Hemispheric Lateralisation

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.

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 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.

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

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.

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.

• 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.