Our divided Our divided brainbrain

For more than a century, clinical evidence has For more than a century, clinical evidence has shown that the brain’s two sides serve differing shown that the brain’s two sides serve differing functions. Accidents, strokes, and tumors in functions. Accidents, strokes, and tumors in the left hemisphere generally impair reading, the left hemisphere generally impair reading, writing, speaking, arithmetic reasoning, and writing, speaking, arithmetic reasoning, and understanding. Similar lesions in the right understanding. Similar lesions in the right hemisphere seldom have such dramatic hemisphere seldom have such dramatic effects.effects.

Patients with speech problems gave early researchers the Patients with speech problems gave early researchers the first clues about how the brain is involved with language. The first clues about how the brain is involved with language. The loss of the ability to speak is called "aphasia." The ancient loss of the ability to speak is called "aphasia." The ancient Greeks noticed that brain damage could cause aphasia. Greeks noticed that brain damage could cause aphasia. Centuries later, in 1836, Marc Dax described a group of Centuries later, in 1836, Marc Dax described a group of patients who could not speak properly. Dax reported that all patients who could not speak properly. Dax reported that all of these patients had damage to the left side of their brain. A of these patients had damage to the left side of their brain. A quarter century later in 1861, Paul Broca described a patient quarter century later in 1861, Paul Broca described a patient who could say only one word..."tan." For this reason, Broca who could say only one word..."tan." For this reason, Broca called this patient "Tan." When Tan died, Broca examined called this patient "Tan." When Tan died, Broca examined his brain and found that there was damage to part of the left his brain and found that there was damage to part of the left frontal cortex. This part of the brain has come to be known frontal cortex. This part of the brain has come to be known as "Broca's Area."as "Broca's Area."

In 1876, In 1876, Karl Karl WernickeWernicke found that damage to a different found that damage to a different part of the brain also caused language problems. This part of the brain also caused language problems. This area of the brain ("Wernicke's Area"), was further back area of the brain ("Wernicke's Area"), was further back and lower in the brain compared to Broca's area. In fact, and lower in the brain compared to Broca's area. In fact, Wernicke's area is in the posterior part of the temporal Wernicke's area is in the posterior part of the temporal lobe. Broca's area and Wernicke's area are connected lobe. Broca's area and Wernicke's area are connected by a bundle of nerve fibers called the arcuate fasciculus. by a bundle of nerve fibers called the arcuate fasciculus. Damage to the arcuate fasciculus causes a disorder Damage to the arcuate fasciculus causes a disorder called conduction aphasia. People with conduction called conduction aphasia. People with conduction aphasia can understand language, but their speech does aphasia can understand language, but their speech does not make sense and they cannot repeat words.not make sense and they cannot repeat words.

Speaking the Written Speaking the Written Word Word

To speak a word that is read, information must first get to To speak a word that is read, information must first get to the primary visual cortex. From the primary visual cortex, the primary visual cortex. From the primary visual cortex, information is transmitted to the posterior speech area, information is transmitted to the posterior speech area, including Wernicke's area. From Wernicke's area, including Wernicke's area. From Wernicke's area, information travels to Broca's area, then to the Primary information travels to Broca's area, then to the Primary Motor Cortex. Motor Cortex.

Speaking the Heard Speaking the Heard Word Word

To speak a word that is heard, information must first get To speak a word that is heard, information must first get to the primary auditory cortex. From the primary auditory to the primary auditory cortex. From the primary auditory cortex, information is transmitted to the posterior speech cortex, information is transmitted to the posterior speech area, including Wernicke's area. From Wernicke's area, area, including Wernicke's area. From Wernicke's area, information travels to Broca's area, then to the Primary information travels to Broca's area, then to the Primary Motor Cortex. Motor Cortex.

