Step-by-step solution file
Psychonomic Bulletin & Review 1998. 5 (4). 644-649 BRIEF
Read both articles attached, and then discuss the following:
- What is the purpose of the article?
- What are the methods used?
- What are the primary results?
- What is the significance of the study?
- Provide a brief criticism of the study including proposals for future research.
Psychonomic Bulletin & Review
1998. 5 (4). 644-649 BRIEF REPORTS
Failure to detect changes to people
during a real-world interaction
DANIELJ. SIMONS Harvard University, Cambridge, Massachusetts
DANIELT. LEVIN Kent State University, Kent, Ohio
Recent research on change detection has documented surprising failures to detect visual changes occurring between views of a scene, suggesting the possibility that visual representations contain few details. Although these studies convincingly demonstrate change blindness for objects in still images and
motion pictures, they may not adequately assess the capacity to represent objects in the real world.
Here we examine and reject the possibility that change blindness in previous studies resulted from passive viewing of 2-Ddisplays. In one experiment, an experimenter initiated a conversation with a pedestrian, and during the interaction, he was surreptitiously replaced by a different experimenter. Only half
of the pedestrians detected the change. Furthermore, successful detection depended on social group
membership; pedestrians from the same social group as the experimenters detected the change but
those from a different social group did not. A second experiment further examined the importance of
this effect of social group. Provided that the meaning of the scene is unchanged, changes to attended
objects can escape detection even when they occur during a natural, real-world interaction. The discussion provides a set of guidelines and suggestions for future research on change blindness.
Despite our impression that we retain the visual details
of our surroundings from one view to the next, we are
surprisingly unable to detect changes to such details. Recently, experiments from a number of laboratories have
shown that people fail to detect substantial changes to photographs of objects and real-world scenes when the ability to detect retinal differences is eliminated (Blackmore,
Brelstaff, Nelson, & Troscianko, 1995; Grimes, 1996;
Henderson, 1997; McConkie & Currie, 1996; O'Regan,
Deubel, Clark, & Rensink, 1997; Pashler, 1988; Phillips,
1974; Rensink, O'Regan, & Clark, 1997; Simons, 1996;
for a review see Simons & Levin, 1997). That is, when
retinally localizable information signaling a change is
masked by an eye movement or a flashed blank screen,
observers have difficulty detecting changes to the visual The authors contributed equally to this report, and authorship order
was determined arbitrarily. Thanks to Leon Rozenblit, Carter Smith,
Julia Noland, and Joy Beck for helping to carry out the experiments and
to Linda Hermer for reading an earlier draft of the manuscript. DJ.S.
was supported by NSF and Jacob K. Javits fellowships, and parts of this
research appeared in his doctoral thesis. Correspondence should be addressed to D. J. Simons, Department of Psychology, Harvard University,
820 William James Hall, 33 Kirkland St., Cambridge, MA 02138 (email: [email protected]) or D. T. Levin, Department of Psychology, Kent State University, P.O. Box 5190, Kent, OH 44242-000 I
(e-mail: [email protected]). Copyright 1998 Psychonomic Society, Inc. details of a scene. These findings of "change blindness"
suggest that observers lack a precise visual representation of their world from one view to the next. Although
we have known for some time that memory for scenes is
often distorted, sometimes quite sparse, subject to suggestions, and influenced by expectations and goals (Bartlett,
1932/1977; Brewer & Treyens, 1981; Loftus, 1979; Nickerson & Adams, 1979), studies of change blindness suggest that such details may not be retained even from one
instant to the next, a claim that is consistent with earlier
studies of the integration of information from successive
fixations (Bridgeman & Mayer, 1983; Dennett, 1991;
Hochberg, 1986; Irwin, 1991; McConkie & Currie, 1996;
Pashler, 1988; Rayner & Pollatsek, 1992).
