Perspectives REASONS FOR CHANGE BLINDNESS. Stephen Mitroff & Daniel J. Simons Harvard University

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1 ADVANCED SEARCH FAQ SEARCH: all of CogNet Perspectives NEWS LIBRARY PERSPECTIVES DISCUSSIONS CONFERENCES CAREERS PROFILES SEMINARS PUBLISHER COLLECTIONS WORKSPACE REASONS FOR CHANGE BLINDNESS ARCHIVE WHAT IS PERSPECTIVES? Stephen Mitroff & Daniel J. Simons Harvard University Imagine that a stranger approaches you to ask for VIEW COMMENTS directions. As you are giving the directions, a group of people walk by, momentarily obstructing your view of POST A COMMENT the stranger. Once they pass, you finish giving directions and move on. Surely you would notice if, when the group passed by, the original stranger were replaced by a different person. Or would you? As it turns out, observers frequently miss changes such as this (Simons & Levin, 1998) as well as other kinds of changes in a wide variety of situations (see Rensink, 2000; Simons, 2000 for reviews. You can view an example of a participant missing this sort of change here: ~viscog/grafs/demos/door.mov; Simons & Levin, 1997). This phenomenon, known as change blindness, has received increasing attention over the past 5 years. As this example illustrates, change blindness occurs even for large, salient changes to scenes during natural viewing. Moreover, it occurs across a range of conditions, including changes made across eye movements (Grimes, 1996), during blinks (O'Regan, Deubel, Clark, & Rensink, 2000), and during cuts in motion pictures (Levin & Simons, 1997; Simons, 1996). Change blindness even occurs when observers expect changes and are explicitly instructed to search for them (Rensink, O'Regan, & Clark, 1997). As long as a visual disruption of some sort (e.g., a saccade, flashed screen, or door) accompanies the changes, observers miss large changes to scenes. Why are changes so difficult to detect? What causes change blindness? Under normal viewing conditions, changes produce transient signals that can draw attention. Change blindness studies are designed to eliminate or block these transient signals by inserting a visual disruption (e.g., a camera cut) when the change occurs. For example, in the commonly-used "flicker task," two slightly different displays quickly alternate back and forth with a blank screen inserted between (creating a 'flickering' appearance). The blank interval serves to disrupt the transient signal that would otherwise result from the change (Rensink et al., 1997). Try to find the change in this example ( With a blank of only 100ms between the original and changed image, detecting the change can be quite difficult, even if you are actively searching for the change. In this example ( demos/flicker_noblank.mov), the blank has been removed. Now changes are easily noticed because the transient signal draws attention. To notice changes when the transient signal has been disrupted, observers must: (1) Form a representation of the first display, (2) Compare that representation to the second display, and (3) Have conscious access to the Page 1 of 5

