Hemispheric lateralisation

• What is brain lateralisation?

  • The two hemispheres are functionally different and certain mental processes and behaviours are mainly controlled by one rather than the other
  • Contralateral wiring: the right brain hemisphere controls the left side of the body and vice versa
  • Ipsilateral wiring: vision is both contralateral and ipsilateral (opposite and same-sided) e.g. the left visual field of both eyes sends info to the right hemisphere and vice versa, enabling depth perception
• What is equipotentiality?

  • Developed by Lashley suggesting that both hemispheres and all areas of the brain are equally able to perform a task and work together
  • Not all functions are lateralised to one hemisphere - more complex cognitive higher processes such as conscious thought and learning often require both
• What is one strength of the hemispheric lateralisation theory?

  • Research support: Fink et al. (1996) used PET scans to identify which brain areas were active during a visual processing tasks
  • When ppts. with connected brains were asked to attend to global elements of an image the right hemisphere was more active, when asked to focus on the finer details, the left hemisphere was more active
  • Suggests that hemispheric lateralisation is a feature of the connected brain as well at the split-brain
• What is one limitation of the hemispheric lateralisation theory?

  • One brain: idea of LH as the analyser and RH as the synthesiser may be wrong
  • Nielsen et al. (2013) analysed brain scans of over 1000 people aged 7-29 and found that people used certain hemispheres for certain tasks but there was no evidence of a dominant side
  • Suggests that the notion of right or left brained people is wrong as there is no dominant side that creates a different personality
• What is a split-brain operation?

  • Surgical separation of the right and left hemisphere by severing the corpus callosum, used to reduce epilepsy
  • During epileptic seizures the brain experiences excessive electrical activity travelling from one hemisphere to the other
• What was Sperry's split-brain research? (1)

  • Sperry (1968) presented 11 split-brain patients using a special set-up where images were shown to their RVFs (processed by LH) and presented to their RVFs (processed by RH) and info could not be shared between hemispheres
  • When an image was presented to the RVF (LH), the participant could describe what they saw but said there was 'nothing there' if this was shown to their LVF (RH) - because their brains are split the RH cannot relay information to the LH's language centres to be able to describe the image
• What was Sperry's split-brain research? (2)

  • Even though participants couldn't give verbal labels to images that were shown to their LVFs (RH), the left hand was able to select an object closely associated with the object presented to their LVF (RH) e.g. would select an ashtray when showed a cigarette
  • Pinup pictures shown to the LVF would elicit an emotional response like a giggle but they would report seeing nothing/ flash of light
  • Supports the idea that certain functions are lateralised - LH is verbal and RH is 'silent but emotional'
• What is one strength of split-brain research?

  • Research support: Gazzaniga (1989) showed that split-brain participants perform better on certain tasks than connected brains - were faster at identifying the odd one out of an array of objects
  • Kingstone et al. (1995) suggested that in normal brains, the LH's better cognitive strategies are 'watered' down by the inferior RH. supporting Sperry's findings that the left and right brain are distinct
• What is one limitation of split-brain research?

  • Generalisation issues: behaviour of Sperry's participants were compared to a neurotypical control group, none of which had epilepsy
  • This confounding variable could mean that any differences between the two groups could be the result of epilepsy rather than the split-brain
  • Means we cannot generalise findings as the unique features of the split-brain ppts. cognitive abilities may have been due to epilepsy, so a cause-effect relationship cannot be established
• What is another strength of hemispheric lateralisation?

  • Increases neural processing capacity: Rogers et al. (2004) found that in a domestic chicken, brain lateralisation is associated with an enhanced ability to perform two tasks simultaneously (finding food and being vigilant for predators)
  • Using only one hemisphere to engage in a task leaves the other hemisphere free to engage in other functions, showing the advantages of brain lateralisation
  • However because this research was conducted on animals, it is impossible to conclude the same of humans
• What is another strength of split-brain research?

  • High control: Sperry’s (1968) procedure was closely controlled
  • Patients were given eye patches, and images were flashed up for a very brief time (fractions of a second), meaning there was no possibility of looking over and using the other visual field
  • This strengthens the internal validity of the studies as we can establish a solid cause-and-effect relationship