By 1960 the left hemisphere was well accepted By 1960 the left hemisphere was well accepted as the “dominant” or “major” hemisphere, and its as the “dominant” or “major” hemisphere, and its silent companion to the right as the silent companion to the right as the “subordinate” or “minor” hemisphere, and the “subordinate” or “minor” hemisphere, and the left, the one easiest to observe and study. The left, the one easiest to observe and study. The other side is there of course, but backstage. (for other side is there of course, but backstage. (for about 1 in 10 people, including one-fourth of all about 1 in 10 people, including one-fourth of all left-handers, speech is processed in the right left-handers, speech is processed in the right hemisphere. But then researchers found that hemisphere. But then researchers found that the “minor” right hemisphere was not so limited the “minor” right hemisphere was not so limited after all. The story of this discovery is a after all. The story of this discovery is a fascinating chapter in psychology’s history.fascinating chapter in psychology’s history.

In 1961, two Los Angeles neurosurgeons, Philip In 1961, two Los Angeles neurosurgeons, Philip Vogel and Joseph Bogen, speculated that major Vogel and Joseph Bogen, speculated that major epileptic seizures were caused by an epileptic seizures were caused by an amplification of abnormal brain activity that amplification of abnormal brain activity that reverberated between the two hemispheres. reverberated between the two hemispheres. They therefore wondered whether they could They therefore wondered whether they could reduce seizures in their patients with reduce seizures in their patients with uncontrollable epilepsy by cutting uncontrollable epilepsy by cutting communication between the hemispheres. To communication between the hemispheres. To do this, Vogel and Bogen would have to sever do this, Vogel and Bogen would have to sever the the corpus callosum, corpus callosum, the wide band of axon the wide band of axon fibers connecting the two hemispheresfibers connecting the two hemispheres

In a normal brain, stimuli entering one hemisphere is In a normal brain, stimuli entering one hemisphere is rapidly communicated by way of the rapidly communicated by way of the corpus corpus callosumcallosum to to the other hemisphere, so the brain functions as a unit. the other hemisphere, so the brain functions as a unit. When the corpus callosum of an individual is severed, When the corpus callosum of an individual is severed, leaving a split brain, the two hemispheres cannot leaving a split brain, the two hemispheres cannot communicate. In some forms of communicate. In some forms of epilepsy epilepsy a seizure will a seizure will start in one hemisphere, triggering a massive discharge of start in one hemisphere, triggering a massive discharge of neurons through the corpus callosum and into the second neurons through the corpus callosum and into the second hemisphere. In an effort to prevent such massive seizures hemisphere. In an effort to prevent such massive seizures in severe epileptics, neurosurgeons can surgically sever in severe epileptics, neurosurgeons can surgically sever the corpus callosum, a procedure called a the corpus callosum, a procedure called a commissurotomy. If one side of the brain can no longer commissurotomy. If one side of the brain can no longer stimulate the other, the likelihood of severe epileptic stimulate the other, the likelihood of severe epileptic seizures is greatly reduced. seizures is greatly reduced.

In a cerebral commissurotomy the surgeon opens In a cerebral commissurotomy the surgeon opens the skull, lays back the brain's coverings and, with the skull, lays back the brain's coverings and, with a tool called a cerebral retractor, exposes the a tool called a cerebral retractor, exposes the corpus callosum between the two hemispheres. corpus callosum between the two hemispheres. The doctor snips through the corpus callosum, The doctor snips through the corpus callosum, severing communication between the severing communication between the hemispheres and preventing the transfer of hemispheres and preventing the transfer of seizures .seizures .

They had tried this experiment with cats then They had tried this experiment with cats then monkeys with no serious ill effects. So Vogel and monkeys with no serious ill effects. So Vogel and Bogen operated. The result? The seizures were Bogen operated. The result? The seizures were all but eliminated and the patients with these all but eliminated and the patients with these split split brainsbrains were surprisingly normal, their were surprisingly normal, their personalities and intellect hardly affected. Waking personalities and intellect hardly affected. Waking from the surgery, one patient even managed to from the surgery, one patient even managed to quip that he had a “splitting headache”quip that he had a “splitting headache”

The Corpus callosumThe Corpus callosum

In general, the right hemisphere interprets In general, the right hemisphere interprets information and controls actions of the left side of information and controls actions of the left side of the body. The left hemisphere interprets information the body. The left hemisphere interprets information and controls actions of the right side of the body. If and controls actions of the right side of the body. If the connection between the hemispheres is the connection between the hemispheres is severed, sensory information cannot pass to the severed, sensory information cannot pass to the correct region of the brain in order for correct region of the brain in order for corresponding response to be made. For example corresponding response to be made. For example callosal apraxiacallosal apraxia is a form of limb apraxia caused is a form of limb apraxia caused by damage to the anterior corpus callosum. When a by damage to the anterior corpus callosum. When a person hears a verbal request to perform a person hears a verbal request to perform a movement, let's say to raise both hands in the air, movement, let's say to raise both hands in the air, the meaning of the speech is analyzed by circuits in the meaning of the speech is analyzed by circuits in the left hemisphere. the left hemisphere.

