Lateralization & The Split Brain
and
Cortical Localization of Language
Ch. 16
Outline
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The Dominant Left Hemisphere
Tests of Cerebral Lateralization
The Split-Brain Experiment
Tests of Split-Brain Patients
Differences Between the Left and Right
Hemispheres
• 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
production
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
laterality
Tests of Cerebral Lateralization
• PET of FMRI techniques have revealed that
there is typically more activity in the left
hemisphere than the right during languagerelated activities
Tests of Cerebral Lateralization
• Many studies have reported a relation
between speech laterality and handedness;
the following general conclusions have been
reached:
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
callosum
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:
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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-corpuscallosum-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
hemisphere
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
brain
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
hand
– Could not pick out the correct object with the
right hand
– Claimed nothing had been presented
Cross-cuing
• 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
green…
Cross-cuing
• 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
Cross-cuing
• 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
hemisphere
• 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
salad)
Conduction Aphasia
• Damage to pathway connecting Broca’s and
Wernicke’s areas called the arcuate
fasciculus
• Comprehension and spontaneous speech are
intact but patient not able to repeat words
they have just heard
Alexia
• 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
Agraphia
• Also due to damage to the left angular
gyrus
• 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
code
• 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
language
• Use Cognitive Neuroscience (brain
imaging) to study relation of brain and
language
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
Neuroscience
• Dyslexia is pathological difficulty in
reading, does not result from general visual,
motor, or intellectual deficits
• Developmental dyslexia- apparent in
childhood
• 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
susceptible
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
word
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
glass
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