Localisation + lateralisation

• localisation of function:

  Localisation of function is the idea that certain functions (e.g. language, memory, etc.) have certain locations within the brain.
• 4 key areas of brain:


  1. motor area: located in frontal lobe and responsible for voluntary movements by sending signals to the muscles in the body
  2. somatosensory area: in parietal lobe and receives incoming sensory info from the skin to produce sensations related to pressure, pain, temperature, etc (robertson)
  3. visual area: in occipital lobe - receives and processes visual info (contralaterally organised)
  4. auditory area: in temporal lobe - processes acoustic info (contralaterally organised)
• localisation of function: language centres (Broca's area)


  Broca discovered while treating patient who could only say Tan but understand everything else.

  He identified that the the left frontal lobe was responsible for speech production - he had damage in left frontal lobe (Experiences Broca's aphasia)
• localisation of function: language centres (Wernicke's area) 


  Wernicke discovered another area of brain involved in understanding language (damage to this = Wernicke's aphasia = able to speak but not able to understand language)

  Wernickes area found in left temporal lobe (involved in understanding language, produce meaningless sentences)

  He also said language involved separate motor and sensory region (motor region in Broca's area) (sensory region in Wernicke's area)
• limitation: localisation


  The claim that functions are localised to certain areas of the brain has been criticised. Lashley proposed equipotentiality theory (suggests that the basic motor and sensory functions are localised, but higher mental functions aren't)- claimed that intact areas of the cortex could take over responsibility for specific cognitive functions following brain injury. This therefore casts doubt on theories ab the localisation of functions, suggesting that functions are not localised to just one region, as other regions can take over specific functions following brain injury.
• Strength: localisation

  There is a wealth of case studies on patients with damage to Broca’s and Wernicke’s areas that have demonstrated their functions. For example, Broca’s aphasia is an impaired ability to produce language; in most cases, this is caused by brain damage in Broca’s area. Wernicke’s aphasia is an impairment of language perception, demonstrating the important role played by this brain region in the comprehension of language.
• However, altho theres evidence from case studies to support the function of the Broca’s area + Wernicke’s area, more recent research has provided contradictory evidence. Dronkers et al. conducted an MRI scan on Tan’s brain, to try to confirm Broca’s findings. a lesion found in Broca’s area but also found that other areas may have contributed to the failure in speech production. These results suggest that the Broca’s area may not be the only region responsible for speech production + the deficits found in patients with Broca’s aphasia could be the result of damage to other neighbouring regions.
• Limiation: localisation


  some psychologists argue that the idea of localisation fails to take into account individual differences. Herasty (1997) found that women have proportionally larger Broca’s and Wernicke’s areas than men, which can perhaps explain the greater ease of language use amongst women. This, however, suggests a level of beta bias in the theory: the differences between men and woman are ignored, and variations in the pattern of activation and the size of areas observed during various language activities are not considered
• hemispheric lateralisation:

  Lateralisation is the idea that the two halves of the brain are functionally different and that each hemisphere has functional specialisations, e.g. the left is dominant for language, and the right excels at visual motor tasks. The two hemispheres are connected through nerve fibres called the corpus callosum, which facilitate interhemispheric communication: allowing the left and right hemispheres to ‘talk to’ one another.
• split-brain research: sperry and gazzinga


  first to investigate hemispheric lateralisation with the use of split-brain patients (individuals whose corpus callosum, which connects the two hemispheres, is cut - separates the 2 hemispheres and this procedure used as treatment for severe epilepsy/seizures)
• sperry and gazzinga split-brain research: aim and method


  aim: to examine the extent to which the two hemispheres are specialised for certain functions.

  Method: An image/word is projected to the patient’s left visual field (which is processed by the right hemisphere) or the right visual field (which is processed by the left hemisphere). When info is presented to one hemisphere in a split-brain patient, the information is not transferred to the other hemisphere (as the corpus callosum is cut).
• sperry and gazzinga procedure:


  conducted experiments: describe what you see tasks, tactile tests, and drawing tasks.

  • describe what you see task: picture was presented to either the left or right visual field and the ppt had to describe what they saw.
  • tactile test: object was placed in the patient’s left or right hand and they had to either describe what they felt or select a similar object from a series of alternate objects.
  • drawing task: ppts were presented with a picture in either their left or right visual field, and they had to simply draw what they saw.
• findings (sperry + gazzinga)


  • describe what u see task
  • pic presented to right visual field (processed by left hemisphere) - patient could describe what they saw, demonstrating superiority of left hemisphere and speech production
  • pic presented to left visual field (processed by right hemisphere) - patient couldnt describe what was shown and often reported that there was nothing present
• findings: sperry and gazzinga


  tactile tests

  • object placed in right hand (processed by left hemisphere) - patient could describe verbally what he felt or selecting similar object from a series of diff objects
  • object placed in left hand (processed by right hemisphere) - patient couldnt describe what they felt and could only make wild guess but left hand could identify from series of diff objects
• findings: sperry and gazzinga:


  drawing tasks:

