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8.3 Brains and behaviour

8.3 Brains and behaviour

 

The human brain

The brain is part of the CNS, its role is to initiate and coordinate activities of the body. It receives input from receptors inside and outside the body, and the information from these is integrated to produce appropriate responses.

Different parts of the brain have different functions:

 

The cortex is the top of the brain, made mostly of cell bodies, synapses and dendrites (grey matter). This is the largest brain region and is divided into the cerebral hemispheres.

 

  • Cerebral hemispheres: Divided into right and left – ability to see, think, learn and feel emotions
  • Mid brain: Relays information to the cerebral hemispheres, auditory information to the temporal lobe and visual information to occipital lobe
  • Corpus callosum: A band of white matter (nerve axons) which connects the two cerebral hemispheres. It also provides connections between the cortex and the brain structures below

 

Each hemisphere is composed of 4 regions:

  • Frontal lobe: Concerned with higher brain functions such as decision making, reasoning, planning, consciousness of emotions, forming associations and ideas. It includes the primary motor cortex which has neurones that connect to the CNS.
  • Parietal lobe: Orientation, movement, sensation, calculation, some types of recognition and memory
  • Occipital lobe: Processing information from the eyes (vision, colour, shape, recognition, perspective)
  • Temporal lobe: Processing auditory information (hearing, sound, recognition, speech) and memory

 

  • Cerebellum: Coordination of movements, balance. It receives information from the primary motor cortex, muscles and joints
  • Medulla oblongata: Regulates involuntary processes – control of the heartbeat (cardiovascular centre) and breathing movements (respiratory centre) and blood pressure
  • Hypothalamus: Thermoregulation (maintaining core body temperature), sleep, hunger, thirst, blood concentration. It connects to the pituitary gland which secretes other hormones
  • Thalamus: Routes incoming sensory information to the correct part of the brain via the axons of the white matter
  • Hippocampus: Laying down long term memory
  • Basal ganglia: A collection of neurones responsible for selecting and initiating stored programmes for movement
  • Brain stem: At the top of the spinal column, it extends from the midbrain to the medulla

 

 

How the human brain works

Another way to map brain activity is CAT scans (computerised axial tomography). This uses narrow-beam X-rays rotated around a patient to pass through the tissue from different angles. Each narrow beam is reduced in strength depending on the tissue density to produce an image of a thin slice of the brain showing dense and less dense areas. A computer builds up a 3D image. CAT scans are cheaper than fMRI but do not show which tissues are active, so can only show structure not function of the brain. They only give frozen pictures. They are useful in the diagnosis and monitor of conditions such as brain tumours where changes in brain structure are visualised.

 

MRI (magnetic resonance imaging) uses a magnetic field and radio waves to detect soft tissues. When placed in a magnetic field, the nuclei of atoms line up with the direction of the magnetic fields. In an MRI scanner, the magnetic field runs down the centre of the tube in which the patient lies. Another magnetic field comes from the high frequency radio waves. These combined fields cause the direction and frequency of the hydrogen nuclei to change, taking energy from the radio waves. When the radio waves are turned off, the hydrogen nuclei return to their original alignment and release the absorbed energy. This is detected and a signal is sent to a computer to analyse it to produce an image. It is used to diagnose tumours, strokes, brain injuries and infections of the brain and spine.

 

One way to investigate the functions of brain parts following oxygen uptake is using fMRI (functional magnetic resonance imaging). A person lies inside a huge cylindrical magnet. The magnetic field affects the nuclei of atoms in the body, causing them to line up in the direction of the filed. This sends a signal to a computer, which builds up an image by mapping the strengths of signals from different brain parts. Deoxyhaemoglobin absorbs the radio wave signal and oxyhaemoglobin does not. Increased neural activity requires increased oxygen demand so an increase in blood flow – there is a large increase in oxyhaemoglobin levels in the blood so less signal is absorbed. The less radio signal, the higher the level of activity – active areas of the brain light up. By comparing images of a person doing different activities, the areas of the brain involved in these tasks can be identified.

 

Visual development

We see because the brain processes the image formed from the retina, using past experience and other sensory inputs. They capacity of the brain to process and interpret the action potentials that arrive along the optic nerve is acquired in early childhood. Newborn babies have little ability to interpret information but this is highly developed by the age of 5 or 6 years. The experiences determine the way brain cells are wired up during development.

