One of the most common optical illusions
is the Full Moon, which looks so enormous
when it first comes up over the horizon. This is clearly
a misinterpretation on the brain's part, because the Moon
is 385 000 km from Earth and therefore always covers the
same number of degrees of arc on the retina—about 0.5—whether
it is at the horizon or directly overhead.
Since the image formed on the retina is thus always the
same, it is the presence of the horizon close to the disc
of the Moon that makes us perceive it as larger. To prove
this to yourself, cut a hole in a piece of paper, then close
one eye, hold the paper up to the other, and look through
the hole at the rising Moon while blocking your view of
the horizon. The Moon will look much smaller—so much
smaller that if you open your other eye, which can see the
horizon as well as the Moon, your two eyes will each see
a different-sized Moon!
From experience, we know that a cloud, an airplane, or a tree
will seem smaller if it is near the horizon than if it is
overhead. This rule applies to all the objects that we deal
with on Earth. Our visual apparatus seems to have been shaped by evolution
on the basis of this reality, so that we are at a loss when
we have to interpret an object like the Moon, which is so
far away that it occupies the same amount of area on our retina
regardless of whether it is near to or far from the horizon.
When the Moon is near the horizon, the brain may tell itself
that if this object's image doesn't become smaller near the
horizon, then it must be really big, and the brain then makes
us perceive the Moon accordingly.
Another way to explain this is to say that to the brain,
the default distance to an object is always less than the
distance to the horizon. Of course, the brain applies this
rule to terrestrial objects, since these are the only ones
it knows, except for the Moon and the Sun. The distance
to these two heavenly bodies does not vary whether they
are at the horizon or the zenith, which confuses our visual
systems. Some refinements to this explanation have been
proposed, and the psychological mechanisms that cause this
illusion with the Moon are still being debated.
WHAT OPTICAL ILLUSIONS SHOW US ABOUT VISUAL
PERCEPTION
Mechanisms that cause
optical illusions have been located in various places along
the nervous system's visual pathways. Some of these mechanisms
arise in the retina, but most of them result
from the way that the images captured with the eyes are reconstructed
by the visual cortex (see box below).
Contrary to what we intuitively believe, the information presented by our senses
does not actually correspond to reality directly. With vision, for example, the
image striking the retina contains vastly more information than is actually transmitted
to the brain by the optic nerve. This makes sense, when you consider that the
125 million photoreceptors in each retina converge onto
100 times fewer ganglion cells.
To compensate
for this massive loss of information and provide us with
visual perceptions that are rich in contrast, colour, and
movement, the brain introduces abstract parameters that
often fill in or amplify the fragments of reality that
it is given to work with. The brain's powers to interpret
visual information in this way are so great that it sometimes
creates an impression of coherence where there is none—in
other words, an optical illusion.
In geometric optical illusions, there
is generally an "inducing element"
that causes the misinterpretation and a "test element" that
is the subject of it. For example, in Zöllner's
illusion (right), the small vertical and horizontal lines
are the inducing element and the long diagonal lines
are the test element.
In the size-relationship illusion, the
proximity of a test element to larger inducing elements
causes the size of the test element to be underestimated.
The opposite occurs with smaller inducing elements, which
cause the size of a test element to be overestimated. The
result is that though two test elements are identical,
they can look different to us, because of the context effect.
The presence of lines suggesting perspective can
also create size illusions. Given two objects of equal
size, if one of them looks farther away because of perspective,
we will perceive it as being larger.
In Zöllner's illusion, the
long lines are parallel even though they look as if they
would intersect one another if they were extended. (If
you don't believe it, place your mouse cursor over the
picture.) The reason for this illusion is that the brain
tries to bring the angles between the short lines and
the long ones closer to 90°, thus
"tilting" the lines toward one another.
Place your mouse cursor over this
picture, and you'll see that the two central circles are
actually the same size.
The effective of perspective is strengthened
here by the checkerboard pattern, which your brain uses
to estimate the size of the two vertical lines. Place your
mouse cursor over this picture, and you'll see that these
two lines are actually the same height.
Two incompatible viewpoints
cleverly combined in one drawing.
Young woman or old woman?
The young woman's chin is the old woman's nose, and
the old woman's eye is the young woman's ear.
Some artistic optical illusions are constructed by combining
two different drawings that lead to incompatible interpretations.
Other artistic optical illusions involve
ambiguity, so that a drawing can be visually interpreted
in at least two ways that are mutually exclusive. Once
an observer has identified the markers for the various
possible interpretations, he or she can move among them
at will. These kinds of illusions in which the observer
goes back and forth between two interpretations of the
same image are similar to illusions in which the figure and the background
are interchangeable.
Motion illusions are
another major category of optical illusions .
Some images can give the illusion that their elements
are moving when you move yourself slightly relative to
them. In the image here, for example, if you stare at
the centre dot, then move your head in toward the screen,
the two circles will start to seem as if they are turning
in opposite directions.
For other motion illusions, you don't even have to move.
The particular arrangement of the graphic elements in
the picture suffices on its own to create the appearance
of movement as you look at it. That is what happens in
the figure here, because the pattern makes it hard for
your eye to determine the contours of the circle in the
centre.
In the image below, the illusion that some
of the wheels are turning occurs only in your peripheral vision:
as soon as you look straight at one of the wheels, it holds still,
but the wheels that are peripheral to it keep turning. Though this
illusion has not been fully explained, we do know that the order
in which the four areas of differing colour and brightness are
placed is decisive. More specifically, the illusory movement seems
to occur from black areas to adjacent areas that are dark but brighter
than the black ones (here, the blue areas), or from white areas
to adjacent areas that are coloured but not so bright as the white
ones (here, the yellow areas).
Source: Akiyoshi Kitaoka, Department of
Psychology, Ritsumeikan University, Kyoto, Japan
In other circumstances,
we can perceive motion when we are simply shown two or more stationary
images with a short enough time interval between them. One familiar
example of this kind of illusion is the phi effect.
In its simplest form, it occurs when an observer who has no reference
markers is alternately shown two points of light that are slightly
separated from each other (when one point goes dark, the other
lights up).
Geometric illusions do
not arise from the retina, because they appear almost as
clearly when the inducing element is placed in front of
one eye and the test element in front of the other. This
indicates that these illusions arise at the point where
the information from the two eyes converges for the first
time, beyond the lateral geniculate nucleus, in the visual cortex.
