Hemispheric lateralisation and split brain research

• Hemispheric lateralisation


  The two halves of the brain are not exactly alike as each hemisphere has functional specialisations located primarily in one half of the brain
• Brain lateralisation


  • Left hemisphere is dominant for language and speech
  • Right hemisphere excels at visual-motor tasks
• Corpus callosum


  Connects the two hemispheres and allows information to be sent to the other hemisphere
• Right hemisphere


  Controls the left side of the body
• Left hemisphere


  Controls the right side of the body
• Advantages of hemispheric lateralisation

  • Using only 1 hemisphere to engage in a particular task leaves the other hemisphere free to engage in another function
  • Little empirical evidence that lateralisation confers any advantage to the functioning of the brain in humans
  • In domestic chickens, brain lateralisation is associated with an enhanced ability to perform two tasks simultaneously
• Disadvantages of hemispheric lateralisation

  • Architects and mathematically gifted tend to have superior right-hemispheric skills but are also more likely to be left handed and suffer higher rates of allergies and immune system problems
  • Significant relationship between handedness and immune system disorders, suggesting the same genetic processes that lead to lateralisation may also affect the immune system development
• Lateralisation changes with age


  Language became more lateralised to the left hemisphere with increasing age in children and adolescents, but after 25 lateralisation decreased with each decade of life
• Split-brain research


  Procedure where the corpus callosum and connecting tissue is removed/severed to create two separate hemispheres that don't communicate with each other
• Split-brain research procedure


  1. Participants have all undergone a commissurotomy

  2. Compared against people with no history of epilepsy

  3. Information is not able to be passed between the hemispheres if one eye is covered

  4. Information can be passed to each hemisphere if both eyes see the stimuli
• Sperry's split-brain research tasks

  1. Describe what you see - images shown to left or right visual field

  2. Tactile test - object placed in left or right hand, participant has to select similar object or describe what they feel

  3. Drawing task - participant presented with picture in left or right visual field and has to draw what they see
• Findings from Describe what you see tasks

  • Left eye (right hemisphere): Patient could not describe what was shown and often reported nothing was present. 
  • Right eye (left hemisphere): Patient could describe what they saw which shows that language is left dominated. 
• Findings from Tactile test task

  • Left eye (right hemisphere): Could not describe what they felt.
  • Could identify and select an appropriate similar item.
  • Right eye (left hemisphere): Could describe what they felt 
  • Could identify and select an appropriate similar item because they can HEAR themselves describe it. 
• Findings from draw what you see task

  • Left eye (right hemisphere): Left hand could draw clearer pictures despite being right-handed.
  • Right dominates visual motor tasks.
  • Right eye (left hemisphere): Right-hand drawings were not as clear as left-hand drawings. 
• Language may not be restricted to the left hemisphere

  Gazzaniga- some of the early discoveries from split-brain research has been disconfirmed by more recent discoveries as damage to the left hemisphere was found to be far more detrimental to language function than the right

  Case studies have demonstrated that this was not necessarily the case- JW developed the capacity to speak out of the right hemisphere with the result that he can now speak about information presented to the left or right brain

  Challenges the claim that the right hemisphere is unable to handle language
• Limitations of split bain research

  • Andrews- many studies are presented with few patients- 3 or 1 single participants
  • Some cases conclusions have been drawn from individuals who either have a confounding physical disorder that made the split-brain procedure necessary or have had a less complete sectioning of the two hemispheres than was originally believed
  • Patients who have had this procedure without these confounding variables are rarely encountered- low population validity