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Why do we have two brain hemispheres?

Why do we have two brain hemispheres?


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Is there any explanation why we have two brain hemispheres? Is there any advantage of having two in contrast to having one (or three / four)?

From what I've read so far (not too much; I'm not a biologist) each half can take the job of the other half when it comes to injuries. Is that correct?

Are there animals which have more than two hemispheres?

edit: I have seen What was the evolutionary reason for cross lateralization of the brain?, but that question seems to be about the question why the left hemisphere controls right body parts and the right hemisphere controls left body parts. My question is why there are two hemispheres at all.


WATCH: You Are Actually Two Brains Living in One Person

People toss around the term "mind-blowing" a lot these days, but when it comes to knowledge that is capable of rupturing your mind, this video from CGP Grey pretty much takes the proverbial cake.

Because, as you're about to find out when you watch the episode above, you are not 'you'. you are actually two brains residing in one person. And one of your brains is just a silent (potentially unwilling) passenger, along for the ride.

Creeped out yet? It's only just getting started.

We've all heard about the left brain/right brain divide, and most of us have been told that it doesn't really exist. And that's partly true - the old-fashioned notion that certain behaviours and traits are controlled solely by one side of the brain doesn't really pan out. But what is real is the fact that there definitely are two distinct brain hemispheres - a left and a right.

These hemispheres each receive half our visual information, and direct half our movement - the left brain controls the right side of our body, the right brain controls the left.

And these two hemispheres communicate through a 'wire', or series of nerves. But, as CGP Grey explains, that wire can be severed, and back in the day, this procedure was performed as a treatment for epilepsy.

This wire-cutting can leave people with what's known as 'split brain', and research on these individuals is, quite frankly, unnerving.

In fact, as CGP Grey breaks it down, it becomes clear pretty quickly that, in people with split brain, the right hemisphere can perform actions that the left hemisphere has no idea about, and has to try to make up a story to explain it.

We don't want to spoil too much, because it's really, really worth watching the video to understand the full extent of what's going on. But we'll paraphrase CGP Grey and just let you know that right now, there are two of you watching this video - one getting its mind blown, and one mentally rolling its eyes at the obviousness of it all.

And if that's the case, then which one is really you? (Oh god, we went there. Next-level creepiness.)

But that's not all. In a follow-up, CGP Grey partnered with some of our favourite film makers over at Kurzgesagt - In a Nutshell to demonstrate that we're not even just two: we're so, so much more than that.

The pile of meat that makes up our bodies is actually trillions of tiny individuals that each has a life of its own. So at what point do they start working together as one, and at what point do they actually become you?


The Two Hemispheres of Our Brain

Our brain is divided into 2 halves, or hemispheres, that are connected to each other by the corpus callosum. These two hemispheres control the motion in and receive sensory inputs from the opposite side of our body. In other words, the left hemisphere controls the right side of our body and also receives sensory inputs from the right side of our body.

The Left Brain

The left hemisphere of our brain handles tasks such as reading, writing, speaking, arithmetic reasoning and understanding. Study shows that when we speak or do arithmetic calculations, activity increases in our left hemisphere. Another characteristics of our left hemisphere is that it tends to process information sequentially, one at a time.

The Right Brain

The right hemisphere of our brain excels in visual perception, understanding spatial relationships, recognizing patterns, music, emotional expressions, etc. It is also good at making inferences. For example, when primed with words such as “foot”, “cry” and “glass”, our right hemisphere will relate these words to “cut”.

Our left hemisphere knows all these words individually but is unable to quickly make inferences from them. Our right hemisphere also lets us perceive the sense of self. People with lesions in the right brain sometimes have difficulty recognizing themselves in the mirror. Unlike the left hemisphere, our right hemisphere tends to process information as a whole.

Brain Lateralization

In a normal brain, any information that enters the left hemisphere will travel across the corpus callosum to the right hemisphere and vice versa. The two halves of the brain work interdependently and information is not being processed solely on right or left. However, having said that, one hemisphere is usually dominant in certain functions. This is called lateralization.

The degree of lateralization varies from one person to another. For example, for right-handed people, the control of language is probably concentrated more in the left hemisphere while for left-handed people, the language centres are more likely located more in the right hemisphere.