  • picture presented to right visual field (processed by left hemisphere) - the picture wasnt drawn clear with right hand (shows superiority of right hemisphere for visual motor tasks)
  • picture presented to left visual field (processed by right hemisphere) - the left hand (controlled by right hemisphere) would draw clearer even tho all ppts were right handed (shows superiority of right hemisphere for visual motor tasks)
• conclusions of sperry and gazzingas research


  The findings of Sperry and gazzinga research highlights a number of key differences between the two hemispheres. Firstly, the left hemisphere is dominant in terms of speech and language. Secondly, the right hemisphere is dominant in terms of visual-motor tasks.
• strength: split-brain research


  brain lateralisation increases neural processing capacity (ability to perform multiple tasks simultaneously). Rogers et al. found that in a domestic chicken, brain lateralisation is associated with an enhanced ability to perform two tasks simultaneously (finding food + being vigilant for predators). Using only one hemisphere to engage in a task leaves the other hemisphere free to engage in other functions. This provides evidence for the advantages of brain lateralisation and demonstrates how it can enhance brain efficiency in cognitive tasks.
• limit: split-brain research


  because this research was carried out on animals, it is impossible to conclude the same of humans. Unfortunately, much of the research into lateralisation is flawed bc split-brain procedure is rarely carried out now, meaning patients are difficult to come by. Such studies often include very few ppts, and often the research takes an idiographic approach. Therefore, any conclusions drawn are representative only of those individuals who had a confounding physical disorder that made the procedure necessary- such results cannot be generalised to the wider population.
• limit: split-brain research


  Furthermore, research has suggested that lateralisation changes with age. Szaflarki et al. (2006) found that language became more lateralised to the left hemisphere with increasing age in children and adolescents, but after the age of 25, lateralisation decreased with each decade of life. This raises questions about lateralisation, such as whether everyone has one hemisphere that is dominant over the other and whether this dominance changes with age.
• limit: split-brain research


  Finally, it could be argued that language may not be restricted to the left hemisphere. Turk et al. (2002) discovered a patient who suffered damage to the left hemisphere but developed the capacity to speak in the right hemisphere, eventually leading to the ability to speak about the information presented to either side of the brain. This suggests that perhaps lateralisation is not fixed and that the brain can adapt following damage to certain areas.
• plasticity:


  The brain is not a static organ, and the functions and processes of the brain can change as a result of experience and injury. Brain plasticity refers to the brain’s ability to change and adapt because of experience. Research has demonstrated that the brain continues to create new neural pathways and alter existing ones in response to changing experiences
• functional recovery: 


  The brain also appears to show evidence of functional recovery: the transfer of functions from a damaged area of the brain after trauma to other undamaged areas. It can do this through a process termed neuronal unmasking where ‘dormant’ synapses (which have not received enough input to be active) open connections to compensate for a nearby damaged area of the brain. This allows new connections in the brain to be activated, thus recovering any damage occurring in specific regions.
• strength: plasticity and functional recovery


  Kuhn et al. found significant increase in grey matter in various regions of the brain after ppts played video games for 30 mins a day for 2-months. Also, Davidson et al. demonstrated permanent change in the brain generated by prolonged meditation: Buddhist monks who meditated frequently had much greater activation of gamma waves (coordinate neural activity) than did students w no experience of meditation. These 2 studies highlight the idea of plasticity + the brain’s ability to adapt as a result of new experience
• strength: plasticity and functional recovery


  There is further research to support the notion of brain plasticity. Maguire et al. found that the posterior hippocampal volume of London taxi drivers’ brains was positively correlated with their time as a taxi driver and that there were significant differences between the taxi drivers’ brains and those of controls. This shows that the brain can permanently change in response to frequent exposure to a particular task.
• limitation: plasticity and functional recovery (maguire taxi)


  Some psychologists suggest that research investigating the plasticity of the brain is limited. E.g. Maguire’s research is biologically reductionist and only examines a single biological factor (size of the hippocampus) in relation to spatial memory. This approach is limited + fails to take into account all the diff biological/cognitive processes involved in spatial navigation which may limit our understanding. Other psychologists suggest that a holistic approach to understanding complex human behaviour may be more appropriate.
• limit: plasticity and functional recovery


  While there is evidence for functional recovery, it is possible that this ability can deteriorate with age. Elbert et al. concluded that the capacity for neural reorganisation is much greater in children than in adults, meaning that neural regeneration is less effective in older brains. This may explain why adults find change more demanding than do young people. Therefore, we must consider individual differences when assessing the likelihood of functional recovery in the brain after trauma.