Hemispheric Lateralisation and Split-brain

• Hemispheric lateralisation


  the idea that the two halves of the brain function differently and certain mental processes and behaviours are mainly controlled by one hemisphere e.g. language.
• Lateralisation of language


  the two main centres of language are located in the left hemisphere(LH) so language is both lateralised and localised.
• functions that aren't lateralised


  Vision, motor and somatosensory areas appear in both hemispheres. The motor area is cross-wired (contralateral), the RH controls movement on the left and the LH controls movement on the right. Vision is both contralateral and ipsilateral. Each eye receives light from the left visual field and right visual field. The LVF of both eyes is connected to the RH and the RVF is connected to the LH. Enabling the eyes to compare perspectives which aid in depth perception.

  
• Split-Brain Research


  Studying people whose connection between the RH and LH (the corpus callosum) has been severed. This was done to reduce epileptic fits. Split-brain research studies how the hemispheres function when they can't communicate with each other.
• Sperry's research procedure


  Eleven split-brain individuals were studied. An image or word would be presented to a participant's RVF and the same or different image/ word would be presented to the LVF. In the 'normal' brain the corpus callosum would share the information, giving a complete picture. However, in a split-brain information cannot be conveyed from one hemisphere to the other.
• Sperry's research findings


  When an image was presented to the RVF they could describe what was seen but not if it was shown to the LVF. Messages are unable to be relayed from the RH to the LH language centres. They could however select a matching object to that shown to the LVF, using their left hand or the object most closely related to the object shown to the LVF. If a pinup picture was shown to the LVF there was an emotional response but the participants would report seeing nothing or just a flash of light.
• Sperry's research conclusion


  The observations show how certain functions are lateralised in the brain and support the view that LH is verbal and RH is 'silent' but emotional.
• Strength of Lateralisation theory

  There is research to show even normal in brains the hemispheres process information differently. Fink et al. used PET scans to identify which areas were active during visual processing tasks. When participants were asked to look at the whole picture, the RH was much more active. When required to focus on a finer detail, 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 normal brain as well as the split-brain.
• Limitation of Lateralisation Theory


  The idea that the LH is the analyser and the Rh the synthesiser may be wrong. There may be different functions but research suggests people do not have a dominant side of their brain which creates a different personality. Neilsen et al (2013) analysed brain scans from over 1000 people aged 7 to 29 and found people used different hemispheres for different tasks. But there was no evidence for dominant sides. E.g. no artist brain vs mathematicians brain. Suggesting the notion of left and right-brained people is wrong.
• Lateralisation versus plasticity


  Lateralisation is adaptive as it enables two tasks to be performed simultaneously with greater efficiency. Rogers et al. showed lateralised chickens could find food while watching for predators but 'normal' chickens couldn't.

  However, neural plasticity could also be seen as adaptive. Following damage through illness or trauma, some functions can be taken over by non-specialised areas in the opposite hemisphere. E.g. language functions can switch sides.
• Recent research support for Split-brain research

  There is support from more recent split-brain research. Luck et al. (1989) showed split brain participants actually perform better than the regular brain in certain tasks. E.g. faster identifying odd ones out in an array of similar objects. In the normal brain, the LH's better cognitive strategies are weakened by the RH. This supports Sperry's findings that the 'left brain' and 'right brain' are distinct.
• Generalisation issues with Sperry's research


  Causal relationships are hard to establish. The behaviour of Sperry's participants was compared to a neurotypical control group. However, none of the control group has epilepsy. This is a major confounding variable. Differences may have been due to epilepsy rather than split brain. This means some of the unique features may have been due to epilepsy.
• Ethics of split-brain research


  The split-brain operation was not performed for the research. So Sperry's participants were not deliberately harmed. Also, all procedures were explained and informed consent was obtained.

  However, the trauma of the operation may have meant participants did not later fully understand the implications of what they agreed to. They were subject to repeat testing over a lengthy period of time and this may have become stressful.