Localisation and Lateralisation of the Brain

• Localisation of function;

  • this is the idea that different parts of the brain perform different tasks and are involved with different parts of the body
  • therefore, when can look at specific areas of the brain and see which physical and psychological functions they are linked to
  • this is sometimes called cortical specialisation
  • if a certain part of the brain gets damaged due to illness or injury the function linked with that area will also be affected
• Motor area;

  • this controls voluntary movement in the opposite side of the body
  • it is important for complex movements and has control over fine movements
  • different parts of the motor area represent different parts of the body. If you electrically stimulate part of the motor cortex it will result in movement of the muscle that it is linked to
  • it is found at the back of the frontal lobe in both hemispheres
• Somatosensory area;

  • this receives sensory input from receptors in the skin, including touch, pain, pressure and temperature from all areas of the surface of the body
  • the amount of somatosensory area devoted to each part of the body is linked to how sensitive that body part needs to be. For example, there is a bigger part of the somatosensory areas given to the hands and the lips
  • it is found at the front of the parietal lobe in both hemispheres
• Visual area;

  • this receives input directly from the eyes. Each eye sends information to the opposite hemisphere of the brain. For example, information from the right visual field is sent to the left hemisphere and vice versa
  • damage to this part of the brain can lead to blindness
  • it is found in the occipital lobe in both hemispheres
• Auditory area;

  • this receives input directly from the ears. It analyses speech based information. Each ear sends information to the opposite hemisphere of the brain. For example, information from the right ear is sent to the left hemisphere and vice versa
  • damage to this part of the brain can lead to hearing loss
  • it is found in the front of the temporal lobe in both hemispheres
• Broca's area;

  • this is responsible for producing language
  • damage to this area results in motor aphasia and the inability to speak or write. However, the patient can still understand language
  • it is found in the left of the frontal lobe (left hemisphere)
• Wernicke's area;

  • this is responsible for understanding language
  • damage to this area results in sensory aphasia and can lead to patients saying nonsense words
  • it is found at the top of the left temporal lobe (left hemisphere)
• Evaluation of brain localisation;

  S - evidence to support (Peterson)

  S - evidence to support (Phineas Gage)

  W - evidence against (Lashley's rats)

  W - brain plasticity
• Research to support from Peterson's brain scans (identify);

  • there is research evidence to support the idea that physical and physiological functions are localised in the brain
• Research to support from Peterson's brain scans (explain);

  • Peterson scanned participants' brains in an fMRI scanner whilst they were doing two different tasks (one was a listening task and one was a reading task). They found that the Wernicke's area was active during a listening task and the Broca's area was active during a reading task
  • This suggests that Broca's and Wernicke's are 2 different areas of the brain that specialise in different aspects of language mastery
• Research to support from Peterson's brain scans (conclusion);

  • this adds to the validity of localisation of function as it provides empirical evidence for different areas of the brain having different functions and specialisms
  • Broca's area is responsible for speech production and Wernicke's area is responsible for the understanding of language
• Research to support from Phineas Gage (identify);

  • a strength is that there is evidence to support localisation of function from the case study of Phineas Gage
• Research to support from Phineas Gage (explain);

  • Phineas Gage was a railway worker in the 19th century who suffered brain damage after a metal tamping rod was sent shooting through his head following an explosion. He made a full recovery from the injury, yet his behaviour changed. Damage to his amygdala resulted in him becoming extremely aggressive and bad-tempered
• Research to support from Phineas Gage (conclusion);

  • this adds to the validity of localised functions as it suggests specific areas of the brain are responsible for specific functions such as personality suggesting the idea is more accurate
  • however, case studies are not generalisable and these past experiences are difficult to verify
• Research to contradict from Lashley (identify);

  • a weakness of localisation is that there is research to contradict that our brains are strictly localised
• Research to contradict from Lashley (explain);

  • Lashley removed areas of the brain (cortex) in rats that were learning a maze. He removed different parts of the brains in all of the different rats and then timed how long it would take from them to complete the maze. He found that there was no difference in the time taken by the rats, regardless of which part of the brain had been removed
  • however the generalisability can be questioned as rats were studied not humans
• Research to contradict from Lashley (conclusion);

  • this is important as it directly contradicts the theory of localisation in the brain. This is known as the holistic theory of the brain
  • Lashley went on to suggest an equipotential theory of the brain, which is the idea that when parts of the brain are damaged then other areas of the brain can compensate for this and taken over their function
• Brain plasticity (identify);

  • a weakness of research into localisation is that there is evidence to contradict the brain being strictly localised
• Brain plasticity (explain);

  • research suggests that when the brain has become damaged and a particular function is lost, the rest of the brain is able to reorganise itself to recover the lost function
  • Danelli conducted a case study where a boy who lost his entire left hemisphere as a child recovered his language abilities by the age of 17. This suggests that areas of the brain are not strictly localised and can adapt in response to brain damage
  • this supports the holistic theory of the brain as other areas of the brain are able to carry out the tasks, suggesting the whole brain works together
• Brain plasticity (conclusion);

