Historical Antecedents of the Wernicke-Geschwind

PowerPoint Presentation for
Biopsychology, 8th Edition
by John P.J. Pinel
Prepared by Jeffrey W. Grimm
Western Washington University
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Chapter 16
Lateralization, Language, and
the Split Brain
The Left Brain and the Right
Brain of Language
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Cerebral Lateralization of
Function



Major differences between the function of
the left and right cerebral hemispheres
Cerebral commissures connect the two
halves of the brain
Split-brain patients have been studied to
understand what happens when these
connections are severed
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FIGURE 16.1 The cerebral hemispheres and
cerebral commissures.
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Discovery of the Specific
Contributions of LeftHemisphere Damage to Aphasia
and Apraxia



Aphasia – deficit in language comprehension
or production due to brain damage, usually
on the left
Broca’s area – left inferior prefrontal cortex,
damage leads to expressive aphasia
Apraxia – difficulty performing movements
when asked to do so out of context, also a
consequence of damage on the left
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Cerebral Lateralization of
Function Continued



Aphasia and apraxia – associated with
damage to left hemisphere
Language and voluntary movement seem to
be controlled by one half of the brain,
usually the left
Suggests that one hemisphere is dominant,
controlling these functions
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Tests of Cerebral
Lateralization

Determining which hemisphere is dominant

Sodium amytal test


Dichotic listening


Anesthetize one hemisphere and check for language
function
Report more digits heard by the dominant half
Functional brain imaging

fMRI or PET used to see which half is active when
performing a language test
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Discovery of the Relation
between Speech Laterality and
Handedness

Left hemisphere is speech dominant in
almost all dextrals (right-handers) and
most sinestrals (left-handers)
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Sex Differences in Brain
Lateralization

Females may use both hemispheres more
often for language tasks than men do
(females may be less lateralized)
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The Split Brain

Corpus callosum – largest cerebral
commissure
 Transfers learned information from one
hemisphere to the other
 When cut, each hemisphere functions
independently
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Groundbreaking Experiment of
Myers and Sperry

Studied split-brain cats

Transected the corpus callosum and optic chiasm
so that visual information could not cross to the
contralateral hemisphere
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FIGURE 16.3 Restricting visual information to one hemisphere in cats. To restrict
visual information to one hemisphere, Myers and Sperry (1) cut the corpus callosum,
(2) cut the optic chiasm, and (3) blindfolded one eye. This restricted the visual
information to the hemisphere ipsilateral to the uncovered eye.
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Split-Brain Cats Continued


Each hemisphere can learn independently
Split-brain cats with one eye patched

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

Learn task as well as controls
No memory or savings demonstrated when the
patch was transferred to other eye
Intact cats or those with an intact corpus
callosum or optic chiasm – learning
transfers between hemispheres
Similar findings with split-brain monkeys
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FIGURE 16.4 Schematic illustration of Myers and Sperry’s (1953) groundbreaking split-brain
experiment. There were four groups: (1) the key experimental group with both the optic chiasm
and corpus callosum transected, (2) a control group with only the optic chiasm transected, (3) a
control group with only the corpus callosum transected, and (4) an unlesioned control group.
The performance of the three control groups did not differ, so they are illustrated together here.
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Commissurotomy in Human
Epileptics

Commissurotomy limits convulsive activity


Sperry and Gazzaniga


Many never have another major convulsion
Developed procedures to test split-brain patients
Differ from split-brain animals in that the two
hemispheres have very different abilities –
most left hemispheres are capable of
speech, while the right aren’t
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FIGURE 16.5 The testing procedure that was used to evaluate the neuropsychological status of
split-brain patients. Visual input goes from each visual field to the contralateral hemisphere; fine
tactile input goes from each hand to the contralateral hemisphere; and each hemisphere
controls the fine motor movements of the contralateral hand.
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Evidence that the
Hemispheres of Split-Brain
Patients Can Function
Independently

Left hemisphere can tell what it has seen, right
hemisphere can only show it

Present a picture to the right visual field (left brain)



Left hemisphere can tell you what it was
Right hand can show you, left hand can’t
Present a picture to the left visual field (right brain)

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Subject will report that they do not know what it was
Left hand can Copyright
show© 2011
you
what it was, right can’t
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Cross-Cuing

Cross-cuing – facial feedback from the other
hemisphere
 For example, the right hemisphere might
make the face frown when the left
hemisphere gives an incorrect spoken
answer
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Doing Two Things at Once

Each hemisphere of a split-brain can learn
independently and simultaneously



Helping-hand phenomenon – presented with two
different visual stimuli, the hand that “knows” may
correct the other
Dual foci of attention – split-brain hemispheres can
search for target item in array faster than intact
controls
Chimeric figures task – only symmetrical version of
right half of faces recognized

Indicates competition between hemispheres
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The Z Lens



