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Biological psychology
Supplementary Chapter to Part 3 ‘Brain and Behaviour’
Dr Kirston Greenop
Psychology, School of Human and Community Development, University of the Witwatersrand
CHAPTER OBJECTIVES
After studying this chapter you should be able to:
• describe the organisation of the nervous system
• explain what efferent and afferent nerve fibres are
• compare the processes of the sympathetic and parasympathetic divisions of the peripheral nervous system
• describe the structure of neurons and synapses
• explain communication within neurons and between neurons
• describe the action and dysfunction of neurotransmitters
• explain the structure and function of the different areas of the hindbrain, midbrain, and forebrain
• apply knowledge about the cerebrum in understanding brain damage
• describe cerebral dominance
• explain what happens in a ‘split-brain’ patient.
Nosipho became aware of the effects of brain damage
when her granny had a stroke. Nosipho’s granny was sixtysix years old and had had medical problems in the past.
The doctors were most worried about her high blood pressure and she had been trying different medications in
order to find the one that would work the best. A neighbour had found her granny unconscious on the floor at her
house and rushed her to hospital. When Nosipho asked
her granny if she remembered what had happened, she
could only remember feeling dizzy while making tea. She
could not remember anything after that.
Nosipho often went to visit her granny in hospital, but
noticed that her behaviour had changed. Nosipho’s granny
would only notice her and talk to her if she was on the
right-hand side of her bed and ignored anyone standing on
the left side of the bed. She also had trouble dressing and
washing herself, as she seemed to ignore the left side of
her body. She only washed the right side of her face and
dressed the right side of her body and the nurse had to do
the rest. A CT scan (a type of X-ray) revealed that her
granny had the stroke in the right occipital-parietal region
at the back of her brain. From her studies in neuropsychology Nosipho could understand why her granny
acted in the strange manner that she had. The injury
resulted in what is known as unilateral neglect, where
patients generally do not pay attention to the left side of
their bodies.
A
Introduction
The nervous system
In the case study on the previous page, Nosipho witnessed the effects of injury to the brain. The brain is
part of the nervous system in the body. To understand what happened to Nosipho’s granny we have
to have an understanding of the psychobiology of
the brain. Knowledge about psychobiology is
important in our everyday lives too. Do you remember feeling angry when someone insulted you, or
afraid while watching a scary movie, or thinking
hard to answer an exam question? All of these
processes are the result of the workings of neuron
cells in the nervous system (Wortman, Loftus &
Weaver, 1999).
This chapter will explain how neurons make up
the nervous system. The individual workings of
each neuron will be explored, as well as the way in
which neurons connect to one another to form communication paths through the body and with the
outside world. The last section in this chapter will
focus on the central nervous system in depth, with a
focus on the brain, the brain structures, how these
structures function, and how they affect behaviour.
Although you are not aware of it, your nervous system is made up of billions of interconnected cells that
are constantly communicating with one another. In
order to understand the different kinds of cells in the
nervous system, you could consider a simple action
such as washing a cup. When you want to wash a
cup you pour some water into a sink, add some soap,
and feel the water to make sure that it is not too hot.
You then hold the cup with one hand and wash it
with the other. Being able to move your hands and
hold the cup properly is the result of moving your
muscles correctly. This happens as a result of the
efferent neurons in the body which are responsible
for contracting muscles and also secreting hormones
from the glands. Feeling the temperature of the water
involves the afferent neurons which receive information from the environment. All these different neurons are then bundled together to form nerves.
Figure 1 shows how the nervous system is organised.
The nervous system consists of the central nervous
system (including the brain and spinal cord) and the
Nervous system
Central nervous system
Spinal cord
Brain
Midbrain
Forebrain
Thalamus
Peripheral nervous system
Afferent
system
Hindbrain
Somatic system
(voluntary muscle
activation)
Autonomic system
(controls smooth
muscle, cardiac
muscle, and glands;
basically involuntary)
Hypothalammus
Sympathetic
(generally activates)
Cerebrum
(cerebral cortex)
Efferent
system
Limbic system
Corpus
callosum
Cerebellum
Pons
Medulla
Reticular formation
(begins at the level of the medulla
and runs up through the brain to
the level of the midbrain)
Figure 1 The organisation of the nervous system (adapted from Passer & Smith, 2001)
2 Psychology an introduction
Parasympathetic
(generally inhibits)
peripheral nervous system. The afferent and efferent
nerve pathways form part of the peripheral nervous
system which constantly communicates with the
central nervous system. The afferent nerve pathway
receives information from the environment through
the sensory receptors and sends this information to
the central nervous system. The efferent nerve pathway takes information from the central nervous
system to the muscles and glands and gives them
directions on how and when to act or move. This
efferent pathway is further divided into the somatic
and autonomic divisions.
The somatic division (‘soma’ means body) controls
all the muscles attached to your bones or skeleton.
These are the ones that allow you to jump, walk,
bend, and crawl, for example. The autonomic division
controls all the other muscles which are attached to
your internal organs and glands in the body. The
muscles attached to the somatic division control
voluntary movement and allow you to have control
over what movement you want to happen. The muscles attached to the autonomic division control
mainly involuntary actions such as your heartbeat
or secretion of hormones, for instance. This latter
process is not completely involuntary though, as
people can often affect their heartbeat by using
relaxation techniques, for example.
The autonomic nervous system is divided into the
sympathetic and parasympathetic nervous systems.
These two sections are involved in the body’s reaction to stress: the fight-or-flight response. The sympathetic nervous system is used to get the body ready
for action (whether this is fighting or preparing for
an injury). This involves:
• dilating or widening the pupils to take in as much
light as possible to see the stressor;
• relaxing the bronchi in the lungs, allowing large
amounts of air to come into the lungs;
• increasing the heart rate so that more oxygen is
pumped around the body;
• closing down the digestive system slightly to make
energy available to other areas of the body; and
• constricting the blood vessels so that blood pressure is increased.
Parasympathetic Division
Sympathetic Division
Dilates pupil
Constricts pupil
Inhibits flow of saliva
Stimulates flow of saliva
Cervical
Dilates bronchi
Slows heart rate
Accelerates heart rate
Thoracic
Constricts bronchi
Inhibits peristalsis and secretion
Stimulates peristalsis
and secretion
Lumbar
Secretion of
adrenaline and
nonadrenaline
Stimulates realease of bile
Stimulates conversion
of glycogen to bile
Sacral
Contracts bladder
Chain of
sympathetic
ganglia
Inhibits bladder contraction
Figure 2 The autonomic system of the peripheral nervous system
Biological psychology 3
Once the body has reacted to the stress by either
fighting or running away, the parasympathetic system inhibits the action and relaxes the body. This is
done by:
• contracting the pupils to normal size;
• contracting the bronchi;
• slowing the heart rate so that you do not constantly have a racing heart;
• reactivating the digestive system; and
• dilating the blood vessels once again.
Neurons and neural
transmission
Neurons
Structure of neurons
Not all neurons look exactly the same as they vary in
shape and size and 2 000 different types of neurons
have been discovered already (Passer & Smith, 2001).
Figure 3 shows the structures of a typical neuron.
You can think of the neuron as a train trip from
Johannesburg to Cape Town. The electric impulse or
message that travels down the neuron is the train on
which you are travelling. The message starts at the
dendrites. This is similar to the many roads that arrive
at Johannesburg central station. The dendrites receive
messages from other neurons. This message then travels through the cell body of the neuron, which keeps
track of all the messages coming from the dendrites.
This is similar to all the people arriving at the station
and finding their way to the appropriate trains. If the
train to Cape Town is selected, the train will travel on
Dendrites
Soma (cell body)
Nucleus
Axon hillock
the railway tracks out of the station directly to Cape
Town. Similarly, the message travels down the neuron
axon which transmits the message to other neurons.
The axon goes straight to the terminal buttons or axon
terminals (this would be Cape Town station for the
train on which you are travelling).
The axon may also be covered with a myelin sheath
which serves to insulate the axon and make the message stronger and faster. In the train example, this is
like parts of the railway track being in a better condition than others. The train travels faster and more
smoothly on the well-maintained parts of the track
around towns, but travels more slowly in the more
deserted areas where the track is poorer. Similarly,
myelinated axons transmit messages faster and more
efficiently then non-myelinated axons. The myelin
sheath develops in the early stages of human development. You have probably seen how children just
older than one year walk – they are unstable and often
fall over. This is because their neurons are still developing myelin (Peterson, 1997). Multiple sclerosis is a
disease which breaks down the myelin sheath and
uncovers parts of the axon. This affects the transmission of messages that travel from the brain to the muscles, causing an interruption in the message and an
effect on movement, for example (Feldman, 2000).
The message is passed onto another neuron at the
axon terminals. Similarly, at Cape Town station the
train passengers get into taxis to reach their final destination. Some substances travel in the opposite direction from the axon terminal buttons to the cell body so
that food and nourishment reaches the cell body.
Certain diseases (for example Lou Gehrig’s disease or
amyotrophic lateral sclerosis) affect this reverse movement and the neuron eventually dies from starvation.
Rabies is an example of a disease that travels in a
reverse direction up the neuron (Feldman, 2000).
Axon
Axon terminals
Myelin sheath
Figure 3 The components of a neuron (adapted from Kalat, 2001)
4 Psychology an introduction
Transmission of nerve impulses
Neurons have an electrical charge inside of them.
The impulse that travels down the neuron does so as
a wave of electrical activity. The neuron is surrounded by ion-filled fluid and contains a slightly
different ion-filled fluid. Ions have an electrical
charge and, similar to all things with electrical
charges, like charges repel each other and unalike
charges attract each other. Some of the ions in the
fluid are chloride (with a negative charge), sodium
(with a positive charge), and potassium (also with a
positive charge). This state of tension between the
ions is called the membrane potential.
When neurons are not doing anything they are
said to be in a resting state. The electrical charge
inside the neuron is at about -70 millivolts (onethousandth of a volt). In this state the gates for sodium (+) are closed and those for potassium (+) and
chloride (-) are slightly open. In this semi-permeable
Sodium channel
Sodium ions
Stimulus
Potassium
channel
Flow of
charge
Axon
membrane
Sodium ions
Potassium
ions
Axon
membrane
Flow of
charge
Figure 4 The action potential in the neuron (sodium ions travel
into the cell as depolarisation occurs and a small amount of
potassium cells travel out of the cell) (adapted from Passer &
Smith, 2001)
membrane state most of the sodium+ and chlorideare kept outside of the neuron and most of the
potassium+ is kept inside the membrane. The inside
of the neuron is mainly negatively charged and the
outside is mainly positively charged in the resting
state. Potassium, a positive charge, is kept inside the
neuron by the positively charged outside of the cell.
As the inside of the cell is negative, chloride, which
is also negative, is kept outside the cell (like charges
repel each other). Sodium, on the other hand, is
positive and is therefore attracted to the inside of the
cell which is predominantly negative. However, a
pump called the sodium-potassium pump continues
to pump sodium out of the cell and potassium into
the cell. Overall, the membrane is not very permeable to sodium. This means that even though sodium is attracted to the inside of the cell, it cannot
cross the membrane.
When something causes the permeability of the
cell to change and the sodium ions to rush into the
cell, the generally negative charge of the cell
becomes more positive. This is called the action
potential.
The graph in Figure 5 illustrates the different
stages of an action potential. The straight line at
-70mv is the cell in the resting potential. The following happens in an action potential:
1. When the threshold of excitation is reached, the
permeability of the cell changes, causing sodium
ions to rush into the cell. This stage is called
depolarisation.
2. As the millivolts increase, the potassium channels open and the potassium ions start to leave
the cell. This continues until the level of 40mv is
reached. At this point an action potential occurs.
3. At the top part of the graph, the sodium channels
become blocked and no more sodium ions can
flow into the cell. At this stage the potassium
gates are fully open and potassium is flowing
freely out of the cell. As this happens, the cell
becomes more negatively charged.
4. When the membrane potential returns to the
state in which it was initially, the potassium gates
also close. The sodium gates reset and wait for
the next depolarisation incident. This is called the
refractory period and the cell cannot fire in this
state.
