Perceptual constancy is
our tendency to see familiar objects as having a constant
shape, size, and colour, regardless of any changes in perspective,
distance, or lighting that they undergo. Our perception
of these objects under such changing conditions is much
closer to the general image of them that we have memorized
than to the actual stimulus reaching our retinas. Thus,
perceptual constancy is what lets you recognize a plate
of vegetables, for instance, regardless of whether you
are looking down on it at your own table, or noticing it
on someone else's table in front of you in a dark restaurant,
or seeing it in side view on a huge billboard several dozen
metres away from you as you drive past it in broad daylight.
French psychologist Jean Piaget showed that this perceptual
constancy is far from innate and is learned in childhood.
As children learn how to grasp objects, they form an idea
of the objects' size and distance, and as children move around,
they learn to recognize objects from various angles and under
various light conditions.
WHAT OPTICAL ILLUSIONS
SHOW US ABOUT VISUAL PERCEPTION
The human visual system analyzes the interactions
between visible electromagnetic waves and the objects in our environment,
extracts information about the world from them, and makes visual
perception possible. Visual perception is considered a dynamic process
that goes far beyond simply replicating the visual information provided
by the retina.
To perceive is to create a figure or shape that does not necessarily appear as
such in the real world but that we can represent mentally so that we can recognize
it under various conditions (for instance, when it is partly hidden). Hence,
by studying the way that the brain fills in missing or ambiguous visual information,
we can learn a lot about the way that we perceive the world. Optical illusions
provide fertile ground for such study, because they involve ambiguous images
that force the brain to make decisions that tell us about how we perceive things.
Most optical
illusions result from processes in the cortex, but some
do originate in the retina. One such illusion
is the Hermann grid shown here, in which gray spots appear
at the intersections of the rows and columns created by
the squares, because of a phenomenon called lateral retinal inhibition. If you stare
directly at one of these intersections, however, the gray
spot disappears, and it looks white, because then you are
using the cells of the fovea, which do much less correction
for an area's surroundings.
Here is another example of an illusion of retinal origin.
Look at the bird for about 20 seconds, then look at the
cage: the bird's silhouette will appear inside it, in red.
The reason this time is the cones, the colour-sensitive receptors in the retina.
As you stare at the bird, the green cones covering its
shape on your retina gradually become desensitized, so
that the other cones begin to dominate. When you then look
at the cage, with its white background, a red bird appears,
because the white minus the green creates a reddish light.
This image that persists when you stop looking at an object
is called a residual image.
Despite appearances, these two circles
are the same shade of grey (place your mouse cursor over
either image to prove it).
These two circles are the same colour,
even though the one on the blue background seems reddish
and the one on the red background seems bluish (if you
don't believe it, place your mouse cursor over the picture).
Thus the background effect also influences colour perception.
Because the human visual system interprets lighting as
coming from above, we perceive the spheres whose upper
part is brighter as emerging from this image, and the
spheres whose upper part is darker as receding into it.
The same phenomenon also applies to the apparent brightness
of a surface, which depends not only on its own luminance
but also on the luminance of the areas surrounding it. As
shown here, given two circles that are the same shade of
grey, the one placed on a dark background will look lighter
and the one placed on a light background will look darker.
One explanation often offered for this phenomenon is that
the brain thinks of the circle on the dark background as
a light disc in a dimly lit location, and the circle on the
light background as a dark disc in a brightly lit location.
This would suffice to make the two circles appear to be different
shades of grey even though they are not.
Another way to explain this phenomenon is to think of our
perception as being adapted to past surrounding stimuli,
so that they constitute a sort of baseline from which subsequent
stimuli are perceived—a bit like the profound silence
you perceive when the refrigerator suddenly stops running.
What you were perceiving as silence before the refrigerator
stopped was actually the steady humming that you have gotten
used to.
The lighting of a scene is an important
factor that the visual system considers to help it identify
objects. As soon as you interpret a visual object as possibly
being three-dimensional, your visual system immediately
tries to determine where the light is coming from, then
uses this information to decode the object's properties.
The most disturbing optical illusions
often combine several phenomena all working in the same direction to
accentuate the error of interpretation that the visual system is
making. In Adelson's checkerboard, below, you would swear that squares
A and B were black and white, but they're actually the same shade
of grey. Here's how it works.
First of all, there is
an illusion of context (see above). Square A is surrounded
by light squares, which make you perceive it as darker. Conversely,
square B is surrounded by dark squares, which make you perceive
it as lighter. Meanwhile, your eyes automatically interpret
the darker area to the left of the cylinder as a shadow,
because the gradations of green on the cylinder suggest that
a light source to its right is making it cast a shadow to
its left. On the basis of past experience, your visual system
assumes that square B would actually be brighter in full
light, since even in "shadow" it seems brighter
than the squares surrounding it.
