Full Biopsychology Notes
Part 1 — The Nervous System and the Endocrine System
The nervous system comprises the brain and spinal cord. The peripheral nervous system (PNS) transmits messages from the environment to the central nervous system (CNS) via sensory neurons and from the CNS to effectors via motor neurons.
The PNS is divided into the autonomic nervous system (controls involuntary functions like heart and breathing rate) and the somatic nervous system (receives sensory information and stimulates effectors via motor neurons).
The autonomic nervous system has sympathetic and parasympathetic branches that work antagonistically during the ‘rest and digest’ response and maintain the fight or flight response.
Example: The sympathetic nervous system increases heart rate, breathing rate, causes vasoconstriction and pupil dilation, while the parasympathetic nervous system decreases heart rate, breathing rate, causes vasodilation and pupil constriction.
The endocrine system uses hormones secreted into the bloodstream from glands, transported towards target cells with complementary receptors. The pituitary gland is the ‘master’ gland because it controls hormone release from other glands. For example:
The thyroid gland releases thyroxine, increasing heart rate and growth rate.
The adrenal gland releases adrenaline, creating physiological arousal for the fight or flight response by increasing activity in the sympathetic nervous system.
Fight or Flight Response:
The body detects a stressor (e.g., a speeding car).
Sensory receptors and neurons in the PNS send this information to the hypothalamus, which coordinates a response and increases activity in the sympathetic branch of the ANS.
Adrenaline is released from the adrenal medulla in the adrenal glands, transported to target effectors via the blood and the endocrine system.
This results in rectum contraction, inhibited saliva production, and increased breathing rate, which creates the physiological response needed to sustain the fight or flight response. This response's adaptive purpose is to enable escape from the stressor, increasing the likelihood of survival.
Once the stressor is no longer a threat, the hypothalamus reduces activity in the sympathetic branch and increases activity in the parasympathetic branch of the ANS. This is the ‘rest and digest’ response, where the parasympathetic branch decreases activity initially increased by the sympathetic branch.
Part 2 — Neurons and Synaptic Transmission
Synaptic transmission is how neurons communicate, relaying information to the CNS via sensory neurons and carrying out responses dictated by the brain through sending information to effectors via motor neurons.
Process of synaptic transmission:
An action potential arrives at the presynaptic membrane, causing depolarisation through the opening of voltage-dependent calcium ion channels and the influx of calcium ions.
The increased calcium ion concentration causes vesicles containing neurotransmitters to fuse with the presynaptic membrane and release their contents into the synaptic cleft through exocytosis.
The neurotransmitters diffuse across the synaptic cleft, down a concentration gradient, and bind to complementary receptors on the postsynaptic membrane. This can result in inhibitory or excitatory effects.
The resulting action potential is transmitted along the axon of the following neuron, creating a cascade of neurotransmission.
Neurotransmitters can be inhibitory or excitatory:
Inhibitory neurotransmitters (e.g., serotonin) reduce the potential difference across the postsynaptic membrane by closing voltage-dependent sodium ion channels, reducing the likelihood of an action potential.
Excitatory neurotransmitters (e.g., dopamine) increase the potential difference across the postsynaptic membrane by opening more voltage-dependent sodium ion channels, increasing the likelihood of an action potential.
Part 3 — Localisation of Function in the Brain
Localisation theory: specific brain areas are responsible for particular processes, behaviours, and activities.
Motor area: Found in the frontal lobe, separated from the auditory area by the central sulcus. Regulates and coordinates movement. Lesions result in an inability to control voluntary fine motor movements.
Auditory area: Located in the temporal lobe on the superior temporal gyrus. Processes auditory information and speech. Lesions cause hearing loss, while damage to specific parts (Wernicke’s area) results in Wernicke’s aphasia.
Visual area: Located in the occipital lobe. Processes visual information.
Somatosensory area: Located in the parietal lobe. Processes information associated with the senses (touch, heat, pressure, etc.). Receives neuronal input from the thalamus that corresponds with the handling of sensation along the lines of touch, pain, temperature and limb position. Lesions result in a loss of ability to denote sensitivity to particular bodily areas.
