Johns Hopkins Gazette: December 12, 1994

Seeing is Not Necessarily Perceiving, Psychology Research Shows
By Emil Venere

If not for the brain s complex system of processing visual
images, people might see reality more like the disconnected bits
and pieces in a Picasso painting.
     Making sense of the visual world requires a mysterious
interplay between the brain and the constant stream of images it
receives. Now a Hopkins researcher has added a new wrinkle to the
understanding of vision.
     Steven Yantis, an associate professor in the Department of
Psychology, is studying the way the mind fills in the gaps when
visual information is missing. 
     One way to illustrate how the system works is to look at a
figure that has two squares with a rectangle between them (see
illustration). The brain assumes that there are actually two
rectangles, one lying on top of and therefore partly covering the
other so that only the ends of the bottom rectangle are visible.
     The same effect emerges by placing a pencil over the space
between two separated rectangles. Because the observer can t see
the gap that is hidden by the pencil, the two rectangles are
perceived as one continuous rectangle.
     "Your visual system is predisposed to interpret the image in
a way that is consistent with how the physical world is
structured," Dr. Yantis said.
     Vision is no simple sense, although it would appear to be.
"People don t think about it they just do it," said Dr. Yantis,
who has won the 1994 Distinguished Scientific Award for Early
Career Contribution to Psychology in the area of perception and
motor performance. The award, which will be issued in August by
the American Psychological Association, is given to outstanding
researchers who have had their doctorates for no longer than nine
     His recent work deals with how the brain interprets images
in motion. An example can be found by visiting a crowded
     "If you are at a baseball game and a player throws a
baseball and you are sitting behind somebody else so that you
actually can t see the ball for some part of its trajectory, you
still perceive it as a single trajectory, as one object, not as
one object that disappears and a different object that appears,"
he said. In other words, the broken trajectory seen by the retina
is experienced as continuous.
     Using objects on a computer screen, Dr. Yantis has taken
that concept one step further. If people are shown a white dot on
the screen and then the dot disappears, reappearing a fraction of
a second later a short distance from its original location, the
dot will be perceived as a single object moving from point A to
point B. The shorter the time interval between its disappearance
and reappearance, the more likely it will be perceived as a
single object moving through space.
     But as the time gap increases, the perception of motion
starts to fade, until the observer finally begins to see two
distinct objects, one at point A and one at point B.
     Researchers have known about this kind of "apparent motion"
for years. It serves as the basis for the appearance of
continuous motion in such things as motion pictures and
     But Dr. Yantis has discovered that if an occluding object,
much like the spectator who blocks the baseball trajectory, is
placed between points A and B, the observer will continue to see
the two dots as a single moving object even with long time gaps
that would normally eliminate the perception of motion.
     In essence, the brain is tricked into thinking that the dots
on the computer screen are the same object moving through space,
because the occluding surface between the dots appears to be
blocking the view of its trajectory.
     "You essentially ca  see  the object behind the occluder, in
your mind s eye," Dr. Yantis said. "Even with a long [time] gap I
can cause these objects to be perceived as stable and continuous,
if I have an occluding surface that can explain the gap."
     His findings, based on a recent experiment, will be detailed
in a paper to be published in an upcoming issue of the journal
Psychological Science.
     Sixteen Hopkins students were used as subjects in the
experiment. The students wore 3-D glasses to create the illusion
that the occluding object (a rectangle) was actually floating in
front of the dots on the computer screen. Each of the two lenses
of the 3-D glasses acted as "shutters," quickly switching between
opaque and clear, so that each eye received a slightly different
view of the screen. That caused the occluding rectangle to appear
to float stereoscopically in front of (or, in some cases, behind)
the dots. 
     When no occlusion was used, the students tended to see the
dots as two distinct objects, flickering in place, as the time
gap was increased to a tenth of a second. When the occluding
surface was present, subjects reported seeing the dot as a single
moving object much more often.
     But if the 3-D glasses were adjusted so that the occluding
surface appeared to float behind the dots, the illusion of
continuous motion was eliminated. This "control condition"
demonstrated that placing the rectangular occlusion in front of
the dots is essential for the illusion.
     The experiment shows how vision is not simply a matter of
the retina sending signals to the brain, like so many pictures
from a video camera.
     And although the "perceptual completion" might seem like
something learned from experience, scientists believe it may be
"hardwired" into the brain at birth, since past research has
found that even very young infants  vision works the same way as
adult vision. The research also rules out reasoning and inference
as an explanation because the effect happens too fast, within a
few tenths of a second. "It is happening at a very low, automatic
level," Dr. Yantis said.

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