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Hemispheric Lateralisation

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Hemispheric Lateralisation

Just as ears and eyes perform different functions, so do the two hemispheres of the brain. This is known as hemispheric lateralisation. The two hemispheres are connected by the corpus callosum (a bridge of nerve fibres that send signals between the hemispheres).

Hemispheric lateralisation is where cognitive functions are divided up and performed by the different hemispheres of the brain; the hemispheres are specialised to perform certain functions and are not entirely alike. So, one region may be responsible for language, whilst another is responsible for movement, and so on.

The brain operates contralaterally. Essentially, the left side of the brain controls the right side of the body, and the right side of the brain controls the left side of the body; therefore, understanding the core concept of hemispheric lateralisation is essential.

This is particularly relevant when we discuss split-brain research further in this article. Splitting the hemispheres causes issues with communication between these two sides of the brain, which interrupts how we process information the hemispheres specialise in.

Broca’s area, residing in the left hemisphere (specifically the left frontal lobe), is an example of hemispheric lateralisation. When damage occurs in this area in the left hemisphere, speech production is affected. However, speech production remains unaffected when the same place in the right hemisphere is damaged.

Another famous example is Wernicke’s area, located in the upper temporal lobe of the left hemisphere.

Wernicke’s area, like Broca’s, is associated with language. Wernicke’s area is responsible for making speech meaningful, as damage to this area results in fluent but meaningless speech. Patients with damage to this area can often speak with the right tones and inflexions that you would find in a typical, healthy speech pattern, but the content of their words doesn’t make sense.

Damage doesn’t always mean a person cannot understand speech, either, so language comprehension involves different areas of the brain, despite Wernicke’s prominence in meaningful speech production.

Split-brain research into hemispheric lateralisation: Sperry (1968)

Considering the two brain hemispheres’ talk’ to each other through the corpus callosum, Sperry (1968) conducted a study on split-brain patients to see how brain function was affected by the split-brain situation.

These patients had undergone surgery, where their corpus callosum was severed to treat severe epilepsy. Sperry tested these patients to see if their abilities were affected now that they had separated hemispheres.

In this study, Sperry (1968):

  • Wanted to assess the specialised functions of each hemisphere and how they operate without the connection.

  • Presented patients with an image to the left visual field and the right visual field. Each image was processed by the opposite hemisphere: the left visual field by the right hemisphere and the right visual field by the left hemisphere.

  • In split-brain patients, the information is not transferred to the other hemisphere because the corpus callosum is severed.

Hemispheric Lateralisation Split-brain processing StudySmarter Split-brain processing visual fields with a key and bone, Tyler Smith, StudySmarter Originals & Wikimedia Commons

  • In patients without a severed corpus callosum, the hemispheres would communicate, and the key and bone would be present in both hemispheres.

  • Sperry conducted different tasks:

    • Describe what you see - an image was presented to the left or right visual field, and they were asked to describe what they saw.

    • Tactile tests - an object was placed in the right or left hand, and they were asked to describe what they felt or pick it out of similar objects.

    • Drawing tasks - patients were given an image on the right or left visual field and asked to draw it.

The results:

Hemispheric Lateralisation Sperry hemispheric deconnection StudySmarterResults of Sperry (1968) hemispheric deconnection, Tyler Smith - StudySmarter Originals

Participants who had seen an image in the right visual field (processed by the left hemisphere, important in speech production) could say what they saw or write it down. If it was presented to the left visual field, they could not say what they saw; however, they could point it out if given the opportunity.

The results of the study suggest that overall:

  • People with split brains have two separate visual inner worlds, each interpreting visual images.

  • There’s a lack of communication/cross-integration - one hemisphere does not know what the other is doing.

  • There seem to be two streams of consciousness, each with memories, perceptions, and impulses.

The conclusions indicate that the left hemisphere is dominant in speech production and language, whilst the right hemisphere is dominant in visual-motor tasks. The disconnection of the hemispheres results in the inability to communicate information, and functions are inhibited or outright impossible to carry out.

For instance, a patient cannot tell you what they saw in their left visual field, as it cannot be communicated to the left hemisphere for language and speech production.