This illusion ultimately works because
every square is surrounded by a well defined "X" structure
consisting of four other squares, which strongly indicates to
your visual system that the square at the centre must be interpreted
as a change in the colour of the surface itself, and not a change
caused by differences in light or shadow.
Far from demonstrating the faults in the human visual system, these
phenomena actually reveal the powerful mechanisms of discernment
that let us isolate and identify objects among the myriad confusing
shapes in the real world.
There are many illusions
in which you perceive a figure standing out from its background
even though this subjective image has no lines defining
its boundaries and displays no differences in brightness
that would let you isolate it from its surroundings.
Once again, the visual system is not passive. It automatically
extends line segments into parts of the drawing where they
are missing, so that your mind imagines that an object has
been placed over the abstract shapes and lines in the drawing.
The perception of such contourless figures thus
reflects some innate properties of the way the visual system is wired.
In another category of illusions that rely on the subjectivity
of contours, the figure can become the background, and
vice versa. The psychologists who developed Gestalt theory
created many such images in which the figure/ground relationship
is ambiguous. These images demonstrated the central tenet
of Gestalt theory: that the whole has global properties
different from those derived from the sum of its parts.
In the first, classic example shown to the right here,
two profiles facing each other delimit a space that can
be seen as a goblet, and your perception can alternate
between the profiles and the goblet. If you focus most
of your attention on the light part of the image, you will
perceive that part as the figure and automatically see
the dark part as simply the background. The reverse is
also true. Thus, perceiving one figure prevents you from
perceiving the other.
It thus seems that to interpret a complex image, your brain
has to identify a main figure and relegate the rest of
the image to the background. Such illusions clearly demonstrate
how your visual system groups and separates the characteristics
of a complex image in order to recognize objects within
it. Some facetious artists take advantage
of these necessary associational mechanisms of perception
to leaves two possibilities open, each of which is just
as plausible as the other.
Because the missing quarters of
these 4 discs all face inward, they create the perception
of a square with a subjective contour.
A circle seems to have been placed
on top of these converging lines, even though there is
no contour defining this circle.
Two profiles, or a goblet?
A saxophone player, or a woman's
face?
What's showing here, the gentlemen's
scalps, or the lady's bosom?
Your visual system takes two-dimensional
images projected onto your two retinas and uses these images to
reconstruct a three-dimensional perception of the world around
you. To perceive the depth in a visual scene,
your visual cortex relies on two kinds of information: the information
that your binocular vision provides by integrating the two slightly
different images from your two eyes, and the information that your
monocular vision provides from the image perceived by each eye
separately.
When an object is close up, you rely mainly on the disparity between
the two images perceived by your two eyes—in other words,
on your binocular vision. The reason is simply
that the closer an object is to your eyes, the greater the difference
in the angles from which your two eyes view it.
But even with monocular vision, you can receive
an impression of depth, because your brain deduces it from several
indicators.
- Interposition is certainly the most common depth
indicator. Whenever one object hides your view of another object
either partly or completely, your brain deduces that the hidden
object is farther away, simply because the other object is in front
of it.
- Atmospheric perspective is created by the dust
particles and water vapour in the air, which cause objects to seem
dimmer and blurrier the farther away they are.
- Texture gradients appear
when you look at surfaces from a certain angle; the texture seems
to become denser and less detailed as your eye moves toward the
part of the surface that is farthest from you.
- Object size is another depth indicator that
your brain refers to constantly. When you don't know the exact
size of two objects, but you do know that they are identical, and
one of them projects a smaller image on your retina, you interpret
it as being farther away. In other words, you judge the two objects'
relative size. Similarly, if you are very sure of the size of a
familiar object, you use its size as a reference to estimate its
distance, since you know that the smaller it appears, the farther
away it is.
- Parallax movement occurs when when you are in
motion yourself, and objects that are at different distances from
you appear to be moving at different speeds. The farther away these
objects are, the smaller the parallax movement.
Despite appearances, the monster being
chased is the same size as the one doing the chasing. Don't
believe it? Place your mouse cursor over the picture.
The first involves drawings like the one to the left,
where two identical objects are placed along converging
lines that imply perspective. The two monsters in this
picture are the same size, so the two images that they
make on your retina are the same size too. But because
your brain assumes that the monster who is closer to the
virtual horizon must be farther away, your brain also assumes
that this monster must be larger, in order to create the
same size image on your retina.
The other exception involves heavenly bodies such as the
Moon, which create the well known illusion of the Full Moon appearing
larger when it first comes up over the horizon.
One important source of
ambiguity for the visual system is that the world is three-dimensional,
but the images that it projects onto your retina are two-dimensional.
Hence differing objects, depending on their distance and
orientation, may occupy the same amount of surface area
on your retina. Your brain therefore becomes confused,
and tries to use other indicators to clarify the situation.
Two such indicators are your own past experience with the
object in question and the experience
of the human species, which is encoded in your
genes.
A given angle projected on
the retina may come from various objects at various angles,
of varying lengths, and with varying orientations in
space.
