- Created by: moll99
- Created on: 27-01-17 10:35
The Nervous System
The nervous system is a specialised network of cells in the human body and is our primary internal communication system. It has two main functions:
- To collect, process and respond to information in the environment.
- To co-ordinate the working of different organs and cells in the body.
The central nervous system (CNS) is made up of the brain and the spinal cord. The brain is the center of all concious awareness. The brains outer layer, the cerebral cortex, is highly developed and is what distinguishes our higher mental functions from that of animals. The spinal cord is an extension of the brain and is responsible for reflex actions.
The peripheral nervous system (PNS) transmits messages via millions of neurones (nerve cells) to and from the CNS. The PNS is sub-divided into:
- Autonomic nervous system: governs vital functions of the body.
- Somatic nervous system: controls muscle movements and receives information from sensory receptors.
The Endocrine System
The endocrine system works alongside the nervous system to control vital functions in the body. The endocrine system works a lot slower than the nervous system but has very widespread and powerful effects.
Various glands in the body produce hormones and hormones are secreted into the bloodstream and affect any cell in the body that has a receptor for that particular hormone.
Most hormones affects cells in several organs or throughout the entire body, leading to many diverse and powerful responses.
The main glands of the endocrine system are: pituiatry gland, thyroid, hypothalamus, parathyroid, adrenals, pancreas, ovaries (females) and testes (male).
The pituiatry gland is known as the major endocrine gland as it is controls the release of hormones from all other endocrine glands in the body.
Fight or Flight
The endocrine system and the autonomic nervous system work with one another, for instance during a stressful event.
When a stressor is percieved the first thing that happens is a part of the brain called the hypothalamus triggers activity in the sympathetic branch. This changes the autonomic from it's normal resting state (parasympathetic state) to the physiologically aroused sympathetic state.
The stress hormone adrenaline is released from the adrenal medulla (part of the adrenal gland) into the bloodstream. This triggers physiological changes in the body (e.g increased heart rate) which creates the physiological arousal necessary for the fight or flight response.
All of this happens in an instant as soon as the threat is detected and is an automatic reaction in the body.
Once the threat has passed, the parasympathetic nervous system returns the body to its resting state. The parasympathetic branch of the ANS works in opposition to the sympathetic nervous system - its actions are antagonistic to the sympathetic system.
Localisation of Function in the Brain
Localisation versus Holistic Theory
Scientists such as Paul Broca and Karl Wernicke discovered that specific areas of the brain are associated with particular physical and psychological functions.
Before these investigations, scientists generally supported the holistic theory of the brain - that all parts of the brain were involved in the processing of thought and action.
In contrast, Broca and Wernicke argued for localisation of function (cortical specialisation). This is the idea that different parts of the brain perform different tasks and are involved with different parts of the body.
It follows then, that if a certain area of the brain becomes damaged through illness or injury, the function associated with that area will also be affected.
The Case of Phineas Cage
Whilst working on a railroad in 1848, Phineas Cage was preparing to blast a section of rock with explosives to create a new railway line.
During this process, Gage dropped his tamping iron onto the rock causing the explosive to ignite. The explosive hurled the metre-length pole through Gage's left cheek, passing behind his left eye and exiting his skull from the top of his head, taking a portion of his brain with it.
Gage survived but the damage to his brain had left a mark on his personality as a lot of his left frontal lobe was destroyed.
Gage changed from someone who was calm and reserved to someone who was quick-tempered and rude. Gage is seen as a landmark case in science as the change in his temperament following the accident suggests that the frontal lobe may be responsible for regulating mood.
This case supports the idea that different parts of our brain perform different tasks and are involved with different parts of the body - the localisation theory.
Hemispheres of the Brain
The brain is divided into two symmetrical halves called the left and right hemispheres. Some of our physical and psychological functions are controlled or dominated by a particular hemisphere.