Gazzaniga (2000)

Gazzaniga (2000) suggested that the corpus callosum (the section of nerve fibres that connect the brain’s two hemispheres) is potentially one of the key components of the ability of the brain to establish hemispheric lateralisation.

This is because:

  • Split-brain patients over the last 40 years allowed for deeper insights into functional areas of the brain.

  • The corpus callosum potentially enabled the development of specialisation and lateralisation.

  • For instance, language functions are localised to the left hemisphere. Initially, when these language functions began to take over sections of the brain to become more complex, the areas taken over would usually lose their original functions that existed there before. They would now be lost at the cost of this new development.

  • However, the corpus callosum served as a great communication link between these systems. Critical features were spared in the opposite side of the brain!

  • One hemisphere could continue to perform previous functions whilst the new functions developed on the other hemisphere flourished.

Funnell et al. (2007)

In this study, Funnell et al. (2007) investigated calculation capabilities of the two cerebral hemispheres in a split-brain patient:

  • Over the course of four experiments, the left hemisphere performed better than the right (confirming reports it is specialised for calculation).

  • In two recognition paradigms, the right performed at chance (it could be due to chance) for all arithmetic operations.

  • In the recall paradigm, the right performed above chance at addition and subtraction but performed at chance for multiplication and division.

  • The right can make approximate guesses when unable to get the right solution.

  • The right hemisphere is more accurate in addition and subtractions with small operands than large operands.

  • The left was equally accurate in approximate and exact addition, whereas the right was more accurate in approximate than exact addition.

The study overall highlights the separate differences in the capabilities of each hemisphere, supporting the idea of hemispheric lateralisation with this evidence.

Evaluation of hemispheric lateralisation and split-brain research

Research into hemispheric lateralisation and the split-brain will be evaluated for its strengths and weaknesses. There are numerous strengths of hemispheric lateralisation, and we will contrast this with the theory's perceived weaknesses.

Strengths

When it comes to the advantages of hemispheric lateralisation and split-brain research, we need to highlight the following:

  • Increase in neural processing capacity: By separating functions and allowing hemispheres to specialise in tasks, it frees the other hemisphere up to do other tasks.

    • Rogers et al. (2004) found that, in domesticated chickens, lateralisation allowed for enhanced ability to perform two separate tasks simultaneously: looking for food and watching for predators, suggesting lateralisation increases brain efficiency in cognitive tasks that demand simultaneous but different hemispheric attention.

    • Rogers (2002) also found that hemispheric lateralisation (specifically behavioural, as referenced above) exists in all other vertebrates with similar needs, so they have individual limb and sensory focuses. Prey capture and foraging are allocated to the left hemisphere, and responses to predators and new stimuli are allocated to the right hemisphere. The right specialises further in expressing fear and aggression.

  • Overall, hemispheric lateralisation exists in many animals for survival reasons and is corroborated by studies.

Weaknesses

  • Research carried out on animals: Due to the studies assessing lateralisation in animals in some cases, we can not definitively say the research applies to humans.

  • Split-brain procedures are rarely carried out now: Different and better treatment options have replaced the need for split-brain surgeries to be carried out. This reduces the population of people to be studied for this phenomenon.

  • Hard to generalise: Due to the idiopathic nature/approach (findings are focused on the individual and cannot be generalised to the population), it is hard to apply the results to everyone.

  • Lateralisation decreases with age: Szaflarski et al. (2006) found in their fMRI study on language lateralisation that a dominant hemisphere’s control increases from ages 5 to 20 years, plateaus (flattens out/remains steady) from ages 20 to 25 years, and decreases between 25 and 70 years. As we age, the brain may allocate tasks to different regions to compensate for declining functioning abilities.

  • Contradicting claims of the right hemisphere: Sperry stated that the right hemisphere was incapable of processing basic language. The case study of JW found that he had developed the ability to speak using the right hemisphere, he could speak about the information given to both sides of the brain/visual fields (Turk et al., 2002). This evidence shows that the brain can adapt.

