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Lateralization of Brain Function

By Olivia Guy-Evans, published May 18, 2021


Lateralization of brain function is the view that functions are performed by distinct regions of the brain. For instance, it is believed that there are different areas of the brain that are responsible for controlling language, formulating memories, and making movements.

If a certain area of the brain becomes damaged, the function associated with that area will also be affected.

It contrasts with the holistic theory of the brain, that all parts of the brain are involved in the processing of thought and action.

The human brain is split into two hemispheres, right and left. They are both joined together by the corpus callosum, a bundle of nerve fibres which is located in the middle of the brain.

Hemispheric lateralization is the idea that each hemisphere is responsible for different functions. Each of these functions are localized to either the right or left side.

The left hemisphere is associated with language functions, such as formulating grammar and vocabulary, and containing different language centres (Broca’s and Wernicke’s area).

The right hemisphere is associated with more visuospatial functions such as visualization, depth perception, and spatial navigation. These left and right functions are the case in the majority of people, especially those who are right-handed.

The brain contains cortices such as the visual, motor, and somatosensory cortices. These cortices are all contralateral, meaning that each hemisphere controls the opposite side of the body.

For example, the motor cortex in the left hemisphere controls the muscle movements of the right arm and leg. Likewise, damage to the right occipital lobe (responsible for vision) can result in loss of sight in the left field of vision.


Language Lateralization

Hemispheric lateralization is the idea that both hemispheres are functionally different and that certain mental processes and behaviors are mainly controlled by one hemisphere rather than the other.

There is evidence of some specialization of function mainly regarding differences in language ability. Beyond that, however, the differences that have been found have been minor. What we do know is that the left hemisphere controls the right half of the body, and the right hemisphere controls the left half of the body.

Broca's area and Wernicke's area in the brain

Broca’s Area

Paul Broca was a French physician and was one of the earlier advocators for the ideas of lateralization of brain function. In 1861, Broca met a patient who he would refer to as ‘Tan’.

At the time, there was a lot of debate as to whether there was localization of function within the brain or if the whole brain was utilized in the performance of every function.

Broca described the patient ‘Tan’ who was named this due to this being the only word they could say. Often this patient would repeat the word twice, saying ‘Tan Tan’.

When ‘Tan’ died, a post-mortem of his brain revealed that there was damage to a part of his left frontal cortex. Broca found that other patients with similar problems to Tan had damage to the same region.

It was concluded that the damage to this region, then given the name ‘Broca’s area’ was the reason for Tan’s language problems. Broca’s area is believed to be located in part of the inferior frontal gyrus in the frontal lobe, on the left side of the majority of people.

This research is large support for the view that the role of language function is localized to the left hemisphere of the brain. Broca’s area has been found to be associated with multiple language functions including language comprehension and being able to articulate words.

This region is also associated with listening as being able to understand words requires articulating them in your head. It has also been suggested to be active during the planning of movements, initiating movement and understanding another’s movement.

Broca’s area may also contain mirror neurons as this area appears to be involved in observing people and imitating them (Amunts & Hari, 2005).

The term Broca’s Aphasia was used to describe the condition of Tan and Broca’s other patients. People who have damage to Broca’s area tend to have suffered brain injury (e.g. through a stroke) which then affects this language area.

The main symptom of Broca’s aphasia is a deficit in the production of language, both spoken and written. A person with damage to this area would likely be unable to articulate words or be able to string a coherent sentence together.

Speaking in an abnormal tone or rhythm can also be a symptom of this damage, as well as speech being repetitive, disordered grammar and a disordered structure of individual words.

Finally, damage can also result in transcortical motor aphasia, meaning the speech is non-fluent and often limited to two words at a time.

Wernicke’s Area

A few years after Broca’s discoveries, in 1876, German neurologist Carl Wernicke identified another region of the brain associated with language.

Wernicke identified that some of his patients were able to speak but were not able to actually comprehend language. When examining the brains of these patients, it was revealed that there were lesions at a junction of the upper temporal lobe in the left hemisphere.

This region was named Wernicke’s area and was described as being an area where heard and seen words are understood as well as words being selected for articulation.

This area also works together with Broca’s area. Wernicke’s area comprehends the language and chooses words, which are then sent to Broca’s area to be articulated.

Wernicke’s area contains motor neurons which are involved in speech comprehension and is surrounded by an area called Geschwind’s territory.

When a person hears words, Wernicke’s area associated the sounds to their meaning, to which neurons in Geschwind’s territory are thought to help by combining the many different properties of words (such as the sound and meaning) to provide fuller comprehension.