Activity on the left-hand side of the body is controlled by the right hemisphere and activity on the right-hand side of the body is controlled by the left hemisphere.
The outer layer of both hemispheres is the cerebral cortex which covers the inner parts of the brain.
The cerebral cortex is about 3mm thick and is what seperates us from other animals because the human cortex is much more developed.
Lobes in the Brain
The cortex of both hemispheres is sub-divided into four lobes, which are named after the bones beneath them - the frontal lobe, the parietal lobe, the occipital lobe and the temporal lobe. Each lobe is associated with a different function.
At the back of the frontal lobe in both hemispheres is the motor area which controls voluntary movement in the opposite side of the body. Damage to this area of the brain may result in a loss of control over fine movements.
At the front of both parietal lobes is the somatosensory area which is seperated by an area called the central sulcus. The somatosensory area is where sensory information from the skin is represented.
In the occipital lobe at the back of the brain is the visual area. Each eye sends information from the right visual field to the left visual cortex and from the left visual field to the right visual cortex.
The temporal loves house the auditory area, which analyses speech-based information.
Language Area of the Brain
Language in most people is restricted to the left side of the brain.
In the 1880's, Paul Broca, a surgeon identified a small area in the left frontal lobe responsible for speech production.
Damage to Broca's area causes Broca's aphasia - characterised by speech that is slow, laborious and lacking in fluency.
Karl Wernicke was describing patients who had no problem producing language but severe difficulties understanding it, such that the speech they produced was fluent but meaningless.
Wernicke identified a region (Wernicke's area) in the left temporal lobe as being responsible for language comprehension which would result in Wernicke's aphasia when damaged.
Patients who have Wernicke's aphasia will often produce nonsense words (neologisms) as part of the content of their speech.
- Brain scan evidence of localisation: Petersen et al (1988) used brain scans to demonstrate how Wernicke's area was active during a listening task and Broca's area was active during reading tasks, suggesting that these areas have different functions. Tulving et al (1994) revealed that semantic and episodic memories reside in different parts of the prefrontal cortex.
- Neurosurgical evidence: Dougherty et al (2002) reported on 44 OCD patients who had undergone a neurosurgical procedure that involves lesioning of the cingulate gyrus. After 32 weeks, a third had met the criteria for successful response and 14 percent for partial response. This strongly suggest that mental disorders are localised.
- Case study evidence: Unique cases of neurological damage support localisation theory such as the case of Phineas Cage.
- Lashley's research: Karl Lashley (1950) suggests that higher cognitive functions are not localised but are distributed in a more holistic way in the brain. He removed areas of the cortex in rats and no area was proven to be more important than any other area in terms of the rats' ability to learn a maze.
- Plasticity: When the brain has become damaged through illness or accident, and a particular function has been compromised or lost, the rest of the brain appears able to reorganise itself in an attempt to recover the lost function. Surviving brain circuits 'chip in' so the same neurological action can be achieved.
The brain would appear to be plastic in the fact that it has the ability to change throughout life.
During infancy, the brain experiences a rapid growth in the number of synaptic connections it has, peaking at approximately 15,000 at age 2-3 years (Gopnick et al, 1999). This equates to about twice as many as there are in the adult brain.
As we age, rarely used connections are deleted and frequently used connections are strengthened - a process known as synaptic pruning.
It was thought that such changes were restricted to the developing brain within childhood, and that the adult brain would remain fixed and static in terms of function and structure.
More recent research suggests that at any time in life, existing neural connections can change or new neural connections can be formed as a result of learning and experience (plasticity).
Research into Plasticity
Eleanor Maguire et al (2000) studied the brains of London taxi drivers and found significantly more volume of grey matter in the posterior hippocampus than in a matched control group. This part of the brain is associated with the development of spatial and navigational skills in humans.
As part of their traning, London cabbies must take a complex test which assesses their recall of the city streets and possible routes. It appears that the result of this learning experience is to alter the structure of the taxi drivers' brains. The longer they had been on the job, the more pronounced was the structural difference.
