Lateralization of brain function is the view that distinct brain regions perform certain functions.
For instance, it is believed that different brain areas 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.
Left brain, Right brain
The human brain is split into two hemispheres, right and left. They are joined together by the corpus callosum, a bundle of nerve fibers located in the middle of the brain.
Hemispheric lateralization is the idea that each hemisphere is responsible for different functions. Each of these functions is 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 centers (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 cases in most people, especially right-handed people.
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 found have been minor. We know 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
Paul Broca was a French physician and one of the earlier advocators for 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 the localization of function within the brain or if the whole brain was utilized in performing 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 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 a part of the inferior frontal gyrus in the frontal lobe, on the left side of the majority of people.
This research largely supports the view that the role of language function is localized to the brain’s left hemisphere.
Broca’s area is associated with multiple language functions, including language comprehension and being able to articulate words.
This region is also associated with listening, as understanding words requires articulating them in your head. It has also been suggested to be active during planning, initiating, 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 a brain injury (e.g., through a stroke) which then affects this language area.
The main symptom of Broca’s aphasia is a deficit in spoken and written language production. 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 an area where heard and seen words are understood and words 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 involved in speech comprehension and is surrounded by an area called Geschwind’s territory.
When a person hears words, Wernicke’s area associates the sounds with 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 to be expressed.
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 have sentences lacking meaning.
Most of the time, people with Wernicke’s aphasia often speak fluently, compared 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
Split-brain research demonstrates that the brain’s two hemispheres can operate independently when the corpus callosum, which connects them, is severed. This reveals lateralization of brain functions, with certain tasks predominantly managed by one hemisphere or another. For instance, the left hemisphere typically handles language and logical processing, while the right is more involved in spatial and holistic processing.
As an outdated treatment for severe epilepsy, the corpus callosum was sliced, meaning the connections between the two hemispheres were 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 a 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 while one eye is covered to test whether the patients can say the word.
They found that the patients could say the word presented on the right visual field, controlled by the left hemisphere and containing 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 chose 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 used faces projected to both visual fields. It was found that faces on the left visual field, thus projecting to the right hemisphere, were recognized, but not through the right visual field to the left hemisphere.
This demonstrates that the right hemisphere may better recognize faces in general.
Language Lateralization
Although it is known that the lateralization of language functions is in the left hemisphere in most people, this lateralization may depend on personal handedness.
Szaflarski et al. (2002) used functional magnetic resonance imaging (fMRI) on individuals who were left-hand dominant while 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 or not lateralization of language function occurs from birth, or if this lateralization develops over time.
Olulade et al. (2020) aimed to study the lateralization of language development by using fMRI on children and adults completing language-based tasks.
The researchers found that in the youngest children (aged 4-6 years old), there was 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, predominately the left hemisphere, develops over time during childhood.
Emotion lateralization
A review of the 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 and recognizing emotions and is associated with feelings of negative emotions.
Whereas the left hemisphere was specialized in dealing with positive emotions. This implied that different functions of emotion 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 while the right is lateralized to negative ones.
Gender Differences
There are several studies that 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. In contrast, females had increased left lateralization of connectivity in the left 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’ typical 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. They found a lot of evidence of a 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 with 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 depends on the sex of the individual.
Phineas Gage (1848)
The theory of brain localization is supported by the famous case study of Phineas Gage (1848), an American 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 localization of functions theory as it shows that control of social behavior is located in the frontal cortex.
References
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.
Further Reading
