Split-brain and hemispheric lateralisation

• Roger Sperry, procedure


  • An image or word projected to ppts right visual field (processed by left hemisphere)
  • The same or different image could be projected to the left visual field
  • The ppts brain could not communicate the information seen to the other hemisphere
  • Sperry studied how this changes ppts functioning
• Sperry findings- describing what you see

  • Picture or object shown to right visual field, ppt could describe what was seen
  • If shown to left visual field, ppt could not describe, sometimes reported nothing was there
  • Language is processed in the left hemisphere, therefore ppts could not process info seen in the left visual field due to right hemisphere processing (and lack of language centres in there)
• Sperry findings- recognition by touch

  • Ppts could not give verbal labels to objects projected in left visual field
  • However they could select a matching or associated object from a grab-bag of different objects using their left hand (right hemisphere processing)
  • E.g. ashtray in response to a picture of cigarette
  • The ppts could not verbally identify what they had seen but could understand what the object was using the right hemisphere
• Sperry findings- composite words

  • 2 words presented simultaneously, one on either side of the visual field
  • E.g. key on left, ring on right
  • Ppt would select key with left hand (left visual field, right hemisphere linked to left hand) and say the word ring
  • Suggests language is processed in left hemisphere and right hemisphere is dominant in drawing tasks
• Sperry- matching faces

  • Ppts were shown an image of a face (different to the left and right)
  • asked to match it to one from a series of other faces.
  • They were also given composite faces (a face made of two different halves)

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• Sperry- matching faces findings

  • First task, picture processed by the left visual field (right hemisphere) was consistently selected, whilst the picture to the left hemisphere was ignored.
  • Second task, (composite faces) the half presented to the right visual field (left hemisphere) dominated the description of the face
  • the half presented to the right hemisphere dominated in terms of selecting a matching picture.
  • This suggests that facial recognition is lateralised to the right hemisphere.
• What is hemispheric lateralisation?

  • The idea that the two hemispheres of the brain are functionally different
  • Certain mental processes and behaviours are mainly controlled by one hemisphere rather than the other
• Limitation of generalisation (AO3)

  • Sample size lacks population validity with only 11 ppts
  • All had a history of epileptic seizures
  • This itself may have acted as a confounding variable that may have caused unique changes in the brain which may have influenced/accounted for findings
  • Additionally some ppts had a larger disconnect between the two hemispheres after their commissurotomy
  • Therefore lacks external validity and generalisability as patients may largely differ and be unrepresentative of the wider population
  • Extreme to assume findings of laterlisation account for everyone
• Strength, real life app(AO3)

  • Sperry and Gazzaniga's research has contributed to the research and explanations of lateralised brain functions
  • As a result, the left hemisphere is more geared towards analytical and verbal tasks (the analyser)
  • The right is more adept for spatial tasks and music (the synthesiser)
  • The right can only produce rudimentary words and phrases but contributes to emotional and holistic content to language
• Strength, standardised methodology (AO3)

  • Sperry would provide participants with a fixation point
  • The image would be projected in a flash of 1/10th of a second
  • This way split-brain ppts could not have time to move their eye across the image and both sides of the brain
  • Allowed Sperry to vary aspects of the basic procedure, ensuring only one hemisphere was receiving information at a time
  • Therefore findings may have high internal validity on HL and are likely due to be because of the IV on DV
• Limitation (AO3)

  • Differences in function may be overstated
  • Sperry is likely to have oversimplified the distinct differences between the two hemispheres
  • Verbal and non verbal labels can sometimes be applied to summarise the differences yet modern neuroscientists would argue that the actual distinction is more complex than this
  • This is because our hemispheres are communicating constantly, relying on each other for everyday tasks
  • Behaviours typically associated to one hemisphere can be effectively performed by the other when required
  • Limitation as therefore HL may not be as direct and distinct as Sperry suggested, reducing explanatory validity