The Invisible Gorilla Strikes Again Sustained Inattentional Blindness in Expert Observers

Abstract

Researchers have shown that people often miss the occurrence of an unexpected yet salient event if they are engaged in a different task, a phenomenon known as inattentional blindness. However, demonstrations of inattentional blindness have typically involved naive observers engaged in an unfamiliar task. What about expert searchers who have spent years honing their ability to detect small abnormalities in specific types of images? We asked 24 radiologists to perform a familiar lung-nodule detection task. A gorilla, 48 times the size of the average nodule, was inserted in the last case that was presented. Eighty-three percent of the radiologists did not see the gorilla. Eye tracking revealed that the majority of those who missed the gorilla looked directly at its location. Thus, even expert searchers, operating in their domain of expertise, are vulnerable to inattentional blindness.

Full text

Psychological Science
24(9) 1848 –1853
© The Author(s) 2013
Reprints and permissions:
sagepub.com/journalsPermissions.nav
DOI: 10.1177/0956797613479386
pss.sagepub.com
Research Report
When one is engaged in a demanding task, attention can
act like a set of blinders, making it possible for salient
stimuli to pass unnoticed right in front of one’s eyes
(Neisser & Becklen, 1975). This phenomenon of sus-
tained inattentional blindness (IB) is best known from
Simons and Chabris’s (1999) study in which observers
attended to a ball-passing game while a human in a
gorilla suit wandered through the field of play. Even
though the gorilla walked through the center of the
scene, a substantial portion of the observers did not
report seeing it (the video can be viewed at http://www
.theinvisiblegorilla.com/videos.html). Moving beyond
such demonstrations, one might ask whether IB still
occurs when the observers are experts who are highly
trained on the primary task. There is some evidence that
expertise mitigates the effect. For example, Memmert
(2006) found a decreased rate of IB among basketball
players who were asked to count the number of passes
in an artificial basketball game. However, when Potchen
(2006) asked radiologists to review cases as if for an
annual exam and showed them chest x-rays with a clavi-
cle (collarbone) removed, roughly 60% failed to notice
the missing bone. Finally, a recent observational report
documented that a misplaced femoral line was not
detected by a variety of health-care professionals who
evaluated the case (Lum, Fairbanks, Pennington, &
Zwemer, 2005).
Both of these instances of apparent IB in the medical
setting occurred when single-slice medical images were
viewed. Modern medical imaging technologies, such as
MRI, computed tomography (CT), and positron-emission
tomography (PET), are increasingly complex: The single
image of a chest x-ray has been replaced with hundreds
of slices in a chest CT scan. It is therefore important to
study whether IB occurs in these modern imaging modal-
ities. These situations are interesting because the observer
actively interacts with the stimulus—for example, scroll-
ing through a stack of images of the lung. This degree of
control may ameliorate the effects of IB because the
searcher is able to return to and further examine any
images that appear unusual.
479386PSSXXX10.1177/0956797613479386Drew et al.Sustained Inattentional Blindness in Expert Observers
research-article2013
Corresponding Author:
Trafton Drew, Visual Attention Lab, Harvard Medical School, 64
Sidney St., Suite 170, Cambridge, MA 02139
E-mail: tdrew1@partners.org
The Invisible Gorilla Strikes Again:
Sustained Inattentional Blindness
in Expert Observers
Trafton Drew, Melissa L.-H. Võ, and Jeremy M. Wolfe
Visual Attention Lab, Harvard Medical School, and Brigham and Women’s Hospital, Boston, Massachusetts
Abstract
Researchers have shown that people often miss the occurrence of an unexpected yet salient event if they are engaged in
a different task, a phenomenon known as inattentional blindness. However, demonstrations of inattentional blindness
have typically involved naive observers engaged in an unfamiliar task. What about expert searchers who have spent
years honing their ability to detect small abnormalities in specific types of images? We asked 24 radiologists to perform
a familiar lung-nodule detection task. A gorilla, 48 times the size of the average nodule, was inserted in the last case
that was presented. Eighty-three percent of the radiologists did not see the gorilla. Eye tracking revealed that the
majority of those who missed the gorilla looked directly at its location. Thus, even expert searchers, operating in their
domain of expertise, are vulnerable to inattentional blindness.
Keywords
visual attention, perception, selective attention
Received 8/1/12; Revision accepted 1/27/13

