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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Learning Objectives
LO1: Summarize early studies of cerebral lateralization of language.
LO2: Describe the method used to demonstrate the hemispheric independence of visual experience in human split-brain patients.
LO3: Summarize major effects of split-brain surgery in human patients.
LO4: Discuss common misunderstandings about lateralization of function in humans.
LO5: Describe major differences between left and right hemisphere function in humans.
LO6: Discuss the evolution of cerebral lateralization of language.
LO7: Describe the Wernicke-Geschwind model.
LO8: Evaluate the Wernicke-Geschwind model by reviewing relevant evidence.
LO9: Summarize the cognitive neuroscience approach to the study of language.
LO10: Discuss 3 things we know about the cortical localization of language.
LO11: Discuss developmental dyslexia.
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4. 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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FIGURE 16.1 The cerebral hemispheres and cerebral commissures.
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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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7. 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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8. Tests of Cerebral Lateralization
Determining Which Hemisphere Is Dominant
Sodium amytal test
Anesthetize one hemisphere 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 performing a language test
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9. Discovery of the Relation between Speech Laterality and Handedness
The left hemisphere is speech dominant in almost all dextrals (right-handers) and most sinestrals (left-handers).
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10. 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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12. 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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14. 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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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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16. 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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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
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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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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20. 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 the target item in an array faster than intact controls can.
Chimeric figures task: only symmetrical version of right half of faces recognized
Indicates competition between hemispheres
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The Z Lens
Researchers are advancing the study of split brains with a contact lens used to restrict visual input to one hemisphere.
Previous studies had to limit viewing time to less than .1 second.
The Z Lens 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 split-brain 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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23. 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.
For example, the case of Peter
His hemispheres often disagreed with each other.
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24. 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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25. 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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26. Examples of Cerebral Lateralization of Function
Left hemisphere: superior in controlling ipsilateral movement
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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29. 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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30. Anatomical Brain Asymmetries (Con’t)
Although asymmetries are seen in language related areas, these regions are not all larger in the left.
Left planum temporale: larger in only 65 percent of human brains
Heschl’s gyri: larger on the right
Two in the right, only one in the left
Frontal operculum: visible surface suggests that the right is larger, but the left has greater volume.
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FIGURE 16.8 Three language areas of the cerebral cortex that have been the focus
of studies on neuroanatomical asymmetry: the frontal operculum, the planum temporale
(Wernicke’s area) and Heschl’s gyrus (primary auditory cortex).
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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, G., Jancke, L., Huang, Y., and Steinmetz, H. (1995). In vivo evidence of structural brain asymmetry in musicians. Science, 267, )
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33. 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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34. 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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35. 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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36. 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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37. 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, which posits that there is overlap between speech comprehension and the 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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38. 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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39. 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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40. 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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41. The Wernicke-Geschwind Model
Norman Geschwind integrated the ideas of Broca, Wernicke, and Dejerine into this theory.
It involves seven components, all of which are in the left hemisphere.
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FIGURE The seven components of the Wernicke-Geschwind model.
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FIGURE 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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44. Wernicke-Geschwind Model: The Evidence
There is a lack of evidence that damage to various parts of the cortex has the 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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45. 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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FIGURE The lack of permanent disruption of language-related abilities after surgical excision of the classic Wernicke-Geschwind language areas. (Based on Penfield, W., & Roberts, L. (1959). Speech and brain mechanisms. Princeton, NJ: Princeton University Press.)
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47. 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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FIGURE The wide distribution of left hemisphere sites where cortical stimulation
either blocked speech or disrupted it. (Based on Penfield & Roberts, 1959.)
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49. Current Status of the Wernicke-Geschwind Model
Empirical evidence supports two elements.
Important roles are played by Broca’s and Wernicke’s—many aphasics have damage in these areas.
Anterior damage associated is with expressive deficits and posterior damage is associated with receptive deficits.
There is 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) are rare.
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50. Cognitive Neuroscience of Language
Premise: Activity in brain areas for specific cognitive processes:
Underlies language-related behaviors
Has functions independent of language
Is likely to be small, widely distributed, and specialized
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51. 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 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 percent of the indicated areas on any particular trial.
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53. Damasio’s PET Study of Naming
Domasio and Colleagues (1996)’s 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 was seen well beyond Wernicke’s area.
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54. 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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55. 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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56. 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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57. 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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58. 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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59. 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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