hemispheric lateralisation

• ‘Split-brain’ operation involves severing the connections between the RH and LH, mainly the corpus callosum. This is a surgical procedure used to reduce epilepsy.
• Sperry devised a system to study how two separated hemispheres deal with, for example, speech and vision.
• Sperry’s procedure:

  An image was projected to a participant’s RVF and the same or a different image was projected to their LVF. In a connected brain the corpus callosum would immediately share the information between both hemispheres giving a complete picture of the visual world.
• Sperry’s findings:

  When a picture of an object was shown to a participant’s RVF the participant could describe what was seen. But they could not do this if the object was shown to the LVF-they said there was nothing there. This is because in the connected brain, messages from the RH are relayed to language centres in the LH.
• Sperry’s findings:

  Although participant‘s could not give verbal labels to objects projected to the LVF, they could select a matching object out of sight using their left hand. If a picture was shown to the LVF there was an emotional reaction-e.g. giggle- but participants usually reported seeing nothing or just a flash of light.
• Fink used PET scans to identify which brain areas were active during a visual processing task. When participants with connected brains were asked to attend to global elements of an image regions of the RH were much more active. When required to focus in on the finer details the specific areas of the LH tended to dominate. As far as visual processing is concerned, hemispheric lateralisation is a feature of the connected brain as well as the split-brain.
• Nielsen analysed brain scans from over 1000 people and did find that people used certain hemispheres for certain tasks. But there was no evidence of a dominant side.
• Gazzaniga showed that split-brain participants actually perform better than connected controls on certain tasks. For example, they were faster at identifying the odd one out in an array of similar objects. In the normal brain, the LH’s better cognitive strategies are ‘watered down’ by the inferior RH. This supports Sperry’s findings that ‘left brain’ and ’right brain’ are distinct.
• Sperry’s split-brain participants were compared to a neurotypical control group. However none of the participants in the control group had epilepsy so any differences observed between the two groups may be the result of the epilepsy rather than the split brain.