Wernicke’s Area: Responsible for speech comprehension, located in the temporal lobe (usually the left). Lesions result in Wernicke’s aphasia, characterised by nonsensical words (syllogisms), no awareness of incorrect word usage, but no issues with pronunciation and intonation.
Broca’s Area: Responsible for speech production, located in the frontal lobe, usually in the left hemisphere. Lesions result in Broca’s aphasia, characterised by difficulty forming complete sentences and understanding sentences, failing to understand word order and who they are directed towards (I, you, we, he, me, etc.).
The left hemisphere is associated with language production and comprehension; therefore, language is a cognitive ability that is both localised and lateralised (to the left hemisphere).
Supporting evidence for localisation of brain function:
Tulving et al. demonstrated using PET scans that semantic memories were recalled from the left prefrontal cortex, while episodic memories were recalled from the right prefrontal cortex. This shows that different brain areas are responsible for different functions, as predicted by localisation theory.
Petersen et al. (1988) found that Wernicke’s area activation is required for listening tasks, whereas Broca’s area is required for reading tasks. This confirms that Wernicke’s area is involved in speech comprehension, while Broca's area is responsible for speech production.
Supporting Case Studies:
Phineas Gage was injured by a blasting rod that damaged his prefrontal cortex. This damage caused a defect in rational decision-making and the processing of emotion, supporting the idea that specific brain areas are responsible for specific functions. However, the subjectivity of conclusions drawn, the unusual sample, and a lack of control over confounding and extraneous variables must be considered.
Contradictory Theory:
A holistic view of brain function suggests that each function requires several brain areas to be activated, and these functions are not restricted to these areas. For example, after removing 20-50% of the cortices belonging to rats, researchers found that no specific brain area or lesion was associated with learning how to traverse through a maze. This suggests that intelligence, or even learning, is too complex and advanced a cognitive ability to be restricted to certain areas of the brain. Therefore, this suggests that localisation theory may provide a better explanation for ‘simple’, rather than complex, brain functions.
Evidence supporting the link between certain brain areas and symptoms of OCD:
Dougherty et al. (2002) studied 44 OCD sufferers who’d undergone lesioning of the cingulate gyrus (cingulotomy) in order to control their symptoms. The researchers found that, after a mean follow-up of 32 months after one or more cingulotomies, 32% met criteria for treatment response, and 14% were partial responders. This suggests that not only are certain brain areas responsible for symptoms of OCD, but that an improved understanding of localisation of brain function has practical applications in the development of more advanced treatments for serious mental disorders.
Part 4 — Plasticity and Functional Recovery of the Brain after Trauma
Plasticity: The brain’s ability to physically and functionally adapt and change in response to trauma, new experiences, and learning. Neuroplasticity was demonstrated by Maguire et al. (2006).
Plasticity opposes the idea of a ‘critical window’ for synaptic and neuronal connection formation during the first 3 years of life, after which no new neuronal connections would be formed (Gopnik et al.).
We control the strength and number of neuronal connections in our brains through synaptic pruning: the process by which extra neurons and synaptic connections are eliminated in order to increase the efficiency of neuronal transmissions.
Maguire et al. found a larger grey matter volume in the mid-posterior hippocampi (and a lower volume in the anterior hippocampi) of London taxi drivers' brains, alongside a positive correlation between increasing grey matter volume and how long the individuals had been taxi drivers. They concluded that a complex spatial representation, which facilitates expert navigation and is associated with greater posterior hippocampal grey matter volume, might come at a cost to new spatial memories and grey matter volume in the anterior hippocampus. This may be because the hippocampus is associated with spatial awareness, an ability which taxi drivers must have when they complete The Knowledge test.
Functional recovery: The ability of the brain to transfer the functions of damaged areas to other healthy parts of the brain, allowing normal functioning to continue. This is enabled through:
The law of equipotentiality (secondary neural circuits surrounding the damaged area become activated).
Axonal sprouting (formation of new synapses and strengthening of axonal connections between damaged and healthy areas).
