Our divided brain

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Our divided brain

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

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

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

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Speaking the Written Word

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

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Speaking the Heard Word

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

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

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

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

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• In a cerebral commissurotomy the surgeon opens
  the skull, lays back the brain's coverings and,
  with a tool called a cerebral retractor, exposes
  the corpus callosum between the two hemispheres.
  The doctor snips through the corpus callosum,
  severing communication between the hemispheres
  and preventing the transfer of seizures .
• They had tried this experiment with cats then
  monkeys with no serious ill effects. So Vogel
  and Bogen operated. The result? The seizures
  were all but eliminated and the patients with
  these split brains were surprisingly normal,
  their personalities and intellect hardly
  affected. Waking from the surgery, one patient
  even managed to quip that he had a splitting
  headache

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The Corpus callosum

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

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• Then, a neural command activates the region of
  the brain that contains the memory of the
  movement, the prefrontal cortex.
• This information is passed to the part of the
  brain that controls the actual movement to be
  performed, the motor cortex.
• The left motor cortex controls the movements of
  the right hand, and the right motor cortex
  controls the movements of the left hand. In order
  for the right motor cortex to be activated so
  that the left hand can be raised, the analysis of
  the verbal command must be passed from the left
  hemisphere to the right side, through the corpus
  callosum. Thus, the right arm can perform the
  requested movement, but the left cannot.

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• So, say a "typical" 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.

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Visual field
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• 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."

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• 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

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• 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

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The undivided brain

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

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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..

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• LeftHemisphere
• Language
• Math
• Logic

• RightHemisphere
• Spatial abilities
• Face recognition
• Visual imagery
• Music

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Dominance

• 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 "right hand
  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...

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Dominant Ear?

• ear's OAE (otoacoustic emission) .
• Researchers measured the babies OAE with two
  types of sound. First, they used rapid clicks and
  then sustained tones. They were surprised to find
  that the left ear provides extra amplification
  for tones like music, while the right ear
  provides extra amplification for rapid sounds
  timed like speech.
• "We were intrigued to discover that the clicks
  triggered more amplification in the baby's right
  ear, while the tones induced more amplification
  in the baby's left ear," said Sininger. "This
  parallels how the brain processes speech and
  music, except the sides are reversed due to the
  brain's cross connections."
• "

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• Our findings demonstrate that auditory processing
  starts in the ear before it is ever seen in the
  brain," said Cone-Wesson. "Even at birth, the ear
  is structured to distinguish between different
  types of sound and to send it to the right place
  in the brain."
• Previous research supports the team's new
  findings. For example, earlier research shows
  that children with impairment in the right ear
  encounter more trouble learning in school than
  children with hearing loss in the left ear.