 

Evidence for a critical period in visual development:

1960s – Hubel and Wiesel – experiments using monkeys and kittens because they are similar to humans. If they prevented light reaching the retina of one or both eyes as the animals matured they did not develop normal visual abilities. Depriving them at different stages of development affected different aspects. Monkeys deprived of light in one eye (monocular deprivation) were blind in that eye. They concluded that there are specific windows during development, known as critical periods/windows during which particular types of visual input are needed for visual capacities to develop.

 

More information has been obtained from the development of human babies. Some are born with cataracts, which prevent light patterns from reaching the retina, depriving normal visual input. These can be removed to give normal vision. However they can have a permanent impairment of their ability to perceive shape such as difficulties in face recognition. However elderly people who develop cataracts in later life have normal vision after they are removed.

Researchers tested their visual abilities as they grow up and looked for correlations between the time of visual deprivation and visual abilities that fail to develop. They found three different critical periods:

  • Sensitive period: Developmental changes in the eyes and brain do not occur if a particular visual input is not experienced
  • Period after sensitive period: Even if visual ability has developed properly, it can be damaged if abnormal visual input is experienced
  • Period after above: When any damage by earlier deprivation can be reversed if normal visual input is given

 

One visual ability is visual acuity – the ability to distinguish objects of small sizes. It is tested by the ability to tell the difference between a plain grey square and one divided by striped. The narrower stripes distinguished, the better the visual acuity.

 

In human babies:

  • A newborn’s visual acuity is very poor
  • Visual acuity improves during the first 6 months and reached adult values at around 5 years of age
  • Babies born with cataracts that are not treated until 9 months have the visual acuity of a newborn baby when they are removed
  • These babies then improve their visual acuity so by 1 year old they have the same of a normal baby
  • They then fall behind in development, so by 5 years old their visual acuity is 3 times worse than normal and they never develop normal adult acuity

 

There are a range of visual critical windows in which deprivation of normal visual input has different effects on development.

 

Similar results have been shown from binocular vision and peripheral vision. For example a young boy who had an eye infection had his eyes bandaged for two weeks and when removed, he had permanently impaired vision.

 

During the critical period:

The axons of the ganglion cells that make up the optic nerve pass out of the eye and extend to several brain areas, including the thalamus. Impulses are then sent along other neurones to the primary visual cortex where information is processed. Before reaching the thalamus, some neurones in each optic nerve branch off to the midbrain. Here they connect to motor neurones involved in controlling the pupil reflex and eye movement. Audio signals also arrive at the midbrain so you can quickly turn your eyes in the direction of a visual or auditory stimulus.

 

Retina à thalamus à visual cortex

 

The human nervous system begins to develop at birth. There is no large increase in the number of brain cells but there is a large increase in brain size due to the elongation of axons, myelination and the development of synapses. Once neurones have stopped dividing, the immature neurones migrate to their final position and wire themselves. Axons lengthen and synapse with the cell bodies of other neurones. Neurones must make correct connections to function properly.

 

Axons of the neurones from the retina grow to the thalamus where they form synapses with neurones. Axons from these thalamus neurones grow towards the visual cortex in the occipital lobe. The visual cortex is made of a column of axons which are overlapping at birth and receive stimulation from the retina. Normally, the critical period produces the distinctive patterns of columns but those that receive input from a light deprived eye become narrower. This is because dendrites and synapses from the light-stimulated eye take up more space in the visual cortex. Axons compete for target cells in the visual cortex and every time a neurone fires onto a target cell, the synapses of another neurone sharing the target cell are weakened and they release less neurotransmitter.

 

Nature v nurture

The effect of genes is nature and the environment (experiences) is nurture. Most behaviour patterns are determined by the interactions of both. Any behaviour shown at birth is innate and is considered to be caused by genes. Newborn babies show reflex reactions such as the startle reflex to a sudden loud noise or when they are dropped a short distance. The baby responds by flinging out their arms and legs and contracting the neck muscles. This response is innate but may be influenced by the experiences of the baby whilst a foetus in the uterus.

 

One way to investigate is using identical twins as they have identical genes. Brain development and behaviour of twins brought up in different families and environments are compared – these differences were caused by the environment.

 

Animal studies confirm that innate behaviour patterns can be modified by experience.