Reformation of blood vessels (as part of the haemodynamic response, where activated areas experience a higher blood deoxygenation level).
Recruiting homologous areas on the opposite side of the brain.
Function is not always lateralised to specific hemispheres!
Examples of functional recovery:
Ramachandran’s research into phantom limb syndrome: caused by sensory input from the face skin ‘invading’ and activating deafferented hand zones in the cortex and thalamus… There appears to be tremendous latent plasticity even in the adult brain. This demonstrates negative plasticity because the neuroplasticity results in painful or negative consequences.
The case of Jodi Miller: her entire right hemisphere was removed to control epileptic seizures. However, through neuroplasticity, she could still control the right side of her body. This demonstrates positive plasticity, because the neuroplasticity results in desirable or positive consequences.
Evidence supporting the positive and negative effects of neuroplasticity:
Ramachandran et al. demonstrated negative plasticity through providing an explanation for phantom limb syndrome in terms of cortical reorganisation in the cortex and thalamus (particularly, the somatosensory area).
Positive plasticity has been demonstrated by the case study of Jodi Miller, who has shown the power of recruiting homologous areas on the opposite side of the brain, axonal sprouting and the reformation of blood vessels. Therefore, there is evidence supporting not only the existence of, but also the uses of plasticity.
Neuroplasticity occurs in animals, too:
Hubel and Weisel (1970) sutured the right eye of kittens, who are blind from birth, for 6 months, opening the eyes and several points and monitoring brain activity in the visual cortex. The researchers found that, although the right eye was closed, there was still activity in the left visual cortex, corresponding to the development of ocular dominance columns. This was demonstrated by how, during the period of high susceptibility in the fourth and fifth weeks, eye closure for as little as 3-4 days leads to a sharp decline in the number of cells that can be driven from both eyes. This therefore supports the idea that areas of the brain receiving no input can take over the function of highly stimulated areas, despite originally having different functions.
Cognitive reserve may increase the rate of functional recovery:
Cognitive reserve is the level of education a person has attained and how long they have been in education. Research suggests that an increased cognitive reserve increases the likelihood of making a disability-free recovery (DFR) after trauma, due to increased rates of neuroplasticity. For example, Schneider et al (2014) found that of the 769 patients studied, 214 achieved DFR after 1 year. Of those, 50.7% had between 12 and 15 years of previous education, and 25.2% had more than 16 years. This suggests that individuals who have been in education for a longer time may have developed the ability to form neuronal connections at a high rate, and therefore experience high levels of functional recovery, demonstrating positive plasticity.
There are limits to spontaneous and functional recovery:
After trauma, the brain activates secondary neural circuits that contribute toward reinstating normal function (law of equipotentiality). The brain can only ‘repair’ itself up to a specific point, after which motor therapy or electrical stimulation is needed to increase recovery rates. For example, Lieperta et al (1998) found that after constraint-induced movement therapy, the motor performance of stroke patients improved significantly. Therefore, this suggests that functional recovery cannot be relied upon to reinstate normal function.
Part 5 — Split-Brain Research into Hemispheric Lateralisation
Hemispheric lateralisation: Each hemisphere (half) of the brain is mainly responsible for certain behaviours, processes, and activities. This contrasts with the holistic theory of brain function, which suggests that function is distributed across the whole brain (i.e., is global).
Each visual field has two sides - left and right. The right hemisphere controls the left side of the body, and vice versa. Therefore, information which we receive from the left visual field is processed by the right hemisphere, which then coordinates a response to affect the left side of the body.
Sperry and Gazzaniga (1968) split-brain research: conducted on 11 epileptic patients who had undergone surgical lesioning of the corpus callosum (cerebral commissurotomy) to control seizures. This procedure prevents information processed by one hemisphere from being relayed to the other, allowing researchers to expose a single hemisphere to certain stimuli and infer the functions of each hemisphere.
Patients had one eye covered, and stimuli were flashed on a screen for one-tenth of a second to prevent both visual fields from being exposed to the information. This research was conducted under strictly controlled conditions using a laboratory experiment.