Draganski et al (2006) who imaged the brains of medical students three months before their final exams. Learning-induced changes were seen to have occured in the posterior hippocampus and the parietal cortex presumably as a result of the exam.
Mechelli et al (2004) also found a larger parietal cortex in the brains of people who were bilingual compared to matched monolingual controls.
Functional Recovery of the Brain
Following physical injury or other forms of trauma such as the experience of a stroke, unaffected areas of the brain are often able to adapt and compensate for those areas that are damaged.
The functional recovery that may occur in the brain after trauma is another example of neural plasticity. Healthy brain areas may take over the functions of those areas that are damaged, destroyed or even missing.
Neuroscientists suggest that this process can occur quickly after trauma (spontaneous recovery) and then slow down after several weeks or months. At this point the individual may require therapy to further their recovery.
What happens during recovery?
The brain is able to rewire and reorganise itself by forming new synaptic connections close to the area of damage. Secondary neural pathways that would not typically be used to carry out certain functions are activated to enable functioning to continue, often in the same way as before (Doidge 2007).
This process is supported by a number of structural changes in the brain including:
- Axonal sprouting: the growth of new nerve endings which connect with other undamaged nerve cells to form new neuronal pathways.
- Reformation of blood vessels.
- Recruitment of homologous areas on the opposite side of the brain to perform specific tasks. An example would be if Broca's area was damaged on the left side of the brain, the right-sided equivalent would carry out its functions. After a period of time, functionality may then shift back to the left side.
- Practical application: Understanding the processes involved in plasticity has contributed to the field of neurorehabilitation. Spontaneous recovery tends to slow down after a number of weeks so forms of physical therapy may be required to maintain improvements in functioning. This shows that, although the brain may have the capacity to 'fix itself' to a point, this process requires further intervention if it is to be completely successful.
- Negative plasticity: The brain's ability to rewire itself can sometimes have maladaptive behavioural consequences. Prolonged drug use has been shwon to result in poorer cognitive functioning as well as an increased risk of dementia later in life (Medina et al 2007). 60-80% of amputees have been known to develop phantom limb syndrome. This is thought to be due to cortical reorganisation in the somatosensory cortex that occurs as a result of limb loss (Ramachandran and Hirstein 1998).
- Age and Plasticity: Functional plasticity tends to reduce with age. The brain has a greater propensity for reorganisation in childhood as it is constantly adapting to new experiences and learning. Ladina Bezzola et al (2012) demonstrated how 40 hours of golf traning produced changes in the neural representation of movement in participants aged 40-60. Neural plasticiy does continue throughout the lfiespan.
- Support from animal studies: Early evidence of neuroplasticity and functional recovery was derived from animal studies. A study by David Hubel and Torten Wiesel (1963) involved sewing one eye of a kitten shut and analysing the brain's cortical responses. The shut eye continued to process information from the open eye.
- The concept of cognitive reserve: Eric Schneider et al (2014) discovered that the more time a brain injury patient spend in education, the greater their chances of a disability-free recovery. Two-fifths of patients studied who achieved DFR had more than 16 years education compared to 10% who had less than 12 years education.
The ability to produce and understand language for most people, is controlled by the left hemisphere.
This suggests that for the majority of us, language is subject to hemispheric lateralisation. The specialised areas associated with language are found in one of the brain's hemispheres rather than both.
The question of whether other neural processes may be organised in this way was investigated in a series of ingenious experiments conducted by Roger Sperry and his colleagues (known as a split-brain research) for which Sperry was awarded the Nobel Price in 1981.
Sperry's (1968) studies involved a unique group of individuals. All of them had undergone an operation called commissurotomy in which the corpus callosum and other tissues which connect the two hemispheres were cut down the middle in order to seperate the two hemispheres and control frequent and severe eplileptic seizures.