Advancing the study of split-brains with a
contact lens to restrict visual input to one
hemisphere
Previous studies had to limit viewing time to
less than .1 second
Can be used to assess each hemisphere’s
understanding of spoken instructions by
limiting essential visual information to one
side of brain
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FIGURE 16.7 The Z lens, which
was developed by Zaidel to study
functional asymmetry in splitbrain patients. It is a contact lens
that is opaque on one side (left or
right), so that visual input
reaches only one hemisphere.
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Dual Mental Functioning and
Conflict in Split-Brain Patients


Usually in split-brain patients the left
hemisphere is dominant in most everyday
activities
For some, the right is dominant and this can
create conflict between hemispheres


For example, the case of Peter
Hemispheres often disagreed with each other
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Independence of Split
Hemispheres: Current
Perspective


Emotional information somehow passed
between hemispheres
Difficult tasks are more likely to enlist
involvement of both hemispheres
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Differences between Left and
Right Hemispheres


For many functions there are no substantial
differences between hemispheres
Key point: Lateralization of function is
statistical rather than absolute
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Examples of Cerebral
Lateralization of Function



Left hemisphere: superior in controlling
ipsilateral movement
Left hemisphere: an “interpreter”
Right hemisphere superiority:



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Spatial ability
Emotion
Musical ability
Some memory tasks
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Table 16.1
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What is Lateralized—Broad
Clusters of Abilities or
Individual Cognitive
Processes?


Broad categories are not lateralized – individual
tasks may be
Better to consider lateralization of constituent
cognitive processes – individual cognitive elements

Example: two spatial tasks – left hemisphere is better
at judging above or below, right at how close two
things are
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Anatomical Brain Asymmetries

Frontal operculum (Broca’s area)
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Planum temporale (Wernicke’s area)

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Near face area of primary motor cortex
Language production
Temporal lobe, posterior lateral fissure
Language comprehension
Primary auditory cortex (Heschl’s gyrus)
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Anatomical Brain Asymmetries
Continued



Although asymmetries are seen in language related
areas, these regions are not all larger in the left
Left planum temporale – larger in only 65% of
human brains
Heschl’s gyri – larger on the right


Two in the right, only one in the left
Frontal operculum – visible surface suggests right
is larger, but left has greater volume
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FIGURE 16.9 The anatomical asymmetry detected in the planum temporale of musicians by
magnetic resonance imaging. In most people, the planum temporale is larger in the left
hemisphere than in the right; this difference was found to be greater in musicians with perfect
pitch than in either musicians without perfect pitch or controls. (Based on Schlaug et al., 1995.)
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Theories of the Evolution of
Cerebral Asymmetry

All theories propose that it’s better to have
brain areas that have similar functions be in
the same hemisphere: Analytic-synthetic
theory



Two modes of thinking, analytic (left) and
synthetic (right)
Vague and essentially untestable
“The darling of pop psychology”
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Theories of the Evolution of
Cerebral Asymmetry Continued

Motor theory

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
Left controls fine movements – speech is just a
category of fine movement
Left damage may produce speech and motor
deficits
Linguistic theory

Primary role of left hemisphere is language
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When Did Cerebral
Lateralization Evolve?



Lateralization of function may have been
present at the beginning of vertebrate
evolution
Right-handedness may have evolved from
a preference for use of the right side of the
body for feeding
Left-hemisphere dominance is present in
species that existed prior to humans

For example: birds, dogs, monkeys
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What Are the Survival
Advantages of Cerebral
Lateralization?


Increased neural efficiency to concentrate
function in one hemisphere
Two cognitive processes may be more
readily performed simultaneously if both are
lateralized to the same hemisphere
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Evolution of Human Language



Nonhuman primates appear to have more ability
in comprehending sounds vs. making vocal calls
This fits with the “motor theory of speech
perception”: posits that there is overlap between
speech comprehension and motor regions
involved in speech production
Chimpanzees have a highly nuanced vocabulary
of hand gestures

May indicate a stage in the development of
human language
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Cortical Localization of
Language: Wernicke-Geschwind
Model


Language localization – place within the
hemisphere of language circuitry
Wernicke-Geschwind Model

The predominant theory of language localization
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Historical Antecedents of the
Wernicke-Geschwind Model

Broca’s area – speech production



Damage leads to expressive aphasia
Normal comprehension; speech is meaningful, but
awkward
Wernicke’s area – speech comprehension


Damage causes receptive aphasia
Poor comprehension; speech sounds normal, but
has no meaning (“word salad”)
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Historical Antecedents of the
Wernicke-Geschwind Model
Continued

Arcuate fasciculus – connects Broca’s and
Wernicke’s areas



Damage causes conduction aphasia (inability to
repeat words just heard)
Comprehension and speech normal
Left angular gyrus – posterior to Wernicke’s
area

Damage causes alexia (inability to read) and
agraphia (inability to write)
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The Wernicke-Geschwind
Model


