NCERT Textbook - Sensory, Attentional and Perceptual Processes | Psychology Class 11 - Humanities/Arts PDF Download

 Page 1


Psychology
60
Chapter
4
• understand the nature of sensory processes,
• explain the processes and types of attention,
• analyse the problems of form and space perception,
• examine the role of socio-cultural factors in perception, and
• reflect on sensory, attentional and perceptual processes in everyday life.
After reading this chapter, you would be able to
Sensory Sensory Sensory Sensory Sensory, A , A , A , A , Attentional and ttentional and ttentional and ttentional and ttentional and
P P P P Perceptual Processes erceptual Processes erceptual Processes erceptual Processes erceptual Processes
Sensory Sensory Sensory Sensory Sensory, A , A , A , A , Attentional and ttentional and ttentional and ttentional and ttentional and
P P P P Perceptual Processes erceptual Processes erceptual Processes erceptual Processes erceptual Processes
The quality of life is determined
by its activities.
– Aristotle
Introduction
Knowing the World
Nature and Varieties of Stimulus
Sense Modalities
Attentional Processes
Selective Attention
Divided Attention (Box 4.1)
Sustained Attention
Span of Attention (Box 4.2)
Attention Deficit Hyperactivity Disorder (Box 4.3)
Perceptual Processes
Processing Approaches in Perception
The Perceiver
Principles of Perceptual Organisation
Perception of Space, Depth, and Distance
Monocular Cues and Binocular Cues
Perceptual Constancies
Illusions
Socio-Cultural Influences on Perception
Key Terms
Summary
Review Questions
Project Ideas
Contents
2024-25
Page 2


Psychology
60
Chapter
4
• understand the nature of sensory processes,
• explain the processes and types of attention,
• analyse the problems of form and space perception,
• examine the role of socio-cultural factors in perception, and
• reflect on sensory, attentional and perceptual processes in everyday life.
After reading this chapter, you would be able to
Sensory Sensory Sensory Sensory Sensory, A , A , A , A , Attentional and ttentional and ttentional and ttentional and ttentional and
P P P P Perceptual Processes erceptual Processes erceptual Processes erceptual Processes erceptual Processes
Sensory Sensory Sensory Sensory Sensory, A , A , A , A , Attentional and ttentional and ttentional and ttentional and ttentional and
P P P P Perceptual Processes erceptual Processes erceptual Processes erceptual Processes erceptual Processes
The quality of life is determined
by its activities.
– Aristotle
Introduction
Knowing the World
Nature and Varieties of Stimulus
Sense Modalities
Attentional Processes
Selective Attention
Divided Attention (Box 4.1)
Sustained Attention
Span of Attention (Box 4.2)
Attention Deficit Hyperactivity Disorder (Box 4.3)
Perceptual Processes
Processing Approaches in Perception
The Perceiver
Principles of Perceptual Organisation
Perception of Space, Depth, and Distance
Monocular Cues and Binocular Cues
Perceptual Constancies
Illusions
Socio-Cultural Influences on Perception
Key Terms
Summary
Review Questions
Project Ideas
Contents
2024-25
Chapter 4 • Sensory, Attentional and Perceptual Processes
61
kinds of information about various objects.
However, in order to be registered, the objects
and their qualities (e.g., size, shape, colour)
must be able to draw our attention. The
registered information must also be sent to the
brain that constructs some meaning out of
them. Thus, our knowledge of the world
around us depends on three basic processes,
called sensation, attention, and perception.
These processes are highly interrelated; hence,
they are often considered as different elements
of the same process, called cognition.
NATURE AND VARIETIES OF STIMULUS
The external environment that surrounds us
contains a wide variety of stimuli. Some of them
can be seen (e.g., a house), while some can be
heard only (e.g., music). There are several
others that we can smell (e.g., fragrance of a
flower) or taste (e.g., sweets). There are still
others that we can experience by touching (e.g.,
softness of a cloth). All these stimuli provide
us with various kinds of information. We have
very specialised sense organs to deal with these
different stimuli. As human beings we are
bestowed with a set of seven sense organs.
These sense organs are also known as sensory
receptors or information gathering systems,
because they receive or gather information
from a variety of sources. Five of these sense
KNOWING THE WORLD
The world in which we live is full of variety of
objects, people, and events. Look at the room
you are sitting in. You will find so many things
around. Just to mention a few, you may see
your table, your chair, your books, your bag,
your watch, pictures on the wall and many
other things. Their sizes, shapes, and colours
are also different. If you move to other rooms
of your house, you will notice several other
new things (e.g., pots and pans, almirah, TV).