For these split-brain patients the main communication line between the two hemispheres was removed which allowed Sperry and his colleagues to see the extent to which the two hemispheres were specialised for certain functions, and whether the hemispheres performed tasks independently of one another.
Sperry devised a general procedure in which an image or word could be projected to a patient's right visual field (processed by the left hemisphere) and the same, or different image, could be projected to the left visual field (processed by the right hemisphere).
In the 'normal' brain, the corpus callosum would immediately share the information between both hemispheres giving a complete picture of the visual world.
However, presenting the image to one hemisphere of a split-brain patient meant that the information could not be conveyed from that hemisphere to the other.
Describing what you see: when a picture of an object was shown to a patient's right visual field, the patient could easily describe what was seen. If the same object was shown to the left visual field, the patient could not describe what was seen and reported there was nothing there.
Recognition by touch: patients could not attach verbal labels to objects projected in the left visual field, they were able to select a matching obkect from a grab-bag of different objects using their left hand. The objects were placed behind a screen so they could not be seen. The left hand was also able to select an obkect that was most closely associated with an object presented to the left visual field. The patient was not able to verbally identify what they had seen but could understand what the object was using the right hemisphere and select the object accordingly.
Composite words: if two words were presented simultaneously, one on either side of the visual field (for example, a key on the left and a picture of a ring on the right), the patient would write with their left hand the word 'key' and say the word 'ring'.
Matching faces: the right hemisphere also appeared dominant in terms of recognising faces. When asked to match a face from a series of other faces, the picture processed by the right hemisphere was consistently selected.
- Demonstrated lateralised brain functions: Sperry's pioneering work into the split-brain phenomenon has produced impressive and sizeable body of research findings. Research suggests that the left hemisphere is the analyser whilst the right hemisphere is the synthesiser - a key contribution to our understanding of brain processes.
- Strengths of the methodology: The experiments involving split-brain patients made use of highly specialised and standardised procedures. Sperry's method of presenting visual information to one hemispheric field at a time was quite ingenious and allowed Sperry to vary aspects of the basic procedure and ensured that only one hemisphere was receiving information at a time.
- Theoretical basis: Sperry's work prompted a theoretical and philosophical debate about the degree of communication between the two hemispheres in everyday functioning and the nature of conciousness. Other researchers argue that the two hemispheres form a highly integreated system and are both involved in most everyday tasks.
- Issues with generalisation: Many researchers have urged caution in their widespread acceptance as split-brain patients constitute such an unusual sample of people. The people in Sperry's study suffered from epilepsy which may have caused unique changes in the brain that may have influenced the findings. The control group Sperry used 11 people who had no history of epilepsy.
- Differences in function may be overstated: One legacy of Sperry's work is literature that overemphasises and oversimplifies the functional distinction between the left and right hemispheres. Modern neuroscientists would contend that the actual distinction is less clear-cut and much more messy than this.
Functional Magnetic Resonance Imaging (fMRI)
fMRI works by detecting the changes in blood oxygenation and flow that occur as a result of neural activity in specific parts of the brain.
When a brain area is more active it consumes more oxygen and to meet this increased demand, blood flow is directed to the active area (known as the haemodynamic response).
fMRI produces 3-dimensional images (activation maps) showing which parts of the brain are involved in a particular mental process and this has important implications for our understanding of localisation of function.
EEGs measure electrical activity within the brain via electrodes that are fixed to an individuals scalp using a skull cap.
The scan recording represents the brainwave patterns that are generated from the action of millions of neurons, providing an overall account of brain activity.
EEG is often used by clinicians as a diagnostic tool as unusual arrhythmic patterns of activity may indicate neurological abnormalities such as epilepsy, tumours or disorders of sleep.
Event-Related Potentials (ERPs)
EEG in its raw form, it is a crude and overly general measure of brain activity. Within EEG data are contained all the neural responses associated with specific sensory, cognitive and motor events.
Researchers have developed a way of teasing out and isolating these responses, using a statistical averaging technique.
