- biological rhythm lasting about 24 hours.
Light, the main exogenous zeitgeber, is detected by the retina
Light information travels via the optic nerve, crossing into left/right hemispheres via the optic chiasm and into the visual cortex
Information is received by suprachiasmatic nucleus (SCN) - main endogenous pacemaker. It contains receptors which are sensitive to light
SCN sends signals to pineal gland via neural pathways
Pineal gland increases production + secretion of melatonin when it is dark
Melatonin induces sleep by inhibiting brain mechanisms for wakefulness
- demonstrated free-running circadian rhythm of about 25 hours. Spent long periods of time in dark caves (two months in caves of the Southern Alps and six months in a Texan cave. Had a regular sleep/wake cycle of about 25 hours.
Aschoff and Wever
- group of participants spent 4 weeks in WW2 bunker deprived of natural light. All but one (whose sleep/wake cycle extended to 29 hours) displayed a circadian rhythm of between 24 and 25 hours.
Suggest that natural sleep/wake cycle may be slightly longer than 24 hours
but is entrained by exogenous zeitgebers associated with our 24-hour day.
Endogenous Pacemakers may have stronger influence than exogenous zeitgebers
- Folkard et al. studied a group of 12 people who lived in a dark cave for 3 weeks, going to bed when the clock said 11:45pm and waking when it said 7:45am. Researchers gradually sped up the clock so an apparent 24 hour day eventually only lasted 22 hours. Only one ppt comfortably adjusted, suggesting existence of a strong free-running circadian rhythm that cannot easily be overridden by changes in external environment.
Use of case studies and small samples
- studies of the sleep/wake cycle often use small samples (Aschoff and Wever) or even one person (Siffre). Not representative of the wider population and therefore we cannot make meaningful generalisations. Siffre also found that his internal clocked slowed down with age, so rhythms may be different for different ages.
Poor control in studies
- participants deprived of natural light still had access to artificial light. In other studies, ppts circadian rhythms have been adjusted using artificial light, so it may have an effect. Researchers may therefore have ignored an important confounding variable.
Practical application to drug treatments
- circadian rhythms coordinate the body's basic processes (heart rate, hormone levels). Research has shown there are times of day where drugs are more effective. Guidelines have been developed for the best timing of dosing for a range of drugs including cancer and epilepsy treatments.
- individual cycles can range from 13 to 65 hours, and so it is difficult to generalise findings from studies. Some people prefer sleeping and waking early ('larks') or later ('owls'). Also age differences. Findings from studies may not represent these people's differences.
Practical application to shift work
- has been found that shift worker experience a lapse in concentration around 6am (circadian trough) so mistakes and accidents are more likely. Link also found between shift work and poor health, three times more likely to develop heart disease. Economic applications of how best to manage worker productivity.
Ways of Studying the Brain
- measures electrical activity. Electrodes placed on surface of skull detect small electrical activity from activity of many millions of neurons. Signals are graphed over a period of time. This visual representation is called an EEG. Amplitude reveals intensity of the activity, frequency reveals speed.
Extremely high temporal resolution
- can detect brain activity at a resolution of a single millisecond.
Information is received from millions of neurons
- it is too generalised, and is difficult to pinpoint the exact source
Invaluable in diagnosing conditions such as epilepsy and understanding the stages of sleep
Event Related Potentials (ERPs)
- what is left when all extraneous brain activity from an EEG are filtered out. Very small voltage changes in the brain, triggered by specific events or stimuli. Very difficult to pick out a specific response to a stimulus amongst all brain activity, so stimulus may be repeated several times and brain responses averaged together. Extraneous activity not related to the stimulus will not occur simultaneously, allowing it to be ignored and highlighting the target activity.
Very specific measurement of neural measurement
- more specific than raw EEG data.
Lack standardisation in methodology
- difficult to confirm findings in ERP studies.