5. In the refractory period the sodium-potassium
pump pushes sodium out of the cell and potassium into the cell. Eventually the cell returns to its
normal state.
(Adapted from Carlson, 2005)
Biological psychology 5
Synaptic transmission
Action potential
Voltage (millivolts)
+40
When the electrical impulse reaches the end of the
neuron it needs to pass the message onto another
neuron. The difficulty is that the terminal buttons
from the first neuron and the dendrites of the second
neuron do not actually meet. Between the two neurons is a space called the synaptic gap or cleft. The
electrical impulse has to pass over this gap to make
sure that the message continues in the next neuron.
Figure 6 illustrates the synaptic gap and synaptic
transmission process.
The action potential is an electrochemical process
as the neural message is electrical and the ions in the
surrounding fluid are chemical. However, the
process of synaptic transmission is a chemical
process.
0
Return to
resting
membrane
potential
Resting
membrane
potential
–70
Refractory period
1
2
3
4
5
Time (milliseconds)
Figure 5 The process of an action potential (adapted from Passer
& Smith, 2001)
The electrical impulse that travels down the neuron
follows an all-or-none law. This law states that the
electrical impulse will either fire or not. Feldman
(2000) compares this to a gun: you either pull the
trigger and the gun fires or you don’t pull the trigger and the gun does not fire – there is no inbetween. Some neurons can fire at different rates to
others. For example, some can fire at 1000 times per
second while others are much less. The intensity of
the stimulus (a loud noise versus a whisper) determines the rate of firing which, in turn, allows us to
tell the difference in intensity of various stimuli
(Feldman, 2000).
Vesicles without
neurotransmitter
travelling back to
cell body
Structure and action of the synapse
The synapse is the region where two neurons meet.
The terminal buttons of the first neuron sit close to
the dendrites of the second neuron. The gap inbetween the terminals and the dendrites is the synaptic gap. There are small sacks called vesicles in the
first neuron that contain neurotransmitters. When a
nerve impulse reaches the terminal buttons, it stimulates the vesicles to release the neurotransmitters
into the synaptic gap. The receptor sites on the second neuron then pick up the neurotransmitters.
However, receptor sites are specialised and will only
receive the neurotransmitters for which they were
designed. For example, in the figure below you can
Nerve
impulse
Neurotransmitter
molecules
Axon terminal
Transmitter
does not fit
receptor
Synaptic gap
Receptor site
Dendrite of
receiving neuron
Transmitter
fits receptor
(a)
Figure 6 Synaptic gap (a) and the process of transmission (b) (adapted from Peterson, 1997)
6 Psychology an introduction
(b)
see that the circular receptor sites can only receive
circular neurotransmitters and not triangular ones.
This is similar to a lock-and-key mechanism: only
one key can fit a lock and open it. Once the receptor
site has accepted the neurotransmitter, it starts or
inhibits an action potential in the second neuron,
depending on the function of the neurotransmitter.
Excitatory neurotransmitters start an action potential
in the following neuron while inhibitory neurotransmitters stop the next action potential.
Once the neurotransmitters have been released
into the synaptic gap and all receptor sites have
received neurotransmitters, there are extra neurotransmitters in the gap. These extra neurotransmitters are either broken down by enzymes or taken
back up into the first neuron’s terminal buttons.
The chapter on psychopharmacology (Chapter 8)
outlines the neurotransmitter process and explains
how drugs can affect neurotransmitters and their
action. Box A.1 illustrates how Ecstasy, an illegal
drug, affects neurotransmitters and their action.
A.1 THE EFFECTS OF ECSTACY
Ecstasy, or E, is an illegal drug which many people
consider to be a safe ‘party’ drug. Ecstasy, or
MDMA (3, 4-methylenedioxymethamphetamine)
is usually taken in pill form and has effects lasting
two to six hours. The immediate effects include
feelings of euphoria, empathy towards others, and
a heightened state of touch. Negative effects can
include jaw clenching, jumpiness, mental problems
such as confusion, anxiety, sleep disruption and
poor judgement, headaches and nausea, as well as
increased heart rate and blood pressure. After a
few days, people report feeling sad, despondent,
and irritable. Prolonged use of Ecstasy results in
the terminal buttons that secrete serotonin (a
mood neurotransmitter) degrading. When these
buttons are affected, people may develop depression, anxiety, and sleep problems. Ecstasy replaces
serotonin, dopamine, and norepinephrine when the
extra neurotransmitters are taken back into the
first cell. Since serotonin, dopamine, and norepinephrine stay in the synaptic gap, they continue to
stimulate the receptor sites. The serotonin axons
are then stimulated to such a degree that they die
off. Once this happens, they cannot be replaced
(Burgess, O’Donohoe & Gill, 2000).
Types of neurotransmitters
Over 50 different neurotransmitters have been identified. This section will concentrate on a few of the
most well-known and researched neurotransmitters.
Dopamine
Dopamine is found in the brain, especially in the limbic system, the cerebellum, and the basal ganglia. It
is involved in thought disorders such as schizophrenia (too much dopamine occurs) and movement disorders such as Parkinson’s disease (too little
dopamine occurs). Parkinson’s disease is a disorder
of movement and causes people to have difficulty
moving, experience tremors when at rest, and have
problems coordinating movement. The drug LDopa provides temporary relief as it mimics or pretends to be dopamine. Drugs used to block
dopamine may alleviate schizophrenia; however,
one must be careful with these drugs because they
may cause Parkinson’s-like symptoms when they
are taken in too large amounts.
Norepinephrine
Norepinephrine is derived from epinephrine (adrenaline). It is involved with arousal and mood, eating,
and sleeping. When the levels of norepinephrine are
too low, people experience depression (Wortman et
al., 1999). Antidepressants, the drugs used to treat
depression, work by either pretending to be a neurotransmitter involved in mood, or blocking the
uptake of neurotransmitters so that the mood neurotransmitters have a better chance to work at the
synaptic gap.
Serotonin
Serotonin is one of the most well-known neurotransmitters and is involved in mood, sleep, eating, and
arousal. Low levels of serotonin result in depression.
The drug Prozac is an SSRI or selective serotonin reuptake inhibitor. This means that it stops serotonin
from being taken back up into the first neuron, causing it to stay in the synaptic gap and stimulate the
next neuron for longer.
Acetylcholine
Acetylcholine or Ach is involved at the level of muscle movement as well as learning and memory.
Wortman et al. (2000) note that certain types of spider venom act at the level of Ach causing the link
between the neuronal message and muscle to be
severed; this results in paralysis and perhaps
Biological psychology 7
Neurotransmitter
Location
Function
Dysfunction
Acetylcholine (Ach)
Brain, spinal cord, PNS
Muscle movement,
cognitive function
including memory
Alzheimer’s disease
Gamma-amino
butyric acid (GABA)
Brain and spinal cord
Eating, aggression,
sleep, arousal
Anxiety, sleep disturbances,
arousal difficulties
Dopamine
Brain, especially basal
ganglia, cerebellum,
limbic system
Movement, mood,
learning, memory
Muscle disorders, mental
disorders, Parkinson’s
disease
Norepinephrine
Brain, especially the
cortex and limbic
system; spinal cord
Eating, sleep, arousal,
emotion
Depression
Serotonin
Especially thalamus and
brain stem
Sleep, arousal
Depression
Table 1 Examples of some neurotransmitters, their functions, and what happens when they are disordered (adapted from Feldman, 2000;
Wortman et al., 1999).
death. People with Alzheimer’s disease have lower
levels of Ach than others. Alzheimer’s disease
involves a gradual degeneration in terms of memory and cognition.
GABA
GABA is an inhibitory neurotransmitter and is
implicated in emotion, anxiety, arousal, and sleep.
Wortman et al. (1999) note that many benzodiazepines (such as Valium, Xanax, and Librium) act
by increasing the effect of GABA. As GABA is
inhibitory, increasing its effects will lower anxiety.
Now that we have explained communication
within and between neurons, we will explain the
larger parts of the nervous system, in particular the
brain (the most complex part of the nervous system
and central nervous system).
Regions of the brain
The brain is about the size of a large grapefruit,
looks like a wrinkly walnut, and has the consistency of porridge. Because of its important functions
and soft consistency, it is the most well-protected
organ in the body. Carlson (2005) notes that this
protection takes the form of cerebrospinal fluid,
meninges, and the skull. The brain floats in cerebrospinal fluid which both nourishes it and protects it from bumps and knocks. The meninges are
the membranes that surround the brain, protecting
8 Psychology an introduction
it and storing the cerebrospinal fluid. Lastly, the
skull surrounds the entire brain creating a thick box
to protect it.
A.2 THE CASE OF HM
HM was a young man who suffered from debilitating epilepsy, often suffering up to ten seizures a
day. His epilepsy became worse and the drugs used
to treat epilepsy were not working. As a result, relationships between the family members became
strained. As surgery at that time was very popular
and no other treatments were working, his doctors
suggested psychosurgery in 1953. The surgeons
decided to operate on HM’s brain and remove the
parts of the brain that were causing the epilepsy.
Subsequent studies of the regions that were
removed did not locate the precise site of epilepsy,
however. These parts, the amygdala and hippocampus, were removed from both sides of his brain
through a silver straw. As a result, HM’s seizures
grew less and were not as intense. But, the side
effects of the surgery were extreme: HM could no
longer remember things, his long-term memory
was impaired, and he could no longer form new
memories. He could remember things from before
the surgery, but nothing afterwards. He would
complete the same crossword puzzle over and over
because he forgot that he had seen it before and he
‘met’ his doctors for the first time every day
(Wortman et al., 1999; Ogden & Corkin, 1991).
The brain is made up of two sides called hemispheres.
Each hemisphere has the same structures, but each
side is responsible for slightly different functions. A
thick band of tissue called the corpus callosum connects the hemispheres and allows communication
between the two hemispheres to occur.
ing such as planning and organising their worlds.
The following categorisation of the brain into the
hindbrain, midbrain, and forebrain refers to when
the regions of the brain evolved as well as their location in the head. The hindbrain is at the bottom near
the back, the midbrain is near the centre of the brain,
and the forebrain is at the top and front of the brain.
Forebrain
Midbrain
Hindbrain
Left hemisphere
Right hemisphere
Figure 8 The brain is divided into the forebrain, midbrain, and
hindbrain (adapted from Peterson, 1997)
Figure 7 The brain is divided into the left hemisphere and the
right hemisphere (adapted from Peterson, 1997)
The hindbrain
One could describe the different parts of the brain by
looking at the regions of the brain in the order in
which they developed during evolution. From this
point of view, you will see that the earliest parts of
our brain to evolve are the ones responsible for our
most important survival skills. Only later in evolution did humans develop the skills of complex think-
The hindbrain is made up of the medulla oblongata,
the pons, the cerebellum, and portions of the reticular formation.
The medulla oblongata is the first structure in the
transition from the spinal cord to the brain. The
medulla is responsible for breathing, circulation,
Cerebral cortex
Parietal lobe
Frontal lobe
Corpus callosum
Occipital lobe
Thalamus
Hypothalamus
Midbrain
Pituitary gland
Pons
Cerebellum
Medulla
Spinal cord
Central canal of the
spinal cord
Figure 9 The major structures of the brain (adapted from Kalat, 1997)
Biological psychology 9
heart functioning, and other involuntary behaviours
such as vomiting, coughing, sneezing, hiccupping,
and blinking if something flies towards your eye.
From this description you can see that any damage
to the medulla could result in death, as breathing or
heart functioning would be affected (Peterson,
1997).
The pons (which means ‘bridge’ in Latin) is
directly above the medulla. Very little is known
about this structure (Westen, 1999), but we do know
that it links the brain and the spine. In essence, the
pons acts like a relay station sending signals from
the spine to the brain and from the brain to the
spine. Peterson (1997) notes that the pons also plays
a role in sleeping and waking. People with narcolepsy have been shown to have strange neural
activity in the pons. Norcolepsy is a disorder in
which a person has no control over sleep and may
fall asleep anywhere and at any time.
