 seconds or longer. 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). Causes and relationship to other phenomenaChange 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. See alsoReferences
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Inattentional blindness, also known as perceptual blindness which is the phenomenon of not being able to see things that are actually there. This can be a result of having no internal frame of reference to perceive the unseen objects, or it can be the result of the mental focus or attention which cause mental distractions. The phenomenon is due to how our minds see and process information. Closely related to the subject of change blindness, is an observed phenomenon of the inability to perceive features in a visual scene when the observer is not attending to them. That is to say that humans have a limited capacity for attention which thus limits the amount of information processed at any particular time. Any otherwise salient feature within the visual field will not be observed if not processed by attention. Also related to this is the phenomena of blind people who later in life gain sight. Their processing of the visual stimuli does not allow them to identify objects easily, effectively they can see but are still perceptually blind. Experiments demonstrating inattentional blindnessThe term inattentional blindness was coined by Arien Mack and Irvin Rock in 1992. It was used as the title of Rock's last text published in 1998 by the MIT Press. The most well known study demonstrating inattentional blindness was conducted by Daniel Simons of the University of Illinois at Urbana-Champaign and Christopher Chabris of Harvard University. Their study, a contemporized version of earlier studies conducted by Ulric Neisser , asked subjects to watch a short video [1] in which two groups of people (wearing black and white t-shirts) pass a basketball back among themselves. The subjects are told to either count the number of passes made by one of the teams, or to keep count of bounce passes vs. aerial passes. In different versions of the video a woman walks through the scene carrying an umbrella, or wearing a full gorilla suit. In one version the woman in the gorilla suit even stops in the middle, faces the camera, and pounds her chest before walking out of the scene. After watching the video the subjects are asked if they saw anything out of the ordinary take place. In most groups 50% of the subjects did not report seeing the gorilla. Simons interprets this by stating that we are mistaken with regard to how important events will automatically draw our attention away from current tasks or goals. This result indicates that the relationship between what is in our visual field and perception is based much more significantly on attention than was previously thought. Another experiment was carried out by Steve Most, Chabis and Scholl. They had objects moving randomly on a computer screen. Participants were instructed to attend to the black objects and ignore the white, or vice versa. After several trials, a red cross unexpectedly appeared and traveled across the display, remaining on the computer screen for five seconds. The results of the experiment showed that even though the cross was distinctive from the black and white objects both in color and shape, about a third of participants nonetheless missed it. They had found that people may be attentionally tuned to certain perceptual dimensions, such as brightness or shape. Current researchA good review is Chun & Marois (2002) The dark side of visual attention. Current Opinion in Neurobiology, Volume 12, Issue 2, 1 April 2002, Pages 184-189 ExploitationsInattentional blindness is exploited by illusionists in the presentation of "magic shows" in the performance of some tricks by focusing the audience attention upon some distractive element, away from elements of the scene under manipulation by the performer. This is called misdirection amongst magicians. See alsoExternal linksWhy lifeguards sometimes can't see bodies at the bottom of a pool? Further readingChange Blindness and Inattentional Blindness Choice BlindnessIn psychology, choice blindness is a phenomenon in which subjects fail to detect conspicuous mismatches between their intended (and expected) choice and the actual outcome. Writing in Science, psychologist Petter Johansson and coworkers describe choice blindness demonstrated in an experiment. The subject is presented with two cards, on which different (female) faces appear. The subject is asked to choose which one he finds more attractive. In the non-manipulated (NM) version, the subject is handed the card that he chose and asked to say why he chose that one. In the manipulated (M) version, the experimenter uses sleight of hand techniques to switch the cards without the subject's knowledge and give the subject the other card. The workers found that most subjects failed to notice the switch, and furthermore justified their decision using post-hoc confabulated evidence. For example, in a M trial, a subject might say "I preferred this one because I prefer blondes" when he had in fact chosen (and pointed to) the dark-haired woman, but was handed a blonde. They point out that his experiment allows one to investigate the relationship between choice and introspection. Johansson concludes that he has found that some normal participants unequivocally produce confabulatory reports when asked to describe the reasons behind their choices and suggests that choice blindness affords some insight into the mechanisms behind truthful report. References
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External linksThis article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Change Blindness" |