Given the richness of our visual world, it is perhaps
unsurprising that we cannot represent all the visual details of every object and instead must focus on a few important objects. Recent models of attention have argued
that observers can fully represent the details of only a few
centrally attended objects in a scene. For example, models
based on object files (e.g., Treisman, 1993) suggest that
we can simultaneously represent several distinct objects
in our environment, updating our representations for
changes in their properties and features. Such models suggest the possibility that representations of centrally attended objects are relatively detailed even if those for peripheral objects are not. 644 CHANGE DETECTION A recent series of studies directly examined the role of
attention in the detection of changes to natural images
(Rensink et al., 1997). In their "flicker paradigm," an original version and a modified version of an image were presented in rapid alternation (240 msec each), with a blank
screen (80-msec duration) interposed between them, producing a flickering appearance. On each trial, subjects
were asked to identify the changing part of the image as
soon as they saw it. Consistent with earlier studies of integration across views (for a review, see Irwin, 1991), observers rarely noticed changes during the first cycle of
alternation and often required many cycles to detect the
change. The change detection process requires observers
to shift their attention among the objects in the scene, actively searching for a change. As predicted by models of
object files, changes to objects that independent raters
consider to be the center of interest of a scene are detected in significantly fewer alternations than changes to
peripheral objects. That is, changes to the details of attended objects are detected more readily.
Clearly, focused attention to an object is helpful and
possibly necessary for change detection, as evidenced by
such "center of interest" effects (O'Regan, Rensink, &
Clark, 1996; Rensink et aI., 1997; Tarr & Aginsky, 1996,
July) and by findings of more successful change detection
when explicit cues specify the location or the type of
change (Aginsky, Tarr, & Rensink, 1997). However, attention may not be sufficient for change detection. In fact,
observers often fail to detect changes even when attention is focused directly on the changing object (Levin &
Simons, 1997; O'Regan et al., 1997; Simons, 1996). In a
recent series ofstudies, we used motion pictures to directly
examine the ability to detect changes to attended objects
(Levin & Simons, 1997). These brief motion pictures depicted a simple action performed by a "single" actor. During the film, the actor was replaced by a different person.
For example, in one film an actor walked through an
empty classroom and began to sit in a chair. The camera
then changed, or "cut," to a closer view and a different
actor completed the action. Even though the actors were
easily discriminable and were the focus of attention, only
33% of the 40 participants reported noticing the change
from one actor to another (Levin & Simons, 1997).
Although the motion picture experiments demonstrate
that attention alone is not sufficient for a complete representation of the visual details of an object, they do not
fully assess our ability to represent objects in the real
world. Motion picture perception is similar in many ways
to perception in the real world, but motion pictures are
still a subset of a complete visual experience (Arnheim,
1933/1966). Most importantly, they are viewed passively
and may not completely engage the processes necessary
for a complete representation of attended objects. Furthermore, cuts from one view to another in motion pictures
may artificially hamper our ability to detect changes. Although cuts are similar in some ways to eye movements, 645 they also instantaneously change the simulated observation point. This artificial jump in viewing position may
somehow disrupt the ability to detect changes even if it
has little effect on our understanding of a scene. Similar
objections might be raised about most studies documenting change blindness (for a discussion, see Simons &
Levin, 1997). In all previous studies of change blindness,
exposure to scenes has been mediated via photographs,
computer displays, or television monitors. Perhaps people can more fully represent the details of a scene when
they are direct participants, interacting with the objects
in the real world.
Here we assess this possibility by taking the study of
change blindness into the real world. Rather than changing
the sole actor in a video, we changed the subjects' conversation partner during a typical daily interaction.
EXPERIMENT 1 In Experiment 1, we created a situation that allowed us
to surreptitiously substitute one individual for another in
the middle of a natural, real-world interaction. The situation we chose was asking directions of a pedestrian on
a college campus.' We temporarily interrupted this interaction by carrying a door between the experimenter and
the pedestrian. While the experimenter was occluded by
the door, another experimenter took his place and continued the interaction after the door had passed. If changedetection failures are based on the passive nature of mediated stimuli, these substitutions should be clearly
Subjects. A total of 15 pedestrians were approached on the campus
of Cornell University. They ranged in approximate age from 20 to 65.