When a person speaks, however, this process happens in reverse as Wernicke’s area will find the right words to correspond to the thoughts that are to be expressed.

The term Wernicke’s Aphasia was coined to describe damage to Wernicke’s area. This is often thought to be damaged via head trauma or disease.

People who experience Wernicke’s aphasia may experience symptoms such as an inability to understand spoken language and speaking using inappropriate words.

Their sentences may not make sense, they may repeat words, make up meaningless words or their sentences lacking any meaning.

The majority of the time, people with Wernicke’s aphasia will often speak fluently, in comparison to Broca’s aphasia where language is non-fluent or broken up.

Some patients may not even be aware that they have an issue with their speech and will believe they are speaking normally.


Research Studies

Split-Brains

The two hemispheres of the brain are joined together by the corpus callosum, which is a thick bundle of millions of nerve fibers.

As an outdated treatment for severe epilepsy, the corpus callosum was sliced, meaning the connections between the two hemispheres was halted.

People who undergo this procedure are known as split-brain patients. In the 1960’s neurobiologist Roger Sperry, conducted experiments on these split-brain patients to test whether there was localization of function in the hemispheres.

Sperry conducted many split-brain experiments, one being the ‘divided field experiment’. An example of this experiment would be to project words on the right and left fields of vision whilst one eye is covered, to test for whether the patients would be able to say the word.

They found that the patients were able to say the word which was presented on the right visual field, which is controlled by the left hemisphere and contains the language centers. The words presented on the left side, controlled by the right hemisphere, could not be spoken.

However, the patients would instead be able to draw the word that was shown on the left side, or pick up the object of the word shown, due to the right hemisphere being able to control motor movements of the left hand.

When asked why the patients choose or drew the objects, they were unable to say, suggesting that the right hemisphere (in most people) is unconscious although the information it holds can affect behavior.

Another study by Gazzaniga (1983) conducted a similar experiment but with the used of faces projected to both visual fields. It was found that faces on the left visual field, thus projecting to the right hemisphere, were recognised, but not through the right visual field to the left hemisphere.

This demonstrates that the right hemisphere may be better at recognizing faces in general.

Language Lateralization

Although it is known that the lateralization of language functions is in the left hemisphere in the majority of people, this lateralization may be dependant on personal handedness.

Szaflarski et al., (2002) used functional magnetic resonance imaging (fMRI) on individuals who were left hand dominant, whilst they completed language acquisition and non-linguistic tasks.

It was found through the fMRI that there was more activation in the right hemisphere of the participants, concluding that they had typical language dominance.

There is a question of whether lateralization of language function is something that occurs from birth, or if this lateralization develops over time.

Olulade et al., (2020) aimed to study the lateralization of language development through using fMRI on children and adults completing language-based tasks.

The researchers found that in the youngest of children (aged 4-6 years old), there were left and right hemispheric activation, so language was not lateralized to one hemisphere.

They also found that right-side activation significantly decreased with age, with over 60% of adults lacking any considerable right activation.

This study suggests that lateralization of language to predominately the left hemisphere, develops over time during childhood.

Emotion lateralization

A review of literature investigating the lateralization of emotion in the brain found that the left and right hemispheres have different functions regarding emotions (Silberman & Weingartner, 1986).

It was suggested that the right hemisphere is better at controlling emotional expressions, recognising emotions, and is associated with feelings of negative emotions.

Whereas the left hemisphere was specialised in dealing with positive emotions. This implies that different functions of emotion and lateralized to each hemisphere.

In support of this view, another study found that patients who had suffered trauma to their left frontal lobe, particularly their prefrontal cortex experienced depression as a result (Paradiso et al., 1999).

Similarly, patients who had suffered damage to their right frontal lobes were found to be more likely to show signs of inappropriate cheerfulness and mania (Starkstein et al., 1989).

This supports the idea that the left hemisphere is lateralized to positive emotions and the right is lateralized to negative emotions.

Gender Differences

There are several studies which support the notion that there are differences in the lateralization of function in the brains of males and females.

Tomasi and Volkow (2012) found that males had increased right lateralization of connectivity in areas of the temporal, frontal, and occipital cortices, whereas females had increased left lateralization of connectivity in the left of the frontal cortex.

It is suggested that differences in the lateralization of males and females’ brains may underlie some of the typical gender differences in cognitive styles.

