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| An Overview of Change Blindness Quoted from: Simons, D. J. (2000). Current Approaches to Change Blindness. Visual Cognition, 7, 1-15. |
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| Imagine
you are watching a movie in which an actor is sitting in a cafeteria
with a jacket slung over his shoulder. The camera then cuts to a close-up
and his jacket is now over the back of his chair. You might think
that everyone would notice this obvious editing mistake. Yet, recent
research on visual memory has found that people are surprisingly poor
at noticing large changes to objects, photographs, and motion pictures
from one instant to the next (see Simons & Levin, 1997 for a review).
Although researchers have long noted the existence of such "change
blindness" (e.g., Bridgeman, Hendry, & Stark, 1975; French, 1953;
Friedman, 1979; Hochberg, 1986; Kuleshov, 1987; McConkie & Zola,
1979; Pashler, 1988; Phillips, 1974), recent demonstrations by John
Grimes and others have led to a renewed interest in the problem of
change detection. The new theoretical ideas and paradigms resulting
from this resurgence in the study of visual memory are the focus of
this special issue. In his demonstration, Grimes (1996) showed observers photographs of natural scenes for a later memory test. While they were studying an image, scanning from one object to another, details of the scene were changed during a saccade. Observers often missed surprisingly large changes (e.g., 2 people exchanging heads). This finding was consistent with earlier work on the failure to integrate information across saccades (e.g., Henderson, 1997; Irwin, 1991; Rayner & Pollatsek, 1983), but in some ways was a more striking demonstration because the changes were so clearly visible to observers when the change occurred during a fixation. Furthermore, Grimes used photographs rather than simple novel objects or letters, thereby bringing demonstrations of change blindness closer to everyday perceptual experience. More recently, several labs have shown that change blindness for objects in natural scenes can occur during a fixation if the effects of a saccade are simulated by disrupting the retinal transient normally accompanying a change. For example, change blindness can occur when a blank screen is inserted between the original and changed image (e.g., Blackmore, Brelstaff, Nelson, & Troscianko, 1995; French, 1953; Gur & Hilgard, 1975; Pashler, 1988; Rensink, O'Regan, & Clark, 1997; Simons, 1996). People also show change blindness when the original and altered image are separated by a "mudsplash" (O'Regan, Rensink, & Clark, in press), by a cut or pan in a motion picture (Hochberg, 1986; Levin & Simons, 1997; Simons, 1996), and even by a real-world disruption (Simons & Levin, 1998). Recent studies build on early work on change detection (e.g., French, 1953; Friedman, 1979; McConkie & Zola, 1979; Pashler, 1988; Phillips, 1974) by systematically examining the role of attention in change detection successes and failures (for a recent review see Simons & Levin, 1997; a compilation of all studies of change detection is under development and is available on the world wide web at http://coglab.wjh.harvard.edu/CB/Cblist.html). Although a number of paradigms have been used to study changed detection, the two most frequently used are the "Flicker" paradigm (Rensink et al., 1997) and the "Forced Choice Detection" paradigm (e.g., Pashler, 1988; Phillips, 1974; Simons, 1996). In the flicker paradigm, an original and modified image are presented in rapid alternation with a blank screen between them. Observers respond as soon as they detect the changing object. Research using this paradigm has produced two primary findings: 1) observers rarely detect changes during the first cycle of alternation, and some changes are not detected even after nearly one minute of alternation (Rensink et al., 1997); and 2) changes to objects in the "center of interest" of a scene are detected more readily than peripheral or "marginal interest" changes (Rensink et al., 1997), suggesting that attention is focused on central objects either more rapidly or more often, thereby allowing faster change detection. In the forced choice detection paradigm, observers only receive one view of each scene before responding, so the total duration of exposure to the initial scene can be controlled more precisely. Furthermore, because only a subset of the images have changes, signal detection analyses can be used and both accuracy and latency can be used as dependent measures. Both the flicker paradigm and the forced choice detection paradigm are intentional change detection tasks in that observers know that changes will occur and actively search the display to find differences. This work demonstrates that observers are change blind even when their primary task is to search for change. Other change detection studies examine detection performance under divided attention conditions. For example, observers in Grimes' (1996) study were aware that changes might occur and were asked to report the changes when they happened, but their primary task was to study the images for a later recognition task. Another recent approach has been to examine change detection with completely incidental encoding - observers view the display without knowing that it might change (see Mack & Rock, 1998 for a discussion of the difference between inattention and divided attention). Many of these studies use motion picture or real-world methodologies, allowing richer insights into the sorts of representations people spontaneously form under natural viewing conditions (see Simons & Levin, 1997 for a review). As with intentional change detection tasks, under incidental encoding conditions, observers are blind to marginal interest changes. For example, when viewing motion picture stimuli, naïve observers consistently miss changes to marginal interest objects occurring across shifts