Only pedestrians walking alone or together with one other person (two
cases) were approached.
Procedure. An experimenter carrying a campus map asked unsuspecting pedestrians for directions to a nearby building (see Figure la).
Pedestrians had a clear view of the experimenter starting from a distance of approximately 20 m as they walked down a sidewalk. After the
experimenter and pedestrian had been talking for 10-15 sec, two other
experimenters carrying a door rudely passed between them. As the door
passed, the first experimenter grabbed the back of the door. and the experimenter who had been carrying that part of the door stayed behind
and continued to ask for directions (Figure Ic). The first experimenter
kept his map during the interruption, and the second experimenter produced an identical copy of the map after the door passed. The door
blocked the pedestrian's view for approximately I sec (Figure Ib). From
the subject's perspective, the door briefly occluded his/her conversation
partner. and when it was gone, a different person was revealed. As the
door passed, subjects typically made eye contact with the second experimenter before continuing to give directions.? The entire interaction
took 2-5 min. The two experimenters wore different clothing and differed in height by approximately 5 cm (Figure Id). Their voices were
also clearly distinguishable.
After a pedestrian finished giving directions. the experimenter told
him/her, "We're doing a study as part of the psychology department [experimenter points to the psychology building next door] of the sorts of
things people pay attention to in the real world. Did you notice anything
unusual at all when that door passed by a minute ago?" Responses were 646 SIMONS AND LEVIN Figure 1. Frames from a video of a subject from Experiment 1. Frames a~ show the sequence ofthe switch. Frame d shows the two
experimenters side by side. noted by the experimenter. and if subjects failed to report the change,
they were directly asked. "Did you notice that I'm not the same person
who approached you to ask for directions?" After answering this question. all subjects were informed about the purpose of the experiment. Results and Discussion
If change blindness results from the passive nature of
mediated stimuli, then these real-world substitutions
should be detected, When asked if they had noticed anything unusual, most pedestrians reported that the people
carrying the door were rude. Yet,despite clear differences
in clothing, appearance, and voice, only 7 ofthe 15 pedestrians reported noticing the change of experimenters.
Those who did not notice the change continued the conversation as if nothing had happened (in fact, some
pedestrians who did notice the change also continued the
conversation!). Pedestrians who did not notice the change
were quite surprised to learn that the person standing in
front of them was different from the one who initiated
the conversation. One pedestrian who reported noticing nothing unusual nonetheless claimed to have noticed the
change when asked directly.
Interestingly, those who noticed the change were all
students of roughly the same age as the experimenters
(approximately 20~30 years old). Those who failed to
detect the change were slightly older than the experimenters (approximately 35-65 years old). One possible
explanation for this difference is that younger pedestrians were more likely to expend effort encoding those
features that would differentiate the experimenters because the experimenters were roughly of their own generation. In contrast, older pedestrians would likely encode the experimenters without focusing on features that
could differentiate the two of them, instead viewing them
as members of a social group other than their own. This
hypothesis draws on findings from social psychology
that members of one's own social group ("in-group") are
treated differently from members of social groups distinctly apart from one's own ("out-group"). Upon encountering a member of an in-group, people tend to focus at- CHANGE DETECTION 647 -~-- Figure 2. The experimenters dressed as construction workers for Experiment 2. tention on individuating features and to pay little attention
to the person's social-group membership. In contrast, for
members of out-groups, people direct more attention to
attributes associated with the out-group as a whole and
generally do not focus on features that distinguish one individual from others in the group (see, e.g., Rothbart &
John, 1985). These differences in processing of members
of in-groups and out-groups extend to many aspects of
cognition. For example, people are likely to assume that
members of out-groups are collectively less variable on a
variety oftraits and variables (Judd & Park, 1988; Linville,
Fischer, & Salovey, 1989). This tendency to code groupspecifying information for members of out-groups can
even determine what represents a visual feature for a particular category (Levin, 1996).
