4. Hemispheric Lateralisation

• • The brain is lateralised into two hemispheres
  • For some functions, the localised areas appear in both hemispheres
• Hemispheres

  • Language is lateralised as it is in the left hemisphere only
  • The LH is the analyser - analyzes sensory information and relates the body to the sensory environment
  • The RH is the synthesiser - contributes emotional context and only produces rudimentary words and phrases
  • Vision is contralateral and ipsilateral
  • The LVF of both eyes is connected to the RH and the RVF to the LH
  • This allows the visual areas to compare slightly different perspectives from each eye and aids depth perception
• Split brain research - Sperry

  • Procedure - 11 epileptic people had a split brain operation and were shown images projected to their RVF and the same/different to their LVF. In the 'normal brain' the corpus callosum would share the image to both hemispheres but for epileptic people, this is not the case
  • Findings - when an image was shown to the RVF, pps could describe what was seen but could not describe anything when shown to the LVF. They said 'nothing was there'
  • This is because messages cannot be relayed between both hemispheres
• • Findings - although pps could not select an object projected to their LVF, they could select a closely associated object to the word eg. an ashtray in response to a cigarette. If a pinup picture was shown to the LVF there was an emotional reaction eg. a giggle but pps reported seeing nothing
  • Conclusions - these observations show how certain functions are lateralised in the brain and support the view that the LH is verbal and the RH is 'silent' but emotional
• Strength - lateralisation in the connected brain

  • 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 (such as looking at a picture of a whole forest) regions of the RH were much more active. When required to focus in on the finer detail (such as individual trees) the specific areas of the LH tended to dominate
  • This suggests that hemispheric lateralisation is a feature of the connected brain as well as the split-brain
• Limitation - contradicting evidence

  • Jared Nielsen et al. (2013) analysed brain scans from over 1000 people aged 7 to 29 years and did find that people used certain hemispheres for certain tasks but there was no evidence of a dominant side
  • This suggests that despite evidence supporting the lateralisation of function, there is a lack of evidence to draw a correlation between lateralisation and personality traits. For instance, the idea that mathematicians are left brained and artists are right brained is not based on factual evidence.
  • Validity of lateralisation as a theory is challenged
• Strength - lateralisation is adaptive

  • Lesley Rogers et al. (2004) showed that chickens with a lateralised brain could find food while watching for predators whereas chickens reared in the dark, whose brain was not lateralised, could not do this
  • This suggests lateralisation is likely to provide an adaptive function – it would not have evolved and been maintained if it didn’t have some value. It is probably that it allows animals to be able to perform two tasks simultaneously with greater efficiency i.e. it enables multitasking
  • Has an evolutionary advantage which increases validity
• Strength - split-brain research

  • Michael Gazzaniga (Luck et al. 1989) 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 than normal controls. In the normal brain, the LH's better cognitive strategies are 'watered down' by the inferior RH
  • This supports Sperry's earlier findings that the 'left brain' and 'right brain' are distinct and have different and unique functions
• Limitation - generalisation issues with split-brain research

  • The behaviour of Sperry's split-brain participants was compared to a neurotypical control group. An issue though is that none of the participants in the control group had epilepsy. This is a major confounding variable. Any differences that were 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
• Limitation - ethics of Sperry's research

  • The split-brain operation was not performed for the purpose of the research, So, in that sense, Sperry's participants were not deliberately harmed. In addition, all procedures were explained to the split-brain participants and their full informed consent was obtained
  • However the trauma of the operation might mean that the participants did not later fully understand the implications of what they had agreed to. They were subject to repeated testing over a lengthy period (years in some cases), and this may have been stressful over time