|Scientific American, Special on Perception, 2008, door
Vilayanur S. Ramachandran en Diane Rogers-Ramachandran||
How blind are we?
We have eyes, yet we do not see
Tussentitel: Have you noticed any gorillas walking by? How can you be sure
Pretend you are a member of an audience watching several people dribbling and
passing a basketball among themselves. Your job is to count the number of times
each player makes a pass to another person during a 60-second period. You find
you need to concentrate, because the ball is flying so quickly. Then, someone
dressed in a gorilla suit ambles across the floor (left).
He walks through the players, turns to face the viewers, thumps his chest and
leaves. Astonishingly, as Daniel J. Simons, now at the University of Illinois,
and Christopher F. Chabris of Harvard University learned when they conducted
this study, 50 percent of people fail to notice the gorilla.
We think of our eyes as video cameras that make a flawless recording of the
world around us, but this demonstration shows how little information we actually
take in at a glance.
The gorilla experiment is the culmination of a long line of related studies on
attention and vision that were begun more than three decades ago by, among many
researchers, Ulric Neisser of Cornell University, Ronald A. Rensink of the
University of British Columbia, Anne Treisman of Princeton University, Harold
Pashler of the University of California, San Diego, and Donald M. MacKay of
Keele University in England.
Researchers refer to the gorilla effect as "inattentional blindness" or "change blindness," which in turn is part of a more
general principle at work in our visual system. Our brain is constantly trying
to construct meaningful narratives from what we see. Things that do not quite
fit the script or that are not relevant to a particular task occupying our
interest are wiped wholesale from consciousness. (Whether such deleted
information is nonetheless processed unconsciously has yet to be investigated.)
A simple example of how the brain's running narrative can interfere with
perception is the children's game "spot the difference" (below left). The two
images are similar enough that the brain assumes they must be identical; it
takes minutes of careful inspection to locate the disparities.
The value of having an underlying brain "story" becomes clear when you consider
how jumbled sensory inputs can be. As you survey the room around you, the image
on your retina is jumping rapidly as various parts of the scene excite different
bits of retina. Yet the world appears stable. Researchers once believed that the
experience of having an unbroken view was entirely created by the brain sending
a copy of the eyemovement command signals originating in the frontal lobes to
the visual centers. The visual areas were thought to be "tipped off" ahead of
time that the jumping image on the retina was caused by eyes moving and not by
the world moving.
But an effect you can demonstrate for yourself at home shows that this cannot be
the entire reason. (Jonathan Miller, an opera director in London, and one of us
[Ramachandran] independently observed the effect in the early 1990s.) Turn a
television set upside down. Gently! Better yet, flip the TV's image optically
with a prism. Alternatively, you can turn the TV sound off and then stand
slightly to the side of the set, looking at the screen with your peripheral
vision. Put the TV on any channel and watch what happens. You will see sudden,
jarring changes and visual jolts.
Next, gaze at the broadcast with the TV right side up, viewing it straight on
and with the sound at normal volume. Now the cuts and pans of the camera flow
smoothly and seamlessly into one another-in fact, you do not even notice them.
Even when the scene switches, say, from one talking head to the other as they
alternate in conversation, you do not see a head transforming or morphing from
one to the other as your mind alternates between each of the two speakers.
Instead you experience your vantage point shifting.
What is going on? The answer is that when the TV is right side up and you can
hear the sound, the brain can construct a sensible narrative. The cuts, pans and
other changes are simply ignored as irrelevant, however gross they might be
In contrast, when the scene is upside down or viewed with peripheral vision and
the sound is off, it is hard for the brain to make meaningful sense out of what
the visual centers perceive, so you start to notice the big changes in the
physical image. This effect is not true just for visual scenes on the boob tube
but also for your entire life's experiences; the unity and coherence of
consciousness is mostly convenient, internally generated fiction.
The scene does not have to be complex for change blindness to occur, either. In
1992 British neurobiologist Colin Blakemore and Ramachandran conducted an
experiment on attendees of a seminar we gave at the Salk Institute for
Biological Studies. We first showed a movie frame containing three abstract,
colored shapes: a red square, a yellow triangle and a blue ctrcle (left
illustration, above). We left this frame up for two seconds, then replaced it
with the same three shapes, which were each shifted in position by a small
degree. The audience observed that all three appeared to flicker or "glitch"
slightly. The big
surprise came when we then swapped one of the three shapes-the circle-with a
different form: a square (right illustration, above). Most people simply did not
notice, except in those few instances when someone accidentally happened to be
focusing all his or her attention on that particular object. Even with three
simple objects, we experience sensory overload and change blindness.
Finally, imagine that you are staring fixedly at a little red X. Slightly off to
the left we briefly show you a cross. All you have to tell us is which is
longer-the cross's vertical or horizontal line. That task is something people
can do effortlessly. Now we surreptitiously introduce a word directly on the
cross during the second that you are judging line lengths. Arien Mack of New
School University and Irvin Rock, then at Rutgers University, discovered that
people will not spot the word.
Maybe you are reading this article in a busy cafe. Have you noticed any gorillas
Given the Simons demonstration, how can you be so sure that none did? We suppose
it depends on how interesting and attention-grabbing you have found this article
to be. M
VILAYANUR S. RAMACHANDRAN and DIANE ROGERSRAMACHANDRAN are at the Center for
Brain and Cognition at the University of California, San Diego. They serve on
Scientific American Mind's board of advisers.
Illustration: Most people simply will not notice if a shape in one movie frame
is changed in the next.
Gorillas In Our Midst: Sustained Inattentlonal Blindness for Dynamic Events.
Daniel J. Slmons and Chrlstopher F. Chabrls in Perception, Vol. 28, pages
1059-1074; 1999. Paper and video clips are available at
Inattentional Blindness. Arlen Mack and Irvln Rock. MIT Press, 2000.
Several papers and a demonstration on attention and failure to see ,change by
Ronald A. Renslnk and his co-authors are available at
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