Lateralisation

• Hemispheric lateralisation


  The idea that each hemisphere (half) of the brain is mainly responsible for certain behaviours, processes and activities

  
• This is in contrast with the holistic theory of brain function, which suggests that function is distributed across the whole brain (i.e. is global)

  
• The right hemisphere controls the left side of the body, and vice versa
• Information which we receive from the left visual field is processed by the right hemisphere, which then coordinates a response to affect the left side of the body

  
• Sperry and Gazzaniga (1968) split-brain research

  1. Conducted on 11 epileptic patients

  2. Underwent surgical lesioning of the corpus callosum (cerebral commissurotomy)

  3. Information processed by one hemisphere cannot be relayed to the other hemisphere

  
• Procedure of split-brain research

  1. One eye covered

  2. Stimulus word flashed onto screen for 1/10 second

  3. Prevented both visual fields being exposed

  
• If the stimulus word was exposed to the right visual field

  It would be processed by the left hemisphere and the patient would say the word
• If the stimulus word was exposed to the left visual field


  It would be processed by the right hemisphere and the patient would write the word using their left hand
• The patient would not be able to give a verbal description of the word, because the right hemisphere contains no language centres
• The right hemisphere appeared to dominate the ability to match a list of faces to a given stimulus
• If two words were presented at the same time, each to one of the visual fields, the patient would say the word presented to the right visual field and write down the word presented to the left visual field
• If an object was placed into the patient's right hand, they would be unable to identify that it is there, because the information would be processed by the left hemisphere which only has language centres, and no visuo-spatial centres
• If an object was placed into the patient's left hand, they would be able to identify the object and choose a similar one from a hidden bag, due to the action of the visuo-spatial centres
• Split-brain research was pivotal in establishing the differences in functions between the two hemispheres, and so opposing the holistic theory of brain function

  
• The left hemisphere was demonstrated as being dominant for language tasks, due to containing language centres, whereas the right hemisphere was demonstrated as being dominant for visuo-spatial tasks
• This suggests that the left hemisphere is the analyser, whereas the right hemisphere is the synthesiser, and so there are marked differences between the two
• Pucetti (1980) criticised Sperry and Gazanigga's work by pointing out that "visual stimuli impinging on the left half of each eye's retina do not go to the right, but to the left cerebral hemisphere (and vice versa), since the retina is concave and each half retina receives light from the contralateral side of the body"

  
• It is important not to jump to conclusions and to appreciate that, through recruitment of homologous areas on the opposite side of the brain, each hemisphere is not restricted to specific functions