The cerebellum (or small brain) is located at the
back of the brain. It is responsible for coordinated
movement, balance, and posture. This structure is
affected when you are drunk. When the police test
people who are drunk, they are determining the
extent to which the cerebellum has been affected by
alcohol, thus indicating the amount they have had to
drink. For example: put your arm out sideways at
shoulder height and face forward; then bend your
arm and, using your index finger, touch your nose.
People who are drunk cannot touch their noses and
often miss or do it very slowly because they have to
concentrate very hard. Westen (1999) also points out
that when boxers are hit many times on the chin
causing the head to snap back, it results in their
movement and reflexes being affected, and they
stagger and may fall (it is often called being ‘punch
drunk’). Recently the cerebellum has also been
shown to be involved in some kinds of learning as
well.
The reticular formation is a structure that begins in
the hindbrain and continues through to the midbrain.
al to watch television. If the reticular formation is
damaged, a permanent state of sleep can result
(Peterson, 1997) or coma (Westen, 1999).
The forebrain
The forebrain was the last area of the brain to develop in the course of evolution and is involved in
many of the activities that we consider to be human
activities: complex cognitive functions, emotions,
and sensory processes. The forebrain comprises the
thalamus, the hypothalamus, the limbic system, the
basal ganglia, and the cerebrum. The cerebrum is
part of the cortex or outer layer of the brain, but the
thalamus, hypothalamus, limbic system, and basal
ganglia are all subcortical structures as they are
below the cortex.
The thalamus
The thalamus is the first structure to process incoming sensory information before relaying it to the
appropriate area of the brain for further processing.
This is similar to the information desk in a shopping
centre – when you walk into a new shopping centre,
you could go to the information desk to find out
where you would find a specific shop. The thalamus
is also active in highlighting or emphasising certain
messages over others.
The hypothalamus
This very small structure is found below the thalamus and is involved in many different activities. The
hypothalamus controls the pituitary gland which is
the main gland affecting all other glands in the body.
This is the link between the nervous system and the
endocrine (gland or hormone) system. The hypothalamus is involved in emotions, regulating body
rhythms for sleep, sexual activity, temperature regulation, hunger, and thirst.
The midbrain
The limbic system
The reticular formation is made up of many neurons
that connect to all the areas of the brain. It is responsible for arousal and sleep/wake consciousness.
Brain arousal is the state of readiness for activity and
varies in intensity. For example, you need a heightened state of arousal to take an exam but less arous-
The limbic system is not a single structure, but
is made up of a few structures to create a ‘system’.
It is involved in emotion, memory, learning, and
motivation.
The septal area in the limbic system is responsible
for pleasure, relief of pain and other bad emotional
experiences, and avoidance of painful stimuli. The
10 Psychology an introduction
Cingulate gyrus
Fornix
Thalamus
Septal nuclei
Olfactory bulb
Amygdala
Hippocampus
Mamillary body
tion areas are involved in the more complex mental
functions. Human beings have the largest association areas. For example, suppose you see a bicycle:
the sensory information about shape, lines, colour,
and movement would come from the eye, through
the thalamus, to the occipital lobe. The neurons in
the primary areas in the occipital lobe are sensitive
to noting specific lines, colours, and movement and
are therefore stimulated by this information. The
visual association area then receives this information and makes meaning from it, determining that it
is a ‘bicycle’. Knowing that those lines, shapes, and
colours represent a bicycle is a result of learning
what a bicycle looks like; if you had never seen one,
you would not be able to say what it is.
Figure 10 The limbic system (adapted from Peterson, 1997)
amygdala is involved in experiencing many emotions, learning, and memory for emotional events.
Most importantly, the amygdala is responsible for
recognising fear in other people and feeling fear. The
hippocampus is responsible for certain kinds of memory (see case of HM in Box A.2).
The basal ganglia
The basal ganglia are involved in movement. When
these structures are damaged, changes in posture,
muscle tone, and normal movements can occur
(Westen, 1999). If one has Parkinson’s disease, the
dopamine neurons start to die. These neurons project to the basal ganglia and, if they no longer exist,
those areas in the basal ganglia that received them
also die. The basal ganglia have also been implicated in mood and memory.
The cerebrum is the last section of the forebrain, but
because it is so complex, we will discuss it separately.
The cerebrum
The cerebrum comprises four lobes: the frontal, temporal, parietal, and occipital lobes. In the cerebrum
there are primary areas and association areas.
Primary areas are those areas of the cerebrum that
process primary or raw sensory information.
Information is received by the sensory receptors
through the thalamus and is directed to the primary
areas. These neurons are more specific than ones in
the association areas. The neurons in the association
areas may be specific, but their functions are learnt
rather than being innate (Westen, 1999). The associa-
Motor cortex
(voluntary movement)
Frontal
lobe
Somatosensory
cortex
(sensation)
Broca’s area
(speech formation)
Parietal lobe
Wernicke’s area (speech
understanding)
Auditory area (hearing)
Occipital lobe
Visual area (sight)
Temporal lobe
Figure 11 The frontal, parietal, temporal, and occipital lobes
(adapted from Peterson, 1997)
The frontal lobes
The frontal lobes are located in the front of the brain
and are responsible for many abilities from higher
cognitive functioning to movement. The frontal lobes
can also be divided into sub-areas. The motor cortex is
the primary area in the frontal lobes and is located at
the back of the frontal lobes. It is responsible for
movement. This area receives information from the
spinal cord, the cerebellum, and the basal ganglia,
and is involved in voluntary movements such as
walking, jumping, running, and threading a needle.
The motor cortex has different parts dedicated to different parts of the body. For example, one part is dedicated to the mouth and another to the hands. Each
area does not have the same sized area dedicated to
it, however. Parts of the body that are sensitive and
that are used frequently because they are complex,
such as the hands, have larger areas dedicated to
them than parts that are not as skilful, such as the
thighs. This map of the body on the motor cortex is
called the homunculus (or “little man”).
Biological psychology 11
The association areas in the frontal lobes are
involved in higher order thinking such as planning,
organisation, personality, abstract thinking, coordinating skilled movements, and memory. If the frontal
lobes are damaged, you may experience difficulties
with these aspects. You may, for example, lose the
ability to think abstractly, to plan and organise
behaviour and activities, to adjust socially, and to
behave appropriately.
A specific area found in the left frontal lobes (in
most people) is involved with language. This is
Broca’s area, named after the man who isolated it.
This area is responsible for the expression of speech
or the motor activities that comprise speech. People
with Broca’s aphasia may not be able to talk but they
can usually still understand speech (Halonen &
Santrock, 1999).
The temporal lobes
The temporal lobes are on the sides of the brain and
are mainly responsible for hearing and language.
The primary areas receive the frequency, amplitude,
and pitch of the sounds and the association areas
combine these into words that we recognise.
Language is also represented in this cortex and
Wernicke’s area is located in the left temporal lobe of
most people. This area is responsible for understanding speech. If you had Wernicke’s aphasia, you
would still be able to speak but it would not make
any sense and you would not understand what others are trying to say. Although hearing and language
are emphasised as the main functions of the temporal lobes, they are also involved in visual association. After information from the visual information
has been processed in the primary areas in the occipital lobes, the visual association areas in the temporal lobes identify what an object is.
The parietal lobes
The parietal lobes are located on top of the head and
contain the somatosensory cortex. This is a band of
brain area that mirrors the motor cortex and also has
a homunculus associated with it. This area receives
sensory information from the body. The parietal
lobes are also responsible for locating where something is in space, the sense of touch, detection of
movement, and noticing how one’s body is located
in space. Damage to this area could result in the neglect syndrome seen in Nosipho’s granny at the
beginning of the chapter.
12 Psychology an introduction
The occipital lobes
These are located at the back of the brain and are
responsible for vision. If someone hits you on the
back of the head and you ‘see stars’, this is because
the primary visual areas in the occipital lobes have
been affected. Damage to the primary visual area
can result in partial or complete blindness. The association areas in the occipital lobes extend to other
areas of the cortex and are responsible for organising
the information from the primary areas into more
complex ‘pictures’ of features of objects.
A.3 CONSCIOUSNESS
An area that has long puzzled brain researchers is the
area of consciousness. What is consciousness? What
parts of the brain or brain activity are necessary for
consciousness? Consciousness mainly means being
aware, alert, and attentive and also includes inner
self-knowledge. From this definition you can see
that consciousness is a very subjective experience
and measuring it has proven to be very difficult. In
terms of brain anatomy, researchers have not found
one specific area of the brain related to consciousness (not least because it is such a hard concept to
define).
The main area of research into brain anatomy and
consciousness is the problem of ‘binding’ – that is,
the way in which the brain takes many different
aspects of information from all over the brain and
‘binds’ them together to form a subjective experience. This implies that different brain regions are
involved, depending on what part of consciousness
you are studying. Also, although some brain areas
may not be working (such as the visual areas), you
would still have consciousness.
Another important area of research in consciousness involves the frontal lobes and in this
research the focus is on executive functioning in
terms of self-reference and self-evaluation. The
reticular activating system is also discussed in
terms of consciousness. Therefore, it is clear that
research on consciousness is determined by the
definition of consciousness and the aspects of consciousness being focused on. In general, every area
in the brain has been shown to link to the study of
consciousness (Zillmer & Spiers, 2001).
Lateralisation of function
Lateralisation refers to the specific functions for which
each hemisphere is responsible, or dominant. The left
hemisphere has been implicated in speech and language in many people, and the right hemisphere has
been implicated in spatial functions (Passer & Smith,
2001). Evidence attained from split-brain patients is
one of the areas that has demonstrated the lateralisation of functions (Halonen & Santrock, 1999). Splitbrain patients have had their corpus callosum severed
to stop debilitating epileptic seizures. Changes in their
behaviour and functioning after the surgery were then
noted by doctors (Passer & Smith, 2001).
Left visual field
Right visual field
Fixation
point
would therefore not be able to tell you what he or she
saw because language is controlled in the left hemisphere. If you gave the object to the patient to feel, he
or she would identify the ice-cream because spatial
awareness is located in the right hemisphere. If you
had shown the patient the flower in the right visual
field which was then transmitted to the left hemisphere, he or she would be able to tell you what was
seen because language is in the left hemisphere, but
would not be able to identify the object by touch.
The endocrine system
This chapter has dealt primarily with the nervous
system as an area of biological psychology. While this
is an extensive and well-researched area, there are
other areas of biology that have an effect on our psychology. One such area is the endocrine system to
which we referred in the discussion about the hypothalamus. The endocrine system is the system of
hormones and glands in the body. Hormones are
chemicals that are secreted by the glands. They travel
through the bloodstream (making them a lot slower
than nerve impulses and also longer-lasting) and
affect the organ to which they were sent. The figure
below illustrates some of the glands in our body.
Pituitary
Thyroid
Severed
corpus callosum
Figure 12 The visual pathway in the brain (adapted from Passer &
Smith, 2001)
Figure 12 illustrates the normal visual pathway in
the brain. Information from the left visual field goes
to the right hemisphere and information from the
right visual field goes to the left hemisphere. The
information crosses over at the optic chiasma. If the
corpus callosum were severed, the information
would not be able to be communicated between the
hemispheres. According to the figure above, if you
show a split-brain patient an ice-cream in their left
visual field, this information will travel to the right
hemisphere. The right hemisphere cannot ‘tell’ the
left hemisphere what it saw, however. The patient
Hypothalamus
Adrenal
cortex
Pancreas
Adrenal
medulla
Ovaries
(female)
Testes
(male)
Figure 13 The endocrine system is made up of the glands in your
body (adapted from Passer & Smith, 2001)
Biological psychology 13
The pituitary gland is known as the master gland
as it regulates and controls all the other glands in
the body. It is close to and connects with the
hypothalamus in the brain. This link between the
nervous system and the endocrine system allows
the two to work in harmony. The pituitary is also
responsible for growth and regulates salt and water
metabolism.
The thyroid is responsible for metabolism. If one
has an under-active thyroid, one is likely to be apathetic, sluggish, put on weight, and feel very
despondent. As a result of this, doctors often first
check a patient’s thyroid functioning before making
a diagnosis of depression. An overactive thyroid
leads to a person being very active and thin.
