The brain is a complex organ, able to perform various functions simultaneously. You may have seen Broca's and Wernicke's areas before, found in the brain's left hemispheres. Famous for their associations with speech production and comprehension, Broca's and Wernicke's areas demonstrate how the brain's hemispheres differ in function and specialise in certain processes. Hemispheric lateralisation of function posits that the brain's two hemispheres specialise in performing different things.
- We will delve into the world of hemispheric lateralisation of function in psychology.
- First, we will define hemispheric lateralisation of function.
- Then, we will explore hemispheric lateralisation in psychology and examine famous studies such as Sperry's (1968) split-brain research into hemispheric lateralisation.
- Finally, we will provide a hemispheric lateralisation evaluation, discussing the advantages of hemispheric lateralisation and its disadvantages.
Hemispheric Lateralisation Definition
Hemispheric lateralisation is the idea that the brain's hemispheres are not entirely alike; they are functionally different. These functions are divided up and performed by the different hemispheres, and the brain's hemispheres specialise in certain functions. For example, Broca's area specialises in speech production in the left hemisphere.
Fig. 1 - The brain has two hemispheres which differ in their dominance of function¹.
The brain has two hemispheres, as we mentioned above. The longitudinal fissure, the brain's most prominent sulcus (groove), separates the two into the right and left hemispheres. Hemispheric lateralisation of function explores how the hemispheres differ in their dominance of function.
Hemispheric lateralisation of function is where cognitive abilities are divided 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.
Communication is integral between the two hemispheres, and they are connected by the corpus callosum (a bridge of nerve fibres that send signals between the hemispheres).
The role of the corpus callosum is particularly relevant when we discuss split-brain research. Splitting the hemispheres causes issues with communication between these two sides of the brain, which interrupts how we process information the hemispheres specialise in.
The brain operates contralaterally; the left side controls the right side of the body, and the right side controls the left side of the body.
Examples of Hemispheric Lateralisation in Psychology
If we study the brain, we can identify the differences in function across the two hemispheres. Two notable examples can be seen in Broca's and Wernicke's areas, which we briefly touched upon before.
Broca’s area, residing in the left hemisphere (specifically the left frontal lobe), is an example of hemispheric lateralisation. When damage occurs in Broca's area, 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. The brain is a complex organ that relies on communication.
The eyes, for instance, require various parts of the occipital lobe to interpret and process visual information, integrating it with other areas of the brain.
Lateralisation: Psychology
Lateralisation in psychology is synonymous with hemispheric lateralisation in that the two hemispheres of the brain are functionally different and, as a result, are not entirely alike. Wernicke's area is a good example of lateralisation in psychology, found in the left hemisphere.
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 'splitting' the brain.
Overall, 11 patients had undergone surgery where their corpus callosum was severed to treat severe epilepsy (commissurotomy). Sperry tested these patients to see if their abilities were affected now that they had separated hemispheres, comparing them to a control group with no disconnected hemisphere.
- We consider the study a quasi/natural experiment, as Sperry did not manipulate the independent variable directly. The dependent variable was their performance in visual and tactile tasks.
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.
Patients covered one eye, and the images were flashed on screens very quickly, so the uncovered eye couldn't move fast enough to compensate.
In split-brain patients, in theory, the information is not transferred to the other hemisphere because the corpus callosum is severed.
Consider the following illustration example. Here, the right and left visual field is presented and shows how information is typically processed in the brain. The right and left visual field process the information contralaterally. In patients without a severed corpus callosum, the hemispheres would communicate, and the key and bone would be present in both hemispheres. With a severed corpus callosum, this would not be the case.
Fig. 2: Split-brain processing visual fields with a key and bone example.
Sperry conducted different tasks:
Visual tests - an image was presented on a screen (tachistoscope) to the left or right visual field, and they were asked to describe what they saw. Participants covered one eye, and the images were presented very quickly so the uncovered eye couldn't move quick enough to compensate. They were asked to look at a fixed point. The left visual field would send information to the right hemisphere and vice versa (remember, both eyes have right and left visual fields processed contralaterally by the brain).
Tactile tests - an object was placed in the right or left hand below the screen, and they were asked to describe what they felt or pick it out of similar objects. Participants could not see the object or their hands below the screen. What is felt in the left hand would be processed by the right hemisphere, and vice versa.
Drawing tasks - patients were given an image on the right or left visual field and asked to draw it.
The results:
- The results indicate for visual tasks that participants could only identify objects again if they were shown to the same visual field as the original one. 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.
- For tactile tests, the resultsshow that patients who held objects in the right hand (processed by the left hemisphere) could describe them verbally or write them down. However, if patients held it in their left hand (processed by the right hemisphere), they could not describe it and often guessed, unaware they were holding anything. If patients held an object in one hand, they could only recognise it again if held by the same hand in the future, but would not recognise it if it was presented to a different hand from the original.
Fig. 3: Results of Sperry's (1968) hemispheric study.
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 impossible to carry out.
Gazzaniga (2000)
Gazzaniga (2000) suggested that the corpus callosum is potentially one of the key components of the ability of the brain to establish hemispheric lateralisation.
This is because:
Over the last 40 years, Split-brain patients 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 of the bilaterally present perceptual system were spared on the opposite side of the brain when language systems in the left hemisphere, for instance, developed.
One hemisphere could continue to perform previous functions whilst the new functions developed on the other hemisphere flourished.
Funnell et al. (2007)
Funnell et al. (2007) investigated the simple calculation capabilities of the two cerebral hemispheres in a split-brain patient in this study.
Throughout 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 with 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 advantages and disadvantages. There are numerous strengths of hemispheric lateralisation, and we will contrast this with the theory's perceived weaknesses.
Advantages of Hemispheric Lateralisation
As we established above, both Gazzaniga (2000) and Funnell et al. (2007) show support for the hemispheric lateralisation of function, demonstrating how the development of the corpus callosum, for instance, allowed for more complex systems to develop without sacrificing original functions.
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.
Disadvantages of Hemispheric Lateralisation
We must also consider the weaknesses and disadvantages of hemispheric lateralisation.
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 of the studies being used to identify the extent to which the hemispheres differ, usually relying on split-brain research (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 (functional magnetic resonance imaging) 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 talk 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 and performed by the different hemispheres of the brain; the hemispheres are specialised in certain functions and are not entirely alike.
- Broca’s and Wernicke's areas, residing in the left hemisphere, are an example of hemispheric lateralisation. They specialise in language.
- Split-brain research involves studies on those who had their corpus callosum severed to treat epilepsy.
- Sperry (1968) 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.
References
- Fig. 1: Diagram of the two brain hemispheres by Polygon data were generated by Database Center for Life Science (DBCLS), CC BY-SA 2.1 JP https://creativecommons.org/licenses/by-sa/2.1/jp/deed.en, via Wikimedia Commons
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