Lateralization of brain function is the view that different regions of the brain perform certain functions.
For example, different areas of the brain are believed to be responsible for controlling language, formulating memories and making movements.
If a certain area of the brain is damaged, the function associated with that area will also be affected.
It contrasts with the holistic brain theory that all parts of the brain are involved in thought and action processing.
In this article
left brain, right brain
The human brain is divided into two hemispheres, right and left. Both are joined by the corpus callosum, a bundle of nerve fibers that sits in the middle of the brain.
Hemispheric lateralization is the idea thateach hemisphere is responsible for different functions. Each of these functions is located on the right or left side.
The left hemisphere is associated with language functions such as grammar and vocabulary formulation and contains different language centers (Broca and Wernicke 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 for most people, especially right-handers.
The brain contains cortices such as the visual, motor, andsomatosensory cortices. These cortices are all contralateral, meaning that each hemisphere controls the opposite side of the body.
For example, hemotor cortexin the left hemisphere, it controls the muscle movements of the right arm and leg. also damage toright occipital lobe(responsible for vision) can cause vision loss in the left visual field.
Hemispheric lateralization is the idea that both hemispheres are functionally different and that certain mental processes and behaviors are primarily controlled by one hemisphere and not the other.
There is evidence of some specialization of function, particularly with regard to differences in language ability. In addition, however, the differences found were 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.
Paul Broca was a French physician and one of the first proponents of ideas of lateralization of brain function. In 1861, Broca met a patient he referred to as "Tan".
At the time, there was much debate about whether there was a location of function within the brain or whether the entire brain was used to perform each function.
Broca described the patient 'Tan', who was called that because it was the only word they could say. Often this patient would repeat the word twice, saying "So So".
When 'Tan' died, an autopsy of his brain revealed that there was damage to a part of his left frontal cortex. Broca found that other patients with problems similar to Tan had damage in the same region.
Damage to this region, later called "Broca's area", was found to be the reason for Tan's language problems. Broca's area is believed to be located in a part of the inferior frontal gyrus in thefrontal lobe, on the left side of most people.
This research broadly supports the view that the role of language function is located in the left hemisphere of the brain.
Broca's area has been found to be associated with multiple language functions, including language comprehension and the ability to articulate words.
This region is also associated with hearing, as understanding words requires articulating them in your head. It was also suggested to be active while planning, initiating and understanding someone else's movement.
drill areait may also contain mirror neurons, as this area appears to be involved in observing and imitating people (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 had a brain injury (for example, due to a stroke) that affects this language area.
The main symptom of Broca's aphasia is a deficit in the production of spoken and written language. A person with damage to this area is unlikely to be able to articulate words or form a coherent sentence.
Speaking in an abnormal pitch or rhythm can also be a symptom of this damage, as can repetitive speech, with confused grammar and confused structure of individual words.
Finally, the damage can also result in transcortical motor aphasia, which means that speech is not fluent and often limited to two words at a time.
A few years after Broca's discoveries, in 1876, the German neurologist Carl Wernicke identified another brain region associated with language.
Wernicke identified that some of his patients could speak but could not really understand language. Examination of the brains of these patients revealed that there were lesions at the junction of the superior temporal lobe in the left hemisphere.
This region has been called Wernicke's area and has been described as an area where heard and seen words are understood, as well as words that are selected for articulation.
This area also works in conjunction with Broca's area.wernicke's areaunderstands language and chooses words, which are sent to Broca's area to be articulated.
Wernicke's area containsmotor neuronsinvolved in speech understanding and is surrounded by an area called Geschwind territory.
When a person hears words, Wernicke's area associates sounds with their meaning, which neurons in Geschwind's territory help combine the many different properties of words (such as sound and meaning) to provide a more complete understanding.
However, when a person speaks, this process occurs in reverse, as Wernicke's area will find the right words to match the thoughts to be expressed.
The term Wernicke's aphasia was coined to describe damage to Wernicke's area. This is often believed to be damaged by head trauma or disease.
People suffering from Wernicke's aphasia may experience symptoms such as an inability to understand spoken language and use inappropriate words.
Your sentences may not make sense. They may repeat words, make up nonsense words, or have sentences that make no sense.
Most of the time, people with Wernicke's aphasia tend to speak fluently, compared to Broca's aphasia, where language is not fluent or fails.
Some patients may not even realize they have a speech impediment and may think they are speaking normally.
The two hemispheres of the brain are connected by the corpus callosum, a thick bundle of millions of nerve fibers.