Then, a neural command activates the region of the Then, a neural command activates the region of the brain that contains the memory of the movement, the brain that contains the memory of the movement, the prefrontal cortex.prefrontal cortex.

This information is passed to the part of the brain that This information is passed to the part of the brain that controls the actual movement to be performed, the controls the actual movement to be performed, the motor cortex. motor cortex.

The left motor cortex controls the movements of the The left motor cortex controls the movements of the right hand, and the right motor cortex controls the right hand, and the right motor cortex controls the movements of the left hand. In order for the right motor movements of the left hand. In order for the right motor cortex to be activated so that the left hand can be cortex to be activated so that the left hand can be raised, the analysis of the verbal command must be raised, the analysis of the verbal command must be passed from the left hemisphere to the right side, passed from the left hemisphere to the right side, through the corpus callosum. Thus, the right arm can through the corpus callosum. Thus, the right arm can perform the requested movement, but the left cannot. perform the requested movement, but the left cannot.

So, say a "typical" split-brain patient is sitting So, say a "typical" split-brain patient is sitting down, looking straight ahead and is focusing down, looking straight ahead and is focusing on a dot in the middle of a screen. Then a on a dot in the middle of a screen. Then a picture of a spoon is flashed to the right of picture of a spoon is flashed to the right of the dot. The visual information about the the dot. The visual information about the spoon crosses in the optic chiasm and ends spoon crosses in the optic chiasm and ends up in the LEFT HEMISPHERE. When the up in the LEFT HEMISPHERE. When the person is asked what the picture was, the person is asked what the picture was, the person has no problem identifying the spoon person has no problem identifying the spoon and says "Spoon." However, if the spoon had and says "Spoon." However, if the spoon had been flashed to the left of the dot (see the been flashed to the left of the dot (see the picture), then the visual information would picture), then the visual information would have traveled to the RIGHT HEMISPHERE. have traveled to the RIGHT HEMISPHERE.

Visual fieldVisual field

Now if the person is asked what the picture was, the Now if the person is asked what the picture was, the person will say that nothing was seen!! But, when this person will say that nothing was seen!! But, when this same person is asked to pick out an object using only same person is asked to pick out an object using only the LEFT hand, this person will correctly pick out the the LEFT hand, this person will correctly pick out the spoon. This is because touch information from the spoon. This is because touch information from the left hand crosses over to the right hemisphere - the left hand crosses over to the right hemisphere - the side that "saw" the spoon. However, if the person is side that "saw" the spoon. However, if the person is again asked what the object is, even when it is in the again asked what the object is, even when it is in the person's hand, the person will NOT be able to say person's hand, the person will NOT be able to say what it is because the right hemisphere cannot "talk." what it is because the right hemisphere cannot "talk." So, the right hemisphere is not stupid, it just has little So, the right hemisphere is not stupid, it just has little ability for language - it is "non-verbal."ability for language - it is "non-verbal."

Another type of experiment Another type of experiment performed with split brain performed with split brain patients uses chimeric patients uses chimeric figures, like this one to the figures, like this one to the right. In this figure, the face right. In this figure, the face on the left is a woman and on the left is a woman and the face on the right is a the face on the right is a man. Therefore, if the patient man. Therefore, if the patient focuses on the dot in the focuses on the dot in the middle of the forehead, the middle of the forehead, the visual information about the visual information about the woman's face will go to the woman's face will go to the right cerebral hemisphere right cerebral hemisphere and information about the and information about the man's face will go to the left man's face will go to the left hemispherehemisphere