Procedure and results:
Describing what you see: If the stimulus word was exposed to the right visual field (processed by the left hemisphere), the patient would say the word because the left hemisphere contains the ‘language centres’ of the brain. However, if the same stimulus word was exposed to the left visual field (processed by the right hemisphere), the patient would write the word using their left hand because the right hemisphere contains the visuo-spatial centres of the brain and allows for the physical act of writing. The patient would not be able to give a verbal description of the word because the right hemisphere contains no language centres.
Matching words or faces: The right hemisphere appeared to dominate the ability to match a list of faces to a given stimulus due to the right hemisphere containing the brain’s visuo-spatial centres, thus allowing for the visual identification and processing of the faces.
Words presented simultaneously: If two words were presented at the same time, each to one of the visual fields, the patient would say the word presented to the right visual field (processed by the left hemisphere with language centres) and write down the word presented to the left visual field (processed by the right hemisphere and containing visuo-spatial centres).
Recognising objects placed into the hands: If an object was placed into the patient’s right hand, they would be unable to identify that it is there because the information is processed by the left hemisphere, which only has language centres and no visuo-spatial centres. Therefore, if an object was placed into the patient’s left hand, they would be able to identify the object and choose a similar one from a hidden bag due to the action of the visuo-spatial centres.
Lack of control with the sample selection: The epileptic patients had been taking anti-epilepsy medications for extended and different periods of time, which may have affected their ability to recognise objects and match words due to causing cerebral neuronal changes. Secondly, although all patients had undergone a commissurotomy, there may have been differences in the exact procedures, e.g. differing extent of the lesioning of the corpus callosum. This would have affected the degree to which the two hemispheres could relay information between themselves. Therefore, these two confounding variables had not been controlled, meaning that the lateralised functions may be examples of unreliable causal conclusions.
Clearly demonstrated lateralisation of function: Split-brain research was pivotal in establishing the differences in functions between the two hemispheres and so opposing the holistic theory of brain function. The left hemisphere was demonstrated as being dominant for language tasks due to containing language centres, whereas the right hemisphere was demonstrated as being dominant for visuo-spatial tasks. Therefore, this suggests that the left hemisphere is the analyser, whereas the right hemisphere is the synthesiser, and so there are marked differences between the two.
Contribution to discussions about lateralisation theories: Such evidence strongly supported the idea of a ‘dual mind’ where the two hemispheres represent two sides of the mind.
The differences in function may not be so clear-cut: With evidence making the drastic distinctions that the left hemisphere is responsible for language (analyser) whilst the right is responsible for visual-spatial tasks (synthesiser), this has given the public the false impression that the two hemispheres are ‘opposite’ in function and that they can receive such labels. However, as suggested by Pucetti (1980), there have been cases of split-brain patients who are left-handed but produce and comprehend speech in the right hemisphere, which opposes the predictions made by lateralisation theory. Therefore, it is important not to jump to conclusions and to appreciate that, through the recruitment of homologous areas on the opposite side of the brain, each hemisphere is not restricted to specific functions.
Part 6 — Ways of Investigating the Brain
Brain’s Electrophysiological responses to specific events (sensory, motor or cognitive event) are isolated by doing a statistical analysis of EEG data. The statistical averaging technique removes all extraneous brain activity from the original EEG recording by filtering it out, leaving only those responses that relate to the presentation of a specific stimulus or performance of a specific task. What is left is event-related potentials – brainwaves that are triggered by particular events. Different types of ERP have been discovered e.g. linked to attention, perception.
Advantages:
Excellent Temporal Resolution- Neural processes are measured more specifically than in an EEG.
Widely used in the measurement of cognitive deficits and functions.
Disadvantages:
Lack of standardisation in ERP methodology in different research studies.
Background noise and extraneous material can be an obstacle.
fMRI scans: Rely on the haemodynamic response. Areas of the brain with high levels of activity have a larger requirement for oxygenated blood, leading to a higher rate of blood deoxygenation. As measured through the bold response, the deoxyhaemoglobin in the blood in these highly active areas absorbs the signal produced by the scan, so such areas appear brightly coloured on the scan.
