hemispheric lateralisation

• hemispheric lateralisation - the assumption that two hemispheres of the brain are functionally different and certain mental processes and behaviours are mainly controlled by one hemisphere rather than the other

  • e.g. language is mainly controlled by the left hemisphere
• • the left hemisphere produces language via language centres (analyser)
  • the right hemisphere places emotional context behind what is said (synthesiser)
• some processes, such as vision, are not lateralised but the brain is contralateral - left controls right, right controls left

  • left visual field connected to right hemisphere
  • right visual field connected to left hemisphere
• split brain research - a series of studies involving people with epilepsy having surgical separation of the two hemispheres of the brain to reduce the severity of their epilepsy, which enabled researchers to test lateral functions of the brain in isolation
• the corpus callosum is a bundle of nerve fibres that join both hemispheres of the brain and allows them to communicate
• sperry's split brain experiment:

  • quasi - natural IV, controlled environment
  • 11 participants
  • all epileptic who had already had their corpus callosum split
• sperry's procedure:

  • participant gazes at fixation point on a screen
  • an image is projected either side of the fixation point into either the RVF or LVF
  • in a normal brain the corpus callosum shares the information from the visual fields between both hemispheres and creates a complete picture of the visual world
• sperry's findings:

  • if image was shown in left visual field, they couldn't describe it - it wasn't there
  • no language centres in the right side of the brain where the information is processed
  • could use left hand to find the object that matched or was associated with the image, when behind a screen
  • right side of brain controls left side of body
• strength - even in connected brains the two hemispheres process information differently

  • Fink et al - PET scans to identify active areas of the brain during visual processing task
  • when participants with connected brains were asked to look at the picture of whole forest, the right hemisphere was active
  • when asked to look at individual trees (finer detail) the left hemisphere was most active
  • suggests that, in terms of visual processing, hemispheric lateralisation is a feature even in connected brains
• limitation - idea of left side being analyser and right being synthesiser may be wrong

  • research suggests that people don't have a dominant side of the brain which determines personality
  • Nielsen et al - analysed brains of 1000+ 7-29 year olds and found people use certain hemispheres for certain tasks but there was no dominant side e.g. no 'artist brain'
  • suggests the notion of 'left-brained' and 'right-brained' people is wrong
• strength - research support from recent split brain studies

  • Gazzaniga - split brained participants performed better than connected control participants in some tasks, e.g. identifying the odd one out
  • in connected brain the left hemispheres better cognitive strategies are dampened by the inferior right hemispheres
  • supports Sperry's claim that 'left-brain' and 'right-brain' are distinct
• limitation - causal relationships hard to establish

  • Sperry's participants compared to neurotypical control group, none of which had epilepsy
  • confounding variable - differences between the two groups could be due to epilepsy not split brain
  • some features of split brain group may have actually been a result of their epilepsy