Split-brain research into hemispheric lateralisation

• hemispheric lateralisation concerns behaviours controlled by just one hemisphere.
• example of hemispheric lateralisation?

  language- it is usually controlled by the left hemisphere
• example of split-brain research:

  sperry sought to demonstrate that the two hemispheres were specialised for certain functions and could perform tasks independently of one another

  normally the hemispheres are connected by the corpus callosum and a few other structures. a commissurotonomy is an operation to cut the corpus callous and is sometimes performed to control epileptic seizures

  speedy studied a group of epileptics who had the operation
• sperry devised a unique procedure to test his split-brain patients

  an image or word is projected to a patient's RVF (processed by the LH) and another image to the LVF (processed by RH)

  in the normal brain, the corpus callosum 'shares' information between both hemispheres. in the split brain, the information cannot be conveyed from the chosen hemisphere to the other
• describing what was seen:

  object shown to:

  RVF= patient easily describes what is seen

  LVF= patient says there's nothin there

  can't describe objects in LVF because RH usually lacks language centres. messages received by RH and normally relayed via the corpus callosum to language centres in LH.
• Recognition by touch:

  objects shown to LVF

  Could not name them but could select a matching object using left hand (connected to RH received information from LVF)

  Left hand could also select an object that was associated with image presented to the LVF (e.g. ashtray selected in response to picture of cigarette)

  in each case, the person couldn't verbally identify what they had seen (because the LH is needed for this) but could 'understand' what the object was (using the RH) and select the corresponding object
• Composite words:

  two words presented on either side of the visual field (e.g. 'key' presented to the left and 'ring' to the right) patient:

  • selects 'key' with left hand (LVF goes to RH linked to left hand)
  • says the word 'ring' (RVF linked to LH with speech centres)
• Matching faces:

  composite picture made up of two different halves of a face was presented (one half to each hemisphere):

  • LH dominated the verbal description
  • RH dominated the selection of a matching picture
• strength of split-brain research: shows lateralised brain functions

  the left hemisphere is analytical and verbal ('the analyser') and the right is adept at spatial tasks and music ('the synthesiser'). the right hemisphere can only produce basic words and phrases but contributes emotional content to language. recent research suggests this distinction may be too simplified and several tasks associated with one hemisphere can also be carried out by the other
• Strength: methodology Sperry used

  sperry's carefully standardised procedure of presenting visual information to one hemispheric field at a time was quite ingenious. participants stared at a fixed point with one eye. an image was flashed up for 0.1 seconds, so the patient had no time to move their eyes over the image and spread the information across both sides of the visual field or both sides of the brain. this allowed sperry to vary aspects of the basic procedure and ensure only one hemisphere received information at a time- a very useful and well-controlled procedure
• limitation: issues with generalisation in relation to sperry's work

  many researchers have said these findings cannot be widely accepted, as split-brain research patients are such an unusual sample of people. only 11 patients took part in all variations and all had a history of seizures. this may have caused unique changes in the brain that influenced the findings. this limits the extent to which the findings can be generalised to normal brains, reducing the validity of the conclusions.
• limitation: differences in hemispheric functions may be overstated

  a legacy of sperry's work is a growing body of pop-psychological literature that oversimplifies and overstates the difference in function between the two hemispheres. modern neuroscientists agues these distinctions are not at all clear-cut. many behaviours that are typically associated with one hemisphere can be performed by the other when situations require. the apparently flexibility of the two hemisphered suggests some of the conclusions drawn by sperry many be too simplistic.