All extraneous brain activity from the original EEG recording is filtered out leaving only those responses that relate to the presentation of a specific stimulus of performance of a specific task.
What remains are event-related potentials: types of brainwave that are triggered by particular events. Research has revealed many different forms of ERP and how these are linked to cognitive processes such as attention and perception.
A technique involving analysis of a person's brain following their death.
In psychological research, individuals whose brains are subject to a post-mortem are likely to be those who have a rare disorder and have experienced unusual deficits in mental processes or behaviour during their lifetime.
Areas of damage within the brain are examined after death as a means of establishing the likely cause of the affliction and the person suffered.
This may also involved the comparison with a neurotypical brain in order to ascertain the extent of the difference.
fMRI - Evaluation
Strengths: one key strength of fMRI is, unlike other scanning techniques such as PET, it does not rely on the use of radiation. If administered correctly it is virtually risk-free, non-invasive and straightforward to use. It also produces images that have very high spatial resolution, depecting detail by the millimetre and providing a clear picture of how brain activity is localised.
Weaknesses: fMRI is expensive compared to other neuroimaging techniques and can only capture a clear image if the person stays perfectly still. It has poor temporal resolution because there is around a 5-second time-lag behind the image on screen and the initial firing of neuronal activity. Finally, fMRI can only measure blood flow in the brain
EEG - Evaluation
Strengths: EEG has proved invaluable in the diagnosis of conditions such as epilepsy, a disorder characterised by random bursts of activity in the brain that can easily be detected on screen. It has contributed much to our understanding of the stages involved in sleep. Unlike fMRI, EEG technology has extremely high temporal resolution. Today's EEG technology can accurately detect brain activity at a resolution of a single millisecond.
Weakness: The main drawback of EEG lies in the generalised nature of the information received. The EEG signal is not useful for pinpointing the exact source of neural activity, and it does not allow researchers to distinguish between activities originating in different but adjacent locations.
ERP - Evaluation
Strengths: These bring much more specificity to the measurement of neural processes than could ever be achieved using raw EEG data. As ERPs are derived from EEG measurements, they have excellent temporal resolution, especially when compared to neuroimaging techniques such as fMRI which has led to their widespread use in the measurement of cognitive functions. Researchers have been able to identify many different types of ERP and describe the precise role of these in cognitive functioning. The P300 component is thought to be involved in the allocation of attentional resources and the maintenance of working memory.
Weaknesses: Critics have pointed to a lack of standardisation in ERP methodology between different research studies which makes it difficult to confirm findings. A further issue is that, in order to establish pure data in ERP studies, background noise and extraneous material must be completely eliminated and this may not always be easy to achieve.
Post-Mortems - Evaluation
Strengths: Post-mortem evidence was vital in providing a foundation for early understanding of key processes in the brain. Paul Broca and Karl Wernicke both relied on post-mortem studies in establishing links between language, brain and behaviour decades before neuroimaging ever became a possibility. Post-mortem studies improve medical knowledge and help generate hypotheses for further study.
Weaknesses: Causation is an issue within these investigations. Observed damage to the brain may not be linked to deficits under review byt to some other unrelated trauma or decay. A further problems is that post-mortem studies raise ethical issues to consent from the patient before death. Patients may not be able to provide informed consent.
All living organisms are subject to biological rhythms and these exert an important influence on the way in which body systems behave.
All biological rhythms are governed by two things: the body's internal biological clocks which are called endogenous pacemakers and external changes in the environment known as exogenous zeitgebers.
Some of these rhythms occur many times during the way (ultradian rhythms) and others take longer than a day to complete (infradian rhythms) and in some cases much longer (circannual rhythms).
Circadian rhythms are rhythms that last for around 24 hours. Two examples include the sleep/wake cycle and core body temperature.
We feel drowsy when it's night-time and alert during the day - this demonstrates the effect of daylight on our sleep/wake cycle.
