Transcript

Hello. Today we are going to learn about a very interesting phenomenon related to
perception called Illusions. Illusions? Really? Do we experience it? And if I
experience it, is there something wrong with me?
An illusion is an incorrect perception. A few very common examples of illusions
are the experience of Mirage on a dry road or perceiving the moon on the horizon
as much larger than the moon in the sky or perceiving a rope at night as a snake,
perceiving trees outside the train as if running in the opposite direction of the train.
Thus we all experience illusions. It is very common to experience illusions. But
there are two forms of incorrect perceptions which we need to distinguish before
we understand illusions in detail –
The first one is called as an
1. Illusion – which is a distorted perception of something which is really there. For
example, a rope is lying on the floor and it is misperceived as a snake. Thus the
stimulus is present in the experience of an illusion.
And the other form is called as a
2. Hallucination - which is a distorted perception of something which is not really
there. For example, hearing the whispers when no one is in the room or when no
one is actually talking. One patient of schizophrenia felt the presence of spirits and
voices talked to him in his head, telling him what to do - this is an example of
hallucination.
There are 3 major differences between Illusion and Hallucination –
1. In the perception of an Illusion the stimulus is present; but it is perceived
incorrectly. In the perception of hallucination the stimulus is absent and it is still
perceived.
2. Illusions are experienced by all of us. It is a very common and normal
phenomenon. Whereas, hallucinations are experienced by people who have certain
mental illnesses.
3. Illusions occur as a result of stimuli in the environment. Whereas hallucinations
originate in the brain, therefore hallucinations are personal.
Now let us understand more about illusions and different types of illusions -
As we have learnt, an illusion is a perception that does not correspond to reality.
People think they see something when the reality is quite different. We can also
think of Illusions as the visual stimuli that fool our eyes.

There are 2 types of illusions –

1. those that occur due to physical processes
2. those that occur due to cognitive processes

The illusion of Mirage is an example of an illusion which occurs due to physical

process. The reflection of the light on the dry road at the distance creates the
illusion of water on the road. This is what we call a mirage.
Countless illusions related to cognitive processes exist. Cognitive processes are the
processes that occur in our brain, for example perception is a cognitive process.
Certain illusions occur due to distortions in perception. Illusions due to cognitive
processes fall into 2 categories –
1. Illusion of Size
2. Illusion of Shape or Area
Now let us understand these 2 categories in detail.
1. Illusion of Size –
Look at the figure shown on your screen –
This is the Gateway Arch monument in St. Louis. Missouri, United States. This St.
Louis Gateway falsely appears taller than it is wide; actually its height and width
are equal. This illusion stems from our tendency to perceive objects which appear
higher in our visual field as more distant, thus more in length.
Researchers have found multiple reasons for illusions. But one explanation is
provided by the theory of Misapplied Constancy. This theory suggests that when
looking at illusions, we interpret certain cues as suggesting that some parts are
farther away than others. Our powerful tendency toward size constancy then comes
into play, with the result that we perceptually distort the length of various lines.
Size constancy is a tendency to interpret an object as always being the same size,
regardless of its distance from the viewer (or regardless of the size of the image
cast on the retina). For example, a friend who is 6 feet tall and who is standing far
away is still perceived as tall even though his image on the retina is smaller.
Now look at the next figure on your screen. This is another example of illusion
related to size. This is called the Ponzo Illusion. Line A and line B are actually of
the same length, but line A which is at a distance appears to be bigger.
Now look at the next picture on your screen. This is called – Müller Lyer Illusion.
In the Müller–Lyer illusion, lines of equal length appear unequal; the line with the
wings pointing outward looks longer than the line with the wings pointing inward.

Now let us understand the second type of illusions related to cognitive processes
which are illusions of shape or area –
A very famous and common example of illusion related to shape is the Moon
Illusion, that is - the moon looks bigger at the horizon (about 30 percent bigger)
than at its highest point in the sky. In reality the size of the moon doesn’t reduce
when it reaches the highest point in the sky. Then why does this illusion occur? In
part, because when the moon is near the horizon, we can see that it is farther away
than trees, houses, and other objects. When it is overhead at its zenith, such cues
are lacking. Thus, the moon appears larger near the horizon because there are cues
available that cause us to perceive that it is very far away. Once again, our
tendency toward size constancy plays a role here.
Like illusions of size or area, shape illusions can influence perception—sometimes
producing some unsettling consequences. Look at the figure presented on your
screen. This illusion is called the Poggendorf illusion.

