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Why a brain's hemisphere controls the opposite side of the body?

Why a brain's hemisphere controls the opposite side of the body?


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As a physics student with very little understanding of biology, in a course about physics foundations diagnostic techniques I have come up with this question. I don't even know if it does make sense or not; in the latter case, please excuse me.

As far as I know, in the human brain the right hemisphere controls the motion of the left side of the body, while the left hemisphere controls the right side. Moreover, the part related to sight is on the back of the brain.

To me, it seems that there is a sort of opposition between the part of the body considered and the area of the brain that have to control it. Is this statement right? Only for human beings or also for animals? Are there any exception to this sort of pattern (if there is anyone)? Is there a logical reason to the fact that we show this characteristic and it couldn't be different from how it is?


"How" is easy to answer (signals from the right side of the body cross the midline and end up in the left cerebral hemisphere; answered well in a previous question). This pattern of wiring appears to have existed since bilateral symmetry first evolved.

"Why" is not a well-defined question; the possible evolutionary advantage gained by having cross-wiring is unclear.

To answer your explicit question: there is no "opposition" in any meaningful sense between the location of a sensory nerve on the body and the location at which the information is processed in the brain. There are a raft of visual areas in the neocortex, spanning from the back but extending forward. The areas that control eye movement are much more frontal ("frontal eye fields"). Before arriving in the neocortex, signals from the eyes pass through the dLGN, a nucleus in the thalamus which is a subcortical structure closer to the middle of the brain than the rear. This flow of visual information is similar across many (most? all?) mammals.


Left Brain, Right Brain: 9 Ways Our Brain Hemispheres Work Together

What are the functions of each brain hemisphere? What does each half of our brains do? Is it true that the left side is the analytic hemisphere and the right side the emotional side of the brain? Is it true that the ‘right brain’ is the creative one and the ‘left brain’ is the logical one? In this article, we will reveal everything you need to know about brain hemispheres.

Brain Hemispheres

We have often been told that the left hemisphere of the brain is the analytic, mathematical, and logical side, the side which is in charge of reasoning. You’ve probably also heard that the right hemisphere of the brain is the emotional, creative side.

In fact, people often use this difference as a way to define personality, referring to people as either left-brained or right-brained. “If you are a creative, sensitive, and passionate person, then you use your right hemisphere more if you are an analytical, organized, and thoughtful person you use your left hemisphere more.” We hear that all the time, so let’s check some facts to see whether there is any truth to this common saying.


“Left-Brained” and “Right-Brained” People

A widespread myth suggests that some people, whose left hemisphere is dominant overall, are more quantitative, logical, and analytical, while right-brained individuals are more emotional, intuitive, and creative. Like many ways of categorizing people, the left brain/right brain dichotomy is appealing, promising to teach individuals about how they think and why. But the reality of hemispheric specialization is much more complex than this popular concept suggests.

Are there “left-brained” people and “right-brained” people?

No. While the brain’s left and right hemispheres do handle distinct functions, their duties aren’t strictly divided according to broad categories like “logical” or “creative.” People can't be split neatly into groups with special strengths based on one hemisphere being more dominant.

How have “left-brained” people and “right-brained” people been described?

The “left-brained” type has been described as logical, analytical, and detail-oriented, while “right-brained” people have been characterized as creative and intuitive—though other supposed traits have been attributed to each. (In any case, the "left-brained" and "right-brained" types are not scientifically supported.)

Where did the myth come from?

The concept of “left-brained” and “right-brained” individuals appears to stem from research in the 1960s on “split-brain” patients, whose corpus callosum (the bridge between hemispheres) had been separated. The research revealed that patients responded in different ways to stimuli such as images, depending on which brain hemisphere perceived them.


A person can either be right-brained or left-brained. It means that one side of the brain is dominant. The left-brained dominant people are methodical and analytical in nature. Those right-brained dominant are creative and artistic.

The left brain and right brain theory was created in the 1960s by a psychologist named Roger W. Sperry.


The left hemisphere of the brain contains parts of the parietal lobe, temporal lobe and the occipital lobe, which make up your language control center. In these lobes, two regions known as the Wernicke area and the Broca area allow you to understand and recognize, read and speak language patterns -- including the ability to learn foreign languages.