Background noise and extraneous material must be completely eliminated
- not always easy to achieve
Excellent temporal resolution
- comes from EEG data
Functional Magnetic Resonance Imaging (fMRI)
- measures changes in brain activity while a person performs a task. When a brain area is active, there is increased demand for energy and therefore oxygen, therefore increased blood flow, which fMRI picks up.High areas of blood flow indicate active areas. Produce powerful magnetic fields that detect the magnetic differences between oxygenated and deoxygenated blood.
- does not rely on the use of radiation and so is safe.
Poor temporal resolution
- 5 second lag between the initial neural activity and the image. Does not measure moment-to-moment brain activity.
High spatial resolution
- shows detail by the millimetre. Provides clear picture of how brain activity is localised.
- used to established neurobiology underlying a particular behaviour (e.g. memory loss). Researchers may examine the brain after death to look for abnormalities or lesions that might explain the behaviour, comparing them to controls.
Causation may be an issue
- observed damage in the brain may not be the cause of the deficits under review, and it is impossible to be certain.
Provided foundations of knowledge of the brain
- Broca and Wernicke both relied on post-mortems.
Ethical issues of consent
- may not be able to provide informed consent
Improve medical knowledge
- help generate hypotheses for further studies
- biological rhythms that happen less frequently than every day.
Female Menstrual Cycle
- about 28 days
Rising levels of oestrogen cause the ovary to develop and release an egg (ovulation)
Progesterone helps the womb lining to thicken, readying the body for pregnancy.
If pregnancy does not occur, the egg is absorbed into the body, the womb lining comes away and leaves the body (menstrual flow)
Exogenous zeitgebers may synchronise them
- Stern and McClintock studied 29 women with irregular period. Pheromones were taken from some at different stages of their cycles, later cleaned with alcohol and rubbed on the upper lips of the other ppts. 68%of women experienced changes to their cycle which brought them closer to that of their 'odour donor'.
Seasonal Affective Disorder (SAD)
Depressive disorder with a seasonal pattern
Often called the 'winter blues' because it is triggered during the winter months when the number of daylight hours becomes shorter.
Called a circannual (yearly) cycle.
May be caused by hormone
- pineal gland secretes melatonin during the night until dawn (increase in light). During winter, the lack of light until later in the morning means secretion goes on for longer. Negative correlation with the production of serotonin in the brain.
Research on menstrual cycle shows evolutionary value
- may have been advantageous for ancestor females to menstruate together and become pregnant around the same time so offspring could be cared for together, increasing their chances of survival.
Practical applications of SAD research
- effective treatment for SAD is phototherapy (lightbox that simulates strong light in morning and evening to reset melatonin levels). Relieves symptoms of up to 60% of sufferers.
Methodology of synchronisation studies
- there are many factors that may change a woman's menstrual cycle that act as confounding variables in research. So any pattern of synchronisation is what we would expect to have occurred by chance. Can't be certain changes observed in Stern and McClintock's study were because of manipulation. Also, sample size is small (generalisability) and method relies on self-report (may lack accuracy).
Use of animal studies
- evidence for the influence of pheromones in animal sexual behaviour is well documented and is the basis for most of our knowledge. However, generalisability to humans is a problem, and evidence for the effects of pheromones on human behaviour remains speculative and inconclusive.
- biological rhythms that occur several times a day
Stages of Sleep
Occurs in 90-minute periods
Divided into 5 stages, each characterised by a different level of brainwave activity.
Stages 1 and 2
- light sleep, person may be easily woken. Brainwaves become slower and more rhythmic, slowing further and sleep becomed deeper. Muscle activity decreases and heart rate and breathing slow down.
Stages 3 and 4
- deep sleep, difficult to rouse. Characterised by slow brainwaves with greater amplitude.
- REM (rapid eye movement) sleep. Fast, jerky activity of eyes. Body paralysed yet brain activity speeds up. Muscles relax and heart rate increases. Breathing rapid and shallow.
Evidence supports different stages in sleep
- Dement and Kleitman monitored sleep patterns of nine ppts in a sleep lab and found evidence of stages of sleep (particularly REM). REM activity correlated with dreaming, and ppts woken during dreaming reported accurate recall of their dreams.