The adrenal glands are made up of the adrenal
medulla and the adrenal cortex. The adrenal cortex
regulates salt and carbohydrate metabolism, while
the adrenal medulla prepares the body for the fightor-flight reaction to stress. You will find more infor-
mation about the endocrine system in the chapter on
stress (Chapter 32).
Lastly, the ovaries (female) and testes (male) are
responsible for sexual behaviour, the development
of the reproductive hormones, and general physical
growth.
Conclusion
This chapter has detailed how certain aspects of our
biology impact on our psychology. The nervous system was discussed in great detail and special reference was made to the central nervous system. The
role of neurons and neural transmission in psychology was explored, as were the various regions of the
brain. Lastly, the endocrine system was discussed as
an additional example of a biological system that
impacts on our psychology.
REFERENCES
Burgess, C., O’Donohoe, A. & Gill, M. (2000). Agony and ecstasy: A
review of MDMA effects and toxicity. European Journal of
Psychiatry, 15, 287–294.
Carlson, N.R. (2005). Foundations of Physiological Psychology (6th edition). Boston: Pearson.
Feldman, R.S. (2000). Essentials of Understanding Psychology (4th edition). Boston: McGraw-Hill.
Halonen, J.S. & Santrock, J.W. (1999). Psychology: Contexts and
Applications (3rd edition). Boston: McGraw-Hill.
Kalat, J.W. (2001). Biological Psychology (7th edition). Belmont,
California: Thomson.
Ogden, J.A. & Corkin, S. (1991). Memories of H.M. In W.C. Arbraham,
14 Psychology an introduction
M. Corballis, & K.G. White (Eds). Memory Mechanisms: A Tribute to
G.V. Goddard (pp. 195–215). N.J.: Lawrence Erlbaum.
Passer, M.W. & Smith, R.E. (2001). Psychology: Frontiers and
Applications. Boston: McGraw-Hill.
Peterson, C. (1997). Psychology: A Biopsychosocial Approach (2nd edition). New York: Longman.
Westen, D. (1999). Mind, Brain and Culture (2nd edition). New York:
John Wiley & Sons.
Wortman, C., Loftus, E. & Weaver, C. (1999). Psychology (5th edition).
Boston: McGraw-Hill.
Zillmer, E.A. & Spiers, M.V. (2001). Principles of Neuropsychology.
United Sates: Wadsworth.
EXERCISES
Multiple choice questions
1.
2.
3.
4.
5.
6.
7.
Neurons receive information from other neurons
through their _________.
a) axons
b) terminal buttons
c) dendrites
d) myelin sheath
The purpose of the myelin sheath is to:
a) insulate the axon against cold
b) insulate the axon so that the neural message
remains strong and fast
c) store fat
d) route the neural impulse
Josi puts her hand in the bath to make sure the
water is not too hot. As she puts her hand into the
water, she feels that the water is too cold; she therefore opens the hot water tap. Which nerve fibres
were responsible for her opening the hot water tap?
a) efferent nerve fibres
b) afferent nerve fibres
c) neurons
d) glial cells
Vinesh is walking down the street one evening and
sees someone following him. He starts feeling
scared, his pupils widen, his heart rate speeds up,
and he feels slightly sick in his stomach. What part
of the nervous system is at work in this example?
a) the central nervous system
b) the nervous system
c) the parasympathetic nervous system
d) the sympathetic nervous system
Vinesh sees that the person following him is actually
a friend of his and he laughs at himself for feeling so
afraid. Slowly he feels his heart rate slow down and
his stomach return to normal. This takes a little
while, however. What part of the nervous system is
responsible for calming Vinesh down?
a) the central nervous system
b) the nervous system
c) the parasympathetic nervous system
d) the sympathetic nervous system
The period when a neuron cannot fire is called:
a) the refractory period
b) the all-or-none law
c) the action potential
d) depolarisation
Gaby is feeling very depressed; she has lost her
appetite and has trouble sleeping. Which neurotransmitter is probably responsible for her state?
a) GABA
b) Serotonin
c) Acetylcholine
d) Endorphins
8. Jacki has a friend who was hit on the back of the
head and is now in a coma. The area of the brain that
was likely to have been damaged is the __________.
a) the forebrain
b) the limbic system
c) the olfactory bulb
d) the pons
9. Thandi was in an accident during which a pipe
entered her brain. Doctors find that some of her subcortical structures were damaged. When she awakes
she has no fear of any danger and does not recognise
fear in other people’s faces. What are of the brain is
likely to have been damaged?
a) the amygdala
b) the thalamus
c) the hippocampus
d) the hypothalamus
10. Kalyani has Parkinson’s disease. From your knowledge of the brain structures and functions, what
area of the brain is likely to be affected?
a) the reticular formation
b) the basal ganglia
c) the cerebellum
d) the cerebrum
Short answer questions
1.
2.
3.
4.
Communication within a neuron is said to be electrochemical, while communication between neurons
is said to be chemical. Explain why this is so.
Which neurotransmitters are involved in mood and
how do they work?
Explain the function of the limbic system and describe
what the result of damage to this area would be.
List the areas of the cerebrum and the structures,
functions, and dysfunctions of each. You may want
to use the following table:
Cerebrum area Location Structure Function Dysfunction
5.
Explain what happens to a patient whose corpus
callosum has been severed. What behaviour are you
likely to see?
Biological psychology 15
Sensation and perception
Supplementary Chapter to Part 3 ‘Brain and Behaviour’
Dr Kirston Greenop
Psychology, School of Human and Community Development, University of the Witwatersrand
CHAPTER OBJECTIVES
After studying this chapter you should be able to:
• define the terms ‘sensation’ and ‘perception’
• explain what ‘psychophysics’ studies
• understand the role of thresholds in the process of sensation and perception
• describe the structure of the eye and the visual pathway to the brain
• contrast theories of colour vision
• explain the forms of visual perception
• describe the structure of the ear and the auditory pathway to the brain
• explain the role of sound waves in hearing
• compare the theories of hearing
• compare the chemical senses of taste and smell
• describe the structure of the tongue and the pathway to the brain
• describe the structure of the nose and the pathway to the brain
• explain why people experience pain differently
• outline the body senses of kinaesthesia and the vestibular sense.
Nosipho is sitting outside on the grass in the middle of
summer. She can feel the warm sun on her skin; she can
smell the flowers that are growing nearby. Nosipho can
also feel the soft grass tickling her. She starts to feel
drowsy and so decides to gently stretch her legs and lie
down. As she looks up she sees her friend coming
towards her through the dark green trees with the bright
yellow flowers. She hears the birds twittering softly and
then hears her friend calling to her. Nosipho sits up and
watches her friend approach. Her friend has brought
some ice-cold water which she drinks so fast that the
cold, fresh taste lingers on her tongue.
The previous paragraph describes a short scene that is
familiar to most people. All of Nosipho’s senses are at
work in the above scene: seeing the trees, flowers, and
the girl, hearing the birds and the girl’s voice, smelling
the plants and flowers, tasting the water, feeling the
grass, and noticing how the body moves in space to lie
down and sit up. These are descriptions of sight, hearing,
taste, smell, touch, as well as the kinaesthetic and
vestibular senses. We use these senses, which we often
take for granted, to make meaning of our world. Our
sense receptors such as eyes, ears, and skin, receive
information from the environment. This information is
then sent to the brain where billions of interconnected
nerve cells fire when they receive the information and
make sense of it. This is essentially what the processes of
sensation and perception entail.
For nearly all people the nerve cells or neurons responsible for sight will fire and you will see or perceive a flower,
for instance. Or the nerve cells for smell will fire and you
may smell baking bread. However, for a very small number of people the sensations they receive may be
processed as an overlapping experience. So, for example,
they may see smells or taste sounds. This is called synesthesia, where one experience overlaps with another
(Cytowic, 1995). A person may hear a particular music
note but perceive it as a sugary taste or experience your
voice as the taste of chocolate and coffee. The cases of
synesthesia are very rare but do illustrate the importance
of studying the different senses and how people make
meaning of sensation. This example also demonstrates
how sensation and perception can affect a person’s
psychology: imagine having an overlapping sense while
everyone else’s senses remain separate. Also imagine what
happens to one’s thoughts, feelings, and behaviour when
drugs, brain damage, or emotion affects perception.
Therefore, understanding sensation and perception is fundamental to understanding how people think, behave, and
feel, and plays an important role in the field of psychology.
B
Introduction
This chapter will describe each sense and explain
what happens as the sensory receptors pick up signals from the environment and send them to the
brain. The usual pathways that these experiences
take in the brain will be explained, as well as what
happens when there is damage or disruption to the
pathways, resulting in the sensation or perception
experience being affected. However, before each
sense is explained in detail, we will define sensation
and perception and explain the field of study of sensation and perception: psychophysics.
Sensation and perception
Definition of sensation and
perception
Sensation is a passive process during which the sensory receptors and the brain receive information
from the environment. Perception, on the other hand,
is a process that entails actively choosing information from sensation, organising it, and interpreting it
to make meaning of our world. Imagine you are
looking at an object. Sensation occurs when the signals hit the eye, move to the brain, and register as
different lines, colours, and shapes. Perception
occurs when you make meaning of all these different shapes and colours and see the object as a rose,
for example. The process of seeing an object and
recognising it as a rose would, therefore, involve the
following steps:
Step 1: Energy signals in the environment hit the
Step 2: specialised receptor cells in the sensory
organs, which turn the energy signal into an
electrochemical impulse (a process called
transduction); the impulse is then sent to
the
Step 3: brain regions responsible (sensation), which
Step 4: make meaning of the message (perception).
The specialised receptor cells in the sensory organs
are only activated or fire when energy in the correct
form hits them. For example, the receptor cells in the
eye will only fire when light waves hit them, not
when sound waves hit them. The energy signals to
which each sense responds are set out in Table 1.
2 Psychology an introduction
Sense
Energy signal
Vision
Light waves
Hearing
Sound waves
Taste
Chemicals
Smell
Chemicals
Touch
Temperature and pressure signals
Kinaesthetic sense
Pressure signals
Vestibular sense
Motion signals
Table 1 Energy signals to which the various senses respond
While sensation involves picking up the bits of signals from the environment, perception involves
making meaning of the information. Making sense
of many small pieces of information depends on
what you already know, what you have learnt, as
well as the social and cultural context in which you
live. For example, suppose you are at a party where
you are surrounded by many people in conversation
with others. If you listen to someone talking to
another person quite a distance away from you, you
may not hear the whole conversation. The pieces of
the conversation you do hear are like the pieces of
sensation you pick up from the environment.
Making sense of the pieces of conversation is similar
to perception. However, your perception may be
incorrect. You may hear ‘dinner … tired … struggle
… murder’. If the person whom you overheard is a
small, dainty lady with lots of make-up and jewellery talking to a similar lady, you may perceive the
conversation to be about her having to make dinner
when she was so tired and that it was ‘murder’. If
the two people in conversation are very large, muscular men with tatoos on their arms, you may think
they are talking about an actual murder. You heard
the same pieces of information (similar to the bits of
sensation signals we receive) and, once you have
made meaning of them, took into account the sociocultural context as well as the appearances of the
people (perception).
This example illustrates that perception is always
subjective as it depends on the person who is interpreting the sensations in order to make meaning of
them. This is a personal process based on past experience, learning, and the environment in which the
person lives. Therefore, while we all make meaning
of our sensations, this meaning depends on who we
are, where we live, and what our past experiences
are. You have probably experienced this when you
heard the same story told by different people: they
all seem to tell a slightly different story, focusing on
different aspects.
Psychophysics
Psychophysics is a special field in psychology that
studies sensations, their limits, and how they are
perceived. Sternberg (2004) explains psychophysics
as the study of the physical energy stimulation of
the sensory organs which results in meaningful psychological experience. This field of study will ask
questions such as, ‘how loud must a sound be in
order for you to hear it’ or ‘how bright must a light
be in order for you to see it’.
