Lateralization The Split Brain

Document Sample
Lateralization The Split Brain Powered By Docstoc
					 Lateralization & The Split Brain
Cortical Localization of Language
• The Dominant Left Hemisphere
• Tests of Cerebral Lateralization
• The Split-Brain Experiment
• Tests of Split-Brain Patients
• Differences Between the Left and Right
• Broca’s Area
• Wernicke’s Area
     Aphasia and Apraxia:
 The Dominant Left Hemisphere
• In 1836, Dax reported that not one of his 40 or so
  patients with speech problems had displayed
  damage restricted to the right hemisphere
• 25 yrs later, Broca reported the results of the
  postmortem examination of two aphasic patients
  (patients with deficits in the use of language that
  are not attributable to general sensory, motor, or
  intellectual dysfunction)…
     Aphasia and Apraxia:
 The Dominant Left Hemisphere
• Both had diffuse left hemisphere damage
  that seemed to be centered in an area of the
  inferior left prefrontal lobe, just in front of
  the primary motor face area
• This became known as Broca’s area that is
  associated with grammar and speech
     Aphasia and Apraxia:
 The Dominant Left Hemisphere
• Liepmann discovered that apraxia
  (difficulty performing movements with
  either side of the body when asked to do so,
  but not when performing them
  spontaneously) was almost always
  associated with left-hemisphere damage
     Aphasia and Apraxia:
 The Dominant Left Hemisphere
• This led to the view that all complex
  activities were performed by the left
  hemisphere; the left and right hemispheres
  thus became known as dominant and
  minor hemispheres, respectively
 Tests of Cerebral Lateralization
• The first evidence of language laterality
  came from comparisons of the effects of left
  and right unilateral lesions; today, the
  sodium amytal test and dichotic listening
  test are commonly used to assess language
 Tests of Cerebral Lateralization
• PET of FMRI techniques have revealed that
  there is typically more activity in the left
  hemisphere than the right during language-
  related activities
 Tests of Cerebral Lateralization
• Many studies have reported a relation
  between speech laterality and handedness;
  the following general conclusions have been
Tests of Cerebral Lateralization
– Nearly all (about 95%) right-handed subjects
  are left-hemisphere dominant for speech;
– most left-handed or ambidextrous subjects
  (about 70%) are also left-hemisphere dominant
  for speech; and
– Early left-hemisphere damage can cause the
  right hemisphere to become dominant for
  speech and the left hand to be preferred
    The Split-Brain Experiment
• In 1953, Myers and Sperry performed an
  experiment on cats that changed the way
  that we think about the brain; and it
  provided a means of comparing the function
  of the two hemispheres
• It was designed to reveal the function of the
  brain’s largest commissure, the corpus
    The Split-Brain Experiment
• Earlier studies failed to reveal any deficits
  in laboratory animals following callosal
  transection, and people born without a
  corpus callosum had been reported to be
  perfectly normal
      The Split-Brain Experiment
• In the Myers and Sperry experiment there
  were four groups of cats:
  –   Corpus callosum severed
  –   Optic chaims severed
  –   corpus callosum and optic chiasm severed
  –   Intact controls
    The Split-Brain Experiment
• In phase 1 of the experiment, all cats
  learned a lever-press pattern discrimination
  task with a patch over one eye; all four
  groups readily learned this simple task
• In phase 2, the patch was switched to the
  other eye…
    The Split-Brain Experiment
• The cats in the optic-chiasm-severed group,
  corpus-callosum-severed group, and control
  kept performance same
• In contrast the optic-chiasm-and-corpus-
  callosum-severed group acted as if the task
  were completely new to them - they had to
  learn it again with no savings
    The Split-Brain Experiment
• We can conclude:
  – The cat forebrain has the capacity to act as two
    separate forebrains, each capable of independent
    learning and of storing its own memories;
  – The function of the corpus callosum is to carry
    information between hemispheres
  – The best strategy for studying corpus callosum function
    is to use a method to limit information to a single
    Tests of Split-Brain Patients
• Commissurotomy is performed on patients
  with life-threatening cases of epilepsy to
  reduce the severity of convulsions by
  restricting epileptic discharges to half of the
   Tests of Split-Brain Patients
• The operation is remarkably effective;
  many commissurotomized epileptic patients
  never experience another major convulsion;
  more remarkably they experience few
  obvious side effects in their daily lives
    Tests of Split-Brain Patients
• The controlled neuropsychological testing
  of these split-brain patients has revealed
  some amazing things about the human brain
• To test split brain patients,visual stimuli are
  flashed to the right or left of a fixation point
  on a screen
• Also tactual information is presented to one
  hand under a ledge or in a bag
   Tests of Split-Brain Patients
• These tests confirmed the conclusion that
  commissurotomized patients have two
  independent streams of consciousness
  Evidence of Two Independent
   Streams of Consciousness
• When an object was presented to the left
  hemisphere, either by touching something
  with the right hand or viewing something in
