hemispheric lateralisation and split-brain research

• hemispheric lateralisation - 


  two hemispheres of brain are functionally different and certain mental processes and behaviours are mainly controlled by one hemisphere rather than the other

  eg language which is localised and lateralised
• language lateralisation -

  • two main language centres are only in LH
  • Broca's area is in left frontal lobe
  • Wernicke's area is in left temporal lobe
  • RH can only produce rudimentary words and phrases but contributes emotional context
  • LH is analyser, RH is synthesiser
• right and left hemispheres -

  • vision, motor and somatosensory areas appear in both hemispheres
  • motor area - brain is cross-wired (contralateral wiring) - RH controls movement on left side of body, LH controls movement on right
• vision -

  • contralateral and ipsilateral (opposite and left sided)
  • each eye receives light from left visual field (LVF) and right visual field (RVF)
  • LVF of both eyes is connected to the RH and the RVF of both eyes is connected to the LH
  • enables visual areas to compare the slightly different perspective from each eye and aids depth perception

  
• split brain research -

  • operation involves severing the connections between the RH and LH
  • surgical procedure to reduce epilepsy
  • during seizure the brain experiences excessive electrical activity which travels from one hemisphere to the other
  • to reduce these fits these connections are cut
  • split brain research studies how the hemispheres function when they can't communicate with each other
• split brain research: Sperry's research procedure -

  • 11 people studied who had a split brain operation
  • image projected to a participant's RVF (processes by the LH) and the same or different image could be projected to the LVF (processes by the LH)
  • in the 'normal' brain, corpus callosum would immediately share the info between both hemispheres giving a complete picture of the visual world
  • presenting the image to one hemisphere of a split brain participant means the info cannot be conveyed from that hemisphere to the other
• split brain research: Sperry's research findings -

  • when pic of object shown to participants RVF (linked to LH) the participant could describe what was seen - couldn't do this if object was shown to LVF (linked to RH)
  • because in the connected brain, messages from the RH are relayed to the language centres in the LH - not possible in the split brain
  • participants couldn't give verbal labels for objects projected to LVF, they could select a matching object out of sight using left hand (linked to RH)
  • left hand could also select a object closely associated with object presented to LVF
• split brain research: Sperry's research conclusion -

  • final finding - if pinup picture shown to LVF, there's an emotional reaction (eg giggle) but participants usually reported seeing nothing or just a flash of light
  • conclusion - observations show how certain functions are lateralised in the brain and support the view that the LH is verbal and the RH is 'silent' but emotional
• evaluation of hemispheric lateralisation: lateralisation in the connected brain (strength) -

  • P: research showing that even in connected brains two hemispheres process information differently
  • E: fink et al (1996) PET scans - when participants with with connected brains were asked to attend to global elements of an image regions of RH were more active - when required to focus in on the finer detail the specific areas of the LH tended to dominate
  • L: suggests that as far as visual processing is concerned hemispheric lateralisation is a feature of the connected brain as well as split-brain
• evaluation of hemispheric lateralisation: one brain (limitation) -

  • P: idea that the LH as analyser and RH as synthesiser may be wrong
  • E: different function in RH and LH but research suggests people don't have a dominant side of their brain
  • E: Nielsen et al (2013) analysed 1000 peoples brain scans aged 7-29 years and found people used certain hemispheres for certain tasks - no evidence of a dominant side
  • L: suggests notion of right or left brained people is wrong
• evaluation of split brain: research support (strength) -

  • P: support from more recent split brain research
  • E: luck et al (1989) showed split brain participants actually perform better than connected controls on certain tasks - eg faster at identifying the odd one out in an array of similar objects than normal controls
  • E: kingstone et al (1995) in normal brain LH's better cognitive strategies are 'watered down' by inferior RH
  • L: supports Sperry's findings of distinct LH and RH
• evaluation of split brain: generalisation issues (limitation) -

  • P: causal relationships are hard to establish
  • E: behaviour of Sperry's split brain participants was compared to neurotypical group
  • E: none of control group had epilepsy - major confounding variable - any differences observed between 2 groups may be result of epilepsy rather than split brain
  • L: means unique features of split brain participants cognitive abilities may be due to epilepsy