For instance, females’ typical linguistic advantage over males may reflect increased left lateralization of language areas. In contrast, males’ typically advantage of visuospatial skills may reflect increased lateralization of right-side visuospatial areas (Clements et al., 2006).

Reber and Tranel (2017) reviewed studies of brain differences in males and females and found a lot of evidence that there was sex-related difference in an area of the brain called the ventral-medial prefrontal cortex (vmPFC), an area associated with decision-making and emotion.

Tranel et al., (2002) found that male patients who had damage to their right vmPFC showed deficits in social, emotional, and decision-making skills than those with left-side damage.

However, the only female patient with right vmPFC damage displayed fewer deficits in all behavioral categories. This evidence implies that lateralization of higher cognitive functions is dependant on the sex of the individual.

Phineas Gage (1848)

The theory of brain localisation is supported by the famous case study of Phineas Gage (1848) who was an America railway construction foreman. During an accident a large iron rod was driven completely through his head, destroying much of his brain's left frontal lobe.

He survived the accident but his personality changed, he became unstable and is reported not to have been able to hold down a job. This supports the localisation of functions theory as it shows that control of social behaviour is located in the frontal cortex.

About the Author

Olivia Guy-Evans obtained her undergraduate degree in Educational Psychology at Edge Hill University in 2015. She then received her master’s degree in Psychology of Education from the University of Bristol in 2019. Olivia has been working as a support worker for adults with learning disabilities in Bristol for the last four years.

How to reference this article:

Guy-Evans, O. (2021, May 18). Lateralization of brain function . Simply Psychology. https://www.simplypsychology.org/brain-lateralization.html

APA Style References

Britannica, T. Editors of Encyclopaedia (2020, January 29). Broca area. Encyclopedia Britannica. https://www.britannica.com/science/Broca-area

Britannica, T. Editors of Encyclopaedia (2020, March 10). Wernicke area. Encyclopedia Britannica. https://www.britannica.com/science/Wernicke-area

Clements, A. M., Rimrodt, S. L., Abel, J. R., Blankner, J. G., Mostofsky, S. H., Pekar, J. J., Denckla, M. B. & Cutting, L. E. (2006). Sex differences in cerebral laterality of language and visuospatial processing. Brain and Language, 98(2), 150-158.

Gazzaniga, M. S., & Smylie, C. S. (1983). Facial recognition and brain asymmetries: Clues to underlying mechanisms. Annals of Neurology: Official Journal of the American Neurological Association and the Child Neurology Society, 13(5), 536-540.

Olulade, O. A., Seydell-Greenwald, A., Chambers, C. E., Turkeltaub, P. E., Dromerick, A. W., Berl, M. M., Gaillard, W. D. & Newport, E. L. (2020). The neural basis of language development: Changes in lateralization over age. Proceedings of the National Academy of Sciences, 117(38), 23477-23483.

Paradiso, S., Johnson, D. L., Andreasen, N. C., O’Leary, D. S., Watkins, G. L., Boles Ponto, L. L., & Hichwa, R. D. (1999). Cerebral blood flow changes associated with attribution of emotional valence to pleasant, unpleasant, and neutral visual stimuli in a PET study of normal subjects. American Journal of Psychiatry, 156(10), 1618-1629.

Reber, J., & Tranel, D. (2017). Sex differences in the functional lateralization of emotion and decision making in the human brain. Journal of Neuroscience Research, 95(1-2), 270-278.

Silberman, E. K., & Weingartner, H. (1986). Hemispheric lateralization of functions related to emotion. Brain and Cognition, 5(3), 322-353.

Sperry, R. W. (1967). Split-brain approach to learning problems. The neu.

Starkstein, S. E., Robinson, R. G., Honig, M. A., Parikh, R. M., Joselyn, J., & Price, T. R. (1989). Mood changes after right-hemisphere lesions. The British Journal of Psychiatry, 155(1), 79-85.

Szaflarski, J. P., Binder, J. R., Possing, E. T., McKiernan, K. A., Ward, B. D., & Hammeke, T. A. (2002). Language lateralization in left-handed and ambidextrous people: fMRI data. Neurology, 59(2), 238-244.

Tomasi, D., & Volkow, N. D. (2012). Laterality patterns of brain functional connectivity: gender effects. Cerebral Cortex, 22(6), 1455-1462.

Tranel, D., Bechara, A., & Denburg, N. L. (2002). Asymmetric functional roles of right and left ventromedial prefrontal cortices in social conduct, decision-making, and emotional processing. Cortex, 38(4), 589-612.

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