1. Prepared by Jeffrey W. Grimm Western Washington University
PowerPoint Presentation for Biopsychology, 9th Edition by John P.J. Pinel
Prepared by Jeffrey W. Grimm
Western Washington University
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2. The Left Brain and the Right Brain
Chapter 16
Lateralization, Language,
and the Split Brain
The Left Brain and the Right Brain
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3. Cerebral Lateralization of Function
There are major differences between the functions of the left and right cerebral hemispheres.
Cerebral commissures connect the two halves of the brain.
Studies of split-brain patients have been instrumental in developing our understanding of what happens when these connections are severed.
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Discovery of the Specific Contributions of Left-Hemisphere 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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5. Cerebral Lateralization of Function (Con’t)
Aphasia and apraxia are associated with damage to left hemisphere.
Language and voluntary movement seem to be controlled by one half of the brain, usually the left.
This suggests that one hemisphere is dominant, controlling these functions.
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6. Tests of Cerebral Lateralization
Determining Which Hemisphere Is Dominant
Sodium amytal test
Anesthetize one hemisphere and check for language function
Functional brain imaging
fMRI or PET used to see which half is active when performing a language test
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7. Sex Differences in Brain Lateralization
Women may use both hemispheres more often for language tasks than men do; women 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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9. 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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10. Split-Brain Cats (Con’t)
Each hemisphere can learn independently.
Split-brain cats with one eye patched:
Learn task as well as controls
No memory or savings demonstrated when the patch was transferred to other eye
In intact cats, or those with an intact corpus callosum or optic chiasm, learning transfers between hemispheres.
There have been similar findings with split-brain monkeys.
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11. Commissurotomy in Human Epileptics
Commissurotomy limits convulsive activity.
Many of those who undergo the procedure never have another major convulsion.
Sperry and Gazzaniga
Developed procedures to test split-brain patients
Split-brain humans differ from split-brain animals in that the two hemispheres of the human brain have very different abilities—most left hemispheres are capable of speech, while the right hemispheres are not.
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Evidence that the Hemispheres of Split-Brain Patients Can Function Independently
The left hemisphere can tell what it has seen; the right hemisphere can only show it.
Present a picture to the right visual field (left brain).
The left hemisphere can tell you what it was.
The right hand can show you; the left hand can’t.
Present a picture to the left visual field (right brain).
Subjects will report that they do not know what it was.
The left hand can show you what it was; the right can’t.
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13. Dual Mental Functioning and Conflict in Split-Brain Patients
In split-brain patients, the left hemisphere is usually dominant in most everyday activities.
For some, the right is dominant, and this can create conflict between hemispheres.
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14. Differences between the 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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15. Examples of Cerebral Lateralization of Function
Left hemisphere: an “interpreter”
Right hemisphere superiority:
Spatial ability
Emotion
Musical ability
Some memory tasks
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Table 16.1
Abilities that Display Cerebral Lateralization of Function
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What is Lateralized: Broad Clusters of Abilities or Individual Cognitive Processes?
Broad categories are not lateralized; individual tasks may be.
It is more useful to consider lateralization of constituent cognitive processes: individual cognitive elements.
E.g., two spatial tasks: the left hemisphere is better at judging above or below; the right is better at determining how close two things are to each other.
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18. Anatomical Brain Asymmetries
Frontal Operculum (Broca’s Area)
Near face area of primary motor cortex
Language production
Planum Temporale (Wernicke’s Area)
Temporal lobe, posterior lateral fissure
Language comprehension
Primary Auditory Cortex (Heschl’s Gyrus)
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19. Theories of the Evolution of Cerebral Asymmetry
All theories propose that it’s better to have brain areas that have similar functions 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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20. Theories of the Evolution of Cerebral Asymmetry (Con’t)
Motor Theory
Left controls fine movements; speech is just a category of fine movement.
Left damage may produce speech and motor deficits.
Linguistic Theory
The primary role of the left hemisphere is language.
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21. 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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22. Historical Antecedents of the Wernicke-Geschwind Model (Con’t)
The arcuate fasciculus connects Broca’s and Wernicke’s areas.
Damage to the arcuate fasciculus causes conduction aphasia (inability to repeat words just heard).
Comprehension and speech are normal.
Left angular gyrus: posterior to Wernicke’s area
Damage causes alexia (inability to read) and agraphia (inability to write).
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FIGURE The seven components of the Wernicke-Geschwind model.
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24. 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 are most likely to produce expressive symptoms.
Large posterior lesions are 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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25. 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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26. Cognitive Neuroscience of Dyslexia
Dyslexia: reading difficulties not due to some other deficit (e.g., vision, intelligence)
Developmental dyslexia becomes apparent when the affected person is learning to read.
Heritability estimate = 50 percent
More common in boys than in girls
Acquired Dyslexia
Due to brain damage
Relatively rare
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27. Developmental Dyslexia: Causes and Neural Mechanisms
Brain differences have been identified, but none seem to play a role in the disorder.
There are multiple types of developmental dyslexia—perhaps there are multiple causes.
Perhaps dyslexia is a deficit of phonological processing rather than of sensorimotor processing.
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28. Developmental Dyslexia (Con’t)
Various subtle visual, auditory, and motor deficits are commonly seen.
There is a genetic component—yet the disorder is also influenced by culture.
More English speakers have reading deficits than do Italian speakers, perhaps because sound-symbol correspondence in English is more complex than it is in Italian.
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29. 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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30. Cognitive Neuroscience of Deep and Surface Dyslexia (Con’t)
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 are seen from those with surface versus deep dyslexia.
Surface: “quail” for “quill”
Deep: “hen” for “chicken”
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31. Cognitive Neuroscience of Deep and Surface Dyslexia (Con’t)
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.
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