Sternberg (2004) also points out that psychophysics is used in our everyday lives. Health
care workers are constantly evaluating how great a
specific stimulation needs to be in order for us to
perceive or detect it. The implication is that if the
level of stimulation required for a person to perceive
or detect it is too high or too low, there may be something wrong with him or her. For example, suppose
a man had his hand crushed in an accident. In measuring whether he can feel pin pricks on his hand, the
doctor or nurse can determine his levels of feeling in
that hand as well as whether his nerves have been
damaged or not. When we go for routine medical
checks, the health care worker asks questions such
as ‘can you feel this’, ‘can you see this’, or ‘can you
hear this’. These are important to determine level of
functioning.
Besides these levels of stimuli, psychophysics
also studies thresholds (needed to detect stimuli),
the ability to discriminate between stimuli, the
errors we make in detection, and how we become
used to all the stimuli around us.
Thresholds
A threshold is the level of energy that a stimulus must
have or display in order for you to detect or perceive
it. Think of this as the threshold you cross when you
walk through a door. When you are outside the door
(below the threshold), you cannot detect who is in
the room but as soon as you cross the threshold you
can see that there are, for example, ten people in the
room. You used energy to cross the threshold and
can now perceive the stimuli of ten people.
Similarly, a stimulus must have energy in order to
cross the threshold and be noticed.
The absolute threshold is the smallest or minimum
amount of energy required for you to detect a stimulus. For example, how much energy needs to be
detected before you know a mosquito is sitting on
your leg? An example from the real world is where
picking up sounds is an important safety requirement. Ole-Herman (2004) investigated the absolute
threshold of sounds and voices presented over loud
speakers and emergency transmission systems so
that one can ensure that they are heard. The voices
needed to pass the absolute threshold of hearing so
that people could hear them.
Signal-detection theory
We do not detect a signal at the same time or in the
same way and sometimes we even get it wrong.
Being able to detect the presence of a stimulus
depends on many factors including the individual,
fatigue, expectations, and past events (Passer &
Smith, 2001). People differ in their decisions on
whether they hear a noise, for example. Some people may guess ‘Yes’ even when they are unsure,
while others may guess ‘No’. This is called a response
B.1 PSYCHOPHYSICS APPLIED
It is not only the medical world that uses psychophysics
– manufacturers use it all the time. For example, imagine you wanted to buy a piece of fish. You walk into a
shop and have to wait a few moments for your eyes to
adjust to the light so that you can find the fish counter.
You look around but cannot see any fish so you decide
to ask an assistant. However, the music is playing so
loudly that the assistant thinks you are looking for a
‘dish’. This frustrating experience would probably make
you leave the shop without buying any fish. People who
design the interiors of shops use psychophysics to
determine what level of light is needed for you to see
the products but not be overwhelmed by the brightness
of the light. The level of background music is also studied so that one can experience it as pleasant but still be
able to hear someone talking to you.
Sensation and perception 3
bias. For example, if someone were asked to say
whether she saw a flash of light she may say ‘Yes’
when it appears. This is called a hit. However, if she
says ‘No’ when the light is present, this is termed a
miss. A false alarm occurs when she says ‘Yes’ even
when the light does not flash. The last option, when
she says ‘No’ when the light does not flash, is called
a correct rejection. These options are set out in Table 2.
Response
No – absent
Signal
Yes – present
Light flashes
Hit
Miss
Light absent
False alarm
Correct rejection
Table 2 Possible outcomes of signal detection (adapted from
Sternberg, 2004)
People do not objectively and accurately report
what they detect. Rather, they decide whether they
have detected something. Imagine that you have
seen a scary movie. While lying in bed at night you
hear the floor creaking. You would probably decide
that you did hear that noise even though you would
not even notice the same noise if it was during the
day. Bourne and Russo (2001) provide the example
of a soldier on patrol. The soldier may notice certain
sounds when out on normal patrol. He would probably notice more sounds if a sniper recently shot his
comrade. These examples show that even when
studied in an objective setting, people are not objective; rather, they make subjective decisions.
Individual differences therefore need to be taken
into account when studying signal detection.
Discriminating between stimuli
Psychophysics not only studies how we detect stimuli, but also how we detect differences between
stimuli. It is important to notice the difference
between stimuli in many situations. Passer and
Smith (2001) provide the example of a piano tuner
who needs to be able to detect the slightest change
in sounds or pitch so that the piano, once tuned,
sounds perfect. People also need to be able to detect
changes in the taste of food, for example, so that
they can tell when the food is going off or is spoiled.
The difference threshold can explain these changes.
This threshold is ‘the line one has to cross’ in order
to tell when stimulus A is different to stimulus B.
However, people may make mistakes in their deci-
4 Psychology an introduction
sion about the difference (see section on signal detection theory). For this reason, the just noticeable difference (jnd) level is the level at which people will
notice a difference 50% of the time and is the minimum level of difference required. The jnd therefore
defines differences.
The German physiologist Weber found that a
stimulus does not change by the same amount all
the time in order for the jnd to be reached. In effect,
if you increase the volume of the radio one level
when it is quiet around you, you will notice the
change; however, if you change it by one level when
there is a large amount of noise around you, you
would not notice the change. Weber’s law, the first
law of psychophysics (Bourne & Russo, 1998), states
that noticing a change depends on the proportion by
which the stimulus has changed. For example, if
food has a very low level of salt (for example a level
of 1) you would need to change this by 20% to notice
a difference (to an overall level of 1.20). If you wanted to notice a change in very salty food (with a level
of 15 for instance), you would also need to increase
this by 20% (to an overall level of 18). Thus, 20% of 1
is 0.2, but 20% of 15 is 3, showing that the change is
not equal, but proportional.
Sensory dimension
Difference threshold (%)
Brightness
1.6
Loudness
8.8
Pitch
0.3
Pressure (on forearm)
13.6
Lifted weight
1.9
Smell (rubbery)
10.4
Taste (salt)
20.0
Table 3 Percentages by which a stimulus has to change before
one can notice the difference (adapted from Bourne & Russo,
1998)
If you look at the above table, you will notice that it
does not take much difference to notice a change in
visual stimuli (1.6%). However, a 20% difference is
required for you to notice a change in a salty taste, or
a 10.4% difference in smell. This is as a result of the
reliance humans have on vision. These jnd levels
would not apply to animals. They would notice
changes in odour much faster than humans as they
rely more on this sense.
Adaptation
You will agree that we can detect stimuli and notice
when these stimuli change. However, if we responded to everything that we noticed in our environment
we would not be able to cope. One has to be able to
‘tune out’ the extra or background noise or stimuli.
For example, you may apply your favourite perfume in the morning. After breakfast you find that
you cannot smell it anymore and therefore re-apply
it. Before you leave the house you notice again that
you can no longer smell the perfume, so once again
you re-apply it. At university you are surprised
when your friend steps back and says that you smell
very strongly of perfume. You don’t notice it
because you have become used to it over time – this
is adaptation.
Adaptation occurs when we are constantly surrounded by a particular stimulus and so start to
block it out. For example, if you walk into a room
and smell a horrible stench, the smell will seem less
strong the longer you stay in the room. Our eyes go
through this process of adaptation all the time as
they adapt to bright or dark light. Adaptation occurs
at the same rate regardless of how recently we
adapted to the stimuli and is a process over which
we have little conscious control (Sternberg, 2001).
For example, if you get into a hot bath the water will
feel very hot initially; as you adapt to the temperature gradually, it will feel colder and you may even
add more hot water after a while. If you get out of
the water at that point, the outside temperature will
seem very cold but you will adjust to that as well. If
you then get back into the bath, it will seem hot
again even if you only stepped out for a short time
to fetch a bar of soap.
Having discussed detection stimuli, changes to
stimuli and adaptation to stimuli, we will discuss
each sensory system individually to illustrate their
unique characteristics and demonstrate what may
happen when the system is disrupted or disordered.
B.2 CASE STUDY
Consider the following case study (adapted from
Shenker, 2005). Think about what might be wrong
with this man.
Mr S, a seventy-five-year-old man, went to his
doctor because he was afraid he was losing his sight.
He said that his vision had become blurry and that
he thought that he perhaps should get stronger
glasses. He was quite worried however, because he
experienced problems with his vision only when he
was reading. When asked to read, he said that he
could see the words but that they did not seem real.
He could catch a ball, walk around without bumping
into the furniture, and identify colours. When his
vision was tested, he could see everything in his left
visual field but nothing in his right visual field. He
couldn’t read any words but he could write them.
When he wrote the words, however, he would then
say, ‘What does it say?’ He couldn’t even read what
he had written. When an MRI was done on his brain,
doctors found that he had had a stroke in his left
occipital lobe which affected some of the fibres of
the back part of the corpus callosum as well.
Mr S has a condition known as alexia (cannot read)
without agraphia (cannot write). In essence this man
can write but cannot read. In order to investigate
why this is the case or what may be happening in his
vision to result in this disturbance, we have to evaluate all the components of the visual system. From
what has already been discussed it seems as if Mr S
can detect stimuli as well as changes to stimuli. As
mentioned above, sensation and perception involve
an energy signal hitting the receptors in the eye, this
signal being sent to the brain, and some meaning
being made out of the signal in the brain. The rest of
this section will follow this pathway to describe the
process of vision.
Vision
The structure of the eye and the
pathway to the brain
In this chapter we will detail the mechanisms underlying vision. Once you understand these mechanisms, you will be able to track the visual system
pathway and apply the disorders of visual processing to them.
In order to evaluate what has happened to Mr S, one
first needs to establish whether the signal or energy
from the environment is at the appropriate level.
One can then investigate whether the structure of
the eye is normal and working correctly in order to
receive this signal.
Sensation and perception 5
INVISIBLE LONG WAVES
VISIBLE LIGHT SPECTRUM
INVISIBLE SHORT WAVES
Infrared rays
(beyond red)
1 500
1 000
Radio
Ultraviolet rays
(beyond violet)
700
600
TV
Microwaves
500
Infra-red
400
U-V
X-rays
300
Gamma Cosmic
rays
rays
Visible to humans
Figure 1 The portion of the electromagnetic spectrum which humans can see (adapted from Coon, 2004)
Light
Light, the energy signal that the eye receives, is in the
form of electromagnetic radiation (Westen, 1999)
which is in the form of wavelengths. Humans can
only detect wavelengths ranging between 350 and
750 nanometers (or billionths of a meter). This is a
small range of the electromagnetic spectrum (Sternberg,
2004). Bees, for example, can see the ultraviolet and
infrared spectrums which we cannot see (Sternberg,
2004). From the spectrum it is apparent that humans
Cornea
can only see a very limited number of all the wavelengths that exist. Figure 1 illustrates the electromagnetic spectrum.
The retina
Specialist cells in the eye pick up light in the form of
wavelengths. However, before wavelengths hit
these cells, they first have to travel through the eye
itself. Figure 2 illustrates the structure of the eye and
the path of light through the eye.
Direction of light
Iris
Pupil
Blind spot
(optic disk)
Lens
Fovea
Fibres of the optic nerve
Ganglion cell
Amacrine cell
Bipolar neuron
Horizontal cell
Retina
Photoreceptor cells:
Cone
Rod
Optic nerve
(to brain)
Retina
Optic nerve
Pigment layer of retina
Choroid layer
Sclera
Figure 2 The structure of the eye and the path light travels to reach the photoreceptor cells (adapted from Coon, 2004)
6 Psychology an introduction
The route that light travels to reach the photoreceptor cells at the back of the eye can be outlined in
four steps:
Route to vision
Step 1:
Light first hits the cornea of the eye. This is the covering on the outside of the eye. The cornea keeps the
shape of the eye, has a protective function, and has
a curved shape.
Step 4:
Next, the bent light that is focused onto the back of
the eye hits the retina. This covers the surface of the
eye and contains the specialised light receptor cells.
These specialised neurons cover the whole of the
back of the eye except one area called the blind spot.
This is a spot in our vision that has no neurons at all
and therefore leaves a gap in our vision. We do not
notice it because our brain fills in the gaps of our
vision. Try the test in Box B.3 to find your blind spot.
B.3 BLIND SPOT
Step 2:
After entering the cornea, light passes through the
pupil. This is the black centre in the eye. The pupil
can increase or decrease in size depending on how
bright or dark the light is at which you are looking.