When a split brain patient is asked When a split brain patient is asked to point to a whole, normal picture to point to a whole, normal picture of the face that was just seen, the of the face that was just seen, the patient will usually pick out the patient will usually pick out the woman's picture (remember, the woman's picture (remember, the information about the woman's information about the woman's face went to the RIGHT cerebral face went to the RIGHT cerebral hemisphere). However, if the hemisphere). However, if the patient is required to say whether patient is required to say whether the picture was a man or a woman, the picture was a man or a woman, the patient will SAY that the picture the patient will SAY that the picture was of a man. Therefore, was of a man. Therefore, depending on what the patient is depending on what the patient is required to do, either the right or required to do, either the right or left hemisphere will dominate. In left hemisphere will dominate. In this case, when speech is not this case, when speech is not required, the right hemisphere will required, the right hemisphere will dominate for recognition of faces dominate for recognition of faces

The undivided brainThe undivided brain

So what about the 99.99 percent of us with So what about the 99.99 percent of us with undivided brains? Have scientist found our undivided brains? Have scientist found our hemispheres to be similarly specialized? Yes hemispheres to be similarly specialized? Yes they have in several different types of studies. they have in several different types of studies. For example, when a person performs a For example, when a person performs a perceptual task, brain waves, blood flow, and perceptual task, brain waves, blood flow, and glucose consumption reveal increased activity glucose consumption reveal increased activity in the right hemisphere; when a person speaks in the right hemisphere; when a person speaks or calculates, activity increases in the left or calculates, activity increases in the left hemisphere.hemisphere.

Cerebral DominanceCerebral Dominance

Each hemisphere of the brain is dominant for other Each hemisphere of the brain is dominant for other behaviors. For example, it appears that the right behaviors. For example, it appears that the right brain is dominant for spatial abilities, face brain is dominant for spatial abilities, face recognition, visual imagery and music. The left recognition, visual imagery and music. The left brain may be more dominant for calculations, math brain may be more dominant for calculations, math and logical abilities. Of course, these are and logical abilities. Of course, these are generalizations and in normal people, the two generalizations and in normal people, the two hemispheres work together, are connected, and hemispheres work together, are connected, and share information through the corpus callosum.. share information through the corpus callosum..

LeftLeftHemisphereHemisphere

Language Language Math Math LogicLogic

RightRightHemisphereHemisphere

Spatial abilities Spatial abilities Face recognition Face recognition Visual imagery Visual imagery MusicMusic

DominanceDominance

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

Dominant Ear?Dominant Ear? ear's OAE (otoacoustic emission) . ear's OAE (otoacoustic emission) . Researchers measured the babies OAE with two types of Researchers measured the babies OAE with two types of

sound. First, they used rapid clicks and then sustained sound. First, they used rapid clicks and then sustained tones. They were surprised to find that the left ear provides tones. They were surprised to find that the left ear provides extra amplification for tones like music, while the right ear extra amplification for tones like music, while the right ear provides extra amplification for rapid sounds timed like provides extra amplification for rapid sounds timed like speech. speech.

"We were intrigued to discover that the clicks triggered "We were intrigued to discover that the clicks triggered more amplification in the baby's right ear, while the tones more amplification in the baby's right ear, while the tones induced more amplification in the baby's left ear," said induced more amplification in the baby's left ear," said Sininger. "This parallels how the brain processes speech Sininger. "This parallels how the brain processes speech and music, except the sides are reversed due to the brain's and music, except the sides are reversed due to the brain's cross connections." cross connections."

""

Our findings demonstrate that auditory Our findings demonstrate that auditory processing starts in the ear before it is ever processing starts in the ear before it is ever seen in the brain," said Cone-Wesson. "Even seen in the brain," said Cone-Wesson. "Even at birth, the ear is structured to distinguish at birth, the ear is structured to distinguish between different types of sound and to send between different types of sound and to send it to the right place in the brain." it to the right place in the brain."

Previous research supports the team's new Previous research supports the team's new findings. For example, earlier research findings. For example, earlier research shows that children with impairment in the shows that children with impairment in the right ear encounter more trouble learning in right ear encounter more trouble learning in school than children with hearing loss in the school than children with hearing loss in the left ear. left ear.