Advantages:
High spatial resolution as up to 4 images can be produced per second.
It can be used while a patient is carrying out a task.
Does not use ionising radiation, and so is safer.
Disadvantage: Poor temporal resolution because there is approximately a 5-second difference between neuronal activity and the produced image.
EEG scans: Use electrodes attached to the scalp to measure and amplify the electric activity across the whole brain (action potentials being transmitted across the axons of neurons).
Advantages:
Useful in investigating the characteristics of the different stages of sleep.
Much higher temporal resolution than fMRI scans, more appropriate for monitoring ongoing cerebral states and activity.
Useful in the diagnosis of epilepsy.
Disadvantages: Lower spatial resolution compared to fMRI scans, with particular difficulty in differentiating activity between adjacent areas.
Post-mortem examinations: Involve a comparison of the patient’s brain with that of a healthy, neurotypical brain. Any differences (e.g., lesions, damage, abnormally large or small areas) are assumed to have caused the neurological problem the patient faced in their lifetime.
Disadvantages:
Incorrectly assumes that differences compared with the neurotypical brain must be the explanation for neurological or cognitive deficits. Prolonged drug use, stress, and genetic factors may be other plausible explanations.
Ethical issues: informed consent cannot always be obtained before the patient dies or from the family.
Advantages: Particularly useful for advancing medical knowledge and being the basis of further research into certain areas of the brain. Led to the identification of Broca’s area and further localisation theory research.
Part 7 — Biological Rhythms: Circadian Rhythms
Biological Rhythms: Periodic biological fluctuations in an organism that correspond to, and are in response to, periodic environmental change. Biological rhythms can be endogenous (controlled by internal clocks, e.g. the suprachiasmatic gyrus) or exogenous (controlled by external, environmental factors, e.g. exposure to sunlight). The three types of biological rhythms are circadian, infradian and ultradian.
Exogenous zeitgebers: External changes in the environment which affect or ‘entrain’ our biological rhythms.
Circadian Rhythms: A type of biological rhythm which completes one full cycle every 24 hours, e.g. the sleep-wake cycle. Like other biological rhythms, it is affected by both endogenous pacemakers and exogenous zeitgebers.
The main example of an exogenous zeitgeber would be light. Changes in light exposure can trigger desynchronisation of a ‘pre-set’ sleep-wake cycle.
Siffre’s cave study (1962): Siffre descended into a cave completely devoid of natural light. He finished his experiment on September 14th, believing it to be August 20th! This demonstrates that prolonged exposure to a strong exogenous zeitgeber, such as light, disrupts the sleep-wake cycle becomes disrupted and there is a disconnection between psychological time and the clock. His sleep-wake cycle did not conform to a cyclical 24-hour period but was around 24 hours and 30 minutes, with Siffre himself determining when to sleep and when to eat. Therefore, this demonstrates that “there was an internal clock independent of the natural terrestrial day/night cycle”. This describes a ‘free-running’ circadian rhythm, i.e. one which is not affected by exogenous zeitgebers.
Aschoff and Wever (1967): Deprived 55 participants of natural light while spending 4 weeks in an underground bunker. The researchers found that “all subjects showed free-running circadian rhythms, with the average periods of wakefulness and sleep ranging from 23.9 to 50.0 hours. 36 subjects remained internally synchronised during the whole experiment”.
These findings demonstrate that although the free-running circadian rhythm is more than 24 hours long, as a society, we have specific exogenous zeitgebers which entrain the rhythm to conform to a 24-hour cycle.
Individual differences in circadian rhythms: Circadian rhythms may not and do not always have to conform to cyclical 24-hour periods. Therefore, this is a real-life example of how the circadian rhythms of teenagers specifically are not always in line with those of adults, and so an appreciation of this can improve educational attainment.