Norman Geschwind integrated the ideas of
Broca, Wernicke, and Dejerine into this
theory
Involves seven components, all of which are
in the left hemisphere
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FIGURE 16.10 The seven components
of the Wernicke-Geschwind model.
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FIGURE 16.11 How the Wernicke-Geschwind model works in a person who is responding to a
heard question and reading aloud. The hypothetical circuit that allows the person to respond to
heard questions is in green; the hypothetical circuit that allows the person to read aloud is in
black.
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rights reserved.
Wernicke-Geschwind Model:
The Evidence

Lack of evidence that damage to various parts
of the cortex has expected effects


Surgery that destroys only Broca’s area has no
lasting effects on speech
Removal of much of Wernicke’s area has no
lasting effects on speech
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Effects of Cortical Damage on
Language Abilities






No aphasic patients have damage restricted to Broca’s or
Wernicke’s areas
Aphasics almost always have damage to subcortical white
matter
Large anterior lesions most likely to produce expressive
symptoms
Large posterior lesions most likely to produce receptive
symptoms
Global aphasia is usually related to massive lesions of
several regions
Aphasics sometimes have damage that does not
encroach on Wernicke-Geschwind areas
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FIGURE 16.13 The lack of permanent disruption of language-related abilities after surgical
excision of the classic Wernicke-Geschwind language areas. (Based on Penfield & Roberts,
1959.)
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rights reserved.
Effects of Electrical
Stimulation to the Cortex on
Language Abilities


Stimulated sites that affected language were
not necessarily within the boundaries of the
Wernicke-Geschwind language areas
There were major differences between
subjects in the organization of language
abilities
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Current Status of the WernickeGeschwind Model

Empirical evidence supports two elements



Important roles played Broca’s and Wernicke’s –
many aphasics have damage in these areas
Anterior damage associated with expressive
deficits and posterior with receptive
No support for more specific predictions



Damage limited to identified areas has little lasting
effect on language
Brain damage in other areas can produce aphasia
Pure aphasias (expressive OR receptive) rare
Copyright © 2009 Allyn & Bacon
Cognitive Neuroscience of
Language

Premise: activity in brain areas for specific
cognitive processes . . .



underlie language-related behaviors
have functions independent of language
are likely to be small, widely distributed, and
specialized
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Functional Brain Imaging and
Localization of Language


Bevalier’s fMRI study of reading – sought to
establish cortical involvement in reading
Reading sentences versus control periods
(strings of consonants)



Areas of activity were tiny and spread out
Active areas varied between subjects and trials
Activity was widespread
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FIGURE 16.16 The areas in which reading-associated increases in activity were observed
in the fMRI study of Bavelier and colleagues (1997). These maps were derived by
averaging the scores of all participants, each of whom displayed patchy increases of
activity in 5–10% of the indicated areas on any particular trial.
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Damasio’s PET Study of
Naming

Domasio and colleagues (1996) PET study of
naming




Images of famous faces, animals, and tools
Activity while judging image orientation subtracted
from activity while naming
Left temporal lobe areas activated by naming
varied with category
Activity seen well beyond Wernicke’s area
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Cognitive Neuroscience of
Dyslexia



Dyslexia – reading difficulties not due to some other
deficit (e.g., vision, intelligence)
Developmental dyslexia – apparent when learning to
read
 Heritability estimate = 50%
 More common in boys than girls
Acquired dyslexia
 Due to brain damage
 Relatively rare
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Developmental Dyslexia:
Causes and Neural Mechanisms



Brain differences identified, but none seems
to play a role in the disorder
Multiple types of developmental dyslexia –
possibly multiple causes
Perhaps a deficit of phonological processing
rather than sensorimotor processing
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Developmental Dyslexia
Continued


Various subtle visual, auditory, and motor
deficits are commonly seen
Genetic component – yet the disorder is also
influenced by culture

More English speakers have reading deficits
than Italian speakers do, perhaps because
sound-symbol correspondence in English is
more complex than in Italian
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Cognitive Neuroscience of
Deep and Surface Dyslexia

Two procedures for reading aloud




Lexical – using stored information about words
Phonetic – sounding out
Surface dyslexia – lexical procedure lost,
can’t recognize words
Deep dyslexia – phonetic procedure lost,
can’t sound out unfamiliar words
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Cognitive Neuroscience of
Deep and Surface Dyslexia
Continued



Surface dyslexia – loss of visual recognition
of words (cannot “look and say”)
Deep (or “phonological”) dyslexia – loss of
ability to “sound out” unfamiliar words or
“nonwords”
Different error patterns for surface and deep


Surface: “quail” for “quill”
Deep: “hen” for “chicken”
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Cognitive Neuroscience of
Deep and Surface Dyslexia
Continued

Deep dyslexia – extensive damage to lefthemisphere language areas

How is it that lexical abilities are spared?



Lexical abilities may be housed in left language areas
that are spared
Lexical abilities may be mediated by the right
hemisphere
Evidence for both exists
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rights reserved.
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