Michael Siffre spent several extended periods understand to study the effects on his own biological rhythms. Deprived of natural light and sound, but with access to adequate food and water, Siffre re-surfaced in mid-September 1962 believing it to be mid-August. His biological rhythm settled down to one that was just beyond the usual, though he did continue to fall asleep and wake up on a regular schedule.
Aschoff and Wever (1976) convinced a group of participants to spend four weeks in a WWII bunker deprived of a natural sunlight. All but one of the participants displayed a circadian rhythm. Both Siffre's experience and the bunker study suggest that the natural sleep/wake cycle may be slightly longer than 24 hours but that it is entrained by exogenous zeitgebers associated with out 24-hour day.
Folkard et al (1985) studied a group of people who lived in a cave for 3 weeks and made their 24-hour a day actually 22 hours by changing their clocks. Only one of the participants was able to comfortably adjust to the new regime, showing the circadian rhythm cannot be overridden.
The Teenage Circadian Rhythm
According to neuroscientists, teenagers' circadian rhythms typically begin two hours after those of adults, so current school start times mean they wake up too early and are trying to focus when their body still needs sleep.
It also means, at bed time, they tend not to be as tired as they should be.
A pilot study was run at Monkseaton High School in North Tyneside in 2010. Dr Paul Kelley was headteacher at the time and after a decade of researching all the available evidence, he decided to put the start of the school day back to 10am over a two-year period.
Kelley stated: "There were very positive outcomes, both academically and in terms of health. Academic results went up, illness went down and the atmosphere of the school changed."
- Practical application to shift work: night workers engaged in shift work experience a period of reduced concentration around 6 in the morning (a circadian trough) meaning mistakes and accidents are more likely (Boivin et al, 1996). Shift workers are also three times more likely to develop heart disease (Knutsson 2003).
- Practical application to drug treatments: circadian rhythms have an effect on pharmacokinetics - the action of drugs on the body and how well they are absorbed and distributed. There are certain peak times during the day or night when drugs are likely to be at their most effective. This has led to the development of guidelines.
- Use of case studies and small samples: studies tend to involve small groups of participants or studies of individuals. The people involved may not be representative of the wider population and this limits the extent to which generalisations can be made.
- Poor control in studies: in the studies, people still had access to artificial light. Charles Czeisler et al (1999) were able to adjust participants' circadian rhythms from 22 to 28 hours using a dim lighting.
- Individual differences: individual cycles can vary, in some cases from 13 to 65 hours. Jeanne Duffy et al (2001) revealed that some people display a natural preference for going to bed early and rising early whereas some people prefer to do the opposite. There are also age differences.
The Menstraul Cycle
The female menstraul cycle is an example of an infradian rhythm, which is governed by monthly changed in hormone levels which regulate ovulations.
The cycle refers to the time between the first day of a woman's period, when the womb lining is shed, to the day before her next period.
The typical cycle take approximately 28 days to complete, but anywhere between 24 and 35 days is generally considered normal.
During each cycle, rising levels of the hormone oestrogen cause the ovary to develop an egg and release it. After ovulation, the hormone progesterone helps the womb lining to grow thicker, readying the body for pregnancy.
If pregnancy does not occur, the egg is absorbed into the body and the womb lining comes away and leaves the body (the menstrual flow).
The menstraul cycle is an endogenous system, evidence suggests that it may be influenced by exogenous factors, such as the cycles of other women.
Kathleen Stern and Martha McClintock (1998) demonstrated how menstraul cycles may synchronise as a result of the influence of female pheromones.
McClintock involved 29 women with a history of irregular periods. Samples of pheromones were gathered from 9 of the women at different stages of their menstraul cycles, via cotton pads in their armpits which were kept their for at least 8 hours.
The pads were treated with alcohol and frozen, to be rubbed on the upper lip of the other participants.
On day one, pads from the start of the menstraul cycle were applied to all 20 women, on day two they were all given a pad from the second day of the cycle and so on.