In its most simple form, language is a code that consists of symbols that can be connected to letters, words and phrases. Your brain allows you to crack that code and relate each of the words, letters and phrases to a specific meaning. You learn the sounds that form the words and put those words in a sequence that the listener will understand. The brain tells your tongue, mouth and voicebox to work on a millisecond-by-millisecond basis, which allows you to speak.


Right Brain Injury vs. Left Brain Injury | Understanding the Impact of Brain Injury on Daily Life

You’ve had a brain injury or stroke and certain things in your daily life are affected, but not everything. You know your injury was on the right side of your brain. So, why are you having trouble with actions on the left side of your body? Here’s a quick explanation of which side of the brain is responsible for what types of thoughts, emotions, and actions. You might be surprised!

Your brain is amazingly complex. It allows you to accomplish all kinds of feats every day, from simply picking up an apple off a table, to singing, reading, speaking, understanding or feeling emotions. The human brain is so complex that scientists are still making new discoveries about how it works.

Your brain is divided into two sides, called hemispheres

The right side of your brain controls the left side of your body, and the left side of your brain controls the right side of your body. Interesting, right? The two sides of your brain are similar to one another for example, the part of your brain that tells your arms and legs to move is the same on the right and left – it just controls different sides of your body.

What’s the difference between the right side and the left side of your brain?

The left side of the brain is concerned with language, number skills, reasoning, scientific skills, spoken language and right-hand control. The left side is the hub of language, where you “assemble” the language (words and sentence structure) you want to communicate. This is not to say that the right side of your brain has no involvement with language the two sides of your brain work together to perform functions such as understanding, reasoning and organizing language.

We know from patterns of brain damage (for example: stroke, traumatic brain injuries or brain tumors), that when the left side of the brain is damaged in the areas that have been associated with speech production and language comprehension, people present with Aphasia (difficulty comprehending and/or expressing language). These language difficulties are (in almost all situations) not seen if the same damage is done on the right side of the brain. So we know that the left side of the brain is very important for language and understanding. Other functions of the left side of the brain are analytical, logic, and computation skills.

The right side of your brain is in charge of visual awareness, imagination, emotions, spatial abilities, face recognition, music awareness, 3D forms, interpreting social cues, and left-hand control. It performs some math, but only rough estimations and comparisons. The brain’s right side also helps us to comprehend visual imagery and make sense of what we see. It plays a role in language, particularly in interpreting context and a person’s tone, staying on topic in a conversation and organizing your thoughts and ideas.

What if my brain injury or stroke is on the LEFT SIDE of my brain?

Injury to the left side of the brain may result in right-sided weakness and the following communication problems:

  • Receptive Language: Problems with understanding spoken or written language (listening and reading)
  • Expressive Language: Problems with expressing spoken or written language
  • Apraxia of Speech: Problems with programming and coordinating the motor movements for speaking
  • Dysarthria: Aspects of the speech system is impacted, which may result in slurred speech or a change in how your voice sounds
  • Computation: Problems with number and math skills
  • Analyzing: Problems with solving complex problems

What if my brain injury or stroke is on the RIGHT SIDE of my brain?

Injury to the right side of the brain may result in left sided weakness and the following cognitive and communication problems:

  • Attention: Difficulty concentrating on a task or focusing on what is said or seen.
  • Left neglect: Problems with attending to things on the left side
  • Visualperception: Visual perception deficits including processing any information on the left visual field
  • Reasoning and problem solving: Difficulty identifying that there is a problem and generating solutions
  • Memory: Difficulty recalling previously learned information and learning new information.
  • Social communication: Difficulty interpreting abstract language such as metaphors, making inferences, understanding jokes, and nonverbal cues.
  • Organization: Difficulty with arranging information and planning, which is often reflected in communication difficulties, such as trouble telling a story with events in the right order, giving directions, or maintaining a topic during conversation.
  • Insight: Difficulty recognizing problems and their impact on daily functioning.
  • Orientation: Difficulty recalling the date, time, or place.