Hemispheric Lateralisation - Key takeaways

  • The brain is separated into two hemispheres, connected by the corpus callosum (a bridge of nerve fibres that send signals between the hemispheres). Lateralisation is the idea that functions are divided up and performed by the different hemispheres of the brain; the hemispheres are specialised in certain functions and are not entirely alike.
  • Broca’s area, residing in the left hemisphere, is an example of hemispheric lateralisation.
  • Split-brain research involves studies on those who had their corpus callosum severed to treat epilepsy.
  • Sperry¹ found that people with split brains have two, separate visual inner worlds, there’s a lack of communication/cross-integration, and there seem to be two streams of consciousness, each with its memories, perceptions, and impulses.
  • Hemispheric lateralisation has some advantages, being that it frees up space for hemispheres to specialise in certain tasks simultaneously. However, most studies are on animals and cannot be translated to humans, split-brain procedures are no longer carried out, and the brain can adapt so hemispheric lateralisation may not be as pertinent.

Frequently Asked Questions about Hemispheric Lateralisation

Hemispheric lateralisation is the idea that functions are divided up and performed by the different hemispheres of the brain; the hemispheres are specialised in certain functions and are not entirely alike.

No, they are similar but have specialised areas and are therefore not identical, both in function and structure.

Split-brain patients see similarly to healthy patients, it is the inner communication between hemispheres that changes how the images are interpreted by the brain. So although the images are ‘seen’, they are not communicated and cannot be perceived properly.

He found that people with split brains have two separate visual inner worlds, each with its interpretation of visual images. There’s a lack of communication/cross-integration - one hemisphere does not know what the other is doing. There seem to be two streams of consciousness, each with its memories, perceptions, and impulses.

It allows for an insight into how the brain communicates and how functions are divided up by the hemispheres, showing how the brain is adapted to specialise and increase the efficiency of cognitive functions.

Final Hemispheric Lateralisation Quiz

Question

What connects the two hemispheres of the brain?

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Answer

The corpus callosum. 


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Question

What is hemispheric lateralisation? 


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Answer

Hemispheric lateralisation is the idea that functions are divided up and performed by the different hemispheres of the brain; the hemispheres are specialised in certain functions and are not entirely alike.

Show question

Question

What area of the brain is a good example of a localised function? 


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Answer

Broca’s area is localised to the left hemisphere.

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Question

What did patients suffer with before their corpus callosum was severed in Sperry's experiment? 


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Answer

Severe epilepsy.

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Question

What three tasks did Sperry have patients do?

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Answer

The patients had to describe what they saw, tactile tests, and draw what they saw.

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Question

What happens when the corpus callosum is severed? 


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Answer

Information is not shared between the right and left hemispheres.

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Question

If an image was presented to the left visual field, what did patients struggle in doing? 


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Answer

Patients were unable to say what they had seen, as the language area of the brain had not been given the information of what the right hemisphere had seen through the left visual field.

Show question

Question

What did Sperry find?

Show answer

Answer

He found that people with split brains have two, separate visual inner worlds, each with its interpretation of visual images. There’s a lack of communication/cross-integration - one hemisphere does not know what the other is doing. There seem to be two streams of consciousness, each with its own memories, perceptions, and impulses. 

Show question

Question

What did Sperry conclude? 


Show answer

Answer

The conclusions indicate that the left hemisphere is dominant in speech production and language, whilst the right hemisphere is dominant in visual-motor tasks.

Show question

Question

Name one strength of hemispheric lateralisation? 


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Answer

It increases neural capacity, allowing for each hemisphere to specialise and attend to tasks simultaneously.

Show question

Question

Name a weakness of hemispheric lateralisation?

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Answer

Lateralisation decreases with age / Research carried out on animals / Split-brain procedures are rarely carried out now / Hard to generalise / Contradicting claims of the right hemisphere 

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Question

What did Rogers et al. (2004) chicken study find? 


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Answer

Chickens with lateralisation enhanced their ability to find food and watch out for predators simultaneously. 


Show question

Question

What did Szaflarski et al. (2006) find?

Show answer

Answer

In fMRI studies on language lateralisation, a dominant hemisphere’s control increases from ages 5 to 20 years, plateaus (flattens out/remains steady) from ages 20 to 25 years, and decreases between 25 and 70 years.

Show question

Question

What did Turk et al. (2002) find?

Show answer

Answer

In the case of JW, they found that he had developed the ability to speak using the right hemisphere, so he could speak about the information given to both sides of the brain/visual fields. 


Show question

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