Pinel basics ch14

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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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?



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


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


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


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


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


The Split Brain


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


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


Testing Split-Brain Patients


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


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.


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


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


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


Examples of Lateralization of Function Right hemisphere superiority

Spatial abilityEmotionMusical abilitySome memory tasks

Left hemisphere – superior in controlling ipsilateral movement


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


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


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





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


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”


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


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


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’


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)




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


Effects of Damage to Areas of Cortex on Language Abilities


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


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.


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.


Functional Brain Imaging and Language


fMRI and Reading

More activity in the left hemisphere than in the right

Activity extended well beyond what Wernicke-Geschwind predicted


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


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


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


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


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


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

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