If you go beyond your house, you will find still
many more things that you generally know
about (trees, animals, buildings). Such
experiences are very common in our day-to-
day life. We hardly have to make any efforts
to know them.
If someone asks you, “How can you say
that these various things exist in your room,
or house, or in the outside environment?”, you
will most probably answer that you see or
experience them all around you. In doing so,
you are trying to tell the person that the
knowledge about various objects becomes
possible with the help of our sense organs (e.g.,
eyes, ears). These organs collect information
not only from the external world, but also from
our own body. The information collected by
our sense organs forms the basis of all our
knowledge. The sense organs register several
While some of our receptors are clearly observable (for example, eyes or ears), others
lie inside our body, and are not observable without the help of electrical or mechanical
devices. This chapter will introduce you to various receptors that collect a variety of
information from the external and internal worlds. You will also know some
important things about attention, which helps us to notice and register the
information that our sense organs carry to us. Different types of attention will be
described along with the factors that influence them. At the end, we will discuss the
process of perception that allows us to understand the world in a meaningful way.
You will also have an opportunity to know how we are sometimes deceived by
certain types of stimuli such as figures and pictures.
Introduction
2024-25
Page 3


Psychology
60
Chapter
4
• understand the nature of sensory processes,
• explain the processes and types of attention,
• analyse the problems of form and space perception,
• examine the role of socio-cultural factors in perception, and
• reflect on sensory, attentional and perceptual processes in everyday life.
After reading this chapter, you would be able to
Sensory Sensory Sensory Sensory Sensory, A , A , A , A , Attentional and ttentional and ttentional and ttentional and ttentional and
P P P P Perceptual Processes erceptual Processes erceptual Processes erceptual Processes erceptual Processes
Sensory Sensory Sensory Sensory Sensory, A , A , A , A , Attentional and ttentional and ttentional and ttentional and ttentional and
P P P P Perceptual Processes erceptual Processes erceptual Processes erceptual Processes erceptual Processes
The quality of life is determined
by its activities.
– Aristotle
Introduction
Knowing the World
Nature and Varieties of Stimulus
Sense Modalities
Attentional Processes
Selective Attention
Divided Attention (Box 4.1)
Sustained Attention
Span of Attention (Box 4.2)
Attention Deficit Hyperactivity Disorder (Box 4.3)
Perceptual Processes
Processing Approaches in Perception
The Perceiver
Principles of Perceptual Organisation
Perception of Space, Depth, and Distance
Monocular Cues and Binocular Cues
Perceptual Constancies
Illusions
Socio-Cultural Influences on Perception
Key Terms
Summary
Review Questions
Project Ideas
Contents
2024-25
Chapter 4 • Sensory, Attentional and Perceptual Processes
61
kinds of information about various objects.
However, in order to be registered, the objects
and their qualities (e.g., size, shape, colour)
must be able to draw our attention. The
registered information must also be sent to the
brain that constructs some meaning out of
them. Thus, our knowledge of the world
around us depends on three basic processes,
called sensation, attention, and perception.
These processes are highly interrelated; hence,
they are often considered as different elements
of the same process, called cognition.
NATURE AND VARIETIES OF STIMULUS
The external environment that surrounds us
contains a wide variety of stimuli. Some of them
can be seen (e.g., a house), while some can be
heard only (e.g., music). There are several
others that we can smell (e.g., fragrance of a
flower) or taste (e.g., sweets). There are still
others that we can experience by touching (e.g.,
softness of a cloth). All these stimuli provide
us with various kinds of information. We have
very specialised sense organs to deal with these
different stimuli. As human beings we are
bestowed with a set of seven sense organs.
These sense organs are also known as sensory
receptors or information gathering systems,
because they receive or gather information
from a variety of sources. Five of these sense
KNOWING THE WORLD
The world in which we live is full of variety of
objects, people, and events. Look at the room
you are sitting in. You will find so many things
around. Just to mention a few, you may see
your table, your chair, your books, your bag,
your watch, pictures on the wall and many
other things. Their sizes, shapes, and colours
are also different. If you move to other rooms
of your house, you will notice several other
new things (e.g., pots and pans, almirah, TV).
If you go beyond your house, you will find still
many more things that you generally know
about (trees, animals, buildings). Such
experiences are very common in our day-to-
day life. We hardly have to make any efforts
to know them.
If someone asks you, “How can you say
that these various things exist in your room,
or house, or in the outside environment?”, you
will most probably answer that you see or
experience them all around you. In doing so,
you are trying to tell the person that the
knowledge about various objects becomes
possible with the help of our sense organs (e.g.,
eyes, ears). These organs collect information
not only from the external world, but also from
our own body. The information collected by
our sense organs forms the basis of all our
knowledge. The sense organs register several
While some of our receptors are clearly observable (for example, eyes or ears), others
lie inside our body, and are not observable without the help of electrical or mechanical
devices. This chapter will introduce you to various receptors that collect a variety of
information from the external and internal worlds. You will also know some
important things about attention, which helps us to notice and register the
information that our sense organs carry to us. Different types of attention will be
described along with the factors that influence them. At the end, we will discuss the
process of perception that allows us to understand the world in a meaningful way.
