What connects the 2 hemispheres what is it made of

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Índice Show

Right Side - Left Side
Cerebral Dominance
Split-Brain Experiments
What is the corpus callosum and what does it do?
Split-brain patients
References:

How many brains do you have - one or two? Actually, this is quite easy to answer...you have only one brain. However, the cerebral hemispheres are divided right down the middle into a right hemisphere and a left hemisphere. Each hemisphere appears to be specialized for some behaviors. The hemispheres communicate with each other through a thick band of 200-250 million nerve fibers called the corpus callosum. (A smaller band of nerve fibers called the anterior commissure also connects parts of the cerebral hemispheres.)

Handedness

Are you right-handed or left-handed? As you probably know, most people (about 90% of the population) are right-handed - they prefer to use their right hand to write, eat and throw a ball. Another way to refer to people who use their right hand is to say that they are dominant. It follows that most of the other 10% of the population is left-handed or "left hand dominant." There are few people who use each hand equally; they are "ambidextrous." (Most people also have a dominant eye and dominant ear...test your "sidedness" here.)

Exactly why people are right-handed or left-handed is somewhat of a mystery. Dr. William Calvin has developed a fascinating theory about the origin of handedness and has written an essay called The Throwing Madonna to explain it.

Right Side - Left Side

The right side of the brain controls muscles on the left side of the body and the left side of the brain controls muscles on the right side of the body. Also, in general, sensory information from the left side of the body crosses over to the right side of the brain and information from the right side of the body crosses over to the left side of the brain. Therefore, damage to one side of the brain will affect the opposite side of the body.

In 95% of right-handers, the left side of the brain is dominant for language. Even in 60-70% of left-handers, the left side of brain is used for language. Back in the 1860s and 1870s, two neurologists (Paul Broca and Karl Wernicke) observed that people who had damage to a particular area on the left side of the brain had speech and language problems. People with damage to these areas on the right side usually did not have any language problems. The two language areas of the brain that are important for language now bear their names: Broca's area and Wernicke's area.

Broca's Area

Wernicke's Area

Images courtesy of Slice of Life.

Cerebral Dominance

Each hemisphere of the brain is dominant for other behaviors. For example, it appears that the right brain is dominant for spatial abilities, face recognition, visual imagery and music. The left brain may be more dominant for calculations, math and logical abilities. Of course, these are generalizations and in normal people, the two hemispheres work together, are connected, and share information through the corpus callosum. Much of what we know about the right and left hemispheres comes from studies in people who have had the corpus callosum split - this surgical operation isolates most of the right hemisphere from the left hemisphere. This type of surgery is performed in patients suffering from epilepsy. The corpus callosum is cut to prevent the spread of the "epileptic seizure" from one hemisphere to the other.

Dominant Functions

Left Hemisphere

Right Hemisphere

Spatial abilities
Face recognition
Visual imagery
Music

Split-Brain Experiments

Roger Sperry (who won the Nobel prize in 1981) and Michael Gazzaniga are two neuroscientists who studied patients who had surgery to cut the corpus callosum. These studies are called "Split-Brain Experiments". After surgery, these people appeared quite "normal" - they could walk, read, talk, play sports and do all the everyday things they did before surgery. Only after careful experiments that isolated information from reaching one hemisphere, could the real effects of the surgery be determined.

Dr. Sperry used a tachistoscope to present visual information to one hemisphere or the other. The tachistoscope requires people to focus on a point in the center of their visual field. Because each half of the visual field projects to the opposite site of the brain (crossing in the optic chiasm), it is possible to project a picture to either the right hemisphere OR the left hemisphere.

So, say a "typical" (language in the LEFT hemisphere) split-brain patient is sitting down, looking straight ahead and is focusing on a dot in the middle of a screen. Then a picture of a spoon is flashed to the right of the dot. The visual information about the spoon crosses in the optic chiasm and ends up in the LEFT HEMISPHERE. When the person is asked what the picture was, the person has no problem identifying the spoon and says "Spoon." However, if the spoon had been flashed to the left of the dot (see the picture), then the visual information would have traveled to the RIGHT HEMISPHERE. Now if the person is asked what the picture was, the person will say that nothing was seen!! But, when this same person is asked to pick out an object using only the LEFT hand, this person will correctly pick out the spoon. This is because touch information from the left hand crosses over to the right hemisphere - the side that "saw" the spoon. However, if the person is again asked what the object is, even when it is in the person's hand, the person will NOT be able to say what it is because the right hemisphere cannot "talk." So, the right hemisphere is not stupid, it just has little ability for language - it is "non-verbal."

Another type of experiment performed with split brain patients uses chimeric figures, like this one to the right. In this figure, the face on the left is a woman and the face on the right is a man. Therefore, if the patient focuses on the dot in the middle of the forehead, the visual information about the woman's face will go to the right cerebral hemisphere and information about the man's face will go to the left hemisphere. When a split brain patient is asked to point to a whole, normal picture of the face that was just seen, the patient will usually pick out the woman's picture (remember, the information about the woman's face went to the RIGHT cerebral hemisphere). However, if the patient is required to say whether the picture was a man or a woman, the patient will SAY that the picture was of a man. Therefore, depending on what the patient is required to do, either the right or left hemisphere will dominate. In this case, when speech is not required, the right hemisphere will dominate for recognition of faces.