If you are looking at an object in bright light, the
pupil will be small because you do not need a lot
of light in order to see the object. However, if you
are looking at an object in the dark or low light, the
pupil will be big so that more light can enter the
eye to enable you to see the object more clearly.
The iris is the coloured part of the eye that surrounds the pupil. Sternberg (2001) explains that
the iris is a circular band of muscles that make the
iris bigger or smaller. The reason why your eye has
a certain colour (blue, green, or brown) is because
this muscle reflects specific types of light beams
away from the eye. We then perceive this as a specific colour.
The cells of the eye
Step 3:
After entering the pupil, light passes through the
lens. This is like the lens on a pair of glasses. The lens
has a bulging shape (just like glasses), but is flexible
and can change the extent of this bulge. The lens
focuses light onto the back of the eye and, in order
for the object we are seeing to be in focus, that light
needs to be bent at a particular angle – it therefore
adapts constantly. Passer and Smith (2001) note that
when you are looking at things closer to you, the
bulge of the lens is made fatter; when you are looking at things that are far away from you, the lens
will be made thinner. If the lens does not focus the
light correctly, we need glasses to correct this. The
lens in the glasses just helps the lens of the eye to
work correctly. In this instance, light will be bent
going through the lens of the glasses and then again
through the lens of the eye. Another interesting feature of the lens is that it turns the image we are seeing backwards and upside down. The brain corrects
this for us later, though.
The retina is very thin; in fact, it is as thin as this
page (Sternberg, 2004). However, it contains all the
cells that pick up light. These include cells called
photoreceptor cells which change the electromagnetic
energy of light into electrochemical energy (the neural impulse) that can be relayed to the brain. Refer to
Chapter 9 for more information about the process of
electrochemical transmission. All the axons of the
cells bundle together and exit the eye at the optic
nerve. Because the optic nerve has axons only and
no photoreceptor cells, this is the blind spot to
which we referred earlier.
There are two kinds of photoreceptors: rods and
cones, thus named because of their distinctive shape
(see Figure 2). Bourne and Russo (1998) explain that
rods and cones each have their own specific type of
photopigment that is affected by a specific wavelength of light. Rods are found all over the retina,
while cones are found mainly in the fovea with their
numbers reducing dramatically outside of the fovea.
The fovea is the area where the best visual acuity or
best vision occurs.
Place the page about 30 centimetres away from your
face. Close your right eye. Now look at the dot on
the left and move the page towards you until the
cross on the right disappears – this happens because
you have made light land on your blind spot.
Figure 3 Blind spot experiment
Sensation and perception 7
There are about 120 million rods and 6 million
cones in the eye (Passer & Smith, 2001). Rods enable
us to see in low and dark light as they are sensitive
to picking up black and white but not colours. The
cones, on the other hand, pick up colours and function best in bright light.
Returning to our case study of Mr S, having
explained the structure of the eye and how the cells
operate, one can see that Mr S does not have a difficulty at this level. If a person were to have a problem
with the structure of their eye or the cells at the back
of the eye, this would result in diminished overall
vision or blindness. Mr S can still see nearly everything and has difficulty with words only. To explain
Mr S’s difficulty, one needs to investigate the message from the eye travelling to the brain.
your right visual field is transmitted to the left
hemisphere, and everything in the left visual field
is transmitted to the right hemisphere. Notice that
each eye picks up images from the right and the left
visual field. The optic chiasm splits the two pathways and at that point each crosses over to the
opposite hemisphere. The messages are then sent to
the occipital lobes which are responsible for the
sensation input and initial processing of the information into visual perception. Depending on the
type of information, this is then sent for further
processing to the temporal lobes (which tell you
what things are) and parietal lobes (which tell you
where things are).
Returning to the case study about Mr S, it now
seems likely that Mr S has no problems with the
general sensations of vision as he can see quite well.
However, he cannot make meaning of written words,
suggesting that he cannot form a perception of
words. This is clearly a difficulty in Mr S’s processing
abilities. However, before coming to this conclusion,
one should also evaluate the specific aspects of
Mr S’s vision. He indicated that he sometimes sees a
fuzzy image of the words. In order to assess this, one
needs to evaluate the quality of his vision (visual
acuity) and his colour vision.
Pathway to the brain
The electrochemical signals follow a route from the
optic nerve through to the back of the brain in the
occipital lobe. From the occipital lobe the message
may then be sent to other areas of the brain for further processing. This route is illustrated in Figure 4.
The picture illustrates that everything you see in
Right visual field
Visual cortex
(occipital lobe)
Left visual field
Optic nerve
Optic chiasma
Optic tract
Lateral geniculate nucleus
Pulvinar nucleus
Nuclei of the thalamus
Figure 4 Visual pathways in the brain (adapted from Sternberg, 2001)
8 Psychology an introduction
Visual acuity
Visual acuity refers to how well you can see objects
and distinguish between objects in the environment.
If you have poor visual acuity, then you may not be
able to see objects at a distance or the fine detail of
an object. Generally, the better your acuity, the better
your vision. Cones affect acuity as they are concentrated in one area of the eye (the fovea).
We can still see in low light areas, however. This
is called dark adaptation and occurs when the rods
become activated in the low illumination light.
Cones operate in bright light and are not of much
use in the dark. The rods, however, function in the
dark but cannot detect colour. When you enter a
dark room, you probably won’t see anything. As
your eyes become used to the dark in the first five to
ten minutes, you will start detecting shapes and
intensity in objects. The rods continue to become
sensitive over a period of half an hour (Bourne &
Russo, 1998).
Returning to our case study, Mr S does not seem
to have difficulties with visual acuity as his vision is
normal for most objects. His colour vision is also
normal as he did not report any difficulty in picking
out different colours.
Colour vision
Seeing colour involves a combination of three factors (Bourne & Russo, 1998). The three properties of
colour are as follows:
1. Hue – this is determined by the wavelength of
light. Sternberg (2004) states that we see the
shortest wavelength as violet and the longest as
red.
2. Saturation – this is determined by how pure the
colour appears or how much it has been combined with white.
3. Brightness – this is determined by the amplitude
of the light wave, which is the amount of light we
see coming from the wavelengths.
The theories of colour vision all attempt to explain
how we can see so many different colours at different brightness levels and saturation levels. These
theories are currently still being debated and we will
highlight the main two theories only.
The trichromatic theory, proposed by Thomas
Young and later modified by Hermann von
Helmholtz (1852), focuses on the primary colours of
red, blue, and green. Young and Helmholtz argued
that as these three primary colours form the basis of
every possible colour we can think of, it seems logical that the photoreceptors in the eye are specialised
to pick up these primary colours. It is, furthermore,
impossible to have receptors for all the colours that
exist. Therefore, these researchers said that we have
three kinds of cone, each sensitive to green, blue, or
red. To achieve the many different colours we can
see, these photoreceptors are activated to different
degrees (Sternberg, 2004). This would be similar to
mixing paint. If you want violet, you would mix a
lot of blue with a little red. When we see violet,
therefore, our blue photoreceptor is highly activated
and our red one less so.
The reason Young and Helmholtz thought their
theory was correct was because some people who
are colour blind are only blind for one colour. Some
people are blind for blue (exceptionally rare) or
green or red.
According to Montgomery (2005), there are about
ten million men in America who are colour blind.
This is 7% of the male population compared to only
0.4% of the female population. This indicates that
there is a genetic basis to colour blindness. This disorder is carried on the X chromosome and, because
women have two of these, they are protected to a
certain extent as the one can compensate for the
other if it is disordered.
The other main theory of colour vision takes a different view. The opponent-process theory states that
we have neurons in our retina that are able to
process pairs of colours. These pairs are red-green,
yellow-blue, and black-white. When stimulated,
these neurons would react to one side of the pair
more than the other, resulting in you seeing more
red than green for example. The pairs are therefore
called opponents because they are opposed or
against each other. This theory can account for the
experience of afterimages (see Box B.4).
B.4 AFTERIMAGES
Find a picture of the American flag. Stare at the
middle of the flag for 30 seconds, then look at a
white piece of paper. You will ‘see’ the flag but in
different colours. When you look at the white
paper you will see a green and black flag rather
than red and white.
Sensation and perception 9
Sternberg (2004) notes that we seem to need both
theories to account for colour vision. Trichromatic
theory is correct when it states that we have three
kinds of cones but the opponent-process theory
is also correct at a higher level of the neuron.
The opponent-process theory can also explain afterimages, which the trichromatic theory cannot.
Visual perception
In the case of Mr S we found that his sensation was
not affected, as his abilities to pick up light waves
and to see colours were all intact. It was suggested
that Mr S probably had a problem with his perception of visual elements. The process of perception
relies on the elements of sensation that enter the
brain as well as memory, past experience, and the
culture in which one lives in order to make the visual world meaningful.
Top–down and bottom–up processing
Bourne and Russo (1998) and Passer and Smith
(2001) distinguish between top–down and bottom–up processing in perception.
Feature detection theory is an example of
bottom–up processing. According to this theory, the
neurons in the retina send information using the
optic nerve to the brain via the thalamus. Of the
many parts of the brain to which information is sent,
the primary visual cortex is the main one. The primary visual cortex is located in the occipital lobe at
the back of the brain. Passer and Smith (2001) note
that research has shown that specific neurons in the
retina make contact with specific regions of the primary visual cortex. There is an almost one-to-one
mapping. Some of these neurons in the primary
visual cortex only fire or respond to certain visual
stimuli. For example, some may only fire for horizontal lines, others for vertical lines. These are called
feature detectors as they ‘look out’ for certain features
or characteristics. We seem to have feature detectors
for many visual elements such as colour, shape, and
motion. When we see something in front of us, these
many feature detectors fire together (or in parallel)
and we integrate the information to form an image.
This is a bottom–up process in that it takes all the elements of a visual array and combines it into something bigger or more meaningful.
Top–down processing works the other way round.
Bourne and Russo (1998) use the following example.
10 Psychology an introduction
What is the middle image? The letter B? Or the
number 13?
2
A
3
C
4
Figure 5 Top–down processing.
If you read the numbers you would expect the object
in the middle to be 13 as it comes after 12, but if you
read from top to bottom you would read the object
as B as it comes after A. Deciding what you see in
this instance is dependent on past experience as well
as learning; so too is top–down processing.
Rather than being competing ideas, top–down
and bottom–up processing are both used in visual
perception. We construct or make our perceptions
based on what we sense as well as what already
exists in our brains (Bourne & Russo, 1998).
Returning to our case study, it appears as if Mr S
can process information in a bottom–up fashion as
visual information activates features in his primary
visual cortex. However, Mr S does not seem to be
able to utilise top–down processing in order to
utilise his past experience and knowledge of words
and letters to identify and read the words.
Form perception
In the 1920s Gestalt Psychologists identified and
explained a set of principles that we use in order to
perceive our world visually. These principles state
that we take the elements which make up an object
and form a meaningful whole from them. Remember
the Gestalt principle of ‘the whole is greater than the
sum of its parts’? The Gestalt laws of organisation
include the following (Feldman, 1999):
• Proximity – those objects closest together are perceived as belonging together. Have a look at the
row of dots in Figure 6(a). Instead of seeing a row
of single dots, you see a row consisting of groups
of two dots. Another example is when you see a
friend sitting on a bench next to a person who
looks a little like your friend, but much older.
When you walk over to your friend you may say
‘hello’ to the other person thinking it is her mother, whereas in reality they do not know each other
at all. You placed them together as a meaningful
unit as a result of proximity; they were close
together.
• Similarity – things that look the same are grouped
together. Have a look at the blocks and crosses in
Figure 6(b). You form a cross with the blocks
because they are similar in form.
• Closure – people ‘close’ or ignore the gaps in
objects to form a meaningful whole. Have a look
at the triangle in Figure 6(c). Even though there
are ‘holes’ in the triangle you complete them to
form a meaningful whole.