2 results of the comparison. Successful change detection requires a comparison between the original and modified display. If the first display is no longer visible, observers must rely on their internal representation, or memory trace, of the first display. They must then compare this representation to the second display, if it is still visible, or to a representation of the second display. Finally, in order to explicitly report the change, observers must be aware of the results of this comparison. Change blindness, therefore, results from a failure of one of these three components of the detection process. Most explanations for change blindness have focused on failures of representation or comparison. That is, people infer the absence of representations from findings of change blindness because without a representation, detection is not possible. However, failed detection does not require the absence of a representation (Simons, 2000). Change blindness could occur even if observers formed a detailed representation of the initial view, provided that they failed to compare that representation to the changed view (Scott-Brown, Baker, & Orbach, 2000). Moreover, change blindness could occur even when observers do represent the initial view and compare it to the changed view if (a) their representation lacks sufficient detail, (b) their representation does not include the changed feature, or (c) they do not become cognizant of the results of the comparison (failed access). If change blindness does result from failed access, then at some level, observers might have registered the presence of a change even if they failed to report it. That is, change blindness reflects the observers ability to report a change, but it may underestimate other forms of change detection. If so, then evidence for such implicit change detection would imply that we represent and compare far more detail from a scene than change blindness findings initially suggested. More sensitive measures than those used in traditional change blindness studies might well reveal greater levels of change detection (Fernandez-Duque & Thornton, 2000; Hollingworth, Williams, & Henderson, 2000; Simons, 1996; Smilek, Eastwood, & Merikle, 2000; Thornton & Fernandez-Duque, in press; Williams & Simons, 2000). The past few years have seen several attempts to find evidence for change detection in the absence of awareness. To do so, experimenters must take care to exclude any change detection resulting from explicit access to the representation, the comparison, and the results of the comparison. Given the appeal of demonstrating the existence of more detailed scene representations, researchers have been eager to conclude that implicit change detection occurs. Although several recent studies have concluded that changes are detected without awareness, none have satisfactorily eliminated possible explicit, or conscious, influences on change detection performance. Once explicit strategies and influences are taken into account, none of the findings to date provide convincing support for implicit change detection (Mitroff & Simons, 2000a; Mitroff & Simons, 2000b). One reasonable way to look for implicit change detection is to explore whether subjects can guess the changed object even when they did not see the change. Intuitively, if observers who report no awareness of a change can guess which of two items had changed better than 50% of the time, we might conclude that they had implicitly detected the change. In fact, under these conditions, observers do guess the change more than 50% of the time (Fernandez-Duque & Thornton, 2000). Yet, explicit knowledge about which items in the display did not change likely influenced the guessing performance; explicit knowledge that one of two items did not change allows for better than 50% selection of the item that changed. In fact, if Page 2 of 5

3 observers are able to determine that one of the items in the display did not change on each trial, their guessing would be almost identical to the empirical results. In several experiments, we find that guessing rates are entirely predicted by this explicit elimination strategy -- after accounting for explicit strategies, guessing performance did not differ from chance performance (Mitroff & Simons, 2000b). Similar explicit strategies can explain most, if not all, of the existing evidence for implicit change detection. The one potential exception comes from analyses of looking time. If the visual system can register the presence of a change even if observers did not report seeing it, then the pattern of fixations on a changed scene might reflect change detection in the absence of awareness. In some experiments, even when observers report no awareness of a change, they sometimes refixate the changed object for a longer duration than if no change had occurred (Hollingworth et al., 2000). Differences in looking time as a function of the presence or absence of a change might well reflect implicit detection. However, in order to determine whether these effects are unquestionably implicit, such studies must first account for several explicit alternatives. First, looking time differences might result from explicit differences in criterion or confidence. When observers are less confident in their change detection, they tend to respond more slowly (Mitroff & Simons, 2000b). They might also tend to look longer if they are less certain of their response. Given that observers in change detection studies tend to have a default bias to respond "no-change" unless they are certain of the change, even if they had consciously detected the change, they might not report it. Consequently, such cases of sub-criterion explicit detection could contribute to longer looking times. Confidence does seem to account for slowed response times for missed changes relative to the same explicit report on trials with no change. Perhaps it can account for the looking time results as well. The absence of compelling evidence for implicit change detection resuscitates the possibility that change blindness implies a failure to represent the scene or to compare those representations. That is, change blindness might well result from the minimal representation of scenes or from the failure to explicitly compare the represented scene with the current view. This conclusion is consistent with two broad principles in scene perception, one that is widely accepted, and one that is more debated: (a) scenes are minimally represented in memory and (b) comparisons of representations require awareness. The argument for minimal representation is consistent with the broad literature on information integration across saccades (e.g., Irwin, 1991) as well as with theoretical arguments within the change blindness literature (e.g., Rensink, 2000). These studies suggest that detailed visual representations are not stored and compared across eye movements or other visual disruptions. Furthermore, the information that is preserved across eye movements tends to be abstracted away from the sensory details (e.g., Irwin, 1991). Minimal representations are efficient in that they do not internally duplicate information that is still available in the environment. In real world perception, changes generally do not occur without producing a transient signal. Hence, in most cases, the visual system can assume stability without a need to compare detailed representations over time. Stability is the norm and changes typically produce a signal that draws attention. A more controversial claim, and one that appears to be supported by change detection evidence, is that change detection requires an explicit comparison of an initial representation to the changed display. Some philosophical positions (e.g., Dulany, 1997) argue that abstract representations cannot Page 3 of 5