  • this is important as it suggests that the brain isn't strictly localised. Different areas of the brain can handle more than one task and can adapt their specialisations. This goes against the idea of localisation
  • this can help to support people to redevelop language skills after a stroke (support neurorehabilitation after brain trauma)
• Hemispheric lateralisation;

  • hemispheric lateralisation is a form of localisation where the two hemispheres of the brain have different specialisations
  • the brain has a left and right hemisphere. They are roughly equal in size and are connected by the corpus callosum
• Hemispheric lateralisation;

  • our brain is contralateral. This means the optic nerve from each eye crosses over into the opposite hemisphere, so our left eye will send signals to the right hemisphere and vice versa
  • the left hemisphere is said to specialise in language processing
  • the right hemisphere is more dominant in visual motor tasks, recognising faces and analysis via touch
• Research into brain lateralisation;

  • although it is accepted that language is lateralised to the left hemisphere research needed to be done to see if other physical and psychological functions and process were also lateralised
  • Sperry (1968) studied a group of 11 people who had undergone treatment for epilepsy. They had all had the same surgical procedure, which was an operation that cut part of their brain
• Commissurotomy;

  • the operation to cut the brain is called a commissurotomy. This operation was done because it has been show to control severe and frequent epileptic seizures
  • it also allowed Sperry to see if the two hemispheres were specialised for certain functions and whether they perform tasks independently of one another
• Corpus callosum;

  • Normally, the corpus callosum would share the information between the two hemispheres, allowing them to communicate with each other
  • however, if the corpus callosum has been severed then the two hemispheres cannot communicate. Something shown to the left eye would not be recognised in the left hemisphere and vice versa
• Sperry's split-brain research - procedure (1968);

  split-brain patients would be shown a picture or a word to their right or left visual field only and be asked to do one of the following tasks;

  • describe what you see tasks
  • tactile tasks
  • draw it tasks
• Describe what you see tasks;

  • images projected to the different visual fields and the participants had to describe what they saw
• Tactile tasks;

  • participants felt an object in either their left or right hand and had to either describe what they were touching verbally or identify a similar object from a selection of possible objects
• Draw it tasks;

  • images were projected to the two different visual fields and the participants had to draw what they had seen
• Sperry's split-brain research - findings/conclusions (1968)

  • describe what you see - split-brain patients could describe an object when it was presented to their right visual field, but not the left visual field suggesting language is processed in the left hemisphere
  • tactile (describe it) tasks - split-brain patients could describe an object when it was presented to their right hand, but not the the left hand, suggesting language is processed is the left hemisphere
• Sperry's split-brain research - findings/conclusions (1968);

  • tactile (find as a similar object) tasks - split-brain patients could feel an object in one hand and then find a similar object suggesting the somatosensory cortex is split across both hemispheres
  • draw it tasks - split-brain patients would draw more accurate drawings using their left hand, even though they were all right-handed suggesting that fine motor skills are processed by the right hemisphere
• Sperry's split-brain research - conclusions (1968)

  general conclusions from split-brain patients about brain lateralisation;

  • language is processed in the left hemisphere
  • the somatosensory cortex is found across both hemispheres
  • fine motor skills are processed in the right hemisphere
• Evaluation of hemispheric lateralisation and split-brain research;

  S - evidence to support (Peterson)

  W - contradictory evidence from Danelli

  W - problems with the research - small sample size

  W - problems with the research - different surgeries
• Research to support from Peterson (identify);

  • there is research evidence to support Sperry's conclusions that some physical and physiological functions are lateralised in the brain
• Research to support from Peterson (explain);

  • Peterson used brain scans to show that the Wernicke's areas was active during a listening task and Broca's area was active during a reading task. Both these language centres are lateralised to the left hemisphere
  • this suggests language functions are lateralised to the left hemisphere, so each hemisphere has different functions
• Research to support from Peterson (conclusion);

  • this adds validity to Sperry's research conclusions because it shows that each hemisphere can work independently of each other to the same extent and have their own specialisms, which are responsible for different functions
• Contradictory evidence from Danelli (identify);

  • there is contradictory evidence about lateralisation from the Danelli case study
  • Danelli conducted a case study of an Italian boy (E.B) who had his entire left hemisphere removed from his brain aged 3. They tracked his development during childhood and found that initially his language and movement was significantly behind children his own age. However, by the age of 15 he had caught up with children his own age and his language abilities were 'normal' despite not having a left hemisphere
• Contradictory evidence from Danelli (explain);

  • Danelli conducted a case study of a boy who had his left hemisphere removed at the age of 3. The boy had made a full recovery by the age of 15 and his language was as good as his peers, suggesting that language isn't strictly localised to the left hemisphere
• Contradictory evidence from Danelli (conclusion);

  • this contradicts Sperry's conclusions because it suggests that the right hemisphere of E.B was able to recover the language function. This case study shows that language isn't strictly localised to the left hemisphere and Sperry's findings don't occur in every individual
• Small sample (identify);

  • a weakness of Sperry's research is that his sample size was very small