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.
• One strength is research showing that even in connected brains the two hemispheres process information differently. For example, Fink et al. 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. This suggests that, as far as visual processing is concerned, hemispheric lateralisation is a feature of the connected brain as well as the split-brain.
• One limitation is the idea that the LH as analyser and RH as synthesiser may be wrong. There may be different functions in the RH and LH, but research suggests people do not have a dominant side of their brain which creates a different personality. Nielsen et al. 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. This suggests that the notion of right- or left-brained people is wrong.
• One strength is support from more recent split-brain research. 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.
• One limitation of Sperry's research is that causal relationships are hard to establish. The behaviour of 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. This means that some of the unique features of the split-brain participants' cognitive abilities might have been due to their epilepsy.
• Hemispheric lateralisation = the idea that the two hemispheres of the brain are functionally different and that certain mental processes and behaviours are are mainly controlled by one hemisphere rather than the other
• In the case of language, the two main centres are only in the LH. So we say that language is lateralised. The RH can only produce rudimentary words and phrases but contributes emotional context. This has led to the suggestion that the LH is the analyser whilst the RH is the synthesiser.
• Many functions are not lateralised. For example vision, motor and somatosensory areas appear in both hemispheres.
• In the case of the motor area the brain is cross-wired - the RH controls movement on the left side of the body whilst the LH controls movements on the right side of the body.
• In the case of vision the situation is even more complex - it is both controlled contralateral and ipsilateral. Each eye receives light from the left visual field and the right visual field. The LVF of both eyes is connected to the RH and the RVF of both eyes is connected to the LH. This enables the visual areas to compare the slightly different perspective from each eye and aids depth perception.