The cerebral hemispheres are well differentiated in the human brain; each receives information and controls the contralateral body part, called the hemifield. That is, the right cerebral hemisphere controls the left hemifield and the left cerebral hemisphere controls the right hemifield.
Although at first glance both hemispheres may seem the same, they actually have anatomical and functional characteristics that differentiate them.
There are numerous researches throughout the history of psychology that have studied these differences. The first studies were carried out comparing the behavior of people with the divided brain, without connection between their hemispheres, and healthy participants.
As technology progressed, more sophisticated tests that included neuroimaging techniques such as functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG) or electroencephalography (EEG) have been used. One of the most used tests today is the Wada test.
Functional differences between the cerebral hemispheres
Throughout history there have been numerous studies to locate which brain area was responsible for each function. The first step to check 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 purpose, studies are usually done with patients with divided brains, who suffer a disconnection between the hemispheres, as well as neuroimaging techniques where the hemisphere is most 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 a divided brain.
While more complex processes such as thought or language, tend to involve greater hemispheric specialization.
Visuospatial processing is responsible for analyzing and understanding how the environment around us is based on the visual information we perceive.
In general, the results obtained in neuropsychological tests, such as the Cubes test on the Weshler Intelligence Scale for Adults ( Wechsler Adult Intelligence Scale, WAIS) indicate that this processing is mainly done in the right hemisphere (Berlucchi, Mangun, & Gazzaniga, 1997).
Although these results are widely accepted in the scientific community, it is true that they do not occur in all cases, since there are people who have found greater activation in the left hemisphere when performing this type of task.
The memory is a fundamental ability in humans because, in addition to help us remember facts and information plays a crucial role in adaptation to the environment and anticipation and action planning.
In general terms, it has been found that the most relevant area for memory is the medial temporal lobe, specifically the hippocampus.
In the studies carried out, the visuospatial memory has been related to the hippocampus of the right hemisphere, and the verbal memory to the left.
One of the best known studies in this regard is that of Maguire et al (2000) conducted 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 did not engage in driving.
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 are usually used, representing emotions such as anger or fear and others with neutral expressions.
Regarding the lateralization of emotions, there are two hypotheses:
- The first hypothesis suggests 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 the processing of emotion is done bilaterally, but each hemisphere is specialized in one type of information. The right hemisphere would be responsible for processing the negative emotions while the left hemisphere would be responsible for the positive ones.
The second hypothesis has not been as contrasted as the first one, since some studies have not found significant differences between the type of emotion and the hemisphere that processes it.
Language is a capacity that is found only in humans, although it is true that other animals also use communication systems.
Possibly this ability is the one that has helped humans evolve the most, since it allows us to represent and express objects that are not present, abstract things such as feelings or to plan complex sequences of actions.
As is widely known, the language is associated mainly with the left hemisphere, although the right hemisphere is also activated when some linguistic tasks are performed, it does so to a lesser extent.
The first studies in which a greater dominance of the left hemisphere was found versus the right in terms of language were those made by Paul Broca and Karl Wernicke. Specifically, they identified the region responsible for the production of the language and the one responsible for its understanding, respectively called the Broca area and the Wernicke area.
From these studies many others have been made to specify which areas form the circuits that are activated when performing different linguistic functions, but in general, it is still considered the dominant hemisphere for language in right-handed people and in most people Left handed is the left one.
The reasoning is, perhaps, the most complex capacity of human beings. In order to make a decision, a reasoning is made based on both the current situation and past experiences.
If we do not know all the variables that influence this decision, an inference is made, that is, we act based on what is most likely to occur as a result of our actions.
Some studies have been conducted to check if there is a dominant hemisphere in this capacity. They have found differences between the hemispheres dependent on the type of reasoning.
When all the variables are known and the reasoning is of a causal type, which variable influences another / s, the most efficient hemisphere is the right one.
While, if you do not know all the variables and you have to make an inference, the dominant hemisphere is the left one.
In summary, it could be said that the left hemisphere is specialized in a more complex reasoning than the right.
When it is not known with certainty what reasoning is appropriate, the complex reasoning carried out by the left hemisphere is usually preceded. Although, on many occasions the correct answer is the simplest.
In a study it was proven that, effectively, humans use more the reasoning of the left hemisphere, although we make more mistakes because of it.
In this study participants were presented with a series of slides with a circle in between, in 75% of the cases a red circle appeared and in 25% a green circle appeared, the order of presentation of the circles was random.
The participants had to press a green button if they thought that the next circle that was going to appear would be green and red if they thought that the next circle would be that color.
The results obtained showed that, although it was totally impossible for the participants to know in what order the circles were going to appear, they kept trying to find a pattern by pressing the green button when they “predicted” that the next circle would be of this color.
Obviously this strategy is not the most appropriate because it takes time to think and many mistakes are made, while if the participants had repeatedly pressed the red button would have made fewer mistakes and, in addition, would have been faster.
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.
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 the left-handed the dominant hemisphere was the right, but nowadays 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 the dominant hemisphere for language is right. 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 Hemispheric
Therefore, it seems that hemispheric dominance is not what determines manual dominance. The most accepted hypothesis defends that this dominance is determined by genetic components, although these are not yet known.
It is often said that women have 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 they have found are differences in the performance of tasks that activate more hemisphere than the other. The most studied skills have been motor, visuospatial, mathematical, perceptive and verbal.
- Motor skills. In general it has been found that the male gender performs motor tasks, such as the launching and reception of objects, more efficiently than the female. It could be thought that this advantage is due to cultural differences between genres 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 controlled predominantly by the left hemisphere.
- Visuospatial skills. It is usual to hear that men have better visuospatial abilities than women, especially if it is about driving a car, but the studies carried out do not support this belief. Although men are better at visuo-spatial orientation tasks, women have an advantage in visuospatial memory tasks. Therefore, in practice, no gender would have an advantage over the other. These abilities are controlled predominantly by the right hemisphere.
- Mathematical skills. Another widespread belief among the population is that men have higher mathematical abilities than women, but this is also not true. The masculine gender performs better the tasks of mathematical reasoning and the feminine the ones of calculation. These abilities are controlled predominantly by the left hemisphere.
- Perceptive skills Studies show that women are more sensitive to all perceptive 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 terms of fluency and verbal memory. These abilities are controlled predominantly by the left hemisphere.
Although these results have been obtained in reliable scientific studies it is important to know that the differences between the genres are smaller than the individual differences. That is, if two people of the same gender were caught at random, it is more likely that there were more differences between them than between two groups of different genders.