Hemispheric Lateralisation and Split Brain Research

• The brain is divided into 2 hemispheres.

  The left / right is responsible for

  • logic / personality
  • math / creativity
  • language / intuition
  • reading / music
  • writing / art
  • analysis / spatial abilities
• Lateralisation: the dominance of one hemisphere of the brain for particular physical and physiological functions.

  
  

  Cerebral Cortex: outer layer of both hemispheres about 3mm thick. This separates us from animals; we have a more developed brain.

  
  

  General Rule: activity in the left side of the body is controlled by the right hemisphere and the right side of the body is controlled by the left hemisphere.
• Motor Cortex :

  • located in frontal love
  • is responsible for voluntary motor movements and speech production
• Somatosensory Cortex

  • located in the parietal lobe
  • is responsible for processing sensory information (touch, heat, pressure)
• Visual Cortex

  • located in the occipital lobe
  • is responsible for processing visual information
• Auditory Cortex

  • located in the temporal lobe
  • is responsible for language comprehension, speech based analysis and memory
• Broca’s Area

  • located in the back left of the frontal lobe
  • is responsible for speech production
  • if this area is damaged, it will cause difficulties in speech production - Broca’s Aphasia
• Wernicke’s Area

  • located in the back left of the temporal lobe
  • is responsible for speech understanding and language comprehension
  • if this area is damaged. it will cause difficulties in comprehension.
• The brain is lateralised - split into two hemispheres. Some functions such as the auditory, visual, motor and somatosensory areas are localised and so appear in the left and right hemisphere.
• The two main language centres Broca's (in frontal cortex ) Werniche's (in the temporal cortex) are in the left hemisphere. The right hemisphere produces rudimentary words but provides emotional context.

  LH= analyser RH=synthesiser. In the motor area the LH controls the right side of the body and vice versa (crosswired). The left visval field of both eyes is connected to the RH and the right visual feld is connected to the LH. This enables the visual areas to compare the slightly different perspective from each eye and aids depth perception. Auditory areas have the same arrangement.
• +Support from 'normal brains‘: PET scans show when participants attend to global events of an image, the RH is more active, when focusing on finer details, the LH is more active - (Fink). This suggests hemispheric lateralisation is a feature of the normal brain as well as the split-brain.
• -Analyser vs synthesiser is wrong: different functions are carried out in the LH vs the RH but people don't tend to have a dominant side, creating a different personality. Nielson analysed 1000 brain scans, finding people use different hemispheres for certain tasks but found no dominance of one side. This suggests the notion of left vs right brained people is wrong.
• +Increases neural processing capacity: Rogers found that (in domesticated chickens) brain lateralisation is associated with an enhanced ability to perform two tasks simultaneously -using only one hemisphere to engage in a task leaves the other hemisphere free to engage in other functions. This provides evidence for the advantage of brain lateralisation and how it can enhance brain efficiency in cognitive tasks.
• -Criticism of increasing neural processing chicken study

  Research was carried out on animals not humans. It’s rarely carried out now due to a lack of patients and the research is flawed. None of the control group in Sperry's research had epilepsy so it’s not generalisable.
• Split brain Research

  Sperry surgically removed the two hemispheres by cutting the corpus callosum. It was used to test severe epilepsy to reduce the ‘electrical storm‘ across hemispheres. 11 split brain participants were studied: an image or work were projected into the right visual field or the left visual field where it would be processed by the opposite side of the brain. Presenting the information to only one hemisphere meant that it couldn’t be conveyed to the other.
• Sperry Split Brain Research

  If an object was shown to the right visual field

  • the participant can only describe what they see (language centre is in left hemisphere

  If an object was shown to the right visual field

  • the participant can’t name the object (no language centres in right hemisphere)
  • participant can select a matching object behind the screen using their left hand
  • can select an object closely associated with the picture
• Sperry Split Brain Research

  A pinup picture was shown to the left visual field and the participant laughed but reported seeing nothing. This demonstrates how certain function are lateralised in the brain.

  Left hemisphere is verbal

  Right hemisphere is ‘silent’ but emotional
• +Hemispheric lateralisation is adaptive:

  Only lateralised chickens better scavenged for food while watching for predators. However, plasticity is also adaptive; Holland found that following damage to the brain, the language function can switch.
• +Luck showed that slit brain participants are better than normal controls. They’re twice as fast at identifying the odd one out in an array of similar objects. In the normal brain, the left hemisphere‘s superior processing abilities are ‘watered down’ by the inferior right hemisphere - Kingstone. This supports Sperry’s earlier findings that the ’left brain‘ and ‘right brain’ are distinct in terms of functions and abilities.
• -Causal relationships are hard to establish;

  In Sperry’s research, the behaviour of the split brain participants was compared to a neurotypical control group. However, none of the control group had epilepsy. Any differences between the groups may be due to epilepsy, not the split brain- confounding variable. This means that some of the unique features of the split brain participants cognitive abilities might have been due to their epilepsy.
• +Sperry’s participants weren’t deliberately harmed and procedures were explained in advance gain informed consent. However, participants may not be realised they would be tested for many years. This suggests there was no deliberate harm but the negative consequences make them unethical.