in camera position, or "cuts" (Levin & Simons, 1997). Findings of change blindness for marginal interest objects in scenes and motion pictures, together with evidence from the flicker paradigm that changes to central objects are detected more readily, lead to the conclusion that attention is necessary for change detection - the details of an object will only be retained if attention is focused on the changing feature. If observers could "take in" an entire scene with a single attentional fixation, they could detect changes anywhere in an image with equal facility. Instead, observers apparently must scan an image, encoding the scene piecemeal (Rensink et al., 1997). In order to retain information about an object or its properties from one view to the next, observers must re-code the information, explicitly comparing the abstracted representation of the initial object to the changed object (Simons, 1996). Objects that are not re-coded are not remembered in any detail. Given the number of potential features and objects in a typical natural scene (effectively an infinite number), many, if not most aspects of a scene will not be preserved across views. Because observers are more likely to focus attention on important objects, they are more likely to notice changes to objects in the center of interest of a scene. Although attention appears to be necessary for change detection, it may not be sufficient. With incidental encoding, observers sometimes miss changes to central objects as well. For example, all observers failed to notice when the central object in a brief motion picture (a soda bottle) was replaced by a box following a brief pan away from the table (Simons, 1996). Furthermore, when naïve observers viewed films of simple action sequences, nearly 2/3 of the them failed to notice when the central actor in the scene was replaced by a different actor (Levin & Simons, 1997; see also Simons, 1996). Change blindness for central objects can occur in the real world as well (Simons & Levin, 1998). In a recent study, one experimenter approached a pedestrian (the subject) to ask for directions. During their conversation, two other people rudely interrupted them by carrying a door between the experimenter and the pedestrian. During the time that the subject's view was obstructed, the first experimenter was replaced by a different experimenter. Only 50 percent of observers noticed the change even though the two experimenters wore different clothing, were different heights and builds, had different haircuts, and had noticeably different voices (Simons & Levin, 1998). Unless observers attend to and encode the specific features that change, they will not detect the difference. Simply attending to an object does not guarantee a complete representation of its features.
Blackmore, S. J., Brelstaff, G., Nelson, K., & Troscianko, T. (1995). Is the richness of our visual world an illusion? Transsaccadic memory for complex scenes. Perception, 24, 1075-1081. Bridgeman, B., Hendry, D., & Stark, L. (1975). Failure to detect displacement of the visual world during saccadic eye movements. Vision Research, 15(6), 719-722. French, R. S. (1953). The discrimination of dot patterns as a function of number and average separation of dots. Journal of Experimental Psychology, 46, 1-9. Friedman, A. (1979). Framing pictures: The role of knowledge in automatized encoding and memory for gist. Journal of Experimental Psychology: General, 108(3), 316-355. Grimes, J. (1996). On the failure to detect changes in scenes across saccades. In K. Akins (Ed.), Perception (Vancouver Studies in Cognitive Science) (Vol. 2, pp. 89-110). New York: Oxford University Press. Gur, R. C., & Hilgard, E. R. (1975). Visual imagery and the discrimination of differences between altered pictures simultaneously and successively presented. British Journal of Psychology, 66(3), 341-345. Henderson, J. M. (1997). Transsaccadic memory and integration during real-world object perception. Psychological Science, 8(1), 51-55. Hochberg, J. (1986). Representation of motion and space in video and cinematic displays. In K. R. Boff, L. Kaufman, & J. P. Thomas (Eds.), Handbook of Perception and Human Performance (Vol. 1: Sensory Processes and Perception, pp. 22.21-22.64). New York: John Wiley and Sons. Irwin, D. E. (1991). Information integration across saccadic eye movements. Cognitive Psychology, 23, 420-456. Kuleshov, L. (1987). Selected Works: Fifty Years in Films (Agrachev, D. Belenkaya, N., Trans.). Moscow: Raduga Publishers. Levin, D. T., & Simons, D. J. (1997). Failure to detect changes to attended objects in motion pictures. Psychonomic Bulletin and Review, 4(4), 501-506. Mack, A., & Rock, I. (1998). Inattentional blindness. Cambridge, MA: MIT Press. McConkie, G. W., & Zola, D. (1979). Is visual information integrated across successive fixations in reading? Perception and Psychophysics, 25(3), 221-224. O'Regan, J. K., Rensink, R. A., & Clark, J. J. (in press). Blindness to scene changes caused by "mudsplashes". Nature. Pashler, H. (1988). Familiarity and visual change detection. Perception and Psychophysics, 44(4), 369-378. Phillips, W. A. (1974). On the distinction between sensory storage and short-term visual memory. Perception and Psychophysics, 16, 283-290. Rayner, K., & Pollatsek, A. (1983). Is visual information integrated across saccades? Perception and Psychophysics, 34(1), 39-48. Rensink, R. A., O'Regan, J. K., & Clark, J. J. (1997). To see or not to see: The need for attention to perceive changes in scenes. Psychological Science, 8, 368-373. Simons, D. J. (1996). In sight, out of mind: When object representations fail. Psychological Science, 7(5), 301-305. Simons, D. J., & Levin, D. T. (1997). Change blindness. Trends in Cognitive Sciences, 1(7), 261-267. Simons, D. J., & Levin, D. T. (1998). Failure to detect changes to people in a real-world interaction. Psychonomic Bulletin and Review, 5(4), 644-649.
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