Applying these differences in the coding of in-groups
and out-groups to the findings of Experiment I, we hypothesize that the younger subjects considered themselves members of the same social group as the experimenters and older subjects considered the experimenters
to be members of an out-group. To test this hypothesis,
we changed the appearance of the experimenters so that
they could be classified as members of an out-group by
the younger subjects. EXPERIMENT 2
To examine the role of social group membership in the
detection of changes, a second experiment was conducted
using the same procedure as the first, but with one critical change: The same two experimenters dressed as construction workers (see Figure 2). The experimenters again wore different clothing: One wore a construction hat with
writing on the front, a large tool belt, and a light blue shirt,
and the other wore a newer hat without writing, no tool
belt, and a black shirt. The experiment was conducted in
the same location as Experiment 1, which happened to be
approximately 50 m from a construction site. As in Experiment I, an experimenter approached a pedestrian to
ask for directions to a building on campus. During the
conversation, the experimenters were switched. Unlike
in the first experiment, all 12 pedestrians who participated in Experiment 2 were from the younger age group
(Cornell graduate or undergraduate students), the group
that had always detected the change in Experiment 1.
The questions asked ofthe subjects were identical to those
of Experiment 1 except that subjects were informed immediately after providing directions that the experimenters
were not actually construction workers but were doing a
study as part of the psychology department. Results and Discussion
In contrast to the younger pedestrians in Experiment I,
all of whom noticed the change, only 4 of the 12 pedestrians in Experiment 2 reported noticing the switch when
asked if they had seen anything unusual. Five subjects
failed to report the change and were surprised to learn of
the switch. An additional 3 subjects reported noticing
nothing unusual but then claimed to have noticed the
switch of experimenters. Unlike pedestrians who clearly
noticed the change, these 3 pedestrians could not accurately describe any of the differences between the experimenters, suggesting that the demands of the task led
them to report noticing the change even though they prob- 648 SIMONS AND LEVIN ably had not. Thus, subjects from the same age group
that had successfully detected the change in Experiment I detected it only 33% of the time in Experiment 2.
When the experimenters appeared to be members of
an out-group, thereby decreasing the likelihood that students would code individuating features, the ability to
detect a change to the centrally attended object in a scene
was dramatically reduced. One subject who failed to detect the change essentially stated our predicted hypothesis: She said that she had just seen a construction worker
and had not coded the properties of the individual. That
is, she quickly categorized the experimenter as a construction worker and did not retain those features that
would allow individuation. Even though the experimenter
was the center of attention, she did not code the visual
details and compare them across views. Instead, she
formed a representation of the category, trading the visual
details of the scene for a more abstract understanding of
its gist or meaning. GENERAL DISCUSSION
These simple experiments build on classic findings offailures of eyewitness identification (e.g., Loftus, 1979) and distortions in memory
(Bartlett, 1932/1977) as well as recent demonstrations of change blindness for objects (Pashler, 1988; Phillips, 1974; Simons, 1996), photographs (Aginsky et al., 1997; Grimes. 1996; O'Regan et aI., 1996;
Rensink et aI., 1997), and motion pictures (Levin & Simons, 1997; Simons, 1996; Simons & Levin, 1997). Yet,unlike earlier demonstrations,
this experiment shows that people may not notice changes to the central
object in a scene even when the change is almost instantaneous and happens in the middle of an ongoing, natural event. Attention alone does
not suffice for change detection, even in the real world. Instead, successful change detection probably requires effortful encoding of precisely those features or properties that will distinguish the original from
the changed object.
One potential objection to our results derives from the pragmatics of
the interaction. Specifically, subjects may have detected the change but
the social demands of the situation precluded them from reporting it.