The Split-brain Experiment

Researchers had found that by cutting the corpus callosum and therefore separating the two hemispheres of the brain of patients who suffer severe seizures decreases the frequency and severity of the attacks. Psychologists Roger Sperry and Michael Gazzaniga studied these patients with split brains and their studies provide a key to understanding the functions served by the two hemispheres.

In one experiment, a blindfolded patient was asked to touch an object with his right hand and then name the object. Since the right side of the body is controlled by the left hemisphere, where the language centres are normally located, the split-brain patient was able to complete the task without problem. However, if the same patient was asked to touch the object with his left hand and then name the object, he was not able to complete the task.

Roger Perry actually won a Nobel prize for his split brain studies. You can read more about the experiments here.


Asymmetry in the Brain

It’s normal and natural that the sides of the brain aren’t identical. Even with something less intellectual, like motor functions, this can be demonstrated by the fact that most people are either right-handed or left-handed, as opposed to being ambidextrous.

This hemispherical division of the brain isn’t unique to humans. Almost all animals have this partitioning of the brain down the middle, and a division of responsibilities along specific regions of the brain.

It’s important to note, however, that despite this division of the brain into halves and sections, there are constant communications between all aspects of the brain. Our perception of reality is not “split” at all. All of these parts of the brain working in harmony with each other is what allow you to experience the world as you do. The cluster of neural fibers called the corpus callosum that connects the left and right sides of the brain is what make it possible to efficiently process and share data between different parts of the brain.

While neurology is a mature field of medical science, the truth is that we still have much to learn. The human brain is a dazzlingly complex piece of biological equipment, with some 86 billion neurons, all firing and receiving synapses from the time we are in the womb until the last minute of life. As the field continues to advance, we will learn more and more about the complexities and evolutionary advantages our split brains offer us. Until then, take a moment to appreciate the gift that nature has given you—then pick up a book and put that brain to use!


Use your Brain for a Change

One of my favourite quotes is by Dr Richard Bandler and it is “When you want new results, it requires new thinking”. Thinking is just something we do. We are always thinking about something. Even when we are asleep our brain is still working.

We are born with this amazing inbuilt computer but unfortunately it didn’t come with a set of instructions. We have to learn how to use it as we go along and mostly we use it out of habits we have formed from watching and listening to others, so if we want to change those habits we have to think differently and to think differently we have to use our brain to get that change.

There is a lot of talk about people being right-brained and left-brained. You might have taken one of those quizzes online to determine which you are. The assumption is that a person who is ‘left-brained’ is more logical, analytical and objective and suitable for jobs such as accountant, solicitor or programmer. The ‘right-brained’ person is said to be more creative, intuitive, thoughtful and subjective and might want to be an actor, graphic designer or therapist.

The reality is that people are not one or the other, they are not ‘right-brained’ or ‘left-brained’. The theory grew out of the work of Nobel Prize winner Roger W Sperry. He discovered that cutting the corpus callosum, the structure that connects the two hemispheres of the brain, reduced or eliminated seizures in people with epilepsy.

However, patients who had undergone the procedure also had new symptoms after the communication pathway between the two sides of the brain was cut. Some patients found that they could no longer name objects that were processed by the right side of the brain, yet they could still name objects processed by the left side of the brain. This led Sperry to suggest that language was controlled by the left side of the brain.

However, later research has shown that the brain is not nearly as neatly divided as this. The brain actually works best when both sides of the brain are engaged at the same time. Neuroscientists have found that the two sides of the brain work together to perform a wide variety of tasks and the two sides communicate through the corpus callosum.

In a study of over 1000 participants by the University of Utah, it was revealed that while activity was sometimes higher on one side or the other in certain critical regions, on average both sides of the brain were essentially equal in their activity. Dr Jeff Anderson, the study’s lead researcher explains “It’s absolutely true that some brain functions occur in one or other side of the brain. Language tends to be on the left, attention more to the right, but people don’t tend to have a stronger left or right sided brain network. It seems to be determined more connection by connection”.

This makes the corpus callosum key in using our brain to evoke and strengthen new thinking. Mid line exercises help to make those connections. We need both sides of our brain working together to create strong neuropathways, making focus and recall much better and easier.

So the next time you take one of those quizzes, know that it’s just a bit of fun. Now I must go and check my horoscope.


