Hemispheric Lateralisation (AQA A Level Psychology)

Hemispheric lateralisation

• The brain is split into two hemispheres:

  • the right hemisphere

  • the left hemisphere 

• The two hemispheres are connected by the corpus callosum, which acts as a bridge between the two hemispheres and allows them to send messages and work together  

• The brain is contralateral 

  • The left side of the body is controlled by the right side of the brain and vice versa

  • A stroke in the left hemisphere will be physically evident on the right side of the body e.g. the right side of the mouth will droop or there may be weakness in the limbs on the right side of the body 

• Hemispheric lateralisation refers to the division of functions between the two hemispheres 

• The left hemisphere 

  • The left hemisphere is generally lateralised (in most people) to the processing of language e.g.

    • Broca's area (speech production) and Wernicke's area (language comprehension) are found in the left hemisphere 

  • A stroke in the left hemisphere is likely to result in impaired speech

• The right hemisphere 

  • The right hemisphere is generally lateralised (in most people) to the processing of visual information, imagination and creativity

  • Spatial information is lateralised to the right hemisphere e.g.

    • one case study of a woman with right hemisphere damage found that she would often get lost even in familiar locations if she wasn't given verbal instructions

Broca's & Wernicke's areas

Broca's area

• Paul Broca (1824- 1880) was a surgeon and neurologist who was particularly interested in speech disorders

• One of Broca's patients was a man named Louis Leborgne, who had suffered from epilepsy throughout childhood 

• Leborgne was hospitalised at the age of 30 and stayed in hospital until he died in 1861 at the age of 51

  • He was transferred to Broca's care toward the end of his life

• Leborgne became known by the nickname 'Tan'

  • This was because his neurological condition meant that he could only produce one spoken word: “Tan”

• Tan suffered from complete paralysis down the right side of his body but he was able to move his left arm and leg with ease

• After Tan died in 1861, Broca performed a brain autopsy

• Broca found a lesion on the left temporal lobe of Tan's brain

  • This was the only damage to the brain; all other areas were intact 

• As the left temporal lobe was the only visible area of damage to Tan's brain, Broca concluded that this was the area that is responsible for speech production

• The term Broca's aphasia is now used as a diagnosis for people who have difficulty producing speech 

Wernicke's Area 

• Around 10 years after the discovery of Broca's area, Carl Wernicke identified a similar language-related problem in which patients were able to speak but were not able to comprehend language

• Wernicke examined the brains of patients who exhibited this specific problem

  • His analysis of their brains revealed that damage to the temporal lobe in the left hemisphere resulted in patients producing speech that was fluent but devoid of meaning

  • The damage in the temporal lobe was close to the auditory area

• The term Wernicke's aphasia is now used as a diagnosis for people who have difficulty understanding language and accessing the correct words

Split-brain research: Sperry

• The corpus callosum connects the left and right hemispheres

  • This connection allows for rapid communication between the two hemispheres 

• A type of epilepsy called tonic close seizures can be treated by severing the corpus callosum in a procedure called a corpus callosotomy

  • The surgery aims to stop erratic electric messages bouncing between hemispheres, reducing the number of seizures

  • Patients report very few side effects from the treatment, however, some have reported that they feel like they are two people inside one body

Sperry's Split Brain Research

• Sperry (1968) studied 11 patients who had a corpus callosotomy as a treatment for their epileptic seizures  

• Sperry's procedure was based on the brain being contralateral

  • Information from the left visual field goes to the right hemisphere and information from the right visual field goes to the left hemisphere

• Sperry projected information into each visual field, controlling which information each visual field accessed

• Each participant was placed in front of a screen and had one of their eyes covered by an eye patch

  • This was done to operationalise the use of either the left or the right visual field

  • A tachistoscope was used to present visual stimuli

• Sperry then sent information to the uncovered eye for a split second as follows:

  • Describe what you see

    • When an image was shown to the participant’s right visual field they could easily describe what was seen (the information had gone to their left hemisphere)

    • When a picture was presented to their left visual field they could not describe it in words, in fact, they often reported that there was nothing there (the information had gone to their right hemisphere)

  • Recognise by touch

    • The participants’ hands were covered and they could not see the objects that were then presented to them

    • Sperry placed a series of objects in the participants’ hands (objects placed in the right hand are processed in the left hemisphere and vice-versa)

    • When an object was placed in the right hand the participant could describe it using speech or writing

    • When an object was placed in the left hand the participant made wild guesses as to what it was and often seemed unaware that they were holding anything, but they could select a matching object from a ‘grab bag’ using their left hand

  • Composite words

    • When two words were presented simultaneously (e.g. key and ring, one word to each visual field), the participant would write the word ‘key’ with their left hand (left-hand goes to right hemisphere linked to the left visual field) and say the word ‘ring’ (right visual field links to left hemisphere)

  • Drawing tasks

    • When participants were asked to draw a picture presented to the right visual field (left hemisphere), the right hand performed worse than the left hand (right hemisphere)

    • When participants were asked to draw a picture presented to the left visual field (right hemisphere), the drawings from the left hand were much better than from the right hand (all the participants were right-handed)

Evaluation of hemispheric lateralisation

Strengths

• This is a well-controlled experiment which involved a range of tasks designed to test hemispheric lateralisation

  • The standardised procedures of each task mean that the research is replicable

  • A replicable procedure can be tested for consistency

  • This means that Sperry's research is high in reliability

• Sperry won the Nobel prize for this research as it provided huge insight into the workings of the brain

  • The findings led many researchers to conclude that severing the corpus callosum causes each hemisphere to acquire its own consciousness

  • This is a strength because it has contributed significantly to understanding the complex and detailed interplay between brain function, cognition and behaviour

Limitations

• Some researchers argue that even in split-brain patients the brain can only produce one consciousness

  • They suggest that the two hemispheres are not separately conscious after surgery

  • Visual perception remains separate, but consciousness is shared across the brain

  • This means that more research should be conducted with split-brain patients to determine what is and is not separate per hemisphere

• The issue with trying to replicate Sperry's research and conduct more studies in this field is that Sperry was working with a small, highly specific sample

  • People who have had a corpus callosotomy represent a restricted population

  • This means that Sperry's findings are difficult to generalise beyond this very limited demographic