CH 16
LATERALIZATION,
LANGUAGE &
THE SPLIT BRAIN
Intro
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Like most everything in our body, the brain is bilateral
Left & right hemispheres are entirely separate except for
the cerebral commisures connecting them
Major differences exist between the functions of the
hemispheres
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Split-brain patients: those whose hemispheres have been
separated
Language is the most lateralized of all cognitive abilities
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Lateralization of function
(Mostly left hemi)
Hemis have different abilities & can function independently
Cerebral Lateralization of Function
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Broca’s area:
 Inferior
prefrontal cortex of the
left hemisphere
 Patients with aphasia (inability to
produce or comprehend language) had damage to this
area
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Apraxia (difficulty performing movements when
asked to do so out of context) almost always
associated with left hemi damage, even though
symptoms are bilateral
Cerebral Dominance
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Cerebral dominance: Idea that one hemi (usually
left) plays the dominant role in controlling all
complex behavioral & cognitive processes
So the left hemi is commonly called the dominant
hemisphere & the right is the minor hemisphere
Tests of Cerebral Lateralization
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Sodium Amytal Test:
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During neurosurgery, inject sodium amytal to anesthetize one
hemisphere of the brain & have patient recite a series of words
When in left hemi, patient becomes mute for a few minutes
When in right hemi, no effect on language
Dichotic Listening Test:
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Audio of #s being read played through headphones, with a different
set of #s going to each ear (simultaneously)
When asked to repeat all the #s, most people say more #s heard in
the right ear
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Indicating left brain hemi for language; contralateral
Functional Brain Imaging:
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PET or fMRI scans
During language tests, more activity is shown in the left hemi
Relation Between Speech Laterality &
Handedness
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Dextrals: right handers
Sinestrals: left handers
Study of handedness, hemisphere
damage & aphasia showed that
the left hemi is dominant for
language for almost all dextrals
& most sinestrals
Sinestrals are more variable in
which hemi controls language
Split Brain
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Corpus callosum: brain tissue that
connects the 2 hemispheres
The largest cerebral commisure
 Contains 200 million axons
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A study using cats with transected (cut) corpus callosums
showed that they were equally able to learn a task using
one hemi as when using both
When tested using the opposite hemi, it was as if they had
never learned it
 Effectively showing the hemis acted as 2 separate brains
 Conclusion: function of corpus callosum is to transmit info
between the hemispheres
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Commissurotomy in Human Epileptics
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Commissurotomy: transecting the corpus callosum
 Done
as a treatment for severe epilepsy to prevent the
spread of the over-stimulated signal from one hemi to the
other
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Tests done by delivering info to one hemi while keeping
it out of the other
 Like
with split-brain animals, split-brain humans seem to
have 2 independent brains, each with its own stream of
consciousness, abilities, memories & emotions
 Unlike the animals, human hemis are unequal in their
abilities to perform certain tasks
 Especially
left hemi is capable of speech, right is not
Hemispheres Functioning Independently
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Reminder: Input from one visual field or movement/feeling from
one hand go to the contralateral hemisphere
Hemispheres Functioning Independently
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Left hemisphere can tell what it has seen, right
hemisphere can show it.
Studies of split-brain patients:
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)
Subject will report that they do not know what it was
Left hand can show you what it was, right can’t
Doing 2 Things at Once
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Your brain can learn 2 different things at once
When shown 2 different pictures (one in each visual
field), patients can reach into 2 bags (one with each
hand) and correctly grab the items they saw
 However,
if you ask them what was in their hands, they
would say 2 of what was shown on the right & be
surprised when they looked at the objects in their hands
and saw 2 different items
Doing 2 Things at Once
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Experiment is repeated, but instead of reaching into
bags, the patient can see the objects in front of
them
When the patient is asked to pick up what was seen
sometimes the helping-hand phenomenon occurs
This is when the right hand goes to pick up what
was seen by the left hemi & the right hemi
“realizes” that is the wrong object (not what the
right hemi saw) & causes the left hand to shoot out
to redirect the right hand towards the correct object
Doing 2 Things at Once
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Because the hemis are effectively seeing twice as
much at once, split brain patients can find a visual
target in a group of items more quickly than healthy
individuals
Chimeric figures test
 Visual
completion
 Scotoma (blind spot)
Split Brain Misc.
