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 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

Cerebral Lateralization of Function Continued  

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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 

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Dichotic listening 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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 

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Sperry and Gazzaniga 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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)  

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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 Copyright show© 2011 you what it was, right can’t Pearson Education, Inc. All rights reserved.

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 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

The Z Lens 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

Independence of Split Hemispheres: Current Perspective 

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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 

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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 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

Table 16.1

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What is Lateralized—Broad Clusters of Abilities or Individual Cognitive Processes? 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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 

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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 

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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 

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Two modes of thinking, analytic (left) and synthetic (right) Vague and essentially untestable “The darling of pop psychology” Copyright © 2011 Pearson Education, Inc. All rights reserved.

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? 

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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? 

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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 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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  

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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  

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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) Copyright © 2011 Pearson Education, Inc. All rights reserved.

The Wernicke-Geschwind Model 

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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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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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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Effects of Electrical Stimulation to the Cortex on Language Abilities 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

Current Status of the WernickeGeschwind Model 

Empirical evidence supports two elements  

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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 

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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 

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Bevalier’s fMRI study of reading – sought to establish cortical involvement in reading Reading sentences versus control periods (strings of consonants)  

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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  

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

Cognitive Neuroscience of Dyslexia 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

Developmental Dyslexia: Causes and Neural Mechanisms 

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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  

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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  

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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” Copyright © 2011 Pearson Education, Inc. All rights reserved.

Cognitive Neuroscience of Deep and Surface Dyslexia Continued 

Deep dyslexia – extensive damage to lefthemisphere language areas 

How is it that lexical abilities are spared? 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.