You will also have an opportunity to know how we are sometimes deceived by
certain types of stimuli such as figures and pictures.
Introduction
2024-25
Psychology
62
organs collect information from the external
world. These are eyes, ears, nose, tongue, and
skin. While our eyes are primarily responsible
for vision, ears for hearing, nose for smell, and
tongue for taste, skin is responsible for the
experiences of touch, warmth, cold, and pain.
Specialised receptors of warmth, cold, and pain
are found inside our skin. Besides these five
external sense organs, we have also got two
deep senses. They are called  kinesthetic and
vestibular systems. They provide us with
important information about our body position
and movement of body parts related to each
other. With these seven sense organs, we
register ten different variety of stimuli. For
example, you may notice whether a light is
bright or dim, whether it is yellow, red or green,
and so on. With sound you may notice whether
it is loud or faint, whether it is melodious or
distracting, and so on. These different qualities
of stimuli are also registered by our sense
organs.
SENSE MODALITIES
Our sense organs provide us with first-hand
information about our external or internal
world. The initial experience of a stimulus or
an object registered by a particular sense
organ is called sensation. It is a process
through which we detect and encode a variety
of physical stimuli. Sensation also refers to
immediate basic experiences of stimulus
attributes, such as “hard”, “warm”, “loud”, and
“blue”, which result from appropriate
stimulation of a sensory organ. Different sense
organs deal with different forms of stimuli and
serve different purposes. Each sense organ is
highly specialised for dealing with a particular
kind of information. Hence, each one of them
is known as a sense modality.
Functional Limitations of Sense Organs
Before we move on to a discussion of sense
organs, it is important to note that our sense
organs function with certain limitations. For
example, our eyes cannot see things which
are very dim or very bright. Similarly our ears
cannot hear very faint or very loud sounds. The
same is true for other sense organs also. As
human beings, we function within a limited
range of stimulation. For being noticed by a
sensory receptor, a stimulus has to be of an
optimal intensity or magnitude. The
relationship between stimuli and the
sensations they evoke has been studied in a
discipline, called psychophysics.
In order to be noticed a stimulus has to
carry a minimum value or weight. The
minimum value of a stimulus required to
activate a given sensory system is called
absolute threshold or absolute limen (AL).
For example, if you add a granule of sugar to
a glass of water, you may not experience any
sweetness in that water. Addition of a second
granule to water may also not make it taste
sweet. But if you go on adding sugar granules
one after another, there will come a point when
you will say that the water is now sweet. The
minimum number of sugar granules required
to say that the water is sweet will be the AL of
sweetness.
It may be noted at this point that the AL is
not a fixed point; instead it varies considerably
across individuals and situations depending
on the people’s organic conditions and their
motivational states. Hence, we have to assess
it on the basis of a number of trials. The
number of sugar granules that may produce
the experience of “sweetness” in water on
50 per cent of occasions will be called the AL
of sweetness. If you add more number of sugar
granules, the chances are greater that the
water will be reported more often as sweet than
plain.
As it is not possible for us to notice all
stimuli, it is also not possible to differentiate
between all stimuli. In order to notice two
stimuli as different from each other, there has
to be some minimum difference between the
value of  those stimuli. The smallest difference
in the value of two stimuli that is necessary to
notice them as different is called difference
threshold or difference limen (DL). To
understand it, we may continue with our
“sugar water” experiment. As we have seen,
2024-25
Page 4


Psychology
60
Chapter
4
• understand the nature of sensory processes,
• explain the processes and types of attention,
• analyse the problems of form and space perception,
• examine the role of socio-cultural factors in perception, and
• reflect on sensory, attentional and perceptual processes in everyday life.
After reading this chapter, you would be able to
Sensory Sensory Sensory Sensory Sensory, A , A , A , A , Attentional and ttentional and ttentional and ttentional and ttentional and
P P P P Perceptual Processes erceptual Processes erceptual Processes erceptual Processes erceptual Processes
Sensory Sensory Sensory Sensory Sensory, A , A , A , A , Attentional and ttentional and ttentional and ttentional and ttentional and
P P P P Perceptual Processes erceptual Processes erceptual Processes erceptual Processes erceptual Processes
The quality of life is determined
by its activities.