(a)
x x x
x
x
x
x x
x x
x
x
x
x x x
(b)
(c)
Figure 6 Diagram of Gestalt laws of organisation
Perceptual constancy
Lahey (2001) states that we see the world as quite
stable and constant. Similar to the way in which the
Gestalt psychologists said we make sense of our
world, the principles of perceptual constancy show
that we use cues in the environment to keep our
world predictable and stable. This stability is in
spite of the fact that the sensations we receive from
the world are constantly changing. As a man walks
away from you, the object on the retina gets smaller
and smaller but you don’t think that the man is
shrinking. Rather, you know he is still the same size
but is moving further away.
There are various kinds of constancy:
• Colour constancy – imagine you are wearing a
bright red jersey. Your friend sees it in bright sun-
light and remarks that it looks nice and bright.
When you go into a lecture theatre which is not as
bright, another friend also remarks on its brightness; when you walk into your darkened bedroom, your mother also says that it is nice and
bright. All these people saw a different brightness
of red in your jersey but all noticed that it was a
bright red. The perception of the colour stayed
the same or constant even though the image on
the retina was not as bright due to different levels
of lighting.
• Size constancy – this refers to the fact that, even
though an object gets smaller on the retina as you
move away from it, you know its size remains the
same. For example, as you walk away from a
friend who is your height, her size will decrease
on your retina; however, you know that she is not
shrinking and that her size remains the same.
• Shape constancy – this is when the shape of something changes on the retina. For example, when a
door opens you know the shape is not changing
in reality even though it appears to change shape.
Therefore, when you look at a closed door it is
rectangular in shape. When it starts to open and
the shape changes to a more trapezoid shape, you
know it is still a rectangle.
If we were relying on bottom–up processing only,
we would think that our friend’s jersey was fading,
that our other friend was shrinking, and that the
door was changing shape right in front of our eyes.
Top–down processing allows us to maintain perceptual constancy. This is due to the fact that we have
learned things about our world through experience
and this learning enables us to make sense of changing stimuli.
Depth perception
Just as we used our past learning experience to
make sense of our ever-changing world we also
need some help in making sense of how things fit
into our world. Depth perception is the ability to perceive the three-dimensional quality of our world.
This is amazing, considering the fact that the retina
is like a flat piece of paper and is, therefore, twodimensional. We use both monocular (one eye) and
binocular (two eyes) cues from the environment to
tell us about depth. Monocular depth cues are the ones
most artists use in their artworks and include the
following:
Sensation and perception 11
• Linear perspective – parallel lines, such as the sides
of a road or a railway track, look as though they
move closer together the further they move away
from you.
• Aerial perspective – things like pollution and dust in
the air affect the quality of light so that objects that
are further away look a little hazy and blue-ish.
• Superposition – things that are closer to you overlap with things that are further away from you.
• Texture gradient – if you stand on a beach you will
be able to see the grains of sand at your feet but
not the grains that are further away.
• Speed of movement – things such as cars that are far
away from you look as though they are moving
more slowly than cars that are closer to you
(Lahey, 2001).
Binocular depth cues include the following:
• Convergence – Hold your finger in front of your
face at an arm’s length. Look at your finger as you
slowly move it towards your face. Can you feel
the muscles in your eyes start to tighten the closer
the finger gets to your face? This information from
the muscles goes to the brain to provide information about distance. The further something is, the
less tension is placed on the eye muscles.
• Retinal disparity – Hold your finger in front of
your face again, about 20 centimetres away. Look
at the finger with one eye open at a time. Notice
how your finger seems to jump from one side to
the other as you focus on it with each eye? This is
due to retinal disparity as each eye is picking up
a different picture of the finger. When the images
from both eyes are put together, depth perception
occurs.
B.4 ILLUSIONS
The following are two examples of illusions:
• The Müller-Lyer illusion – Look at Figures 7 (a)
and (b) below. If asked to judge which line is
longer, most people say the one with the inverted
arrows on the end. However, both lines are the
same length.
• The Ponzo illusion – Which line in figure 7(c) is
longer, the one at the top or the bottom of the
converging lines? Because we think of converging
lines as representing distance we see the furthest
line as longer, but they are both the same length.
(a)
(b)
Visual illusions
Santrock (2003) notes that our perception is most
often correct. However, when we pick up signals
from the environment and come to an incorrect perception, this is called an illusion. See Box B.4 for
some examples of illusions.
Perceptual deficits
Chapter 11 deals extensively with perceptual
deficits and you can refer to this chapter for more
information about it. The case of Mr S also illustrates
what happens when there is a deficit in perception.
While Mr S’s sensory processing abilities were intact,
his ability to process information into a meaningful
12 Psychology an introduction
(c)
Figure 7 The Müller-Lyer illusion (a) and (b) and the Ponzo
illusion (c)
whole, such as words, was disordered. Any deficit of
perception is called an agnosia. Visual-object agnosia
occurs when people can sense the visual field but
cannot identify or put a name to an object. Another
agnosia is prosopagnosia where people cannot recognise faces (Sternberg, 2004).
One can imagine how difficult it is to deal with a
disorder of vision, but hearing is the other sense
upon which we rely heavily.
Hearing and sound waves
After vision, hearing is probably the sense upon
which we rely the most. The energy signals that
come from the environment to our ears are in the
form of sound waves. Sound waves are actually pressure waves. Think about a very large speaker: when
someone increases the volume of a speaker to a very
loud level, objects in front of the speaker move as
the air moves (Passer & Smith, 2001).
Sound has three characteristics: amplitude, frequency, and timbre:
• Frequency – this is the number of waves that occur
per second. This is measured as cycles per second
or hertz (Hz). When you increase the number of
cycles per second or Hz, the pitch of the sound
increases (Passer & Smith, 2001). The sound produced by a high-pitched whistle would have
more cycles per second than the sound produced
by a big, bass drum.
• Amplitude – this relates to the size of the sound
waves, that is, how big or how small they are. The
size of the wave determines loudness and is
measured as decibels (db). For example, the
sound waves produced by a man shouting would
be much bigger than those produced by a child
whispering.
• Timbre – this relates to the quality of the sound.
For example, the notes on a piano would have a
different quality to an explosion (Sternberg, 2004).
The structure of the ear
Figure 9 shows the structure of the ear.
The route that sound waves take through the ear
can be explained in a step-by-step process. The ear is
divided into the outer, middle, and inner ear.
The outer ear:
Step 1: The sound waves are collected by the pinna
or the outer parts of the ear; that is, the
parts that you can see.
Step 2: The soundwaves then move down the
auditory canal to the eardrum.
Step 3: The sound waves make the eardrum
vibrate. Higher frequencies lead to faster
vibrations.
Middle ear:
Step 4: The middle ear has three bones that collect
the vibrations.
Step 5: These bones, the malleus, incus, and stapes,
increase the vibrations and send them to
the inner ear or the cochlea.
Inner ear:
Step 6: The vibrations reach the oval window
which is the start of the cochlea.
Step 7: The cochlea is made up of three channels
separated by membranes. One of the
membranes is the basilar membrane and has
Amplitude
Baseline
Wavelength
(one cycle)
(a) Long-wavelength (low-frequency) sound
Amplitude
Baseline
Wavelength
(one cycle)
(b) Short-wavelength (high frequency) sound
Figure 8 Illustration of frequency waves and amplitude (adapted from Sternberg, 2004)
Sensation and perception 13
Outer ear
Middle ear
Inner ear
Malleus
(hammer)
Incus (anvil)
Semicircular canals (vestibular apparatus)
Pinna
Acoustic nerve
Basilar membrane
with protruding hair
cells
Sound waves
Fluid
Eustachian tube to
throat
Oval window (where stapes attaches)
Stapes (stirrup)
Auditory canal
Eardrum
(tympani)
Figure 9 The structure of the ear (adapted from Sternberg, 2001)
small hairs on it. These hairs float in the
fluid of the cochlea and are our auditory
receptors. The vibration moves parts of
these hairs.
Step 8: The movement of the hairs starts the electrochemical message (neural transmission)
that is then sent to the brain.
Pathway to the brain
Information from the cochlea starts the electrochemical message that is sent via the auditory nerve to the
brain. Sternberg (2004) notes that the path of the
auditory nerve goes to the medulla oblongata, then to
the midbrain, through the thalamus, and finally to the
auditory cortex (in the temporal lobes).
Theories of hearing
neurons to fire at a higher rate, thus registering
that it is a loud sound. This is similar to looking at
an open field of grass or veld. If a high wind
(which is like a loud noise) blows over the grass
(which are like the hair cells), then it will bend
more. Therefore, even if you are in a house looking out the window, you know there is a high
wind because the grass is bent over so far.
• The second indicator that a sound is a loud one is
that specific neurons have a higher threshold for
firing. If the amplitude is high and the sound
wave is high, this will cross the threshold and
make these specific neurons fire. If the neurons
fire, this sends a message that the sound is loud.
Imagine the veld once again. These specific receptors are like small trees. They will only bend in the
wind (or sound amplitude) if the wind (amplitude) is strong enough.
Together with theories of loudness, there are also
theories explaining how we hear pitch.
Loudness
Passer and Smith (2001) explain that loudness is
transmitted to the auditory nerve in two ways.
• Firstly, loud sounds have high amplitude; this
high amplitude sound wave makes the hair cells
on the basilar membrane bend more, causing the
14 Psychology an introduction
Place theory
According to Santrock (2003), place theory states that
we hear pitch because the vibrations caused by each
frequency makes a specific place on the basilar
membrane vibrate. For example, high frequency
waves cause the area close to the oval window in the
cochlea to vibrate, while low frequency sounds
cause the basilar membrane at the end of the cochlea
to vibrate. However, because the vibration starts at
the oval window and moves down the cochlea, this
theory does not explain how we hear low frequency
sounds very well as the vibration seems to exist
throughout the cochlea, which is not very specific.
Other influences need to be considered.
Frequency theory
Frequency theory attempts to address the shortcomings of place theory. According to frequency
theory, our ability to distinguish different pitches is
related to the number of times the auditory nerve
fires. The nerve will fire more often for higher
sounds than for lower ones (Santrock, 2003). This
theory is useful in that it can now explain low
frequency sounds quite well. However, there is a
difficulty in that the neurons fire in the same ratio as
the frequency level (Hz); while our neurons can only
fire at 1 000 times per second, we can hear pitches at
20 000 Hz.
According to Sternberg (2003), researchers have
proposed the volley principle to explain how we can
hear these very high sounds. They argue that neurons work together when they are stimulated by
high frequency sounds. When the frequency vibration of this high sound enters the cochlea, the neurons act in a cooperative group, alternating the firing.
Therefore, while one neuron is resting, the other neuron fires. In this way a rate of rapid firing is possible.
You can think of it as a row of men filling a hole with
sand using spades. If one man were doing it, the
process would be very slow. However, if they all
stand in a line, one man may be throwing sand into
the hole while another man may be lifting more soil;
in that way, when the first man turns to get more soil,
the second man can throw his soil into the hole.
It thus appears that some combination of both
place theory and frequency theory is necessary to
explain pitch.
Locating sounds
Having explained how we hear sounds, it is also
important to explain how we know where sounds
are coming from. Animals are much better than
humans at locating sounds. A dog’s ears, for example, are shaped so that they form a tunnel for the
sounds to travel down. The ears therefore trap the
sound and dogs can move their ears to find where
the best sound is coming from.
Human ears cannot move around to find the best
position to hear the sounds. Humans rely on something called a sound shadow to locate a sound.
Imagine you are in a house for the first time and a
phone is ringing. You want to answer it but you
don’t know where the phone is. You will need to
locate the phone by sound. The sound of the ringing
phone will reach the ear closest to the sound faster
than the other ear. The ear closest to the noise will
hear the ringing first (and at a slightly higher intensity) and your head will block the sound waves
travelling to your other ear to a certain degree, causing a shadow and a lowering of the intensity of the
sound. This shadow results in a very small delay in
the sound reaching the furthest ear and a slight drop
in sound intensity. This is because the sound has to
travel an extra distance to reach the furthest ear. Our
brain can use these two pieces of information to tell
where the sound is coming from. It is more complicated if the sound reaches both ears at the same time
because there is no shadow to tell us where the
sound is. When this happens, people tend to move
their heads one way or the other to create a sound
shadow and so try to locate the sound.