 

Development of the brain

The effects of strokes:

  • Drain damaged caused by a stroke can cause problems with speaking, understanding speech, reading and writing
  • Some patients can recover some abilities showing the potential of neurones to change in structure and function (neural plasticity)
  • The brain structure remains flexible even in later life and can respond to changed in the environment

 

Depth perception:

  • Close objects – depend on presence of cells in visual cortex that obtain information from both eyes – visual field is seen from 2 angles – stereoscopic vision, allows relative position of objects to be perceived
  • Distant objects – the images on the retinas are similar – visual cues and past experienced used to interpret the images

 

Cross cultural studies

People from different cultures may not share the same beliefs and behaviours. Carpentered world hypothesis – those who live in a world dominated by straight lines and right angles perceive depth cues differently to those who live in a circular culture. When surrounded by straight buildings unconsciously we tend to interpret images with acute and obtuse angles as right angles. People who live in a circular culture with few straight lines or right angles have little experience of interpreting acute and obtuse angles on the retina as representations of right angles – they are rarely fooled by optical illusions.

 

Studies with newborn babies:

The visual cliff – babies are encouraged to crawl across a transparent table, which is a visual cliff. Patterns placed below the glass create the appearance of a steep drop. If the perception of depth is innate the babies should be aware even if they have not previously experienced this stimulus. Young babies were reluctant to crawl over the ‘cliff’ even when the mothers encouraged them.

 

Learning and memory

The nervous system changes when there are changes in the synapses that underpin learning and memory changes. Memory is in different parts of the cortex and short-and long term memory is controlled by different parts of the brain.

 

Habituation

Learning is when organisms modify their behaviour as a result of experience. One of the simplest types of learning is habituation, defined as a decrease in the intensity of a response when the same stimulus is given repeatedly. For example humans show habituation when hearing a loud bang repeatedly.

 

Snails withdraw their body when it is touched on the shell. This response helps avoid damage by predators. If it is touched repeatedly and nothing unpleasant happens, it stops withdrawing its body. This is useful because it avoids energy being wasted on an unnecessary action and enables the snail to stay fully active.

 

How is habituation achieved?

With repeated stimulation, calcium ion channels become less responsive:

  1. Less calcium ions cross presynaptic membrane into presynaptic neurone
  2. Fewer synaptic vesicles fuse with presynaptic membrane
  3. Less neurotransmitter released into synaptic cleft
  4. Less sodium ion channels on posysynaptic membrane open
  5. Less sodium ions flow into postsynaptic membrane
  6. Less or no action potential is triggered in postsynaptic motor neurone

 

For example, sea slugs have less neurones than humans so their neurobiology is simpler than that of humans. They also have large accessible neurones so those involved in behaviours can be identified. The sea slug breathes through a gill in a cavity on the upper side of its body and water is expelled through a siphon tube. If the siphon is touched, the gill is withdrawn into the cavity – a protective reflex. Sea slugs are habituated to waves which stimulate the siphon. After a few minutes of repeated stimulation, the siphon no longer withdraws. Habituation allows animals to ignore unimportant stimuli so that limited sensory, attention and memory can be concentrated in more threatening or rewarding stimuli.

 

Practical – investigating habituation in pond snails:

  1. Collect pond snails of the same species and place them in the same tank and leave for a few days to acclimatise
  2. Place a snail in a dish and leave to rest for 5 minutes until active
  3. Using a small implement, gently touch the snail between the tentacles. The snail will withdraw and then slowly extend again.
  4. Repeat the stimulus several times, with set intervals of less than one minute. Record the time for the tentacle to be returned to its fully extended position
  5. Plot a graph of time against number of stimuli given

 

Sensitisation: Sensitisation is the opposite of habituation, when an animal develops an enhanced response to a stimulus. For example if a predator attacks sea slugs, they become sensitised to other changes in its environment and responds strongly. There is a greater calcium ion uptake, more neurotransmitter released, greater depolarisation and a higher frequency of action potentials.

 

Animal testing – ethical issues

ForAgainst
May be the only way to fully test new drugs and substances or find out about an aspect of physiology or behaviour which may lead to less suffering of humans and animalsWe have no right to submit animals to procedures that may cause discomfort or make their lives unpleasant
Only done when necessary – humans have a greater right to lifeThere is no need to use animals in research as there are other ways of conducting the same investigations
Only way to study how a drug affects the whole bodyAnimals are different to humans – no certainty that drugs tested on animals will have the same effect on humans

 

Institutions in the UK that test on animals follow the same codes of conduct:

  • Limit the use of animals to circumstances where there is no alternative method (such as using cells grown in tissue culture)
  • Only allow research after thorough scrutiny of the proposal, which must show no other method is possible and the animal welfare will be given high priority at all times
  • All people involved, including scientists, are given fully training in ensuring the health and wellbeing of the animals

 

Utilitarianism – the belief that the right action is the one that maximises the overall happiness – a utilitarian framework allows certain animals to be used in medical experiments provided the overall expected benefits are greater than the harms.