– Aristotle
Introduction
Knowing the World
Nature and Varieties of Stimulus
Sense Modalities
Attentional Processes
Selective Attention
Divided Attention (Box 4.1)
Sustained Attention
Span of Attention (Box 4.2)
Attention Deficit Hyperactivity Disorder (Box 4.3)
Perceptual Processes
Processing Approaches in Perception
The Perceiver
Principles of Perceptual Organisation
Perception of Space, Depth, and Distance
Monocular Cues and Binocular Cues
Perceptual Constancies
Illusions
Socio-Cultural Influences on Perception
Key Terms
Summary
Review Questions
Project Ideas
Contents
2024-25
Chapter 4 • Sensory, Attentional and Perceptual Processes
61
kinds of information about various objects.
However, in order to be registered, the objects
and their qualities (e.g., size, shape, colour)
must be able to draw our attention. The
registered information must also be sent to the
brain that constructs some meaning out of
them. Thus, our knowledge of the world
around us depends on three basic processes,
called sensation, attention, and perception.
These processes are highly interrelated; hence,
they are often considered as different elements
of the same process, called cognition.
NATURE AND VARIETIES OF STIMULUS
The external environment that surrounds us
contains a wide variety of stimuli. Some of them
can be seen (e.g., a house), while some can be
heard only (e.g., music). There are several
others that we can smell (e.g., fragrance of a
flower) or taste (e.g., sweets). There are still
others that we can experience by touching (e.g.,
softness of a cloth). All these stimuli provide
us with various kinds of information. We have
very specialised sense organs to deal with these
different stimuli. As human beings we are
bestowed with a set of seven sense organs.
These sense organs are also known as sensory
receptors or information gathering systems,
because they receive or gather information
from a variety of sources. Five of these sense
KNOWING THE WORLD
The world in which we live is full of variety of
objects, people, and events. Look at the room
you are sitting in. You will find so many things
around. Just to mention a few, you may see
your table, your chair, your books, your bag,
your watch, pictures on the wall and many
other things. Their sizes, shapes, and colours
are also different. If you move to other rooms
of your house, you will notice several other
new things (e.g., pots and pans, almirah, TV).
If you go beyond your house, you will find still
many more things that you generally know
about (trees, animals, buildings). Such
experiences are very common in our day-to-
day life. We hardly have to make any efforts
to know them.
If someone asks you, “How can you say
that these various things exist in your room,
or house, or in the outside environment?”, you
will most probably answer that you see or
experience them all around you. In doing so,
you are trying to tell the person that the
knowledge about various objects becomes
possible with the help of our sense organs (e.g.,
eyes, ears). These organs collect information
not only from the external world, but also from
our own body. The information collected by
our sense organs forms the basis of all our
knowledge. The sense organs register several
While some of our receptors are clearly observable (for example, eyes or ears), others
lie inside our body, and are not observable without the help of electrical or mechanical
devices. This chapter will introduce you to various receptors that collect a variety of
information from the external and internal worlds. You will also know some
important things about attention, which helps us to notice and register the
information that our sense organs carry to us. Different types of attention will be
described along with the factors that influence them. At the end, we will discuss the
process of perception that allows us to understand the world in a meaningful way.
You will also have an opportunity to know how we are sometimes deceived by
certain types of stimuli such as figures and pictures.
Introduction
2024-25
Psychology
62
organs collect information from the external
world. These are eyes, ears, nose, tongue, and
skin. While our eyes are primarily responsible
for vision, ears for hearing, nose for smell, and
tongue for taste, skin is responsible for the
experiences of touch, warmth, cold, and pain.
Specialised receptors of warmth, cold, and pain
are found inside our skin. Besides these five
external sense organs, we have also got two
deep senses. They are called  kinesthetic and
vestibular systems. They provide us with
important information about our body position
and movement of body parts related to each
other. With these seven sense organs, we
register ten different variety of stimuli. For
example, you may notice whether a light is
bright or dim, whether it is yellow, red or green,
and so on. With sound you may notice whether
it is loud or faint, whether it is melodious or
distracting, and so on. These different qualities
of stimuli are also registered by our sense
organs.
SENSE MODALITIES
Our sense organs provide us with first-hand
information about our external or internal
world. The initial experience of a stimulus or
an object registered by a particular sense
organ is called sensation. It is a process
through which we detect and encode a variety
of physical stimuli. Sensation also refers to
immediate basic experiences of stimulus
attributes, such as “hard”, “warm”, “loud”, and
“blue”, which result from appropriate
stimulation of a sensory organ. Different sense
organs deal with different forms of stimuli and
serve different purposes. Each sense organ is
highly specialised for dealing with a particular
kind of information. Hence, each one of them
is known as a sense modality.