  the right visual field, the subject could:
Evidence of Two Independent
 Streams of Consciousness
– Pick out the correct object with the right hand
– Could not pick out the correct object with the
  left hand
– Could name the correct object
  Evidence of Two Independent
   Streams of Consciousness
• When an object was presented to the right
  hemisphere, either by touching something
  with the left hand or viewing something in
  the left visual field, the subject could:
Evidence of Two Independent
 Streams of Consciousness
– Could pick out the correct object with the left
– Could not pick out the correct object with the
  right hand
– Claimed nothing had been presented
• Represents communication between
  hemispheres via a nonneural route
• For example: a red or green light is flashed
  in the left visual field; the split-brain patient
  was then asked to name the color: red or
• Most split-brain patients get 50% correct on this
  task (guessing, by chance); however one patient
  performed almost perfectly
• When the performance of this subject was
  carefully monitored, it was noticed that on the
  trials when the patient initially said (left
  hemisphere) the incorrect color, his head shook
  and the patient then changed their guess to the
  other color
• Apparently, the right-hemisphere (who
  knew the correct answer) heard the
  incorrect guess of the left hemisphere, and
  signaled to the left hemisphere that it was
  wrong by shaking the person’s head; when
  only first guesses were counted,
  performance fell to 50%
   Learning Two Things at Once
• Split-brain patients are capable of learning two
  things at once
• If a split-brain patient is visually presented two
  objects at the same time - let’s say a pencil in the
  LVF and apple in the RVF - s/he can reach into
  two different bags at the same time, one with each
  hand, and pull out the two objects - a pencil in the
  left-hand and apple in the right
    Helping-Hand Phenomenon
• Occurs when the two hemispheres are presented
  with different information about the correct choice
  and then are asked to reach out and pick up the
  correct object from a collection in full view
• Usually the right hand will reach out to pick out
  what the left hemisphere saw, but the right
  hemisphere seeing what it thinks is an error being
  made causes the left hand to grab the right hand
  and pull it over to the other object
Split-Brain Video
 (shown in class)
     Differences Between
The Left and Right Hemispheres
• Language is the most lateralized of all
  abilities; the left-hemisphere is better than
  the right at most language-related tasks
• however, the right hemisphere proved to
  be able to understand single written and
  spoken words; also right-hemisphere detects
  prosody and discourse
     Differences Between
The Left and Right Hemispheres
• The right hemisphere proved better than
  the left at a variety of tasks involving
  spatial ability, emotional stimuli and
  musical tasks
     Differences Between
The Left and Right Hemispheres
• The two hemispheres seem to engage different
  types of memory processing; LH attempts to place
  its experience in a larger context (relation of
  parts that make up the whole), while the RH
  attends strictly to the Gestalt perceptual
  characteristics of the stimulus (parts or whole but
  not relation between)
• This is usually termed analytical (LH) versus
  holistic (RH)
     Differences Between
The Left and Right Hemispheres
• Thus the RH should not be regarded as the
  minor hemisphere; it has different abilities,
  not less important ones
     Differences Between
The Left and Right Hemispheres
• There are also anatomical asymmetries in
  the human brain; for example the planum
  temporale and frontal operculum (language
  related areas) are larger in LH
• However, Heschl’s gyrus (also language
  related) in larger in RH
     Differences Between
The Left and Right Hemispheres
• (not in book) left-handers seem to have
  symmetrical planum temporales, suffer less
  severely from LH aphasia, and suffer more
  severely from RH aphasia
• This suggests left-handers may have a more
  diffuse representation of language and is
  evident in differential grammatical
  strategies in sentence processing
         Three Theories of
        Cerebral Asymmetry
• Analytic-synthetic theory
• Motor theory
• Linguistic theory
     Analytic-Synthetic Theory
• Suggests that there are two fundamentally
  different modes of thinking, an analytic mode
  (LH) and synthetic mode (RH), and that the
  neural circuitry for each is fundamentally different
• LH (pieces of the whole) operates in logical,
  sequential, analytic fashion
• RH (the whole) makes immediate, overall
  synthetic judgments
                 Motor Theory
• Posits that LH is specialized for fine motor
  movement of which speech is but one example
• Two lines of evidence:
   – Lesions of the LH disrupt facial movements more than
     do RH lesions, even when they are not related to speech
   – Degree of disruption of nonverbal facial movements is
     positively correlated with the degree of aphasia
           Linguistic Theory
• Based on the view that the primary function
  of the LH is language; this is based on
  studies of deaf people who communicate
  using ASL; this ability is lost if these people
  suffer damage to the LH, even when they
  are able to make the movements required
• (or is this just showing ASL is a language,
  and that language is highly analytical?)
