hemispheric lateralisation and split brain research

• Hemispheric lateralisation (HL)

  • The brain is lateralised ie 2 sides (hemispheres)
  • some functions are localised and appear in both left and right hemispheres
  • Eg = auditory, visual, motor, somatosensory areas
  • 2 main language centres are in the LH (for most people) = Broca’s area (left frontal lobe) and Wernicke’s area (left temporal lobe)
• contralateral: (HL)

  • contralateral = information from the left side of the body is processed by the right hemisphere and vice versa 
  • Eg = in the motor area the RH controls the left side of the body
  • LVF of both eyes is connected to the RH AND the RVF of both eyes is connected to the LH
  • Enables the visual areas to compare the slightly different perspective from each eye and aids depth perception
  • Same arrangement for auditory areas
  • While visual input to each hemisphere primarily comes from the contralateral (opposite) visual field, both hemispheres also receive and process information from the ipsilateral (same side) visual field, although to a lesser extenT
• Strength = evidence of lateralised brain functions in ‘normal’ brains

  • PET scans shown when ’normal’ participants attend to global elements on an image, the RH is more active
  • But when required to focus on finer details the specific areas of the LH tend to dominate
  • Suggests that hemispheric lateralisation is a feature of the normal brain as well as the split brain
• limitation =idea of analyser vs synthesiser brain may be wrong

  • There may be different functions in the RH and LH but research suggests people do not have a dominant side, creating a different personality
  • Nielsen et al = analysed 1000 brain scans, finding people did use certain hemispheres for certain tasks but no dominance
  • Suggests that the notion of right- or left- brained people is wrong (eg ‘artist’ brain)
• extra evaluation = lateralisation vs plasticity

  • Lateralisation = is adaptive, enabling 2 simultaneous tasks with greater efficiency
  • Eg = only lateralised chickens better ate finding food while watching for predators
  • BUT = neural plasticity is also adaptive = after damage to the brain, language function can ‘switch sides’
  • suggests that lateralisation is first preference but ultimately plasticity is more important because it deals with loss of lateralisation
• Sperry, split brain research (procedure)

  • ‘Spli-brain’ = 2 hemispheres surgically separated by cutting the connections (eg corpus callosum)
  • Used to treat severe epilepsy to reduce the ‘electrical storm’ across hemispheres
  • 11 split-brain participants were studied
  • Image of word projected to RVF (processed by LH)
  • Same or different image or word projected to LVF (processed by the RH)
  • presenting the image to one hemisphere meant that information could not be conveyed from that hemisphere to the other
• Sperry, split brain research (findings/conclusions)

  • Object shown to RVF:
  • pp can describe what is seen (language centres in LH)
  • Object shown to LVF:
  • Cannot name object (no language centres in RH)
  • Can select matching object behind screen using left hand
  • Can select object closely associated with picture
  • pps giggled when shown picture but reported seeing nothing
  • demonstrates how certain functions are lateralised in the brain
  • Shows that LH is verbal and the RH is ‘silent’ but emotional
• strength = support from more recent split-brain studies

  • Luck et al = showed that split-brain pps are ‘better’ than normal controls
  • Eg = twice as fast at identifying the odd one out in an array of similar objects
  • In the normal brain = the LH’s superior processing abilities are ‘watered down’ by the inferior RH
  • This supports Sperry’s earlier findings that the ‘left brain’ abd ‘right brain’ are distinct in terms of functions and abilities
• limitation = causal relationships are hard to establish

  • In Sperry’s research = the behaviour of the split-brain participants was compared to a neurotypical control group
  • BUT = none of the control group had epilepsy
  • Any differences between the groups may be due to epilepsy not the split-brain (confounding variable)
  • Means that some of the unique features of the split-brain particpants’ cognitive abilities might have been due to their epilepsy
• extra evaluation = ethics

  • Sperry’s participants were not deliberately harmed and procedures were explained in advance to gain informed consent
  • BUT = pps may not have understood they would be tested for many years AND participation was very stressful
  • Suggests that there was no deliberate harm but the negative consequences make the study unethical
• Limitation = Sperry’s research oversimplifies brain processes

  • Sperry’s research often leads to the exaggeration and oversimplification of the different functions of the left and right hemispheres
  • In reality, functions associated with one hemisphere (e.g. the left) can be carried out or shared by the other hemisphere when necessary
• Strength = split brain research is reliable

  • Uses well-controlled, standardised procedures
  • => findings are reliable and replicable
• Assumptions of the left and right hemisphere:

  • Left hemisphere:
  • generally handles language, logic and analytical tasks
  • Right hemisphere:
  • more involved in visual-spatial processing, creativity, imagination and emotional understanding