As an outdated treatment for severe epilepsy, the corpus callosum was cut, meaning the connections between the two hemispheres were severed.
People who undergo this procedure are known as split-brain patients. In the 1960s, neurobiologist Roger Sperry performed experiments on these split-brain patients to test whether there was localization of function to the hemispheres.
Sperry has performed many split-brain experiments, one of which is the "split-field experiment". An example of this experiment would be projecting words into the right and left fields of vision while covering one eye to test whether patients can say the word.
They found that patients could speak the presented word in the right visual field, which is controlled by the left hemisphere and contains the language centers. Words presented on the left side, controlled by the right hemisphere, could not be pronounced.
However, patients can draw the displayed word on the left side or pick up the displayed word object because the right hemisphere can control the motor movements of the left hand.
When asked why the patients chose or drew the objects, they were unable to answer, suggesting that the right hemisphere (in most people) is unconscious, although the information it contains can affect behavior.
Another study by Gazzaniga (1983) performed a similar experiment but used faces projected into both visual fields. Faces in the left visual field were recognized, projecting to the right hemisphere, but not across the right visual field to the left hemisphere.
This shows that the right hemisphere may be better at recognizing faces in general.
Although lateralization of language functions is known to occur in the left hemisphere in most people, this lateralization may depend on personal ability.
Szaflarski et al. (2002) used functional magnetic resonance imaging (fMRI) on left-hand dominant subjects when completing language acquisition and non-linguistic tasks.
It was verified through the fMRI that there was greater activation in the right hemisphere of the participants, concluding that they presented typical domain of language.
There is the question of whether or not lateralization of language function occurs from birth, or whether this lateralization develops over time.
Olulade et al. (2020) aimed to study the lateralization of language development using fMRI in children and adults completing language-based tasks.
The researchers found that in the youngest children (aged 4 to 6 years) there was left and right hemispheric activation, so language was not lateralized to one hemisphere.
They also found that right-sided activation significantly decreased with age, with more than 60% of adults without significant right-sided activation.
This study suggests that language lateralization, predominantly of the left hemisphere, develops over time during childhood.
Lateralization of emotions
A review of the literature investigating the lateralization of emotion in the brain found that the left and right hemispheres have different functions with respect to emotions (Silberman & Weingartner, 1986).
It has been suggested that the right hemisphere is better at controlling emotional expressions and recognizing emotions and is associated with feelings of negative emotions.
While the left hemisphere was specialized in dealing with positive emotions. This implied that different functions of emotion were lateralized for each hemisphere.
In support of this view, another study found that patients who suffered trauma to the left frontal lobe, particularly the prefrontal cortex, experienced depression as a result (Paradiso et al., 1999).
Likewise, patients who suffered damage to their right frontal lobes were more likely to show signs of inappropriate joy and play.mania(Starkstein e col., 1989).
This supports the idea that the left hemisphere is lateralized for positive emotions, while the right hemisphere is lateralized for negative emotions.
There are several studies that support the idea that there are differences in the lateralization of function in the brains of men and women.
Tomasi and Volkow (2012) found that men had greater right lateralization of connectivity in areas of the temporal, frontal, and occipital cortices. In contrast, women had increased left lateralization of connectivity in the left frontal cortex.
It is suggested that differences in the lateralization of men's and women's brains may underlie some of the gender-typical differences in cognitive styles.
For example, the typical linguistic advantage of women over men may reflect greater left lateralization of language areas. On the other hand, the typical male advantage in visuospatial skills may reflect greater lateralization of right-side visuospatial areas (Clements et al., 2006).
Reber and Tranel (2017) reviewed studies on brain differences in men and women. They found plenty of evidence for a sex-related difference in an area of the brain called the ventral-medial prefrontal cortex (vmPFC), an area associated with decision-making and emotions.
Tranel et al. (2002) found that male patients with damage to the right vmPFC had deficits in social, emotional, and decision-making skills than those with damage to the left side.
However, the only patient with a lesion in the right vmPFC had fewer deficits in all behavioral categories. This evidence implies that the lateralization of higher cognitive functions depends on the individual's gender.
Phineas Gage (1848)
The brain localization theory is supported by the famous case study ofPhineas Gage (1848), who was an American railroad construction foreman. During an accident, a large iron rod went completely through his head, destroying much of the left frontal lobe of his brain.
He survives the crash, but his personality has changed; he became unstable and reportedly couldn't hold down a job. This supports the function location theory, as it shows that the control of social behavior lies in thefrontal cortex.
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