McClintock found that 68% of women experienced changes to their cycle which brought them close to the cycle of their 'odour donor'.
Seasonal Affective Disorder (SAD)
SAD is a depressive disorder which has a seasonal pattern of onset, and is diagnosed as a mental disorder in DSM-5. The main symptoms of SAD are persistent low mood alongside a general lack of activity and interest in life.
SAD is often referred to as the winter blues as the symptoms are triggered during the winter months when the number of daylight hours becomes shorter.
SAD is a particular type of infradian rhythm called a circannual rhythm as it is subject to a yearly cycle. It can also be classed as a circadian rhythm as the experience of SAD may be due to the disruption of the sleep/wake cycle.
The hormone melatonin is implicated in the cause of SAD. During the night the pineal gland secretes melatonin until dawn when there is an increase in light. During winter, the lack of light in the morning means this secretion process continues for longer.
This is thought to have a knock-on effect on the production of serotonin in the brain - a chemical that has been linked to the onset of depressive symptoms.
One of the most intensely researched ultradian rhythms is the stages of sleep - the sleep cycle.
Psychologists have identifed five distinct stages of sleep that altogether span approximately 90 minutes and continues through the course of the night. Each of these stages is characterised by a different level of brain wave activity which can be monitored using an EEG.
- Stage One and Two: light sleep where the person may be easily woken. Brainwave patterns start to become slower and more rhythmic (alpha waves), becoming even slower as the sleep becomes deeper (theta waves).
- Stages Three and Four: involve delta waves which are slower still and have a greater amplitude than earlier wave patterns. This is deep sleep or slow wave sleep and it is difficult to rouse someone at this point.
- Stage Five, REM sleep: the body is paralysed yet brain activity speeds up significantly in a manner that resembles the awake brain. REM stands for rapid eye movement to denote the fast activity of the eyes under the eyelids at this point. Research has suggested that REM activity during sleep is highly correlated with the experience of dreaming.
- Evolutionary basis of menstraul cycle: menstraul synchrony is thought to have an evolutionary value, as it may have been advantageous for females to fall pregnant at the same time. Jeffrey Schank (2004) argued that the avoidance of synchrony would appear to be most adaptive evolitonary strategy.
- Methodological limitations in synchronisation studies: commentators argue that there are many factors that may effect change in a woman's menstraul cycle, including stress, changes in diet, exercise etc. which may act as confounding variables. Trevathan et al (1993) failed to find any evidence of menstraul synchrony in all-female samples.
- Evidence supports the idea of distinct stages in sleep: Dement and Kleitman (1957) monitored the sleep patterns of nine adult participants in a sleep lab. Brainwave activity was recorded on en EEG and the researchers controlled for the effects of caffeine and alcohol. REM activity during sleep was highly correlated with the experience of dreamin and brain activity varied according to how vivid dreams were.
- Animal studies: much of the knowledge of the effects of pheromones on behaviour is derived from animal studies. The role of pheromones in animal sexual selection is well-documented. Evidence for the effects in human behaviour remains speculative and inconclusive.
- Practical application: one of the most effective treatments for SAD is phototherapy. This is a lightbox that simulates very strong light in the morning and evening. This relieves the symptoms in up to 60% of sufferers (Eastman et al, 1998).
The Suprachiasmatic Nucleus (SCN)
SCN is a tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain and is one of the primary endogenous pacemakers in mammilian species.
It is influential in maintaining circadian rhythms such as the sleep/wake cycle.
Nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the visual area of the cerebral cortex.
The SCN lies just above the optic chiasm and receives information about light directly from this structure.
This continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep.
Animal Studies and the SCN
Patricia DeCoursey et al (2000) destroyed the SCN connections in the brains of 30 chipmunkes who were then returned to their natural habitat and observed for 80 days.