Additional resources for understanding the impact of stroke or injury to your brain

Brain injury exercises on demand

Constant Therapy is a cognitive & speech therapy app that helps people with brain injury, stroke and aphasia recover key speech and cognitive skills – like memory, math, reading, writing, counting change, and more. Built and tested by a team of top neuroscientists and clinicians at Boston University, Constant Therapy has proven effective in multiple peer-reviewed, published research studies.

Both clinicians and patients can download and use Constant Therapy.


What Is A Cerebral Hemisphere?

The brain, which is found inside the skull, is the epicenter of the nervous system. Its biggest part is called the cerebrum, which is located at the top of the head.

The cerebrum is divided into two parts which are called cerebral hemispheres. A hemisphere is half of a sphere.

So, every person’s brain has two hemispheres, each controlling the opposite side of the body. The right hemisphere of the cerebrum controls the functions on the left side of the body. And vice versa.

Therefore, cerebral hemispheres are crucial for our wellbeing as they support cognitive functions and store memories.

Where Are The Cerebral Hemispheres Located?

As you now know, the cerebrum is the biggest part of the brain and takes up to two-thirds of the whole organ. It’s located at the top of the head, specifically at the left and right sides of the brain stem.


Why does the left hemisphere of the brain control the right side of the body (and vice versa)?

Why does each side of the brain control the opposite side of the body? Is there some juncture of nerves somewhere in the base of my brain that flips the sides? Is there a logical reason that nature decided that this would be so?

It seems to me, that if you were to become Two-Face, and one side of your body were mangled and you had brain damage as well, youɽ want the other half to have a fully working brain.

I had a long discussion with a Nobel-prize winning neuroscientist about this.

His impression was that it was all based on vision. Your eye creates a real image on your retina, and real images are inverted. So, up is down and left is right. This inverting of the image would be present in any eye with any kind of spatial resolution.

So, the left side of the 'retina' (or whatever you want to call the primitive photosensor) was getting information from the right side of visual space. As brains developed, the brain followed suit.

The inverted organization later occurred with motor and sensory systems.

But this presumes that all motor control emerged after the eyes did.

Many nerves in all systems of the body cross the midline and go to the other side. This is called "decussation." The reason why the left brain controls the right body is because in the major motor pathway for most of your body, the the left brain nerves will cross and control the right side of the body. One of the major places this occurs for motor control is at the pyramidal decussation. This however is not limited to just motor control- sensory input like touch and pain also decussates, although at different places and in different manners.

As to why there is a logical reason for this, well that's a very open and debated question. I don't know what most people think right now, but craig has presented one of the ideas.


Causes of split-brain syndrome

The primary cause of split-brain syndrome is intentional severing of the corpus callosum, partially or completely, through a surgical procedure known as corpus callosotomy. Rarely performed in the 21st century (having been replaced largely by drug treatments and other procedures), this operation is reserved as a last measure of treatment for extreme and uncontrollable forms of epilepsy in which violent seizures spread from one side of the brain to the other. By preventing the propagation of seizure activity across the hemispheres, corpus callosotomy can greatly improve the patient’s quality of life. However, following the operation, patients develop acute hemispheric disconnection symptoms that last for days or weeks and chronic symptoms that often are permanent.

Less-common causes of split-brain syndrome include stroke, infectious lesion, tumour, or ruptured artery. Many of these events result in varying degrees of spontaneous damage to the corpus callosum. The syndrome can also be caused by multiple sclerosis and in rare instances by agenesis of the corpus callosum, in which the connection fails to develop or develops incompletely. (Lesions in the corpus callosum also occur in patients with Marchiafava-Bignami disease, a rare alcoholism-related condition, but the more global brain damage associated with this disease leads to stupor, seizures, and coma, rather than the features typical of split-brain syndrome.)


Why Gray Matter Volume in All 4 Brain Hemispheres Matters

Losing gray matter volume asymmetrically in both cerebral hemispheres or in the right cerebellar hemisphere may be linked to Alzheimer's disease, accumulating evidence suggests.

Losing gray matter volume asymmetrically in both cerebral hemispheres or in the right cerebellar hemisphere may be linked to Alzheimer's disease, accumulating evidence suggests.



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