Author(s)

Kate graduated from Scripps College in 2009 with a Bachelor of Arts degree in Neuroscience, completing the cellular and molecular track with honors. As an undergraduate, she studied long-term plasticity in models of Parkinson&rsquos disease in a neurobiology lab at University of California, Los Angeles. She continued this research as lab manager before entering the University of Southern California Neuroscience graduate program in 2011 and then transferring to UCLA in 2013. She completed her PhD in 2017, where her research focused on understanding the communication between neurons in the eye. Kate founded Knowing Neurons in 2011, and her passion for creative science communication has continued to grow.


Benefits Of Balancing Your Left & Right Brain

Your brain has two hemispheres, left & right:

Left hemisphere thinking: Generally more sequential, linear, logical, practical, mathematical, analytical, scientific, and time-oriented.

Right hemisphere thinking: More non-linear, intuitive, abstract, big-picture focused, creative, and space-oriented.

Most people use one hemisphere more than the other, creating an imbalance .

Numerous electroencephalograph (EEG) studies have shown that humanity's greatest philosophers, thinkers, inventors , and artists use both brain hemispheres together, in unison .

Meditation works to balance both hemispheres of the brain, forcing them to work in harmony. Scientists call this " whole brain synchronization " and when achieved, your brain experiences extremely beneficial changes in hemispheric blood flow and chemistry.


The Importance of a Lateralized Brain

The more we discover about lateralization, the more essential it appears to be to brain structure and function, and the more of our behavior it helps to explain. The same, as we have seen, is proving true in other vertebrates. We see now that lateralization goes far beyond any simple dominance of one hemisphere over the other, as was once thought. Lateralization may vary in strength, and even direction, during development it may vary in different tasks and in different types of neural processing and it may vary in more social versus less social species. Indeed, given the seeming ubiquity of lateralized brain function in vertebrates, we must wonder why it took so long to discover it in nonhuman species. The answer lies in that initial decision, more than a century ago, to appropriate lateralization for ourselves, as evidence of our self-proclaimed mental superiority. It is an attitude that persists in some quarters to this day, with researchers on humans who are reluctant to accept the mounting evidence that nonhuman species are lateralized, or that their forms of lateralization have any equivalence to our own.

We need have no fears. Recognizing the existence of brain lateralization in nonhuman animals gives us more powerful ways of investigating its function, evolution, causation, and development. As we understand the complexities of the different forms of lateralization in other species, how the nature and influence of lateralization changes with development, and how it differs in different tasks and contexts, we will deepen our understanding of many patterns of thinking and behaving. That, in turn, will greatly enhance our appreciation of lateralization and its effects, subtle as well as broad, in those tantalizingly complex creatures—ourselves.


Hemisphere

Encyclopedic entry. Any circle drawn around the Earth divides it into two equal halves called hemispheres. There are generally considered to be four hemispheres: Northern, Southern, Eastern, and Western.

Earth Science, Geography, Physical Geography

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Any circle drawn around the Earth divides it into two equal halves called hemispheres. There are generally considered to be four hemispheres: Northern, Southern, Eastern, and Western.

The Equator, or line of 0 degrees latitude, divides the Earth into the Northern and Southern hemispheres. The Northern Hemisphere contains North America, the northern part of South America, Europe, the northern two-thirds of Africa, and most of Asia. The Southern Hemisphere contains most of South America, one-third of Africa, Australia, Antarctica, and some Asian islands.

There are differences in the climates of the Northern and Southern hemispheres because of the Earth's seasonal tilt toward and away from the sun. In the Northern Hemisphere, the warmer summer months are from June through September. In the Southern Hemisphere, summer begins in December and ends in March.

The Earth can also be divided into hemispheres along meridians, or lines of longitude. The prime meridian, or 0 degrees longitude, and the International Date Line, 180 degrees longitude, divide the Earth into Eastern and Western hemispheres. Many geographers consider the 20 degree west line of longitude and the 160 degree east line of longitude as the Eastern and Western hemispheres. This calculation is made so that Africa and Europe are not split.

