Hemispheric lateralisation and split brain research

• hemispheric lateralisation= idea that the two hemispheres of the brain are functionally different and that certain mental processes and behaviours are mainly controlled by one hemisphere rather than the other. Eg: language
• Left and right hemispheres

  • Language centres in the left hemisphere- Broca's area in left frontal lobe and Wernicke's area in left temporal lobes. So language is lateralized.
  • The right hemisphere can only produce rudimentary words and phrases but contributes emotional context.
  • Suggests the left hemisphere is the analyser and right hemisphere is the synthesiser.
• Vision, motor and somatosensory areas aren't lateralised as they appear in both hemispheres.
• Motor area= brain is cross-wired (contralateral wiring)
• Vision= both contralateral and ipsilateral (opposite and same-sided)

  • Each eye receives light from left visual field and right visual field. The LVF of both eyes is connected to the right hemisphere, and the RVF of both eyes is connected to the left hemisphere.
  • This enables the visual areas to compare the slightly different perspective from each eye and aids depth of perception.
• Auditory= arrangement for auditory input to auditory area and the disparity from the two inputs helps us locate the source of sounds.
• Evaluation- lateralisation in the connected brain

  Strength= research showing that even in connected brains the 2 hemispheres process information differently.

  • Fink used PET scan to identify which brain areas were active during a visual task. When participants with connected brains were asked to attend to global elements of an image, regions of the RH were much more active. When required to focus on finer detail the specific areas of the LH tended to dominate
  • Suggests that visual processing, hemispheric lateralisation is a feature of the connected brain as well as the split brain.
• Evaluation- one brain

  Limitation= idea that the LH as analyser and RH as synthesiser may be wrong.

  • May be different functions in RH and LH but research suggests people do not have a dominant side of their brain which creates a different personality.
  • Nielsen analysed brain scans from over 1000 people aged 7-29 and did find that people used certain hemispheres for certain tasks (lateralisation) but there was no evidence of a dominant side.
  • Suggests the notion of right or left brained people is wrong.
• Split brain research

  = Enabled researchers to test lateral functions of the brain in isolation.
• • split-brain operation involves severing the connection between the RH and LH= corpus callosum
  • Used to reduce epilepsy
  • During an epileptic seizure the brain experiences excessive electrical activity which travels from one hemisphere to the other.
  • To reduce fits these connections are cut, 'splitting' the brain in 2 half's.
• Sperry's research

  = Devised a system to study how two separated hemispheres deal with speech and vision.
• Sperry's findings

  • When a picture of an object was shown to participants RVF the participant could describe what was seen. But they couldn't do this if the object was shown in the LVF.
  • Because in the connected brain, messages from RH are relayed to language centres in LH, but this isn't possible in the split-brain.
  • However participants could select a matching object out of sight using their left hand.
  • The left hand was also able to select an object that was closely associated with object present in LVF.
  • Shows how certain functions are lateralised in the brain.
• Evaluation- research support

  Gazzaniga showed that split-brain participants 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. In normal brain, the LH better cognitive strategies are 'watered down' by the inferior RH.
  • Supports Sperry's findings that the left and right brain are distinct.
• Evaluation- generalisation issues

  Limitation= causal relationship are hard to establish

  • Behaviour of Sperry's split brain participants was compared to a neurotypical control group.
  • Issue= none of participants in control group had epilepsy= confounding variable.
  • Any differences observed between the two groups may be the result of epilepsy rather than the split-brain.