Sustained Inattentional Blindness in Expert Observers 1849
Moreover, whereas Potchen (2006) showed that radi-
ologists could miss the unexpected absence of a stimu-
lus, we wanted to know if they would miss the presence
of a readily detectable, highly anomalous item while per-
forming a task within their realm of expertise. In an hom-
age to Simons and Chabris’s (1999) study, we made that
item a gorilla. We compared the performance of radiolo-
gists with that of naive observers.
Design and Procedure
In CT lung-cancer screening, radiologists search a recon-
structed “stack” of axial slices of the lung for nodules that
appear as small light circles (Aberle et al., 2011). In
Experiment 1, 24 radiologists (mean age = 48, range =
28–70) had up to 3 min to freely scroll through each of
five chest CTs, searching for nodules as we tracked their
eye position. The five trials contained an average of 10
nodules, and the observers were instructed to click on
nodule locations with the computer mouse. In the final
trial, we inserted a gorilla with a white outline into the
lung (see Fig. 1). A typical stack of images from a chest
CT contains 100 to 500 slices. In the current study, the
stack that contained the gorilla had 239 slices.
Nine radiologists were tested at Brigham and Women’s
Hospital in Boston, Massachusetts, and 15 expert examin-
ers from the American Board of Radiology were tested at
a meeting of that organization in Louisville, Kentucky.
The gorilla measured 29 × 50 mm. Because of equipment
differences, the image size was slightly different at the
two sites, and consequently the size of the gorilla differed
slightly (Boston: 0.9 × 0.5 degrees of visual angle;
Louisville: 1.3 × 0.65 degrees of visual angle). To avoid
large onset transients, we had the gorilla fade into and
out of visibility over five 2-mm-thick slices (Fig. 1). The
total volume of the rectangular box that could hold the
gorilla was more than 7,400 mm3, roughly the size of a
box of matches. The gorilla was centered in depth near a
lung nodule such that both were clearly visible when the
gorilla was at maximum opacity. That is, if someone
pointed at the correct location in the static image and
asked you, “What is that?” you would have no trouble
answering, “That is a gorilla.” In the scans used in this
study, which were taken from the Lung Image Database
Consortium (Armato et al., 2011), the average volume of
the lung nodules was 153 mm3. Thus, the gorilla was
more than 48 times the size of the average nodule in the
images (see Fig. 2a).
Experiment 2 replicated Experiment 1 with 25 naive
observers (mean age = 33.7, range = 19–55), who had no
medical training. Prior to the experiment, the experi-
menter spent roughly 10 min teaching these observers
how to identify lung nodules. This experiment began
with a practice trial, during which the experimenter took
time to point out several nodules. The experimenter then
encouraged the observer to try to find nodules on his or
her own. Once the observer was able to detect at least
one nodule, the practice trial was concluded, and the
experimental trials began. As in Experiment 1, a subset
(12) of observers completed the study on a slightly
smaller screen. We observed no difference in gorilla or
nodule detection as a result of equipment differences.
Experiment 3 was a control experiment intended to
ensure that the gorilla was, in fact, visible. Twelve naive
observers (mean age = 37.3, range = 21–54) were shown
movies that progressed from the top to the bottom of the
same chest CT case that was used as the final trial in
Experiments 1 and 2. The gorilla was inserted into the
movies in the same location on 50% of the 20 trials, and
observers were asked to judge whether the gorilla was
present or absent on each trial. A circular cue indicated
the possible location of the gorilla on each trial. The
movies were presented at a rate of 35 or 70 ms per frame
(manipulated within subjects).
Fig. 1. Illustration of the slices showing the gorilla in the final trial of Experiments 1 and 2. The opacity of the gorilla increased from 50% to 100%
and then decreased back down to 50% over the course of 5 slices within a stack of 239.