The confounding effect of artificial light: As demonstrated by Czeisler et al (1999), artificial lighting can create shifts in circadian rhythms by up to 6 hours. Siffre’s research was conducted at a time when researchers believed that artificial lighting had no effect on biological rhythms. The use of artificial light meant that over 2 months, Siffre could have entrained his own circadian rhythm through signalling sleeping and waking times by using the light, meaning that the conclusions made about his ‘free-running’ circadian rhythm may not be entirely accurate.
Detrimental impacts on health in shift workers: Shift work was associated with obesity, high triglycerides, and low concentrations of HDL cholesterol, which the researchers suggest may demonstrate a link between the desynchronisation of circadian sleep-wake cycles in shift workers and the consequent disruptions in the biological control of metabolism (and therefore core body temperature). This suggests that there may be practical uses in an improved understanding of the effects of desynchronisation. This in turn has economic implications, in terms of companies that employ shift-workers making the effort to revise their policies in order to reduce days taken off sick.
Use of case studies and small samples in isolation investigations: Although Siffre conducted multiple isolation studies, his results may not be able to be generalised to the wider population, especially as individual differences in the duration and stages of circadian rhythms have been shown, hence his results may lack ecological validity. The same can be said for other isolation studies, such as Aschoff and Wever, where large numbers of participants are unlikely to want to participate. This again limits the extent to which the findings represent the experiences of the general population.
Part 8 — Biological Rhythms: Infradian and Ultradian Rhythms
Infradian Rhythm: One of 3 types of biological rhythms, with a frequency of one complete cycle occurring less than once every 24 hours. Such rhythms are entrained by endogenous pacemakers and exogenous zeitgebers. Notable examples of infradian rhythms include the menstrual cycle and SAD (seasonal affective disorder).
McClintock et al (1998): Demonstrated menstrual cycle synchronisation amongst 29 women who all had irregular periods. The pheromones from 9 of the women were collected through the use of a pad under the armpit, and then rubbed onto the upper lip of the remaining 20 women, corresponding to the specific days of their cycle. The researchers found that recipients had shorter cycles when receiving axillary compounds produced by donors in the follicular phase of the menstrual cycle and longer cycles when receiving ovulatory compounds.
Seasonal affective disorder: an example of the influence of endogenous pacemakers on the circadian sleep-wake cycle. SAD is an infradian disorder caused by disruption to the sleep-wake cycle, and commonly occurs in the winter. Longer nights mean more melatonin is secreted from the pituitary gland, via the endocrine system, which changes the production of melatonin, leading to feelings of loneliness and depression.
Ultradian Rhythm: One of 3 types of biological rhythms, with a frequency of one complete cycle occurring more than once every 24 hours. A notable example is the stages of sleep, where a full sleep cycle takes 90 minutes to complete.
Stages 1 and 2 represent the ‘sleep escalator’ where the participant can easily be awoken, stages 3 and 4 coincide with deeper and slower delta waves (compared to theta waves during the sleep escalator), whilst stage 5 represents REM sleep. REM sleep is closely associated with dreaming and characterised by movement inhibition and a sensory blockade. The stages of sleep have been demonstrated by Dement and Kleitman.
The stages of sleep have clearly been demonstrated: Using EEG scans, the researchers found that “discrete periods of rapid eye movement potentials were recorded without exception during each of 126 nights of undisturbed sleep”. Since participants were able to accurately recall their dreams when awoken during REM sleep, the assumption was made that dreaming is associated with REM sleep. Therefore, there is clear evidence supporting the idea of a distinct set of sleep stages.
Menstrual synchronisation is not always present in all-female samples: There are external (extraneous) variables which may affect the timing and duration of their menstrual cycles. Therefore, this raises doubts about the strength of the influence of pheromones as an exogenous zeitgeber which can entrain infradian rhythms.
Dispute over the chemical and hormonal basis of SAD: Goth et al (1999) found that when treating sufferers of SAD with either Vitamin D supplements or broad-spectrum phototherapy, patients who’d been given vitamin D supplements also experienced a 74% improvement in their depression measures. Since phototherapy involves exposure to bright light in order to increase the rate at which the pineal gland secretes melatonin, this implies that melatonin and serotonin levels have little part to play in the development of SAD and in the entrainment of circadian sleep-wake cycles.
