Split-brain

• Hemispheric Lateralisation


  Refers to the idea that each hemisphere of the brain is mainly responsible for certain behaviours, processes, and activities.

  
  

  eg: neural mechanisms for some functions are localised primarily to one half of the brain

  ie: language centres (Broca's area and Wernicke's area) are found in the left hemisphere only.

  
  

  This is in contrast with the holistic theory of brain function - which suggests that function is distributed across the whole brain.
• Contralateral Organisation


  Each hemisphere controls functioning and information processing from the opposite side of the body.

  
  

  The left motor cortex controls voluntary movements on the right side of the body and vice versa.

  
  

  Visual information from left eye is processed by the right visual cortex.
• Corpus Callosum


  A bundle of nerve fibers connecting the two brain hemispheres and carrying messages between them.

  
  

  - allows information received by one hemisphere to be sent to the other hemisphere (facilitate interhemispheric communication)
• Sherry and Gazzaniga (1967) - Aim

  To examine the extent to which the two hemispheres are specialised for certain functions.

  
  

  They were the first to investigate hemispheric lateralisation with the use of split-brain patients.
• Sperry and Gazzaniga (1967) - Sample


  11 male split-brain patients - people whose corpus callosum has been intentionally severed as a treatment for severe epilepsy, through a procedure called a cerebral commissurotomy.
• Sperry and Gazzaniga (1967) - Procedure

  The subject has one eye covered (so that information wouldn't be received by both eyes) and looks at a fixation point on a translucent screen.

  
  

  Visual Stimuli tests and Tactile Stimuli tests
• Visual Stimuli tests


  A slide is projected onto a screen a rate of 1 picture per 1/10 second.

  
  

  It is either projected to the patient's right visual field (processed by the left hemisphere) or the patient's left visual field (processed by the right hemisphere).

  
  

  As their corpus callosum is cut, the information presented to one hemisphere is not transferred to the other hemisphere.

  
  

  Pp's had to simply describe what they had seen.
• Tactile Stimuli tests


  Objects are presented to the left or right hand behind a screen.

  
  

  Pp's had to either describe what they felt, or select a similar object from a series of alternate objects.
• Drawing task


  Pp's were presented with a picture in either their left or right visual field, and they simply had to draw what they saw.
• Findings - Visual Stimuli tests

  When an image was exposed to the right visual field, and so was processed by the left hemisphere, participants could verbally describe what they saw.

  
  

  This is because the left hemisphere is associated with the 'language centres' and so allows for speech.

  
  

  When an image was exposed to the left visual field, and so was processed by the right hemisphere, pp's couldn't give a verbal description of what they saw - often reported that there was nothing present.
• Findings - Tactile Stimuli tests

  Object placed in right hand - could verbally describe what they felt, or they could identify the object by selecting a similar appropriate object.

  
  

  Object placed in left hand - pp's couldn't verbally describe what they felt, but the left had could choose a similar appropriate object due to the action of visuospatial-spatial centres in the right hemisphere.
• Findings - drawing task

  Picture placed in the right visual field - when the right hand would attempt to draw a picture, the picture was never as clear as the left hand.

  
  

  Pictured presented to the left visual field - the left-hand (controlled by the right hemisphere) would consistently draw clearer and better pictures than the right hand (even though all pp's were right handed).

  
  

  This demonstrates the superiority of the right hemisphere (where the visuospatial-spatial centres are located) for visual, motor tasks.
• Sperry and Gazzaniga - conclusion

  Their findings highlight a number of key differences between the 2 hemispheres.

  
  

  Firstly, the left hemisphere is dominant in terms of speech and language. Secondly, the right hemisphere is dominant in terms of visual-motor tasks.
• Methodology (P)


  One advantage of this research is its use of controlled, and standardised procedures.
• Methodology (E)


  For example, the the image was only projected in the pp's left or right visual field for 1/10th of a second to prevent both eyes from seeing it and sharing visual information.
• Methodology (L)


  This meant they could draw valid conclusions, increasing the study's internal validity.
• Clearly demonstrated lateralisation of function:

  Split-brain research is pivotal in establishing differences in functions between the two hemispheres and so opposing the holistic theory of brain function.

  
  

  The left hemisphere was demonstrated as dominant for language tasks - as it contains the language centres.

  
  

  The right hemisphere was demonstrated as dominant for visuo-spatial tasks.

  
  

  This suggests that the left hemisphere is the analyser, whereas the right hemisphere is the synthesiser.
• Limitation of this research (P)

  In this study, there was a lack of control with the sample selection.
• Limitation of this research (E)

  For example, this study used patients with a history of epileptic seizures. They'd also been taking anti-epilepsy medications for different extended periods of time.

  
  

  This could've had a unique consequence on their ability to perform certain tasks - causing cerebral neuronal changes.
• Limitation of this research (E)

  Also, although all patients had undergone a cerebral commisurotonomy, there may have been differences in the exact procedures (eg: different extent of lesion in of the corpus callosum).

  
  

  This would've affected the degree to which the 2 hemispheres could relay information between themselves.
• Limitation of this research (L)

  Therefore, any conclusions drawn are representative only of a small group of individuals who had a confounding physical disorder which made the procedure necessary.

  
  

  This reduces the population validity of the study, as the findings can't be generalised to the experience of all split-brain patients - idiographic approach.
• Rogers et al (P)


  A main advantage of brain lateralisation is that it increases neural processing capacity (the ability to perform multiple tasks simultaneously).
• Rogers et al (E)


  Rogers et al found that in domestic chickens, brain lateralisation is associated with an enhanced ability to perform 2 tasks simultaneously (findings food and being vigilant for predators).
• Rogers et al (E)


  Using only 1 hemisphere to engage in a task leaves the other hemisphere free to engage in other functions.
• Rogers et al (E)


  This provides evidence for the advantages of brain lateralisation and demonstrates how it can enhance brain efficiency in cognitive tasks.