pinel basics ch14
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Copyright © 2006 by Allyn and Bacon
Chapter 14Lateralization, Language,
and the Split Brain
The Left Brain and the Right Brain of LanguageThis multimedia product and its contents are
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Copyright © 2006 by Allyn and Bacon
Lateralization of Function
Major differences between the function of the left and right cerebral hemispheres
Cerebral commissures connect the 2 halves of the brain
Split-brain patients – what happens when the connections are severed?
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Cerebral Lateralization of Function
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 so out of context – also a consequence of damage on the left
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Cerebral Lateralization of Function
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
Anesthetize one and check for language function Dichotic listening
Report more digits heard by the dominant half Functional brain imaging
fMRI or PET used to see which half is active when doing a language test
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Speech Laterality, Handedness, and Sex Dextrals: right-handers Sinestrals: left-handers Left-hemisphere dominant in almost
all dextrals and most sinestrals Females may use both hemispheres
more often for language tasks than men do – less lateralized
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The Split Brain
Corpus callosum – largest cerebral commissureTransfers learned information from one
hemisphere to the otherWhen cut, each hemisphere functions
independently Studying split-brain cats – transect corpus
callosum and optic chiasm so that visual information can’t cross
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The Split Brain
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Split-Brain Cats
Each hemisphere can learn independently Split-brain cats with one eye patched
Learn task as well as controlsNo memory or savings demonstrated when
the patch was transferred 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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Commissurotomy in Human Epileptics Commissurotomy limits convulsive activity
many never have another major convulsion Sperry & Gazzaniga
developed procedures to test split-brain patients
Differ from split-brain animals in that the 2 hemispheres have very different abilities – most left hemispheres are capable of speech, while the right aren’t
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Testing Split-Brain Patients
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Hemispheres of Split-Brain Patients Function Independently 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
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Cross-Cuing
Allows the two hemispheres of a split-brain subject to communicate with each other indirectly.
Gazzaniga’s color test showed that neurological patients make use of various strategies, including interpreting their own physical cues, to correct their answers.
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Learning 2 Things at Once
Each hemisphere of a split-brain can learn independently and simultaneously
Presented with 2 different visual stimuli Helping-hand phenomenon – the hand
that “knows” may correct the other Chimeric figures task
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Dual Mental Functioning and Conflict in Split-Brain Patients In most split-brain patients, the left
hemisphere seems to control most activities.
In a few patients, the right hemisphere takes a more active role in controlling behavior, which can create conflicts between the left and right hemispheres.
Peter, the Split-Brain Patient
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Differences Between the Hemispheres For many functions the hemispheres
do not differ and where there are differences, these tend to be minimal
Lateralization of function is statistical, not absolute
Right hemisphere has some language abilities
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Examples of Lateralization of Function Right hemisphere superiority
Spatial abilityEmotionMusical abilitySome memory tasks
Left hemisphere – superior in controlling ipsilateral movement
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Right hemisphere superiority
Spatial ability Better at matching 3-D image with 2-D
Emotion Better at perceiving facial expressions and mood
Musical ability Better at perception of melodies
Some memory tasks Nonverbal material Learning tasks where context doesn’t matter
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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 – left is better at judging above or below, right at how close 2 things are
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Anatomical Brain Asymmetries
Planum temporale (Wernicke’s Area)Temporal lobe, posterior lateral fissureLanguage comprehension
Heschl’s gyrus - primary auditory cortex Frontal operculum (Broca’s Area)
Near face area of primary motor cortexLanguage production
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Anatomical Brain Asymmetries
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Anatomical Brain Asymmetries
Although asymmetries are seen in language related areas, these regions are not all larger in the left
Left planum temporal – larger in 65% Heschl’s gyri – larger on the right
2 in the right, only 1 in left Frontal operculum – visible surface suggests
right is larger, but there is greater volume on left
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Theories of Cerebral Aysmmetry All propose that it’s better to have brain
areas that do similar things be in the same hemisphere
Analytic-synthetic theory2 modes of thinking, analytic (left) and
synthetic (right)Vague and essentially untestable“the darling of pop psychology”
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Theories of Cerebral Aysmmetry Motor theory
Left controls fine movements – speech is just a category of movement
Left damage may produce speech and motor deficits
Linguistic theoryPrimary role of left is language
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Evolution of Lateralization of Function Nonhuman primates tend to use their right
hand for certain tasks Hand preference is under genetic
influence in chimps Indicates tool use was not the major factor
in the evolution of lateralization Lateralization of aspects of communcation
and emotion also seen in other species
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Antecedents of the Wernicke-Geschwind Model Language localization – intrahemispheric
organization of language circuitry Broca’s area – production
Damage > expressive aphasia Normal comprehension, speech is meaningful – but
awkward Wernicke’s area – comprehension
Damage > receptive aphasia Poor comprehension, speech sounds normal – but
has no meaning – ‘word salad’
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Antecedents of the Wernicke-Geschwind Model Arcuate fasciculus – connects Broca’s and
Wernicke’sDamage > conduction aphasiaComprehension and speech normal
Unable to repeat Left angular gyrus – posterior to
Wernicke’s areaDamage > alexia (inability to read) and
agraphia (inability to write)
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Evaluation of the Wernicke-Geschwind Model Can it predict the deficits produced by damage
to various parts of the cortex? Surgery that destroys only Broca’s area has no
lasting effects on speech Removal of much of Wernicke’s area does not
have any lasting effects on speech Some argue that failure to support the model is
due to pathology-related reorganization
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Effects of Damage to Areas of Cortex on Language Abilities
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Current Status of the Wernicke-Geschwind Model Empirical evidence supports 2 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
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Cognitive Neuroscience Approach to Language Language-related behaviors are mediated
by activity in brain areas involved in the specific cognitive processes required for the behaviors.
Brain areas involved in language have other functions.
Brain areas involved in language are likely to be small, widely distributed, and specialized.
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Functional Brain Imaging and Language fMRI and reading – Bavelier determines
the extent of cortical involvement in reading
Use a sensitive fMRI machine to record activity during reading of sentencesAreas of activity were tiny and spread out.Active areas varied between subjects and
trials.Activity was widespread.
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Functional Brain Imaging and Language
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fMRI and Reading
More activity in the left hemisphere than in the right
Activity extended well beyond what Wernicke-Geschwind predicted
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Functional Brain Imaging and Language PET and naming
Images of famous faces, animals, and toolsActivity 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 Approach and Dyslexia Dyslexia – reading difficulties not
due to some other deficit Developmental dyslexia – apparent
when learning to read Acquired dyslexia – due to brain
damage
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Developmental Dyslexia
Brain differences identified, but none seems to play a role in the disorder
Multiple types of developmental dyslexia – possibly multiple causes
Differences could be due to reading problems, not the cause of difficulties
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Developmental Dyslexia
Various subtle visual, auditory, and motor deficits are commonly seen
Are these deficits the primary problem – do they cause the dyslexia?
Genetic component – yet the disorder is also influenced by culture
More English speakers are dyslexic than Italian – due to English being more complex
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Acquired Dyslexia - Deep and Surface 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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Acquired Dyslexia - Deep and Surface Where’s the damage? Deep dyslexia – extensive damage to left-
hemisphere 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