This chapter has mainly focused on the senses of
hearing and vision as we rely so heavily on these
senses. For comprehensiveness, the other senses will
be discussed below but in a more condensed form.
The chemical senses: taste and
smell
While hearing and vision are the result of energy
waves creating an electrochemical neural impulse,
taste and smell require a substance to be dissolved
in chemicals in order for neural transmission to
occur. Taste and smell require that pieces of food or
molecules dissolve in either saliva in the mouth or
the mucus in the nose in order for neurons to fire.
Taste
In order to taste, molecules need to dissolve in the
saliva. We are able to taste sweetness, bitterness,
saltiness, and sourness. An additional taste of
monosodium glutamate is sometimes also added.
Sternberg (2004) notes that while our threshold for
Sensation and perception 15
Taste bud
Hairlike
ending
of taste
receptor
Sensory nerve
Figure 10 Structure of the tongue and the pathway to the brain (adapted from Coon, 2004)
taste is low, the just noticeable difference (jnd) is
often very high. Imagine sitting blindfolded while
someone places different things in your mouth to
taste. People often identify the taste incorrectly or
cannot detect when the taste has changed slightly.
From the tongue to the brain
The tongue has thousands of taste buds that only last
about ten days as more taste buds are created continuously. Each taste bud has a finger-like extension
at the top that is sensitive to the chemicals surrounding it. When this protrusion detects a chemical, it sends a message to the brain by making the
neuron fire. The message travels from the neurons to
the thalamus and then to the somatosensory cortex.
Some of the information also goes to the hypothalamus and limbic system (two areas of the brain
involved in emotion).
Most people understand that their sense of taste
is not very sophisticated or sensitive. Remember
when you had a cold and your nose was blocked? At
such times your sense of taste seems to almost disappear. This is because much of what we taste
depends on being able to smell it.
Smell
While we need our sense of smell to be able to taste
a full range of food, our sense of smell is also not
very good in relation to other animals. However, we
often attach many emotions to smell. It is likely that
you have been in a certain place and smelt some-
16 Psychology an introduction
thing familiar that brought back many feelings and
emotions. We smell something when molecules in
the air are dissolved in the mucus in the nose.
From the nose to the brain
Molecules in the air enter the nose through the
nostrils when we inhale them. Here they are transferred to the olfactory epithelium or the membrane
of the nose that secretes mucus. This area is just
below and behind the eyes. On the olfactory
epithelium the molecules activate the olfactory
receptor cells. These cells last four to eight weeks
only and then re-grow. When the receptor cells are
activated, the specialised neurons fire. The information from the neurons then comes together in
the olfactory nerve which leaves the nose and
enters the brain through the skull. This route from
the nose directly to the specific part of the brain
responsible for processing smell is unique.
Information from the other senses travels from the
sense organs to the thalamus (which is like a conductor telling the information where to go) and
then to the area in the brain responsible for processing. Thus, the olfactory nerves go straight to
the olfactory bulb which is in the temporal lobes.
Some information also goes to the hypothalamus
and the limbic system which is possibly why smells
often elicit emotions.
Anosmia occurs when you lose your sense of
smell. This is a rare phenomenon that can occur after
a head injury. People with anosmia often report a
lack of interest in food as their ability to smell and
taste it has reduced.
Afferent fibres of olfactory nerve
Olfactory bulb
To cerebral cortex
Cribriform plate of
ethmoid bone
Olfactory nerve fibres
Basal cell
Supporting cell
Receptor cell (bipolar)
Cilia
Nasal mucous membrane
Nasal cavity
Figure 11 Structure of the nose and pathway to the brain (adapted from Coon, 2004)
The skin senses
The skin’s senses are necessary for us to feel pain,
temperature, and pressure. These senses serve a survival function and, if we did not have them, we may
endanger ourselves. Imagine you could not tell
when you are feeling pain. You may touch a hot
stove plate and only realise it when you smell something burning, or you may not feel the pain of an
inflamed appendix that is ready to burst. Pain is our
body’s way of telling us that we are in danger.
Structure of the skin and pain
Most of the skin’s senses rely on the specialised
receptor cells that are found in the skin. Some parts of
our bodies have more receptors than others and thus
are more sensitive. There are many more receptors
in your fingers compared to your back, for instance
(Feldman, 1999).
Of all the skin’s senses (touch, pressure, temperature, and pain) it is pain that often causes us the
most distress. The experience of pain is exceptionally distressing to people and many situations make
this perception worse. Feldman (1999) notes that
research has shown that some instances, such as
going to the dentist, are related to the experience of
more severe pain. There are also cases when people
should feel pain, but do not. For example, soldiers
have reported not feeling very severe injuries on the
battle grounds but the same type of injuries received
in a surgical procedure are perceived at a higher
pain level (Sternbach, 1987, cited in Feldman, 1999).
This can be explained by the gate-control theory of
pain (Melzack & Wall, 1965; Wall & Melzack, 1989).
According to this theory, receptors in the skin send
a message to the brain when they are activated,
causing one to feel pain. This message opens the
‘gate’ to the brain. However, we have other receptors that can close the gate and so reduce the pain.
This can be done in two ways:
1. Receptors can create impulses that take over or
overwhelm the pain pathway. This happens
when you hurt yourself and then rub the site.
This rubbing overwhelms the pain pathway and
alleviates the pain.
2. The second way to shut the gate is through thinking it shut or using psychological factors. This is
what happened to the soldiers to whom we
referred earlier. The soldiers’ perception of the
pain was influenced by the fact that they were
relieved to be alive; as a result they did not notice
the pain.
This theory explains how psychological factors can
have an effect on our body and how our emotions
may affect our experience of pain.
The body senses
Our ability to know where we are in space, how our
bodies move, as well as our ability to ensure that we
don’t lose our balance are all taken care of by the
body senses.
Sensation and perception 17
Kinaesthesia
Sternberg (2004) describes kinaesthesia as the sense
that monitors the body’s position by noting the
skeleton’s position and movement. The body is able
to do this because it has receptors in the joints, muscles, tendons, and skin that monitor movements of
the skeleton. The neural impulses created from this
movement go to the brain. Specifically, information
travels to the somatosensory cortex and the cerebellum
which are responsible for coordinated movement.
We need our kinaesthesic sense in order to move
well and to maintain our balance. This, together
with the vestibular sense is used by a person doing
a handstand, for example.
The vestibular sense
The vestibular sense is responsible for our sense of
balance and resides in the inner ear. Our ears have
semi-circular canals which are three fluid-filled
tubes in the inner ear. You can think of these as three
bottles of water that lie on their side. As you walk,
the fluid moves from side to side as the head
changes angle or rotates. There are small crystals in
the semi-circular canals called otoliths. These are
responsible for sensing the movement of our bodies
when we move forwards, backwards, fast or slow,
and up and down. Gravity also plays a role in this
regard (Feldman, 1999).
Conclusion
This chapter has demonstrated the importance of sensation and perception to our psychological functioning. Although vision and hearing were explained in
great detail and the other senses in lesser detail, the
importance of all our senses was noted. As Sternberg
(2004, p. 165) says, ‘when [our senses] are damaged or
lacking, life as we know it is radically different. Our
senses are our gateways to our thoughts, feelings, and
ideas. They provide bridges from the external world,
through our bodies, to our minds’.
REFERENCES
Bourne, L.E. & Russo, N.F. (1998). Psychology: Behaviour in Context.
New York: Norton.
Coon, D. (2004). Introduction to Psychology: Gateways to Mind and
Behavior. Belmont, California: Thomson.
Cytowic, R.E. (1995). Synesthesia: Phenomenology and
Neuropsychology. Psyche, 2(10).
Feldman, R.S. (1999). Understanding Psychology (5th edition). Boston:
McGraw-Hill.
Lahey, B.B. (2001). Psychology: An Introduction (7th edition). Boston:
McGraw-Hill.
Melzack, P.D. & Wall, R. (1965). Pain mechanisms: A new theory.
Science, 150, pp. 971–979.
Montgomery, G. (2005). Colour blindness: more prevalent among
males. Seeing, Hearing, and Smelling the World.
http://www.hhmi.org/sesnes/b130.html (accessed 21 June 2005).
18 Psychology an introduction
Ole-Herman, B. (2004). Measure speech intelligibility with a sound
level meter. Sound and Vibration, www.findarticles.com/p/articles
/mi_qa4075/is_200410/ai_n9469295/print (accessed 20 July
2005).
Passer, M.W. & Smith, R.E. (2001). Psychology: Frontiers and
Applications. Boston: McGraw-Hill.
Santrock, J.W. (2003). Psychology (7th edition). Boston: McGraw-Hill.
Shenker, J.I. (2005). Teaching biology in a psychology class. www.psychologicalscience.org/observer/getArticle.cmf?id=1766 (accessed 4
June 2005).
Sternberg, R. (2004). Psychology (4th edition). Orlando: Thomson.
Wall, P.D. & Melzack, R. (1989). Textbook of Pain. New York: Churchill
Livingston.
Westen, D. (1999). Psychology: Mind, Brain, & Culture (2nd edition).
New York: John Wiley & Sons.
EXERCISES
Multiple choice questions
1.
2.
3.
4.
5.
6.
Which one of the following statements about sensation and perception is false (wrong)?
a) overall sensation is a passive process and perception is an active process
b) perception is always an objective process of
stimulus input and processing
c) sensation occurs when specific energy signals
hit specific sensory cells
d) we make meaning of our world through perception
The minimum amount of energy required for you to
detect a stimulus is called:
a) the threshold
b) the best threshold
c) the difference threshold
d) the absolute threshold
Nosipho is taking part in a psychology experiment.
She has been asked to tell the experimenter when
she sees a light on the screen in front of her. If she
says ‘yes’ when no light appears, this is called a:
a) false alarm
b) hit
c) miss
d) correct rejection
Suppose you live in the middle of a big city most of
the year but you leave the city to visit family in a rural
area with only a few houses for one week of the year.
When you return to the city you notice that there is
a strong odour or smell of smoke and car fumes. You
comment to your friend that the city smells very bad
all of a sudden but she does not know what you are
talking about. This experience can be explained by:
a) adaptation
b) the just noticeable difference
c) response bias
d) sensation
Rods are responsible for _________, while cones are
responsible for __________.
a) subjective experience; objective experience
b) colour vision; black and white vision
c) day vision; colour vision
d) dim light vision; bright light vision
You see a friend walking towards you. As he gets
closer, the image on your retina gets bigger and bigger, but you don’t get a fright and think that your
friend is turning into a giant. This is because of:
a) size constancy
b) form perception
c) similarity
d) colour constancy
7. Place theory of hearing states that:
a) we hear pitch depending on the number of
times the auditory nerve fires
b) the volley principle is important
c) pitch depends on where the basilar membrane
vibrates
d) the sound shadow locates sounds
8. The sense of smell is unique because the olfactory
pathway to the brain ________.
a) goes through the limbic system
b) goes through the thalamus
c) goes straight to the olfactory bulb in the brain
d) does not enter the brain
9. Taste and smell are different to hearing and vision
because:
a) taste and smell are less important
b) taste and smell are chemical senses
c) taste and smell are connected to survival
d) they have no jnd
10. You are feeling very sick with flu; you have a blocked
nose and feel light-headed. The doctor says you may
have an ear infection which causes you to feel lightheaded because:
a) the vestibular sense resides in the inner ear
b) no blood is getting to your brain
c) you cannot see properly because your eyes are
watering
d) the vestibular sense is connected to the nose
Short answer questions
1.
2.
3.
4.
5.
Explain to a friend why they have to turn the volume
knob of the radio up more to hear a difference in the
sound when it is already loud. In your answer explain
the just noticeable difference theory.
Compare and contrast the trichromatic and opponent process theories of colour vision.
Explain the depth cues used by artists to give the
illusion of three-dimensional space on a flat piece of
paper.
In hearing, is place theory more correct than frequency theory?
Your friends are having an argument about whether
men and women experience more pain. Some say
that men would never be able to handle the pain
associated with childbirth, while others say that men
fight in wars and experience very severe forms of
pain. What would you tell them about the sensation
and perception of pain?
Sensation and perception 19