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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