The sleep/wake cycle of the chipmunks disappeared and by the end of the study a significant proportion of them had been killed by predators (presumably because they were awake and vulnerable when they should have been asleep).
Martin Ralph et al (1990) bred 'mutant' hamsters with a 20-hour sleep/wake cycle. When SCN cells from the foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters, the cycles of the second group defaulted to 20 hours.
Both of these studies emphasis the role of the SCN in establishing and maintaining the circadian sleep/wake cycle.
The Pineal Gland and Melatonin
The SCN passes the information on day length and light that is receives to the pineal gland, a pea-like structure in the brain just behind the hypothalamus.
During the night, the pineal gland increases the production of melatonin - a chemical that induces sleep and is inhibited during periods of wakefulness.
Melationin has also been suggested as a causal factor in seasonal affective disorder.
Delayed Sleep-Phase Disorder
DSPD is a circadian rhythm sleep disorder affecting the timing of sleep, peak period of alertness, the core body temperature rhythm, hormonal and other daily rhythms.
People with DSPD generally fall asleep some hours after midnight and have difficulty waking up in the morning. Patients can sleep well and have a normal need for sleep but find it very difficult to wake up in time for a typical school or work day.
Twenty patients who suffered from DSPD were involved in an investigation. Researchers randomly allocated the 20 patients into two groups: Group A (treatment group) and Group B (control group).
Group A were given a course of drugs that increase melatonin production for 6 weeks at bed-time (around 11pm). Group B were given a placebo for the same period of time.
At the end of the six-week period, all the participants were assessed on a number of self-report measures. These examined their performance at work, their attention levels during the day and their relationship with their family.
Exogenous zeitgebers are external factors in the environment that reset our biological clocks through a process known as entrainment.
The free running biological clock that controls the sleep/wake cycle continues to 'tick' in a distinct cyclical pattern. Sleeping and wakefulness would seem to be determined by an interaction of internal and external factors.
Light: can reset the body's main endogenous pacemaker and plays a role in the maintenence of the sleep/wake cycle. Light also has influence on key processes in the body such as hormone secretion and blood circulation. Campbell and Murphy (1998) demonstrated that light may be detected by skin receptor sites on the body. Fiften participants were woken at various times and a light pad was shone on the back of their knees. This caused a deviation in the sleep/wake cycle of up to 3 hours.
Social Cues: at about 6 weeks of age the circadian rhythms begin and by 16 weeks, most babies are entrained. Schedules imposed by parents are likely to be a key influence, including mealtimes and bedtimes. Research also suggests that adapting to local times for eating and sleep is an effective way of entraining cidadian rhythms and beating jet lag.
- Beyond the master clock: there are numerous circadian rhythms in many organs and cells of the body. There are called peripheral oscillators and are found in the adrenal gland, oesaphagus, lungs, liver, pancreas, spleen, thymus and skin. Francesca Damiola et al (2000) demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected. This suggests there may be many other complex influences on the sleep/wake cycle.
- Ethics in animal studies: there are problems with ethics of animal studies such as in the DeCoursey et al study as the animals were exposed to considerable harm when they were returned to their natural habitat.
- Influence of exogenous zeitgebers may be overstated: Laughton Miles et al (1977) recount the story of a young man who was blind from birth with a circadian rhythm of 24.9 hours. Despite exposure to social cues, his sleep/wake cycle could not be adjusted. This suggests that there are occasions when exogenous zeitgebers may have little influence on our internal rhythm.
- Methodological issues: psychologists have been critical of the manner in which Campbell and Murphy's study was conducted. Isolating one exogenous zeitgeber in this way does not give us insight into the many other zeitgebers that influence the sleep/wake cycle.
- Interactionist system: only in certain circumstances are endogenous pacemakers free-running and unaffected by the influence of exogenous zeitgebers. Total isolation studies are extremely rare and could be judged as lacking validity. Pacemakers and zeitgebers interact and it may make little sense to seperate the two for the purpose of research.