Hemispheric lateralisation and split brain research

• Hemispheric lateralisation

  The brain is lateralised i.e. two sides (hemispheres)
• Localised

  Some functions are localised and appear in both left and right hemispheres

  e.g. auditory, visual, motor, somatosensory areas
• Localised and lateralised

  Two main language centres are in the left hemisphere e.g. broca's area is located in the left frontal lobe and wernicke's area is located in the left temporal lobe.

  Right hemisphere produces rudimentary words but provides emotional context
• Contralateral

  In the motor area, the right hemisphere controls the left side of the body and vice versa
• Contralateral and ipsilateral

  Left visual field of both eyes is connected to the right hemisphere and the right visual field of both eyes is connected to the left hemisphere.

  This enables the visual areas to compare the slightly different perspectives from each eye and aids depth perception.
• Split brain research - Sperry (procedure)


  Split brain = two hemispheres are surgically separated by cutting down the corpus collosum. This is used to treat severe epilepsy to reduce the electrical storm across hemispheres.

  Eleven split brain participants were studied. An image or word projected to the right visual field (processed by the left hemisphere), and same or different image was projected to the left visual field (processed by the right hemisphere). Presenting the image to one hemisphere meant the information could not be conveyed from that hemisphere to another.
• Split brain research - Sperry (findings)


  When an object was shown to the right visual field, the participants could describe what they had seen as the language centres are in the left hemisphere

  When an object was shown to the left visual field, the participants could not name the object but they could select it behind a screen using their left hand, select object associated with the picture

  This demonstrates how certain functions are lateralised in the brain and shows that the left hemisphere is verbal and the right is silent but emotional
• Evaluation

  One strength is evidence of lateralised brain functions in 'normal' brains. PET scans show when 'normal' participants attend to global elements of an image, the right hemisphere is more active. When required to focus on finer detail the specific areas of the left hemisphere tend to dominate. This suggests that hemispheric lateralisation is a feature of the normal brain as well as the split brain.
• Evaluation

  One limitation is lateralisation versus plasticity. Lateralisation is adaptive, enabling two simultaneous tasks with greater efficiency. On the other hand, neural plasticity is also adaptive. After damage to brain, language functions can switch sides. This seems to suggest that lateralisation is first preference but ultimately plasticity is more important as it deals with loss of lateralisation.
• Evaluation - split brain

  One limitation is that casual relationships are hard to establish. In Sperry's split brain research, the behaviour of the participants were compared to a neurotypical control group. However, none of the control group had epilepsy. Any differences between the groups may be due to epilepsy not the split brain. This means that some of the unique features of split brain participants cognitive abilities may have been due to their epilepsy.
• Evaluation - split brain

  One limitation is ethical issues. Sperry's participants were not deliberately harmed and procedures were explained in advance to gain informed consent. However, participants may not have understood they would be tested for many years, and participation was stressful. This suggests that there was no deliberate harm but the negative consequences make the study unethical.