Split-brain research into hemispheric laterisation

Hemispheric lateralisation

  • The ability to produce and understand language, is controlled by the left hemisphere. 
  • This means language is subject to hemispheric lateralisation
  • This means specialised areas associated with language are found in one of the brain's hemispheres rather than both
  • Roger Sperry conduced studies on split-brain to see if other neural processes may be organised in this way
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Split-brain research

  • Sperry's studies involved a unique group of individuals, all whom had undergone the same surgical procedure
  • An operation called a commissurotomy 
  • This is when the corpus callosum and other tissues which connect the two hemispheres were cut down the middle in order to seperate the two hemispheres and control frequent and severe epileptic seizures
  • This means for these patients that the main communication line between the hemispheres was removed
  • This then allowed Sperry to see the extent to which the two hemispheres were specialised for certain functions, and whether the hemispheres performed tasks independently of one another 
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Procedure

  • An image or word was projected to a paitent's right visual field (processed by the left hemisphere)
  • The same or different, image or word was projected to the left visual field (processed by the right hemisphere)
  • In a normal brain, the corpus callosum would immediately share the information between both hemispheres giving a complete picture of the visual world
  • However, presenting the image to one hemisphere of a split-brain patient meant that the information could not be conveyed from that hemisphere to the other
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Key findings

Describing what you see

  • When an object was shown to a patient's right visual field, the patient could easily describe what was seen
  • If the same object was shown to the left visual field, the patient could not describe what was seen, and sometimes reported that nothing was there
  • Language is associated with the left hemisphere, therefore the patient's inability to describe objects in the left visual field (processed in the right hemisphere) was because of the lack of language centres in the right hemisphere
  • In a normal brain, messages from the right hemisphere would have relayed to the language centres in the left hemisphere
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Key findings

Recognition by touch

  • The patients were able to select a matching object from a grab-bag of different objects using their left hand (linked to the right hemisphere)
  • Objects was behind a screen
  • The left hand was able to select an object that was most closely associated with an object presented to the left visual field
  • E.G. an ashtray was selected in response to a picture of a cigarette
  • The patient was not able to verbally identify what they had seen but could understand what the object was using the right hemisphere and select the corresponding object
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Key findings

Composite words

  • When 2 words were presented, one on either side of the visual field. E.G. key on left and ring on right
  • Patient would write with their left hand the word key and say the word ring
  • Right hemisphere is superior in drawing tasks
  • An investigation of 400 right-handed people revealed 380 of them has left hemisphere dominance for language 
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Key findings

Matching faces

  • Right hemisphere is dominant in terms of recognising faces
  • Patients were asked to match a face from a series of other faces
  • Picture processed by the right hemisphere (left visual field)) was selected, whilst the picture presented to the left hemisphere was ignored
  • When a picture made up of 2 different halves of a face was presented
  • The left hemisphere dominated in verbal description
  • The right hemisphere dominated in matching the picture
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Evaluation of lateralisation

Demonstrated lateralised brain functions

  • Sperry/s work shows that the left hemisphere is more geared towards analytic and verbal tasks
  • Also, the right hemisphere is more geared towards spatial tasks and music
  • Right hemisphere can only produced rudimentary words/phrases but contributes emotional and holistic content to language
  • Research suggests that the LH is the analyser whilst the RH is the synthesiser
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Evaluation of lateralisation

Strengths of the methodology

  • Used highly-specialised and standardised procedures 
  • Sperry made sure that the patients had a short amount of time to look at the picture 
  • The image to one hemisphere was displayed up for 1/10th of a second, this made sure that the patient would not have time to move their eye across the image
  • This suggests that he developed a very useful and well-controlled procedure 
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Evaluation of lateralisation

Theoretical basis

  • Roland Pucetti suggested that the 2 hemispheres are so functionally different that they represent a form of duality in the brain - that in effect we all have two minds
  • Other researchers have argued that, the 2 hemispheres form a highly integrated system and are both involved in most everyday tasks
  • Sperry's work prompted a theoretical and philosophical debate about the communication between the 2 hemispheres in everyday functioning and nature of consciousness
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Evaluation of lateralisation

Issues with generalisation

  • Split-brain patients are a unusual sample of people
  • There were only 11 who took part in the study, where they all had the basic procedure done and had a history of epileptic seizures
  • This means this findings of this study can't be generalised to the rest of the population
  • Also, it is also argued that the surgery these patients had, may have caused changes in the brain that may have influenced the findings
  • Also, some patients had experienced more disconnection of the 2 hemispheres as part of their surgical procedure than others
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Evaluation of lateralisation

Differences in function may be overstated

  • Sperry overemphasises and oversimplifies the functional distinctions between the RH and LH
  • Yes, the two hemispheres have different function in some aspects, however in a normal brain the 2 hemispheres are in constant communication when performing everyday tasks
  • Many of the behaviours typically associated with one hemisphere can be performed by the other when the situation requires it
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