In this illusion, a line disappears at an angle behind a solid figure, reappearing at

the other side — at what seems to be an incorrect position.
There is a real world example of this illusion reported by Coren and Girgus. In
1965 two airplanes were about to arrive in New York City, and because of the
Poggendorf illusion, they perceived that they were on a collision course. Both
pilots changed their path to correct for what they perceived as an error, and thus
the planes collided. The result was four deaths and forty-nine injuries—all because
of an illusion.
In the next figure on your screen, you can see some more examples of illusions of
area or shape.

In these figures the horizontal lines appear to be bent in the middle; whereas they
are not bent.
Now let us understand few more types of illusions –
Let us do one small activity. Please take a white sheet of paper for this activity.
Just keep it infront of you on your desk. Now look at this picture on your screen of
a colorful flower for a little while, say for a minute. Stare at the flower for a
minute.
And now look away at the sheet of the paper. You will see an afterimage of the
flower on the plain white paper. This is one of the very common illusions that we
experience. This illusion occurs due to opponent processes in the retina or LGN of
the thalamus after light information has been detected by the rods and cones.
Illusions are sometimes based on early sensory processes, as we have seen in this
example, or sometimes due to subsequent sensory processing or higher level
assumptions made by the brain’s visual systems. Another illusion based on early
sensory processes is known as The Hermann Grid.
Look at the matrix of squares presented on your screen. Do you notice anything
interesting as you look at different parts of the figure? Particularly at the
intersection of white lines? You probably see gray blobs or diamonds that fade
away or disappear completely when you try to look directly at them. This is the
Hermann Grid.
One explanation for this illusion is attributed to the responses of the neurons in the
primary visual cortex. These neurons respond best to bars of light of a specific
orientation and therefore we see gray blobs at the matrix.

Illusions of Motion
Another type of illusions is illusions of motion. Sometimes people perceive an
object as moving when it is actually still.
One example of this is called the Autokinetic Effect. If you look at a small,
stationary light in a darkened room, it will appear to move or drift because there
are no surrounding cues to indicate that the light is not moving.
Another example of illusion of motion is stroboscopic motion. Some of you might
have played this game in childhood – drawing a series of still pictures on pages of
a notebook and running them rapidly. This gives us perception of movement in
those pictures. This technique is used in motion pictures.
We can also see movement in static images. Look at the pictures on your screen –
What do you see? Do you see movements? Yes, right? This is an illusion of
motion.
There are various explanations for this type of motion illusion, ranging from
factors that depend on the image’s luminance and/or color arrangement to possibly
slight differences in the time it takes the brain to process this information.
Another very commonly used illusion is Phi Phenomenon, in which lights turned
on in sequence appear to move. This is used in decorations in many festivals in
India. This is also used to indicate direction of a hotel.
One important point we need to keep in mind when we are understanding illusions
is culture. Culture plays a role in the experience of illusions. Marshall Segall and
colleagues found that people in western cultures are far more susceptible to the
Müller–Lyer illusion, because they live in a world of buildings with lots of straight
lines and corners. People from non western cultures, like Zulus, who live in round
huts with few corners, rarely experience this illusion.
Illusions are not limited to visual processes. Indeed, there are numerous examples
of illusions for our other sensory modalities, including touch and audition. You can
demonstrate for yourself the illusion of touch temperature. First, place one hand in
a container of hot water and the other hand in cold water. Then place both hands in
a container of lukewarm water. What do you feel? Most people experience a
dramatic difference in perceived temperature between the two hands; the hand
initially placed in hot water feels the lukewarm water as cool, whereas the hand
initially placed in cold water feels it as hot. How do we explain this illusion? When
we touch an object, the temperature of the area of our skin in contact with it shifts
toward that of the object’s surface. So when we perceive an object to be warm or
cool, our experience stems partly from the temperature difference between the
object and our skin, not solely from the actual temperature of the object.
Now let us reflect on what we have learnt so far. Take a paper and pen and answer
the following questions.
Q.1. state whether following statements are true or false
a) Illusions and Hallucinations are exactly the same phenomena.
b) Illusions are very commonly experienced.
c) In Hallucination, the stimulus is present; but is perceived incorrectly.
d) In this picture line A is bigger than line B.

e) In this picture, black line is the continuation of a red line.

Now check your answers –
Statement a) False
Statement b) True
Statement c) False
Statement d) False
Statement e) True
Q.2) Name the illusions.
Read the following examples and write the type of the illusion they are example of
a) At the entrance of the wedding lawn, the lights on the welcoming arch were
dancing to the tunes of music.
b) The vertical lines in both the pictures are equal in size.
Check your answers.
The correct answer of first example is illusion of Movement, specifically Phi
Phenomenon
The correct answer of the second example is the Müller–Lyer illusion.

Thus in this video, we have learnt what is meant by illusion, differences between
illusion and hallucination and different types of illusion. Thank you.