Functional Limitations of Sense Organs
Before we move on to a discussion of sense
organs, it is important to note that our sense
organs function with certain limitations. For
example, our eyes cannot see things which
are very dim or very bright. Similarly our ears
cannot hear very faint or very loud sounds. The
same is true for other sense organs also. As
human beings, we function within a limited
range of stimulation. For being noticed by a
sensory receptor, a stimulus has to be of an
optimal intensity or magnitude. The
relationship between stimuli and the
sensations they evoke has been studied in a
discipline, called psychophysics.
In order to be noticed a stimulus has to
carry a minimum value or weight. The
minimum value of a stimulus required to
activate a given sensory system is called
absolute threshold or absolute limen (AL).
For example, if you add a granule of sugar to
a glass of water, you may not experience any
sweetness in that water. Addition of a second
granule to water may also not make it taste
sweet. But if you go on adding sugar granules
one after another, there will come a point when
you will say that the water is now sweet. The
minimum number of sugar granules required
to say that the water is sweet will be the AL of
sweetness.
It may be noted at this point that the AL is
not a fixed point; instead it varies considerably
across individuals and situations depending
on the people’s organic conditions and their
motivational states. Hence, we have to assess
it on the basis of a number of trials. The
number of sugar granules that may produce
the experience of “sweetness” in water on
50 per cent of occasions will be called the AL
of sweetness. If you add more number of sugar
granules, the chances are greater that the
water will be reported more often as sweet than
plain.
As it is not possible for us to notice all
stimuli, it is also not possible to differentiate
between all stimuli. In order to notice two
stimuli as different from each other, there has
to be some minimum difference between the
value of  those stimuli. The smallest difference
in the value of two stimuli that is necessary to
notice them as different is called difference
threshold or difference limen (DL). To
understand it, we may continue with our
“sugar water” experiment. As we have seen,
2024-25
Chapter 4 • Sensory, Attentional and Perceptual Processes
63
the plain water is experienced as sweet after
the addition of certain number of sugar
granules. Let us remember this sweetness. The
next question is: how many sugar granules will
be needed in the water in order to experience
its sweetness as different from the previous
sweetness. Go on adding sugar granules one
after another tasting the water each time. After
addition of a few granules, you will notice at a
point that the water is now sweeter than the
previous one. The number of sugar granules
added to the water to generate an experience
of sweetness that is different from the previous
sweetness on 50 per cent of the occasions will
be called the DL of sweetness. Thus, difference
threshold is the minimum amount of change
in a physical stimulus that is capable of
producing a sensation difference on 50 per cent
of the trials.
You may realise by now that understanding
of sensations is not possible without
understanding the AL and DL of different types
of stimuli (for example, visual, auditory), but
that is not enough. Sensory processes do not
depend only on the stimulus characteristics.
Sense organs and the neural pathways
connecting them to various brain centers also
play a vital role in this process. A sense organ
receives the stimulus and encodes it as an
electrical impulse. For being noticed this
electrical impulse must reach the higher brain
centers. Any structural or functional defect or
damage in the receptor organ, its neural
pathway, or the concerned brain area may lead
to a partial or complete loss of sensation.
ATTENTIONAL PROCESSES
In the previous section we have discussed some
sensory modalities that help us in collecting
information from the external world and also
from our internal system. A large number of
stimuli impinge upon our sense organs
simultaneously, but we do not notice all of
them at the same time. Only a selected few of
them are noticed. For example, when you enter
your classroom you encounter several things
in it, such as doors, walls, windows, paintings
on walls, tables, chairs, students, schoolbags,
water bottles, and so on, but you selectively
focus only on one or two of them at one time.
The process through which certain stimuli are
selected from a group of others is generally
referred to as attention.
At this point it may be noted that besides
selection, attention also refers to several other
properties like alertness, concentration, and
search. Alertness refers to an individual’s
readiness to deal with stimuli that appear
before her/him. While participating in a race
in your school, you might have seen the
participants on the starting line in an alert state
waiting for the whistle to blow in order to run.
Concentration refers to focusing of awareness
on certain specific objects while excluding
others for the moment. For example, in the
classroom, a student concentrates on the
teacher’s lecture and ignores all sorts of noises
coming from different corners of the school. In
search an observer looks for some specified
subset of objects among a set of objects. For
example, when you go to fetch your younger
sister and brother from the school, you just
look for them among innumerable boys and
girls. All these activities require some kind of
effort on the part of people. Attention in this
sense refers to “effort allocation”.
Attention has a focus as well as a fringe.
When the field of awareness is centered on a
particular object or event, it is called focus or
the focal point of attention. On the contrary,
when the objects or events are away from the
center of awareness and one is only vaguely
aware of them, they are said to be at the fringe
of attention.