1850 Drew et al.
Results
Experiment 1
The nodule detection task was challenging, even for
expert radiologists. The overall nodule detection rate was
55%. While engaged in this task, the radiologists freely
scrolled through the slices containing the gorilla an aver-
age of 4.3 times. At the end of the final case, we asked a
series of questions to determine whether they had noticed
the gorilla: “Did the final trial seem any different than any
of the other trials?” “Did you notice anything unusual on
the final trial?” and, finally, “Did you see a gorilla on the
final trial?” Twenty of the 24 radiologists failed to report
seeing a gorilla. This was not due to the gorilla being dif-
ficult to perceive: All 24 radiologists reported seeing the
gorilla when they were asked if they noticed anything
unusual in Figure 1 after completing the experiment (see
also the results for Experiment 3).
The radiologists had ample opportunity to find the
gorilla. On average, those who missed the gorilla spent
5.8 s viewing the five slices containing it (range = 1.1–12
s) and spent an average of 329 ms looking at the gorilla’s
location. Furthermore, eye tracking revealed that of the
20 radiologists who did not report the gorilla, 12 looked
directly at the gorilla’s location when it was visible. The
mean dwell time on the gorilla in this group was 547 ms.
Figure 2b shows the eye positions of a radiologist who
clearly fixated the gorilla but did not report it.
Experiment 2
None of the 25 naive observers reported noticing the
gorilla. As was the case with the radiologists in Experiment
1, all of the naive observers reported seeing the gorilla
when shown Figure 1. The results support the idea
(Memmert, 2006) that experts are somewhat less prone to
IB than novices are (Fisher’s exact test: p = .0497; see Fig.
3a). However, unlike in Memmert’s study, our two groups
showed a sizable difference in performance on the pri-
mary task. As expected, radiologists were much better at
detecting lung nodules (mean detection rate = 55%) than
were naive observers (12%), t(47) = 12.3, p < .001 (see
Fig. 3b).
Eye movement data followed the pattern seen with the
radiologists. The naive observers spent an average of
4.9 s searching the frames in which the gorilla was visible
and an average of 157 ms looking at the gorilla’s location.
Although both measures showed that radiologists who
missed the gorilla spent slightly more time searching in
its vicinity than did the naive observers, neither differ-
ence was significant, t(43) = 1.26, p = .22, and t(43) =
1.23, p = .22, respectively. Of the 25 naive observers, 9
looked at the gorilla’s location. The mean dwell time on
the gorilla in the latter group was 435 ms.
Experiment 3
Although all observers in Experiments 1 and 2 reported
seeing the gorilla when shown Figure 1 at the end of the
experiment, given the very high rate of IB in both stud-
ies, there was some concern that the gorilla was too dif-
ficult to detect when embedded within a stack of chest
CT images. We tested this possibility in Experiment 3.
The movies played at a fast or slower frame rate such that
the gorilla was visible for 175 or 350 ms, respectively—
substantially less time than the 4.9 s that the average
Fig. 2. Computed-tomography image containing the embedded gorilla
(a) and eye-position plot of a radiologist who did not report seeing the
gorilla (b). In (b), the circles represent eye positions recorded at 1-ms
intervals.