             Broca’s Area
• Inferior left prefrontal lobe in left
• Damage leads to deficits primarily speech
  production (problems with expression) and
  also grammatical comprehension
             Wernicke’s Area
• Left temporal lobe, just posterior to the primary
  auditory cortex
• Damage leads to deficits to semantic language
  comprehension (problems with reception) and
  speech is imcomprehensible, despite having
  correct grammar, rhythm an intonation (word
        Conduction Aphasia
• Damage to pathway connecting Broca’s and
  Wernicke’s areas called the arcuate
• Comprehension and spontaneous speech are
  intact but patient not able to repeat words
  they have just heard
• Damage to the left angular gyrus (area of
  left temporal and parietal cortex jost
  posterior to Wernicke’s)
• Inability to read despite intact language
  comprehension and production
• Also due to damage to the left angular
• Inability to write despite intact language
  comprehension and production
• Involvement of LAG in alexia and agraphia
  show its responsible for language related
  visual input
Broca’s Aphasic Video
   (shown in class)
    Wernicke-Geshwind Model
• Seven components in Left hemisphere:
  primary visual cortex, angular gyrus,
  primary auditory cortex, Wernicke’s area,
  arucate fasciculus, Broca’s area, and
  primary motor cortex
 Responding to a heard question
• Primary auditory cortex to Wernicke’s area
  where comprehended
• To respond, concept generated in
  Wernicke’s area, goes via arcuate fasciculus
  to Broca’s area, then to primary motor
  cortex and articulatory areas (face, lip, and
  tongue muscles, voice box, and muscles
  assoicated with lungs)
             Reading aloud
• Primary visual cortex to left angular gyrus,
  which transmits visual code to auditory
• Then to Wernicke’s area to arcuate
  fasciculus to Broca’s to primary motor
  cortex to articulatory areas
   Evidence against W-G Model
• Damage to these boundaries has little lasting effect
  on language
• Damage to other brain areas can produce aphasia
• Broca’s and Wernicke’s aphasia are rarely “pure”
  - aphasia is both receptive and expressive
• Major individual differences for cortical
  localization for language
      Cognitive Neuroscience
      Approach to Language
• Cannot perform lesion studies because
  humans are only known species with
• Use Cognitive Neuroscience (brain
  imaging) to study relation of brain and
      Cognitive Neuroscience
      Approach to Language
(1) Each of of the components in W-G model
    can be broken down further into
    constituent cognitive processes
  (1) Phonological analysis (sounds)
  (2) Grammatical analysis (structure)
  (3) Semantic analysis (meaning)
      Cognitive Neuroscience
      Approach to Language
(2) Areas of brain involved in language are
  not solely dedicated to language; many of
  the constituent cognitive processes also play
  roles in other behavior
Example - some areas involved in short-term
  memory and visual pattern recognition are
  involved in reading, too
      Cognitive Neuroscience
      Approach to Language
(3) W-G model assumes that brain areas
  involved in language are large,
  circumscribed, and homogenous but Cog
  neuro assumes they are small, widely
  distributed, and specialized
       Dyslexia and Cognitive
• Dyslexia is pathological difficulty in
  reading, does not result from general visual,
  motor, or intellectual deficits
• Developmental dyslexia- apparent in
• Acquired dyslexia - damage in individuals
  who were already capable of reading
      Developmental Dyslexia
(1) Differences between brains of dyslexic
  and non-dyslexic readers have been
  reported, however none seems to play a
  critical role
  Example - dyslexics do not display the
  asymmetry of the planum temporale
      Developmental Dyslexia
(2) Several types of dylexias and thus likely to
  have different causes and brain areas
      Developmental Dyslexia
(3) Difficult to determine cause-and-effect of
  brain abnormalities
Are these abnormalities the cause of dylexia
  or the result of lack of reading experience
  (brain develops differently as a result of
  different experience)?
          Acquired Dyslexia
• Two strategies for reading aloud:
  – Lexical procedure - based on specific stored
    information that has been acquired about
    written words - looks at it, recognizes it and
    says it
  – Phonetic procedure - looks at words,
    recognizes letters, sounds them out and says
           Acquired Dyslexia
• Surface Dyslexia - patients lose ability to to
  pronounce words based on the specific memories
  of the words (they lose their lexical procedure)
• Can pronounce non-words - wug
Example: can pronoun words consistent with rules (
  fish, river, or glass) but can’t pronounce unusual
  words (have, lose, and steak are like cave, hose,
  and beak)
         Acquired Dyslexia
• Deep Dyslexia - patients lose ability to
  apply common rules of pronunciation ( they
  lose their phonetic procedure)
• Can’t pronounce non-words
Example: can say phonetically unusual words
  like aisle and yacht but not pronunciation
  rule-consistent words like fish, river, or
          Acquired Dyslexia
• Where are lexical and phonetic processes in
  the brain?
• Deep dyslexia (lose phonetic procedure)
  due to damage in LH
• Surface dyslexia (lose lexical) due to partial
  LH damage or RH damage