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For most split brain patients, the left hemi tends to control
most of everyday activities
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Split brain hemis mostly act independently, but they can
interact via brain stem
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However, in some cases, the right hemi has a will of its own &
will create conflicts with the left hemi
Individuals can vary on hemispheric independence
Emotional info about a picture presented to right hemi
can be transferred to left hemi which can communicate
the feeling, even when it doesn’t know what the picture
was
More complex tasks tend to involve both hemis
Elderly display less lateralization of function
Differences between Left & Right Hemis
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Many functions have no difference between the hemis
When there are differences, they tend to be a slight
bias in favor of one hemi, NOT a
clear cut, absolute difference
Functions do not reside exclusively
in one hemi or the other
Language is the most lateralized
cognitive ability, but even it is not
totally absent from the right hemi
 Right
hemi language skills like that of a preschooler
Cerebral Lateralization of Function
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Superiority of left hemi in controlling ipsilateral movement
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Feeling an object in hand & deciding which
2-D image shows what it would look like
unfolded
Specialization of right hemi for emotion
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but
Superiority of right hemi in spatial ability
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Most movement controlled contralaterally,
some ipsilateral & left is better at it
Better at identifying facial expressions of emotion
Superior musical ability of right hemi
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Dichotic listening test with musical tunes, better able to identify
with left ear
Cerebral Lateralization of Function
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Hemispheric differences in memory
Both hemis involved in memory, but differ in which is best at
certain tests
 Left hemi specialized for episodic memory
 Left hemi for memory of verbal info
 Right hemi for nonverbal info
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The hemis approach cognitive tasks in different ways
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Left hemisphere acts as the interpreter; continuously assessing
patterns of events and trying to make sense of them
Left hemi dominant for language, but right is better at
perceiving intonation of speech & identifying the speaker
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Example of how these functional lateralizations are not
absolute
Anatomical Asymmetries of the Brain
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Frontal operculum
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Planum temporale
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In temporal lobe; called Wernicke’s area
Involved in comprehension of language
Larger in left hemi, but only in 65% of brains
Heschl’s gyrus
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In frontal lobe
In left hemi it is the location of Broca’s area
In temporal lobe; primary auditory cortex
Larger in right hemi, often 2 gyri in right & only 1 in left
Difficult to define the exact border/size of these structures
Evolution of Cerebral Lateralization
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Analytic-Synthetic Theory:
 Left
hemi operates in an analytical, logical, computerlike
way; analyzing stimulus info input sequentially, collecting
extracting relevant info & attaching a verbal label
 Right hemi synthesizes; concerned with overall stimulus
configuration and organizes & processes info in terms of
wholes
 Mostly pop psychology; difficult to test empirically
Evolution of Cerebral Lateralization
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Motor Theory:
 Left
hemi is specialized for speech because it is a type of
fine motor movements
 Doesn’t explain why motor function would
have become lateralized
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Linguistic Theory:
 Primary
role of left hemi is language
 Deaf people with left hemi damage have difficulty using
sign language, but not pantomime
Evolution of Cerebral Lateralization
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All classes of vertebrates have a right side
preference for feeding
Once hands evolved (monkeys & apes), there was a
right hand preference for feeding and other
complex behaviors
Left hemi bias for communication in non-human
species
 Ex:
birdsong, dogs & monkeys for calls of conspecifics
Advantages of Cerebral Lateralization
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Advantageous for areas of the brain that perform
similar functions to be located in the same hemi
May be more efficient for neurons performing a
particular function to be concentrated in one hemi
2 different kinds of cognitive processes can be more
easily performed simultaneously if they are
lateralized to diff hemis
Cortical Localization of Language
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Language localization refers to the location with the
hemis of the circuits that participate in languagerelated activities
Wernicke-Gerschwind Model
 The
predominant theory of language localization
Cortical Localization of Language
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Broca’s area controls speech production
Wernicke’s area controls language comprehension
Broca’s aphasia:
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Lesions of Broca’s area hypothesized to produce aphasia with symptoms that
are primarily expressive
Normal comprehension of written/spoken language
Speech that retains meaningfulness despite being slow, labored, disjointed &
poorly articulated)
Wernicke’s aphasia:
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Lesions of Wernicke’s area produce aphasia with symptoms that are
primarily receptive
Poor comprehension of both written/spoken language
Speech that is meaningless but still retains superficial structure, rhythm &
normal intonation
Word salad
Cortical Localization of Language
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Conduction aphasia:
Caused by damage to the pathway connecting Broca’s &
Wernicke’s area (arcuate fasciculus)
 Mostly intact comprehension & speech, with difficulty
repeating words they just heard
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Angular gyrus controls comprehending language-related
visual input
Area of left temporal & parietal cortex just posterior to
Wernicke’s area
