Hemispheric lateralisation

• Define hemispheric lateralisation.


  Hemispheric lateralisation refers to the idea that certain functions are governed by one hemisphere of the brain. E.g. the left hemisphere

  specialises in language.
• Outline split brain research as a way of investigating hemispheric lateralisation.


  Sperry’s split brain research studied hemispheric lateralisation by comparing 11 split-brain patients to 11 non-epileptic control participants with intact corpus callosum.

  His tests typically involved presenting participants with a split visual field while asking them to focus on a dot in the centre of the screen.

  An image would then be presented in either their left visual field or right visual field.
• Outline split brain research as a way of investigating hemispheric lateralisation.


  In the ‘describe what you see’ test, all participants had to say what image they had seen in order to investigate which hemisphere was responsible for language i.e. if the split-brain patients could only say what they saw when the image was processed by the left hemisphere, it could be concluded that the left hemisphere is responsible for speech production.
• Outline split brain research as a way of investigating hemispheric lateralisation. 


  In the recognition by touch test, participants were asked to select the object they had seen using their left hand (controlled by right hemisphere) or right hand (controlled by left hemisphere).
• Outline split brain research as a way of investigating hemispheric lateralisation.


  This aimed to investigate which hemisphere controls movement on each side of the body i.e. if they could only select an object processed by the right hemisphere with their left hand, it could be concluded that the left hemisphere controls the right side of the

  body.
• Outline the findings of split brain research


  In Sperry’s describe what you see test, the control group could say what they saw regardless of what hemisphere processed the image.

  In contrast, the split-brain patients could only say what they saw when the image was processed by the left hemisphere.

  This suggests that the left hemisphere is responsible for speech production.
• Outline the findings of split brain research.


  In Sperry’s recognition by touch test, the control group could pick the object with either hand regardless of what hemisphere processed the object.

  In contrast, the split-brain patients could only select the object with the hand opposite to the hemisphere that processed it e.g. if it was processed by the left hemisphere, they could only select it with their right hand.

  This suggests that each hemisphere is responsible for movement on the opposite side of the body.
• Outline the hemispheric lateralisation of the motor cortex.


  The motor cortex (responsible for voluntary motor movement) is located in both hemispheres and is contralateral.

  This means that the motor cortex in the left hemisphere controls voluntary motor movement on the right side of the body while the motor cortex in the right hemisphere controls voluntary motor movement on the left side of the body.
• Outline the hemispheric lateralisation of the somatosensory cortex.


  The somatosensory cortex (responsible for processing touch sensations) is located in both hemispheres and is contralateral.

  This means that the somatosensory cortex in the left hemisphere processes touch sensations on the right side of the body while the somatosensory cortex in the right hemisphere processes touch sensations on the left side of the body.
• Outline the hemispheric lateralisation of the auditory cortex. 


  The auditory cortex (responsible for auditory perception) is located in both hemispheres and is contralateral.

  This means that the auditory cortex in the left hemisphere processes auditory information primarily from the right ear while the auditory cortex in the right hemisphere processes auditory information primarily from the left ear.
• Outline the hemispheric lateralisation of the visual cortex.


  The visual cortex (responsible for visual perception) is located in both hemispheres and is contralateral.

  This means that the visual cortex in the left hemisphere processes visual information in the right visual field while the visual cortex in the right hemisphere processes visual information in the left visual field.
• Outline the hemispheric lateralisation of Broca’s area.


  Broca’s area (responsible for speech production) is lateralised to the left hemisphere.

  This means that the left hemisphere specialises in speech production.
• Outline the hemispheric lateralisation of Wernicke’s area.


  Wernicke’s area (responsible for language comprehension) is lateralised to the left hemisphere.

  This means that the left hemisphere specialises in language comprehension.
• Evaluate split brain research: standardised procedures.


  In the ‘describe what you see test’ all participants were presented with a split visual field and all were asked to focus on a dot in the centre of the screen. All participants were also presented with the same stimuli for the same amount of time. This is a strength because it helped to prevent confounding variables such as eye movement from affecting the results. It also meant that the research was easy to replicate to check the reliability of the findings into hemispheric lateralisation.
• Evaluate split brain research: contradictory findings. 


  Later research found that split brain patients could say what they saw regardless of which hemisphere had processed the image. This contradicts Sperry’s earlier finding that split brain patients could only say what they saw when the image was processed by the left hemisphere.

  This research was conducted many years after the split brain patients underwent surgery so it may be that the brain had the chance to reorganise itself. Split-brain research is only useful if conducted shortly after the patients had their corpus callosum severed.
• Evaluate split brain research: confounding variables. 


  E.g. unlike the controls, split brain patients had a history of epilepsy and drug treatments. This is a limitation because this, rather than the separate functioning of their hemispheres, may have resulted in their abnormal behaviour in the tests.
• Evaluate split brain research: small sample sizes. 


  E.g. Sperry’s research involved 11 patients. It is also difficult to find new participants as the surgical procedure of cutting the corpus callosum is now rarely used as a treatment for epilepsy. This is a limitation because it limits the generalisability and replicability of split brain research.
• Evaluate hemispheric lateralisation: supporting evidence from split brain research.


  E.g. in Sperry’s ‘describe what you see’ test, split brain patients could only say what they had seen if the image was processed by the left hemisphere. This is a strength as it supports the proposal that the left hemisphere specialises in language.
• Evaluate hemispheric lateralisation: supporting evidence. 


  E.g. in Sperry’s ‘recognition by touch’ test, split brain patients could only select the object with the hand opposite to the hemisphere that processed it. This is a strength as it supports the contralateral nature of the motor cortex i.e. how the motor cortex in the right hemisphere controls movement on the left side of the body and vice versa.
• Evaluate hemispheric lateralisation: contradictory findings. 


  Later split brain research found that split brain patients could say what they saw regardless of which hemisphere had processed the image.

  This research was conducted many years after the split brain patients underwent surgery so it may be that the brain had the chance to reorganise itself. This means that while the hemispheres may initially be dominant for a function, the other hemisphere can take on that function if needed.
• Evaluate hemispheric lateralisation: individual differences regarding which function is lateralised to which hemisphere.


  For example, research has found that left-handed people are more likely to have a right hemisphere that specialises in language as opposed to the usual left hemisphere specialisation. In addition, lateralisation changes with age, with most tasks becoming less lateralised in adulthood. This is a limitation because it suggests a degree of flexibility in hemispheric lateralisation.