Vision and hearing are generally believed to be
the two most highly prized senses. What would
your life be if you lost any one of your senses?
Which sense would you find more traumatic to
lose? Why? Think and write down.
What if you could magically improve the
performance of one of your senses, which sense
would you choose to improve? Why? Could you
improve the performance of this one sense without
magic? Think and write down.
Discuss with your teacher.
Activity Activity Activity Activity Activity     4.1
2024-25
Page 5


Psychology
60
Chapter
4
• understand the nature of sensory processes,
• explain the processes and types of attention,
• analyse the problems of form and space perception,
• examine the role of socio-cultural factors in perception, and
• reflect on sensory, attentional and perceptual processes in everyday life.
After reading this chapter, you would be able to
Sensory Sensory Sensory Sensory Sensory, A , A , A , A , Attentional and ttentional and ttentional and ttentional and ttentional and
P P P P Perceptual Processes erceptual Processes erceptual Processes erceptual Processes erceptual Processes
Sensory Sensory Sensory Sensory Sensory, A , A , A , A , Attentional and ttentional and ttentional and ttentional and ttentional and
P P P P Perceptual Processes erceptual Processes erceptual Processes erceptual Processes erceptual Processes
The quality of life is determined
by its activities.
– Aristotle
Introduction
Knowing the World
Nature and Varieties of Stimulus
Sense Modalities
Attentional Processes
Selective Attention
Divided Attention (Box 4.1)
Sustained Attention
Span of Attention (Box 4.2)
Attention Deficit Hyperactivity Disorder (Box 4.3)
Perceptual Processes
Processing Approaches in Perception
The Perceiver
Principles of Perceptual Organisation
Perception of Space, Depth, and Distance
Monocular Cues and Binocular Cues
Perceptual Constancies
Illusions
Socio-Cultural Influences on Perception
Key Terms
Summary
Review Questions
Project Ideas
Contents
2024-25
Chapter 4 • Sensory, Attentional and Perceptual Processes
61
kinds of information about various objects.
However, in order to be registered, the objects
and their qualities (e.g., size, shape, colour)
must be able to draw our attention. The
registered information must also be sent to the
brain that constructs some meaning out of
them. Thus, our knowledge of the world
around us depends on three basic processes,
called sensation, attention, and perception.
These processes are highly interrelated; hence,
they are often considered as different elements
of the same process, called cognition.
NATURE AND VARIETIES OF STIMULUS
The external environment that surrounds us
contains a wide variety of stimuli. Some of them
can be seen (e.g., a house), while some can be
heard only (e.g., music). There are several
others that we can smell (e.g., fragrance of a
flower) or taste (e.g., sweets). There are still
others that we can experience by touching (e.g.,
softness of a cloth). All these stimuli provide
us with various kinds of information. We have
very specialised sense organs to deal with these
different stimuli. As human beings we are
bestowed with a set of seven sense organs.
These sense organs are also known as sensory
receptors or information gathering systems,
because they receive or gather information
from a variety of sources. Five of these sense
KNOWING THE WORLD
The world in which we live is full of variety of
objects, people, and events. Look at the room
you are sitting in. You will find so many things
around. Just to mention a few, you may see
your table, your chair, your books, your bag,
your watch, pictures on the wall and many
other things. Their sizes, shapes, and colours
are also different. If you move to other rooms
of your house, you will notice several other
new things (e.g., pots and pans, almirah, TV).
If you go beyond your house, you will find still
many more things that you generally know
about (trees, animals, buildings). Such
experiences are very common in our day-to-
day life. We hardly have to make any efforts
to know them.
If someone asks you, “How can you say
that these various things exist in your room,
or house, or in the outside environment?”, you
will most probably answer that you see or
experience them all around you. In doing so,
you are trying to tell the person that the
knowledge about various objects becomes
possible with the help of our sense organs (e.g.,
eyes, ears). These organs collect information
not only from the external world, but also from
our own body. The information collected by
our sense organs forms the basis of all our
knowledge. The sense organs register several
While some of our receptors are clearly observable (for example, eyes or ears), others
lie inside our body, and are not observable without the help of electrical or mechanical
devices. This chapter will introduce you to various receptors that collect a variety of
information from the external and internal worlds. You will also know some
important things about attention, which helps us to notice and register the
information that our sense organs carry to us. Different types of attention will be
described along with the factors that influence them. At the end, we will discuss the
process of perception that allows us to understand the world in a meaningful way.
You will also have an opportunity to know how we are sometimes deceived by
certain types of stimuli such as figures and pictures.