 Damage to this area can cause alexia (inability to read) &
agraphia (inability to write)
 No difficulty speaking or understanding speech
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Wernicke-Geschwind Model
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7 components (all in left hemi):
Primary visual cortex
Angular gyrus
Primary auditory cortex
Wernicke’s area
Arcuate fasciculus
Broca’s area
Primary motor cortex
Wernicke-Geschwind Model
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During a conversation, auditory info is received by
primary auditory cortex & sent to Wernicke’s area,
where they are comprehended
To respond, Wernicke’s area generates the neural
representation of the thought underlying the reply
& transmits it to Broca’s area (via left arcuate
fasciculus) where it activates the appropriate
program of articulation that fires the neurons of
primary motor cortex & then muscles of articulation
Wernicke-Geschwind Model
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When reading aloud, signal received by primary
visual cortex is transmitted to left angular gyrus,
which translates visual form of word into its auditory
code & transmits it to Wernicke’s area for
comprehension
Wernicke’s area then triggers responses in arcuate
fasciculus, Broca’s area & motor cortex to elicit
speech sounds
Problems with the Wernicke-Geschwind Model
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Many aspects of this theory are oversimplifications
Removal of Broca’s area but no surrounding area has
no lasting effects on speech
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Speech problems may be due to swelling of surrounding
area
No permanent speech difficulties with lesions to arcuate
fasciculus
No permanent alexia or agraphia with lesions of
angular gyrus
Much of Wernicke’s area can be removed with no long
term language deficits
Problems with the Wernicke-Geschwind
Model
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More recent data has shown:
No aphasic patients have damage restricted to just Broca’s or
Wernicke’s area
Aphasic patients almost always have significant damage to
subcortical white matter
Large anterior lesions are more likely to produce expressive
symptoms; large posterior lesions more likely to produce
receptive symptoms
Global aphasia (severe disruption of all language related
abilities) is usually related to massive lesions of anterior cortex,
posterior cortex & underlying white matter
Aphasic patients sometimes have brain damage not near the
Wernicke-Geschwind areas
Further Study of the WernickeGeschwind Model
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More detailed info about areas related to language
function by testing with electrical stimulation (as opposed
to damage/lesions)
Sites at which stimulation blocked/disrupted speech are
scattered throughout frontal, temporal & parietal cortex
(not just restricted to Wernicke-Geshwind areas)
No specific areas that caused specific speech
disturbances (ex: pronunciation, naming objects)
Right hemi stimulation almost never disrupted speech
Major differences among individuals & their neural
organization of language abilities
Current State of Wernicke-Geschwind
Model
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2.
Original model supported in 2 ways
Broca’s & Wernicke’s areas play important roles in
language
Tendency for aphasias associated with anterior
damage to involve expressive deficits & posterior
damage to involve receptive deficits
Current State of Wernicke-Geschwind
Model
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Original model not supported
Aphasia typically associated with widespread damage,
not just to Wernicke-Geschwind areas
Aphasia can result even when damage does not involve
any Wernicke-Geschwind areas
Broca’s & Wernicke’s aphasias rarely exist in pure forms;
aphasia almost always involves both expressive &
receptive symptoms
Major differences in locations of cortical language areas
in different individuals
Generally, the theory has been abandoned by researchers
Cognitive Neuroscience of Language
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Currently used in language research
Defined by 3 premises:
Premise 1: Each complex language related process
(speech, comprehension, reading) is the combo of
several constituent cognitive processes, which may be
organized separately in different parts of the brain.
Premise 2: Areas of the brain involved in language
are not dedicated solely to that purpose
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Ex: language areas can also function in memory
Premise 3: Brain areas for language are small,
widely distributed & specialized
Functional Brain Imagine & Localization
of Language
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fMRI study showed that areas of the brain involved
in silent reading were patchy, variable among
patients & not limited to classic WernickeGeschwind areas
 Far
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more activity in left hemi
PET study of activity in temporal lobe while naming
objects in categories
 Involved
brain areas outside classic WernickeGeschwind areas
 Area activated depended on category of the objects
Cognitive Neuroscience of Dyslexia
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1.
Dyslexia: pathological difficulty in reading, not
resulting from general visual, motor, or intellectual
deficits
2 types:
Developmental:
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Becomes apparent as child learns to read
5-11% of English speaking kids
2-3x higher in boys than girls
Acquired:
2.
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Caused by brain damage in people who could already
read
Rare
Developmental Dyslexia
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Has a major genetic component (50% heritability)
No single kind of brain pathology has been found
in all cases
Disorder has various forms, likely with different
neural correlates
Results from disturbance of phonological processing
(representation & comprehension of speech sounds)
Acquired Dyslexia
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1.
2 types:
Surface:
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Inability to pronounce words based on their memories of
the words (lexical procedure) but can still apply rules of
pronunciation in reading (phonetic procedure)
Difficulty pronouncing words that don’t follow common
rules of pronunciation (ex: have, lose, steak) & will often
incorrectly pronounce them based on rules (ex: rhyming
with cave, hose, beak)
Deep:
2.
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Inability to apply rules of pronunciation in their reading
(lost phonetic procedure)
Incapable of pronouncing nonwords