4 be compared in the absence of awareness, implying that any comparison mechanism used to detect changes in the absence of a transient will require an intentional, explicit comparison. According to this view, implicit change detection in the absence of a transient signal should not be possible. If change detection is to serve as a useful tool for examining visual cognition, it is essential to understand both why changes are missed and how some changes are successfully detected. The absence of solid evidence for implicit change detection (Mitroff & Simons, 2000a; Mitroff & Simons, 2000b) suggests that change blindness may result from the absence of representations or the failure to explicitly compare representations. Of course, any evidence for implicit change detection that were not subject to explicit, strategic influences would refute this claim. Until such evidence arises, findings of change blindness suggest a critical role for attention and for awareness in the perception of visual scenes. Some useful resources: The Change Detection Database: change VisCog.net - a visual cognition resource site: The visual cognition lab demo page: lab/demos.html aaa References Dulany, D. E. (1997). Consciousness in the explicit (deliberative) and implicit (evocative). In J. D. Cohen & J. W. Schooler (Eds.), Scientific approaches to consciousness (pp ). Mahwah, NJ: Lawrence Erlbaum. Fernandez-Duque, D., & Thornton, I. M. (2000). Change detection without awareness: Do explicit reports underestimate the representation of change in the visual system? Visual Cognition, 7(1/2/3), 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 ). New York: Oxford University Press. Hollingworth, A., Williams, C. C., & Henderson, J. M. (2000). The representation of natural scenes is not limited to currently attended visual information. Michigan State University Eye Movement Laboratory Technical Report, 1, 1-7. Irwin, D. E. (1991). Information integration across saccadic eye movements. Cognitive Psychology, 23, Levin, D. T., & Simons, D. J. (1997). Failure to detect changes to attended objects in motion pictures. Psychonomic Bulletin and Review, 4(4), Mitroff, S. R., & Simons, D. J. (2000a). Changes are not localized before they are explicitly detected. Poster presented at ARVO, Ft. Lauderdale. Mitroff, S. R., & Simons, D. J. (2000b). Without explicit detection, change localization is chance localization. Poster presented at OPAM, New Orleans. O'Regan, J. K., Deubel, H., Clark, J. J., & Rensink, R. A. (2000). Picture changes during blinks: Looking without seeing and seeing without looking. Visual Cognition, 7, Rensink, R. A. (2000). The dynamic representation of scenes. Visual Cognition, 7, 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(5), Scott-Brown, K. C., Baker, M. R., & Orbach, H. S. (2000). Comparison blindness. Visual Cognition, 7, Simons, D. J. (1996). In sight, out of mind: When object representations fail. Page 4 of 5

5 Psychological Science, 7(5), Simons, D. J. (2000). Current approaches to change blindness. Visual Cognition, 7, Simons, D. J., & Levin, D. T. (1997). Change blindness. Trends in Cognitive Sciences, 1(7), Simons, D. J., & Levin, D. T. (1998). Failure to detect changes to people in a real-world interaction. Psychonomic Bulletin and Review, 5(4), Smilek, D., Eastwood, J. D., & Merikle, P. M. (2000). Does unattended information facilitate change detection? Journal of Experimental Psychology: Human Perception and Performance, 26(2), Thornton, I. M., & Fernandez-Duque, D. (in press). An implicit measure of undetected change. Spatial Vision. Williams, P., & Simons, D. J. (2000). Detecting changes in novel, complex three-dimensional objects. Visual Cognition, 7, Acknowledgments: Thanks to Steven Franconeri, Steven Most, and Pamela Yee for comments on earlier drafts of this article. Stephen Mit roff was supported by an NSF graduate fellowship and Daniel Simons was supported by NSF grant #BCS and by an Alfred P. Sloan Foundation Research Fellowship. Correspondence can be addressed to either author by mitroff@wjh.harvard.edu or dsimons@wjh.harvard.edu. For more examples of change blindness, go to demos.html. For more information about change blindness, visit the change detection database: back to top Legal Page 5 of 5