The idea of Eastern and Western hemispheres has become politically and historically significant since European nations began colonizing North America and South America. In this context, the Eastern Hemisphere is sometimes called the "Old World," and the Western Hemisphere is called the "New World." However, the Western Hemisphere is a purely geographic term and should not be confused with other mentions of the "western" world, which is often used to describe parts of Europe, North America and other world regions that share some economic, social, and cultural values.

Map by National Geographic

Internal Hemisphere
The word "hemisphere" is usually used to refer to halves of the Earth, but it is also used to identify the halves of the brain. The brain is divided down the middle into the right and left hemispheres. Each brain hemisphere is considered to be specialized for certain behaviors. For example, the right hemisphere of the brain controls muscles on the left side of the body, and the left hemisphere of the brain controls muscles on the right side of the body. The left side of the brain is also dominant for language skills, mathematical calculations, and logic. The right side of the brain is dominant for visual imagery, music, and face recognition.


We May Have Been Wrong About How The Left And Right Brain Control Different Sides of The Body

We're usually told that the left side of the brain controls the right side of the body, and vice versa, but new research shows there's a lot more to it than that.

For decades, scientists have been finding evidence in both animals and humans to suggest that it's not just the contralateral (opposing side) brain hemisphere that plays a role in body movement, but also the ipsilateral (same side) hemisphere.

As it stands, though, the extent to which the ipsilateral brain hemisphere helps regulate movement in limbs and digits on the same, shared side of the body has never been well understood. Now researchers have broken new ground in identifying the relationship between cortical activity and ipsilateral movement.

For the first time, researchers at Washington University in St. Louis have demonstrated that 3D arm movement kinematics – encompassing limb speed, velocity, and position – can be decoded from human electrocorticographic (ECoG) signals ipsilateral to the moving limb.

"These results clarify our understanding that the ipsilateral hemisphere robustly contributes to motor execution and supports that the information of complex movements is more bihemispherically represented in humans than has been previously understood," the authors write in their paper.

Before now, evidence of movement kinematics decoded from the ipsilateral hemisphere was relatively limited.

To go deeper, lead researcher and neuroscientists Eric C. Leuthardt recruited four epilepsy patients (three male, one female) undergoing a separate ECoG procedure to pinpoint the source of their condition.

With their brains implanted with ECoG electrodes – which could register their neural activity in both the left and right hemispheres of the brain – Leuthardt wanted to measure both contralateral and ipsilateral activity as the participants moved their arms around in a three-dimensional reaching exercise.

Using a machine learning algorithm to decode the neural signals, the researchers found that movement kinematics are distributed bilaterally across cortical hemispheres, with evidence of ipsilateral arm reaches being decoded with comparable accuracy to contralateral reaches.

The researchers say this doesn't necessarily mean that the ipsilateral hemisphere is the dominant cause of physical movement in limbs closest to it, but the fact that so much information is parsed by the ipsilateral hemisphere is in itself significant.

"While the ability to decode ipsilateral limb kinematics does not establish a causal role of the ipsilateral hemisphere for movement execution, a representation of specific movement features, such as kinematics, is a necessary condition for the ipsilateral hemisphere to play a causal role in movement execution," the authors explain.

"This is the first study to show that specific kinematics of arm movements are bihemispherically represented on a time-point by time-point basis."

Obviously, we need to keep our expectations in check, because the current study involved only four patients (with intractable epilepsy), and there's a lot more research to be done before we understand what exactly is going on here between both sides of the brain (and body).

But the researchers say their findings provide an important advancement that could one day point to new potential treatments for stroke patients, whose condition may affect one side of their brain, but not the other.

"The ability to use ECoG to decode kinematics of the same-sided hand also underscores the possibility for a stroke survivor to use signals from their unaffected hemisphere to control a brain-computer interface (BCI)," the researchers explain.

Beyond that, it's possible that given both hemispheres of the brain appear to register the same cortical representation of movement kinematic information, one days these findings might help patients re-learn to control their bodies as a part of an ipsilateral-based rehabilitation treatment.

It's early days, but it's exciting stuff to learn that could benefit thousands of people in the future.

"Collectively, this study demonstrates that 3D kinematics of ipsilateral arm movements are encoded in human ECoG signals," the researchers write.

"These results strengthen evidence that the ipsilateral hemisphere plays a role in planning and executing voluntary motor movements with important implications for neuroprosthetic and neuro-rehabilitation applications."