This possibility is substantially diminished by the subjects in each experiment who reported noticing nothing unusual but then reported
noticing the switch. Although these subjects probably did not notice the
change, the social demands of the situation encouraged them to report
having noticed the switch when asked directly. Thus, the demands of
the situation seem biased to increase reports of the switch rather than to
Another possible objection is that the task of giving directions distracted subjects from focusing their attention on the experimenters. That
is, subjects were focused on the map rather than their conversational
partner. Anecdotally at least, subjects appeared focused on the interaction and the conversation, often making eye contact with the experimenters, hearing their voices, and taking turns in a conversation. Although we believe the results are not specific to this situation, ongoing
experiments using a different type of interaction are directly examining
the possible distraction caused by the map and possible disruptions to
the representation of the first experimenter caused by the unusual nature of the interruption.
A more fundamental question involves assessing the similarity of the
experimenters. Clearly, no one would be surprised if pedestrians failed
to notice a substitution of identically dressed identical twins. The inability to notice small changes is unsurprising because such changes
naturally occur between views. For example, people rarely notice variation in the position and orientation of moveable objects such as body parts (Levin & Simons, 1997). If we constantly noticed such changes,
they would likely detract from our ability to focus on other, more important aspects of our visual world. Change detection as a method relies on the tendency of our visual system to assume an unchanging
world. The fact that we do not expect one person to be replaced by another during an interaction may contribute to our inability to detect such
changes. A critical question for future research is why some changes
are more likely to be detected than others. Clearly we would be quite
surprised ifsubjects missed a switch between enormously different people (e.g., a switch from a 4 ft 9 in. female of one race to a 6 ft 5 in. male
of another). The change in this case would alter not only the visual details of the person, but also their category membership. If, as suggested
by other recent findings of change blindness, we retain only abstracted
information and not visual details from one view to the next, changes
to category membership may well be detectable. Abstraction of category information is clearly central to coding other people (e.g., the effects of in-group and out-group discussed earlier) and may underlie the
representation of other objects across views as well.
What, then, separates inconsequential changes to details from
changes that are worth noting? Although there is no easy answer to this
question, we would like to propose several guidelines or heuristics for
identifying consequential changes for future studies of change blindness. These guidelines, used individually or together, can help constrain
the generation of significant changes to scenes.
First, significant changes to a scene should be easily verbalizable,
and often verbalized (see Simons, 1996). Changes that are easily verbalized likely cross a category boundary, making them more likely to be
detected. The best example of this principle is the change in the color
of the experimenter's shirt in Experiment 2. Both shirt colors (blue and
black) have basic color names.
Second, the original and changed objects should be easily discriminable in simultaneous viewing. Everyone is familiar with the comicspage game of finding differences between two extremely similar images. In such cases, the change is camouflaged, making it difficult to
detect even when both the original and changed version are present. In
our experiment, as in most studies of change blindness (see Simons &
Levin, 1997), changes generally meet this criterion (e.g., the difference
in shirt colors is plainly visible in Figure 2).
Third, changes should affect the immediate functional needs of the
perceiver. For example, changes to the spatial configuration of objects
or their parts can be significant, even if they are not easy to verbalize.
Spatial layout information is crucial to navigation and other immediate
needs of the organism. For our experiments, variation in the configuration of facial features is precisely the information used in identifying
other people; hence the person change should be readily detectable.
Fourth, naive subjects should predict successful change detection. If
change blindness is counterintuitive, we can be certain that the change is
not trivial. For our experiments, individuals unfamiliar with our research
consistently predicted that the change of experimenters would be plainly
detectable. To examine this possibility for our experiments, we informally
polled a class of 50 introductory psychology students by reading them
the following description of our event: "You are walking on the Cornell
campus and a man with a puzzled look asks you to help him find Olin library. Youstop and give him directions. While you are giving directions,
two people carrying a door rudely walk between you and the lost pedestrian. After the door has passed, the person you were giving directions to
is now a different person wearing different clothes." By a show of hands,
they claimed without exception that they would detect the change.
By applying these four heuristics, researchers can be fairly certain
that a change is detectable and that change blindness would be an important finding....
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