Introduction
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organs collect information from the external
world. These are eyes, ears, nose, tongue, and
skin. While our eyes are primarily responsible
for vision, ears for hearing, nose for smell, and
tongue for taste, skin is responsible for the
experiences of touch, warmth, cold, and pain.
Specialised receptors of warmth, cold, and pain
are found inside our skin. Besides these five
external sense organs, we have also got two
deep senses. They are called  kinesthetic and
vestibular systems. They provide us with
important information about our body position
and movement of body parts related to each
other. With these seven sense organs, we
register ten different variety of stimuli. For
example, you may notice whether a light is
bright or dim, whether it is yellow, red or green,
and so on. With sound you may notice whether
it is loud or faint, whether it is melodious or
distracting, and so on. These different qualities
of stimuli are also registered by our sense
organs.
SENSE MODALITIES
Our sense organs provide us with first-hand
information about our external or internal
world. The initial experience of a stimulus or
an object registered by a particular sense
organ is called sensation. It is a process
through which we detect and encode a variety
of physical stimuli. Sensation also refers to
immediate basic experiences of stimulus
attributes, such as “hard”, “warm”, “loud”, and
“blue”, which result from appropriate
stimulation of a sensory organ. Different sense
organs deal with different forms of stimuli and
serve different purposes. Each sense organ is
highly specialised for dealing with a particular
kind of information. Hence, each one of them
is known as a sense modality.
Functional Limitations of Sense Organs
Before we move on to a discussion of sense
organs, it is important to note that our sense
organs function with certain limitations. For
example, our eyes cannot see things which
are very dim or very bright. Similarly our ears
cannot hear very faint or very loud sounds. The
same is true for other sense organs also. As
human beings, we function within a limited
range of stimulation. For being noticed by a
sensory receptor, a stimulus has to be of an
optimal intensity or magnitude. The
relationship between stimuli and the
sensations they evoke has been studied in a
discipline, called psychophysics.
In order to be noticed a stimulus has to
carry a minimum value or weight. The
minimum value of a stimulus required to
activate a given sensory system is called
absolute threshold or absolute limen (AL).
For example, if you add a granule of sugar to
a glass of water, you may not experience any
sweetness in that water. Addition of a second
granule to water may also not make it taste
sweet. But if you go on adding sugar granules
one after another, there will come a point when
you will say that the water is now sweet. The
minimum number of sugar granules required
to say that the water is sweet will be the AL of
sweetness.
It may be noted at this point that the AL is
not a fixed point; instead it varies considerably
across individuals and situations depending
on the people’s organic conditions and their
motivational states. Hence, we have to assess
it on the basis of a number of trials. The
number of sugar granules that may produce
the experience of “sweetness” in water on
50 per cent of occasions will be called the AL
of sweetness. If you add more number of sugar
granules, the chances are greater that the
water will be reported more often as sweet than
plain.
As it is not possible for us to notice all
stimuli, it is also not possible to differentiate
between all stimuli. In order to notice two
stimuli as different from each other, there has
to be some minimum difference between the
value of  those stimuli. The smallest difference
in the value of two stimuli that is necessary to
notice them as different is called difference
threshold or difference limen (DL). To
understand it, we may continue with our
“sugar water” experiment. As we have seen,
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63
the plain water is experienced as sweet after
the addition of certain number of sugar
granules. Let us remember this sweetness. The
next question is: how many sugar granules will
be needed in the water in order to experience
its sweetness as different from the previous
sweetness. Go on adding sugar granules one
after another tasting the water each time. After
addition of a few granules, you will notice at a
point that the water is now sweeter than the
previous one. The number of sugar granules
added to the water to generate an experience
of sweetness that is different from the previous
sweetness on 50 per cent of the occasions will
be called the DL of sweetness. Thus, difference
threshold is the minimum amount of change
in a physical stimulus that is capable of
producing a sensation difference on 50 per cent
of the trials.
You may realise by now that understanding
of sensations is not possible without
understanding the AL and DL of different types
of stimuli (for example, visual, auditory), but
that is not enough. Sensory processes do not
depend only on the stimulus characteristics.
Sense organs and the neural pathways
connecting them to various brain centers also
play a vital role in this process. A sense organ
receives the stimulus and encodes it as an
electrical impulse. For being noticed this
electrical impulse must reach the higher brain
centers. Any structural or functional defect or
damage in the receptor organ, its neural
pathway, or the concerned brain area may lead
to a partial or complete loss of sensation.
ATTENTIONAL PROCESSES
In the previous section we have discussed some
sensory modalities that help us in collecting
information from the external world and also
from our internal system. A large number of
stimuli impinge upon our sense organs
simultaneously, but we do not notice all of
them at the same time. Only a selected few of
them are noticed. For example, when you enter
your classroom you encounter several things
in it, such as doors, walls, windows, paintings
on walls, tables, chairs, students, schoolbags,
water bottles, and so on, but you selectively
focus only on one or two of them at one time.