Sustained Inattentional Blindness in Expert Observers 1851
naive observer in Experiment 2 spent searching frames in
which the gorilla was present. Despite this large differ-
ence, performance on the detection task was near ceiling
(88% correct). Accuracy was not affected by the frame
rate, t(11) = 1.1, p = .18 (see Fig. 3c).
Discussion
In Experiment 1, 20 of 24 expert radiologists failed to
note a gorilla, the size of a matchbook, embedded in a
stack of CT images of the lungs. This is a clear illustration
that radiologists, though they are expert searchers, are
not immune to the effects of IB even when searching
medical images within their domain of expertise. Potchen
(2006) showed that radiologists could miss the absence
of an entire bone. Results from laboratory search tasks
have shown that it is harder to detect the absence of
something than to detect its presence (Treisman &
Souther, 1985). Our data show that under certain circum-
stances, experts can also miss the presence of a large,
anomalous stimulus. In fact, there is some clinical evi-
dence for errors of this sort in radiology. Lum et al. (2005)
reported a case study in which multiple emergency radi-
ologists failed to detect a misplaced femoral-line guide
wire that was mistakenly left in a patient and was clearly
visible on three different chest CT scans. Although these
scans were viewed by radiologists, emergency physi-
cians, internists, and intensivists, the guide wire was not
detected for 5 days. Clearly, radiologists can miss an
abnormality that is retrospectively visible when the
abnormality is unexpected.
It is reassuring that our experts exhibited somewhat
lower rates of IB than naive observers, as was reported
by Memmert (2006). In that earlier study, expertise was
defined as extensive basketball experience, and IB was
measured during an artificial task in which two groups of
individuals passed a ball back and forth while moving
randomly about a small area. The observers were asked
to count the number of passes completed by one group.
In this rather abnormal basketball game, the rate of IB
Radiologists Naive Observers
0
25
50
75
100
Inattentional-Blindness Rate (%)
a
Fast Slow
0
25
50
75
100
Presentation Rate
Gorilla Detection Rate (%)
c
Radiologists Naive Observers
0
25
50
75
100
Nodule Detection Rate (%)
b
Fig. 3. Experimental results. The graph in (a) shows the rate of inattentional blindness (i.e., the percentage of observers who
did not report seeing the gorilla) among the radiologists in Experiment 1 and the naive observers in Experiment 2. The graph
in (b) shows the percentage of nodules that were correctly marked by these same observers. The graph in (c) shows the rate at
which observers in Experiment 3 detected the gorilla as a function of presentation rate (fast: 35 ms/frame; slow: 70 ms/frame).
Error bars represent standard errors of the mean.