Animal studies supporting the role of pheromones: Therefore, this suggests that endogenous pacemakers have a critical role in the entraining of biological rhythms in animals, whose findings can then be generalised to humans.
Part 9 — Endogenous Pacemakers and Exogenous Zeitgebers
Endogenous Pacemakers: Internal bodily regulators of biological rhythms, affecting or ‘entraining’ such biological rhythms to conform to certain cyclical periods, e.g. one cycle every 24 hours for circadian rhythms.
The suprachiasmatic nucleus (SCN) receives information about daylight and day length from the eyes, where such information has been processed by the visual area in the occipital lobe and relayed to the SCN via the optic chiasm (from one hemisphere to the other). The SCN then processes this information and triggers different rates of release of melatonin from the pineal gland. Increased melatonin release triggers decreased serotonin production, creating feelings of sleepiness during the nighttime, where there is little exposure to light. Conversely, during the daytime, when there is high exposure to light, the SCN triggers the pineal gland to release less melatonin over a longer period of time, resulting in increased serotonin production, creating feelings of wakefulness.
The effect of the SCN was demonstrated by DeCoursey et al (2000), who surgically lesioned the SCNs of 30 chipmunks and compared their circadian rhythms in their natural habitat with controls. The researchers found that the vast majority of the experimental group had been killed within the first 80 days after being returned to their habitat, and “episodes of nocturnal movement were detected within the permanent dens by radio telemetry data logging, especially in supra-chiasmatic nucleus-lesioned animals”.
Ralph et al: Extracted SCN cells from hamsters that showed abnormal sleep-wake cycles and inserted these cells into healthy hamster foetuses. The researchers found that “the restored rhythms always exhibited the period of the donor genotype, regardless of the direction of the transplant or genotype of the host. The basic period of the overt circadian rhythm, therefore, is determined by cells of the suprachiasmatic region”.
Exogenous Zeitgebers: External environmental changes, affecting or ‘entraining’ biological rhythms to conform to certain cyclical time periods.
Social cues are examples of exogenous zeitgebers which entrain biological rhythms. These include set meal times and bedtimes, which signify when to wake up and when to fall asleep. This means that in order to avoid jet lag, it is useful to accustom yourself to the set sleeping and eating times of your destination, to avoid desynchronisation of an already ‘pre-set’ circadian rhythm.
A second example of the effect of an exogenous zeitgeber would be the role of light in entraining the sleep-wake cycle, as demonstrated by Siffre et al (1967) and Campbell and Murphy (who produced deviations of 3 hours in the participant’s sleep-wake cycle by shining light onto pads on the back of their knees, showing that light does not always need to be detected by the eyes in order to entrain biological rhythms).
The influence of the SCN may be overestimated. Damiola et al. demonstrated that the circadian rhythm of mouse liver cells could be influenced to experience a 12-hour discrepancy, leaving the SCN unaffected. These so-called ‘peripheral oscillators’, also present in the adrenal gland and lungs, are collections or systems of cells which act independently of the SCN, each having its own biological rhythm. Therefore, the SCN is not as important as once thought.
There are considerable ethical issues with the use of animals in such research, particularly if they are deliberately put in harm’s way as was the case with Decoursey et al, thus breaching the BPS ethical guideline of protection from psychological and physical harm. Although this does not impact the utility or validity of the findings, a cost-benefit analysis would have to be conducted to assess whether such ethical costs outweigh the benefits of an improved understanding of exogenous zeitgebers and endogenous pacemakers. A second problem occurs with the limitations of generalising findings from animal studies to humans, particularly due to differences in physiology and the number/types of circadian rhythms, thus limiting the ecological validity of such findings.
There have also been recorded cases where exogenous zeitgebers and endogenous pacemakers have failed to entrain or alter circadian rhythms, as demonstrated by Miles et al (1977) who reported the case of a man with a sleep-wake cycle of 24.9 hours, which could not be changed through the use of either stimulants or sedatives. Therefore, this suggests that the influence of exogenous and endogenous factors may be overestimated!