Before different types of brain surgery, it is important to identify which cerebral hemisphere is dominant for language so that the neurosurgeon can avoid damaging speech areas. One way to test which hemisphere is dominant for language is with a procedure called the Wada Test. During this test, a fast acting anesthetic called sodium amytal (amobarbital) is injected into the right or left carotid artery. The right artery supplies the right cerebral hemisphere and the left artery supplies the left cerebral hemisphere. Therefore, either the right or left hemisphere can be "put to sleep" temporarily. If the left hemisphere is put to sleep in people who have language ability in the left hemisphere, then when asked to speak, they cannot. However, if the right hemisphere is put to sleep, then these people will be able to speak and answer questions. (Remember too that because the right hemisphere controls muscles on the left side, people will not be able to move the left side of their bodies.)

Another way to test for language representation in the brain is to electrically stimulate the cerebral cortex. A neurosurgeon can place an electrode on various areas of the exposed brain of an awake patient during surgery. The patient can say what he or she feels and thinks. Placement of the electrode on the brain does NOT hurt because the brain itself does not have any receptors for pain. In people who have left side dominance for language, electrical stimulation of various locations on the left cerebral cortex will interfere with speech.

More information:

Corpus callosum (in green).

The corpus callosum is a large, C-shaped nerve fiber bundle found beneath the cerebral cortex.It stretches across the midline of the brain, connecting the left and right cerebral hemispheres. It makes up the largest collection of white matter tissue found in the brain.

What is the corpus callosum and what does it do?

To understand the role of the corpus callosum, it is first important to remember that the brain is divided into two cerebral hemispheres (right and left). The hemispheres are made distinct from one another by a long groove called the medial longitudinal fissure. On a large scale, the two hemispheres are nearly identical, but on a microscopic and functional level there are some differences.

When information like sensory data is sent to the brain it is typically received first in one hemisphere. For example, when you type on your keyboard, information about the feel of the keys is sent up from your right hand to the primary somatosensory cortex on the left side of your brain. That information, however, must then be shared with the right side of your brain as well. That's where the corpus callosum comes into play. It is a large bundle of fibers that connects the left and right hemispheres, and it carries information received in one hemisphere over to the other.

Split-brain patients

In the second half of the twentieth century, Roger Sperry, Michael Gazzaniga, and others studied patients whose corpus callosum had been severed in a procedure called a corpus callosotomy. The procedure is normally undertaken as a last-resort treatment of epilepsy, as it can stop seizures from spreading from one hemisphere of the brain to another. The patients became known as split-brain patients.

Surprisingly, a corpus callosotomy can be completed without severe side effects; the side effects that do appear are often language related. Sperry and Gazzaniga explored language deficits in callosotomy patients in detail. In the process, they learned some interesting things about how language centers are distributed across the cerebral hemispheres and how the corpus callosum facilitates communication between the two sides of the brain.

Sperry and Gazzaniga presented split-brain patients with visual stimuli, but only to one eye at a time. For example, they would present an image of a flower to the right eye, but cover the left eye. They found that split-brain patients, when presented with a visual image to only their left eye, could not name the object shown in the image.

Watch this 2-Minute Neuroscience video to learn more about the corpus callosum.

Sperry and colleagues hypothesized that this occurred because visual information for the majority of the visual field travels to the opposite side of the brain to be processed. If the object is shown to the left eye, most of the information travels to the right side of the brain. Normally, this information would then be shared with the opposite hemisphere by way of the corpus callosum. The researchers suggested that split-brain individuals could not name the object if it was shown only to the left eye because the visual information was not reaching the left side of the brain, which is where our important language centers are located.

Much of what you've heard about one cerebral hemisphere being dominant in the management of a particular skill or capacity is probably exaggerated. For example, someone who is creative doesn't likely have an overall bias toward thinking with the right side of her brain. Instead, most skills are spread fairly evenly throughout both hemispheres. Language, however, appears to be an exception. In most people, speech is generated in the left hemisphere, and thus the left hemisphere is considered to be the dominant hemisphere for language.

Thus, according to Sperry and Gazzaniga, because language centers are located in the left side of the brain, when an image is presented to the left eye of a split-brain patient, the patient's language areas are not privy to the visual information. The information travels to the right hemisphere but does not cross back over to the left due to the severed corpus callosum. So, the ability to place a name to the object is limited.

These experiments helped to demonstrate the importance of the left hemisphere in language processing as well as the importance of the corpus callosum in bridging the two cerebral hemispheres. However, they also demonstrated the versatility and resiliency of the brain, as in most split-brain patients other tracts like the anterior commissure still carry enough information between the cerebral hemispheres to allow overall functionality to be somewhat normal.

References:

Gazzaniga MS. Forty-five years of split-brain research and still going strong. Nat Rev Neurosci. 2005 Aug;6(8):653-9. doi: 10.1038/nrn1723. PMID: 16062172.

Nolte J. The Human Brain: An Introduction to its Functional Anatomy. 6th ed. Philadelphia, PA. Elsevier; 2009.