1852 Drew et al.
was lower for the experts than for observers with less
basketball experience. In the current study, high rates of
IB were obtained with a task and stimulus materials that
were very familiar to our expert observers: searching a
chest CT scan for signs of lung cancer.
Experts may perform slightly better on this IB task
than naive observers do because their attentional capac-
ity is less completely occupied by the primary task.
Simons and Jensen (2009) recently showed that the rate
of IB decreased when the primary task (counting the
number of times an object bounced) was made easier.
Along similar lines, there is evidence that training on a
specific task reduces the subsequent IB rate (Richards,
Hannon, & Derakshan, 2010). In our study, the radiolo-
gists certainly had much more experience on the specific
primary task, and were clearly better at it. Both factors
are likely to have contributed to the reduced rate of IB
observed in our experts. Nevertheless, even though the
radiologists were slightly better than the naive observers,
their miss rate of 83% indicates a striking level of IB.
Why do radiologists sometimes fail to detect such
large anomalies? Of course, as is critical in all IB demon-
strations, the radiologists were not looking for the unex-
pected stimulus. In most previous demonstrations of IB,
observers engaged in a primary task that was unrelated
to detection of the unexpected stimulus (e.g., counting
the number of passes or bounces, as in Most et al., 2001;
Richards et al., 2010; Simons & Chabris, 1999; Simons &
Jensen, 2009). Here, too, though detection of aberrant
structures in the lung would be a standard component of
the radiologist’s task, observers were not looking for
gorillas. Presumably, they would have done much better
at detecting the gorilla had they been told to be prepared
for such a target. Moreover, the observers were searching
for small, light nodules. Previous work with naive observ-
ers has shown that IB is modulated by the degree of
match between the designated targets and the unex-
pected item (Most et al., 2001). This suggests that our
observers might have fared better if we had used an
albino gorilla that better matched the luminance polarity
of the designated targets. Counterintuitively, perhaps a
smaller gorilla would have been more frequently detected
because it would have more closely matched the size of
the lung nodules.
Our results could be seen as an example of a phenom-
enon known as satisfaction of search, in which detection
of one stimulus interferes with detection of subsequent
stimuli (e.g., Berbaum et al., 1998). We placed the gorilla
on a slice that contained a nodule that was detected by
71% of the radiologists. Perhaps the observed rate of IB
was inflated by the presence of this nodule. Without run-
ning an additional experiment examining the detection
rate for the gorilla in the absence of the nodule, it
is difficult to be certain what role the presence of the
nodule played. However, if satisfaction of search truly
drove the IB effect, we would expect that radiologists
who missed the nodule would have been more likely to
detect the gorilla and that radiologists who found the
nodule would have been more likely to miss the gorilla.
Neither of these predictions held true: Of the 7 radiolo-
gists who missed the nodule, none detected the gorilla.
Furthermore, all of the radiologists who detected the
gorilla also detected the nodule on the same slice.
It would be a mistake to regard these results as an
indictment of radiologists. As a group, they are highly
skilled practitioners of a very demanding class of visual
search tasks. The message of the present set of results is
that even this high level of expertise does not immunize
individuals against inherent limitations of human atten-
tion and perception. Researchers should seek better
understanding of these limits, so that medical and other
man-made search tasks could be designed in ways that
reduce the consequences of these limitations.
Author Contributions
T. Drew developed the study concept. All authors contributed
to the study design. T. Drew collected and analyzed the data.
T. Drew wrote the manuscript in collaboration with J. M. Wolfe
and M. L.-H. Võ. All authors approved the final version of the
manuscript for submission.
Declaration of Conflicting Interests
The authors declared that they had no conflicts of interest with
respect to their authorship or the publication of this article.
References
Aberle, D. R., Adams, A. M., Berg, C. D., Black, W. C., Clapp,
J. D., Fagerstrom, R. M., . . . Sicks, J. D. (2011). Reduced
lung-cancer mortality with low-dose computed tomographic
screening. New England Journal of Medicine, 365, 395–
409.
Armato, S. G., III, McLennan, G., Bidaut, L., McNitt-Gray,
M. F., Meyer, C. R., Reeves, A. P., . . . Croft, B. Y. (2011).
The Lung Image Database Consortium (LIDC) and Image
Database Resource Initiative (IDRI): A completed reference
database of lung nodules on CT scans. Medical Physics, 38,
915–931.
Berbaum, K. S., Franken, E. A., Dorfman, D. D., Miller, E. M.,
Caldwell, R. T., Kuehn, D. M., & Berbaum, M. L. (1998).
Role of faulty visual search in the satisfaction of search
effect in chest radiography. Academic Radiology, 5,
9–19.
Lum, T. E., Fairbanks, R. J., Pennington, E. C., & Zwemer,
F. L. (2005). Profiles in patient safety: Misplaced femoral line
guidewire and multiple failures to detect the foreign body
on chest radiography. Academic Emergency Medicine, 12,
658–662.

Sustained Inattentional Blindness in Expert Observers 1853
Memmert, D. (2006). The effects of eye movements, age, and
expertise on inattentional blindness. Consciousness and
Cognition, 15, 620–627.
Most, S. B., Simons, D. J., Scholl, B. J., Jimenez, R., Clifford, E.,
& Chabris, C. F. (2001). How not to be seen: The contribu-
tion of similarity and selective ignoring to sustained inat-
tentional blindness. Psychological Science, 12, 9–17.
Neisser, U., & Becklen, R. (1975). Selective looking: Attending to
visually specified events. Cognitive Psychology, 7, 480–494.
Potchen, E. J. (2006). Measuring observer performance in chest
radiology: Some experiences. Journal of the American
College of Radiology, 3, 423–432.
Richards, A., Hannon, E. M., & Derakshan, N. (2010). Predicting
and manipulating the incidence of inattentional blindness.
Psychological Research, 74, 513–523.
Simons, D. J., & Chabris, C. F. (1999). Gorillas in our midst:
Sustained inattentional blindness for dynamic events.
Perception, 28, 1059–1074.
Simons, D. J., & Jensen, M. S. (2009). The effects of individual
differences and task difficulty on inattentional blindness.
Psychonomic Bulletin & Review, 16, 398–403.
Treisman, A., & Souther, J. (1985). Search asymmetry: A diag-
nostic for preattentive processing of separable features.
Journal of Experimental Psychology: General, 114, 285–310.