The process through which certain stimuli are
selected from a group of others is generally
referred to as attention.
At this point it may be noted that besides
selection, attention also refers to several other
properties like alertness, concentration, and
search. Alertness refers to an individual’s
readiness to deal with stimuli that appear
before her/him. While participating in a race
in your school, you might have seen the
participants on the starting line in an alert state
waiting for the whistle to blow in order to run.
Concentration refers to focusing of awareness
on certain specific objects while excluding
others for the moment. For example, in the
classroom, a student concentrates on the
teacher’s lecture and ignores all sorts of noises
coming from different corners of the school. In
search an observer looks for some specified
subset of objects among a set of objects. For
example, when you go to fetch your younger
sister and brother from the school, you just
look for them among innumerable boys and
girls. All these activities require some kind of
effort on the part of people. Attention in this
sense refers to “effort allocation”.
Attention has a focus as well as a fringe.
When the field of awareness is centered on a
particular object or event, it is called focus or
the focal point of attention. On the contrary,
when the objects or events are away from the
center of awareness and one is only vaguely
aware of them, they are said to be at the fringe
of attention.
Vision and hearing are generally believed to be
the two most highly prized senses. What would
your life be if you lost any one of your senses?
Which sense would you find more traumatic to
lose? Why? Think and write down.
What if you could magically improve the
performance of one of your senses, which sense
would you choose to improve? Why? Could you
improve the performance of this one sense without
magic? Think and write down.
Discuss with your teacher.
Activity Activity Activity Activity Activity     4.1
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In day-to-day life we attend to several things at
the same time. You must have seen people driving
a car and talking to a friend, or attending to phone
calls on a mobile set, or putting on sunglasses, or
listening to music. If we watch them closely, we
will notice that they are still allocating more effort
to driving than to other activities, even though
some attention is given to other activities. It
indicates that on certain occasions attention can
be allocated to more than one thing at the same
Box Box Box Box Box 4.1 Divided Attention Divided Attention Divided Attention Divided Attention Divided Attention
time. However, this becomes possible only with highly
practiced activities, because they become almost
automatic and require less attention to perform than
new or slightly practiced activities.
Automatic processing has three main
characteristics; (i) It occurs without intention, (ii) It takes
place unconsciously, and (iii) It involves very little (or
no) thought processes (e.g., we can read words or tie
our shoelaces without giving any thought to these
activities).
Attention has been classified in a number
of ways. A process-oriented view divides it into
two types, namely selective and sustained.
We will briefly discuss the main features of
these types of attention. Sometimes we can
also attend to two different things at the same
time. When this happens, it is called divided
attention. Box 4.1  describes when and how
the division of attention is possible.
Selective Attention
Selective attention is concerned mainly with the
selection of a limited number of stimuli or
objects from a large number of stimuli. We
have already indicated that our perceptual
system has a limited capacity to receive and
process information. This means that it can
deal only with a few stimuli at a given moment
of time. The question is, which of those stimuli
will get selected and processed? Psychologists
have identified a number of factors that
determine the selection of stimuli.
bright, and moving stimuli easily catch our
attention. Stimuli, which are novel and
moderately complex, also easily get into our
Factors Affecting Selective Attention
Several factors influence selective attention.
These generally relate to the characteristics of
stimuli and the characteristics of individuals.
They are generally classified as “external” and
“internal” factors.
External factors are related to the features
of stimuli. Other things held constant, the size,
intensity, and motion of stimuli appear to be
important determinants of attention. Large,
focus. Studies indicate that human
photographs are more likely to be attended to
than the photographs of inanimate objects.
Similarly, rhythmic auditory stimuli are more
readily attended to than verbal narrations.
Sudden and intense stimuli have a wonderful
capacity to draw attention.
Internal factors lie within the individual.
These may be divided into two main categories,
viz. motivational factors and cognitive factors.
Motivational factors relate to our biological
or social needs. When we are hungry, we notice
even a faint smell of food. A student taking an
examination is likely to focus on a teacher’s
instructions more than other students.
Cognitive factors include factors like interest,
attitude, and preparatory set. Objects or events,
which appear interesting, are readily attended
by individuals. Similarly we pay quick
attention to certain objects or events to which
we are favourably disposed. Preparatory set
generates a mental state to act in a certain way
and readiness of the individual to respond to
one kind of stimuli and not to others.
Theories of Selective Attention
A number of theories have been developed to
explain the process of selective attention. We
will briefly discuss three of these theories.
Filter theory was developed by Broadbent
(1956). According to this theory, many stimuli
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