BRAIN HEMISPHERES: DIFFERENCES, FUNCTIONS AND PARTS - NEUROPSYCHOLOGY - 2024

The cerebral hemispheres are well differentiated in the human brain; each receives information and controls the contralateral part of the body, called the hemifield. That is, the right cerebral hemisphere controls the left hemifield and the left cerebral hemisphere the right hemifield. Although at first glance both hemispheres may appear the same, in reality they have anatomical and functional characteristics that differentiate them.

There are numerous investigations throughout the history of psychology that have studied these differences. The first studies were carried out comparing the behavior of people with divided brains, without connection between their hemispheres, and healthy participants.

As technology has advanced, more sophisticated tests have been used, including neuroimaging techniques such as functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG) or electroencephalography (EEG). One of the most used tests today is the Wada test.

Functional differences between the cerebral hemispheres

Throughout history, numerous studies have been made to locate which brain area was responsible for each function. The first step in checking where a function is located is usually to find out if it is present in both hemispheres or only in one of them.

For this, studies are usually carried out with patients with divided brain, who suffer a disconnection between the hemispheres, in addition to neuroimaging techniques where it is verified which hemisphere is more active while performing a task.

In general, it has been found that the most basic functions, such as perception and attention, are usually carried out with the participation of practically the entire brain, even in patients with divided brain.

While more complex processes such as thought or language, they usually involve greater hemispheric specialization.

Visuospatial Processing

Visuospatial processing is in charge of analyzing and understanding what the environment around us is like based on the visual information we perceive.

In general, the results obtained in neuropsychological tests, such as the Cubes test on the Weshler Adult Intelligence Scale (WAIS) indicate that this processing is performed mainly in the right hemisphere.

Although these results are quite accepted in the scientific community, it is true that they do not occur in all cases, since there are people in whom greater activation has been found in the left hemisphere when performing this type of task.

Memory

Memory is a fundamental capacity in human beings since, in addition to helping us to remember facts and information, it plays a crucial role in adapting to the environment and anticipating and planning actions.

In studies carried out, visuospatial memory has been associated with the hippocampus of the right hemisphere, and verbal memory with that of the left.

One of the best known studies in this regard is that of Maguire et al (2000) carried out with London taxi drivers. In this study it was shown that taxi drivers with more years of experience had a greater right hippocampus than participants who were not dedicated to driving.

Emotion

The perception and production of emotions is one of the most studied processes in psychology, and it seems that it is shared both in humans and in other advanced mammals such as primates.

To study the processing of emotions, images of faces representing emotions such as anger or fear and others with neutral expressions are used. Regarding the lateralization of emotions, there are two hypotheses:

• The first hypothesis states that the right hemisphere is dominant in terms of the recognition of emotional information. Although both hemispheres are activated when perceiving emotions, it seems that the right is more effective than the left, especially when recognizing emotions in familiar faces.
• The second hypothesis considers that emotion processing is carried out bilaterally, but each hemisphere is specialized in one type of information. The right hemisphere would be in charge of processing negative emotions while the left would be in charge of positive ones.

The second hypothesis has not been as proven as the first, since some studies have not found significant differences between the type of emotion and the hemisphere that processes it.

Language

Language is a capacity found only in humans, although it is true that other animals also use communication systems.

Possibly this ability is the one that has most helped humans to evolve, since it allows us to represent and express objects that are not present, abstract things such as feelings or plan complex sequences of actions.

As is widely known, language is mainly associated with the left hemisphere, although the right hemisphere is also activated when performing some linguistic tasks, it does so to a lesser extent.

The first studies in which a greater dominance of the left versus the right hemisphere was found in terms of language were those carried out by Paul Broca and Karl Wernicke. Specifically, they identified the region responsible for the production of language and the one responsible for its understanding, respectively called Broca's area and Wernicke's area.

From these studies, many others have been carried out to specify which areas form the circuits that are activated when performing different linguistic functions, but in general, it is still considered that the dominant hemisphere for language in right-handed people and in most people left-handed is the left.

Reasoning

Reasoning is perhaps the most complex capacity of human beings. To make a decision, reasoning is carried out based on both the current situation and past experiences.

If all the variables that influence this decision are not known, an inference is made, that is, we act based on what is most likely to happen as a consequence of our actions.

Some studies have been done to see if there is a dominant hemisphere in terms of this ability. In them, differences have been found between the hemispheres depending on the type of reasoning.

When all the variables are known and the reasoning is causal, which variable influences another / s, the most efficient hemisphere is the right one. Whereas, if all the variables are not known and an inference must be made, the dominant hemisphere is the left.

In short, it could be said that the left hemisphere is specialized in more complex reasoning than the right.

Individual differences

The main functional differences between the hemispheres have already been explained, but these differences are not present in the same way in all individuals. Hemispheric specialization depends on factors such as manual dominance or gender.

Manual dominance

Most people are right-handed, that is, they use the right hand more for motor functions, while only 10% of the population is left-handed.

Formerly it was believed that in right-handed people the dominant hemisphere for language was the left, while in left-handed people the dominant hemisphere was the right, but today it is known that this is not the case.

In 95% of right-handed people it occurs in this way, while only in 5% of right-handed people is the dominant hemisphere for language the right one. In left-handed people the percentages are much more similar, in 70% of the cases the dominant hemisphere for language is the left, in 15% for the right and in the remaining 15% the hemispheres are activated equally, there is no dominance hemispherical.

Therefore, it appears that hemispheric dominance is not what determines manual dominance. The most widely accepted hypothesis is that this dominance is determined by genetic components, although these are not yet known.

Gender

It is often said that women have a more developed right hemisphere than men, but this is just a popular belief. The truth is that the studies carried out to date have not found significant differences in the activation of the gender-dependent hemispheres.

What has been found are differences in the performance of tasks that activate one hemisphere more than the other. The most studied skills have been motor, visuospatial, mathematical, perceptual and verbal.

• Motor skills. In general, it has been found that the male gender performs motor tasks, such as throwing and receiving objects, more efficiently than the female. It might be thought that this advantage is due to cultural differences between the genders rather than brain structure from birth, but these differences can be observed from 3 years, and even in other species such as chimpanzees. These abilities are predominantly controlled by the left hemisphere.
• Visuospatial skills. It is common to hear that men have better visuospatial skills than women, especially when it comes to driving a car, but the studies carried out do not support this belief. Although men are better at visuospatial orientation tasks, women have an advantage in visuospatial memory tasks. Therefore, in practice, neither gender would have an advantage over the other. These abilities are predominantly controlled by the right hemisphere.
• Mathematical skills. Another widely held belief among the population is that men have greater mathematical abilities than women, but this is not true either. The male gender performs better the tasks of mathematical reasoning and the female those of calculation. These abilities are predominantly controlled by the left hemisphere.
• Perceptual skills. Studies show that women are more sensitive to all perceptual stimuli except visual ones. That is, they detect stimuli that are imperceptible to the male gender, and also do so more quickly. These abilities are controlled by both hemispheres.
• Verbal skills. Several studies have shown that women are better than men in verbal fluency and memory. These abilities are predominantly controlled by the left hemisphere.

Although these results have been obtained in reliable scientific studies, it is important to know that the differences between the genders are smaller than the individual differences. That is, if two people of the same gender were taken at random, it is likely that there would be more differences between them than between two groups of different genders.

References

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