Hemispheric lateralisation and split brain research

• Hemispheric lateralisation - localisation and lateralisation


  Brain is lateralised - two sides of brain (hemispheres)

  For some functions the localised areas appear in both hemispheres eg vision, visual area is in left and right occipital lobe - located in left hemisphere and right hemisphere respectively.
• Left and right hemispheres

  In case of language two main centres are only in LH (for most)

  Brocas area left frontal lobe and Wernickes area left temporal lobe- so we say language is lateralised (performed by one hemisphere rather than the other). RH can only produce rudimentary words and phrases but contributes emotional context to what is being said - LH is analyser and RH is synthesiser.
• Left and right hemispheres

  Many functions not laterisalised eg vision, motor area and somatosensory area in both hemispheres (not lateralised). In case of motor area the brain is cross wired - RH controls movement on left side of body and LH controls movement on right

  Vision - both contralateral and ipsilateral (opposite and same sided). Each eye receives light from left visual field and right visual field. LVF of both eyes is connected to RH and RVF of both eyes connected to LH. Enables visual areas to compare slightly different perspective from each eye and aids depth perception
• Split brain research


  Involves severing connections between RH and LH mainly corpus collosum.

  Surgical procedure used to reduce epilepsy

  Studies how hemispheres function when they can't communicate with each other.
• Split brain research - Sperry 


  11 ppl who had a split brain operation were studied using a special set up where image could be projected to ppt RVF and same or different image could be projected to LVF.

  In normal brain the corpus collosum would immediately share info between both hemispheres giving complete picture of visual world BUT presenting image to one hemisphere of a split brain ppt meant info could not be conveyed from that hemisphere to the other.
• Sperry - Findings 


  When picture of object shown to ppt RVF (linked to LH) the ppt could only describe what was seen but could not do this if object was shown to the LVF (RH) they said nothing was there.

  ppts could not give verbal labels to objects projected to LVF they could select a matching object out of sight using their left hand.

  Left hand able to select object most closely associated with object presented to the LVF eg ashtray selected in response to cig.

  If pinup picture was shown to LVF there was an emotional reaction but ppts usually reported seeing nothing or just a flash of light
• --> shows how certain functions are lateralised in brain and support view that LH is verbal and RH is silent but emotional
• Split brain research - W - generalisation issues

  casual relationships hard to establish - behaviour of Sperry's ppts compares to neurotypical control group. Issue is none of ppts in control group had epilepsy - major confounding variable - any differences observed in two groups may be result of epilepsy rather than split brain

  Lacks validity
• Split brain - S - Research support

  Gazzaniga - split brains better at identifying odd one out in array of similar objects than normal controls.

  Normal brain - LH's better cognitive strategies are watered down by inferior RH

  --> left brain and right brain are distinct
• One brain - W -hemispheric lateralisation

  LH as analyser and RH as synthesiser may eb wrong - may be different functions in LH and RH but research suggests people do not have a dominant side of the brain which creates a different personality

  Nielson analysed brain scans from over 1000 people ages 7-29 and found people used certain hemispheres for certain tasks but there was no evidence of a dominant side

  --> notion of being right or left brained is wrong
• Lateralisation in connected brain - S -

  Research shows even in connected brains two hemispheres process info differently eg Germ Fink used PET scans to identify which brain areas were active during visual processing task. When ppts with connected brain were asked to attend to global elements of image eg whole forest such regions of RH were much more active, when required to focus on finer detail such as individual trees specific areas of LH tended to dominate

  --> least as far as visual processing concerned, hemispheric lateralisation is feature of connected brain as well as split brain.