Lateralization & The Split Brain and Cortical Localization of Language



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Lateralization & The Split Brain and Cortical Localization of Language

  • Ch. 16

Outline

  • The Dominant Left Hemisphere
  • Tests of Cerebral Lateralization
  • The Split-Brain Experiment
  • Tests of Split-Brain Patients
  • Differences Between the Left and Right Hemispheres
  • Broca’s Area
  • Wernicke’s Area

Aphasia and Apraxia: The Dominant Left Hemisphere

  • In 1836, Dax reported that not one of his 40 or so patients with speech problems had displayed damage restricted to the right hemisphere
  • 25 yrs later, Broca reported the results of the postmortem examination of two aphasic patients (patients with deficits in the use of language that are not attributable to general sensory, motor, or intellectual dysfunction)…

Aphasia and Apraxia: The Dominant Left Hemisphere

  • Both had diffuse left hemisphere damage that seemed to be centered in an area of the inferior left prefrontal lobe, just in front of the primary motor face area
  • This became known as Broca’s area that is associated with grammar and speech production

Aphasia and Apraxia: The Dominant Left Hemisphere

  • Liepmann discovered that apraxia (difficulty performing movements with either side of the body when asked to do so, but not when performing them spontaneously) was almost always associated with left-hemisphere damage

Aphasia and Apraxia: The Dominant Left Hemisphere

  • This led to the view that all complex activities were performed by the left hemisphere; the left and right hemispheres thus became known as dominant and minor hemispheres, respectively

Tests of Cerebral Lateralization

  • The first evidence of language laterality came from comparisons of the effects of left and right unilateral lesions; today, the sodium amytal test and dichotic listening test are commonly used to assess language laterality

Tests of Cerebral Lateralization

  • PET of FMRI techniques have revealed that there is typically more activity in the left hemisphere than the right during language-related activities

Tests of Cerebral Lateralization

  • Many studies have reported a relation between speech laterality and handedness; the following general conclusions have been reached:

Tests of Cerebral Lateralization

    • Nearly all (about 95%) right-handed subjects are left-hemisphere dominant for speech;
    • most left-handed or ambidextrous subjects (about 70%) are also left-hemisphere dominant for speech; and
    • Early left-hemisphere damage can cause the right hemisphere to become dominant for speech and the left hand to be preferred

The Split-Brain Experiment

  • In 1953, Myers and Sperry performed an experiment on cats that changed the way that we think about the brain; and it provided a means of comparing the function of the two hemispheres
  • It was designed to reveal the function of the brain’s largest commissure, the corpus callosum

The Split-Brain Experiment

  • Earlier studies failed to reveal any deficits in laboratory animals following callosal transection, and people born without a corpus callosum had been reported to be perfectly normal

The Split-Brain Experiment

  • In the Myers and Sperry experiment there were four groups of cats:
    • Corpus callosum severed
    • Optic chaims severed
    • corpus callosum and optic chiasm severed
    • Intact controls

The Split-Brain Experiment

  • In phase 1 of the experiment, all cats learned a lever-press pattern discrimination task with a patch over one eye; all four groups readily learned this simple task
  • In phase 2, the patch was switched to the other eye…

The Split-Brain Experiment

  • The cats in the optic-chiasm-severed group, corpus-callosum-severed group, and control kept performance same
  • In contrast the optic-chiasm-and-corpus-callosum-severed group acted as if the task were completely new to them - they had to learn it again with no savings

The Split-Brain Experiment

  • We can conclude:
    • The cat forebrain has the capacity to act as two separate forebrains, each capable of independent learning and of storing its own memories;
    • The function of the corpus callosum is to carry information between hemispheres
    • The best strategy for studying corpus callosum function is to use a method to limit information to a single hemisphere

Tests of Split-Brain Patients

  • Commissurotomy is performed on patients with life-threatening cases of epilepsy to reduce the severity of convulsions by restricting epileptic discharges to half of the brain

Tests of Split-Brain Patients

  • The operation is remarkably effective; many commissurotomized epileptic patients never experience another major convulsion; more remarkably they experience few obvious side effects in their daily lives

Tests of Split-Brain Patients

  • The controlled neuropsychological testing of these split-brain patients has revealed some amazing things about the human brain
  • To test split brain patients,visual stimuli are flashed to the right or left of a fixation point on a screen
  • Also tactual information is presented to one hand under a ledge or in a bag

Tests of Split-Brain Patients

  • These tests confirmed the conclusion that commissurotomized patients have two independent streams of consciousness

Evidence of Two Independent Streams of Consciousness

  • When an object was presented to the left hemisphere, either by touching something with the right hand or viewing something in the right visual field, the subject could:

Evidence of Two Independent Streams of Consciousness

    • Pick out the correct object with the right hand
    • Could not pick out the correct object with the left hand
    • Could name the correct object

Evidence of Two Independent Streams of Consciousness

  • When an object was presented to the right hemisphere, either by touching something with the left hand or viewing something in the left visual field, the subject could:

Evidence of Two Independent Streams of Consciousness

    • Could pick out the correct object with the left hand
    • Could not pick out the correct object with the right hand
    • Claimed nothing had been presented

Cross-cuing

  • Represents communication between hemispheres via a nonneural route
  • For example: a red or green light is flashed in the left visual field; the split-brain patient was then asked to name the color: red or green…

Cross-cuing

  • Most split-brain patients get 50% correct on this task (guessing, by chance); however one patient performed almost perfectly
  • When the performance of this subject was carefully monitored, it was noticed that on the trials when the patient initially said (left hemisphere) the incorrect color, his head shook and the patient then changed their guess to the other color

Cross-cuing

  • Apparently, the right-hemisphere (who knew the correct answer) heard the incorrect guess of the left hemisphere, and signaled to the left hemisphere that it was wrong by shaking the person’s head; when only first guesses were counted, performance fell to 50%

Learning Two Things at Once

  • Split-brain patients are capable of learning two things at once
  • If a split-brain patient is visually presented two objects at the same time - let’s say a pencil in the LVF and apple in the RVF - s/he can reach into two different bags at the same time, one with each hand, and pull out the two objects - a pencil in the left-hand and apple in the right

Helping-Hand Phenomenon

  • Occurs when the two hemispheres are presented with different information about the correct choice and then are asked to reach out and pick up the correct object from a collection in full view
  • Usually the right hand will reach out to pick out what the left hemisphere saw, but the right hemisphere seeing what it thinks is an error being made causes the left hand to grab the right hand and pull it over to the other object

Split-Brain Video (shown in class)

Differences Between The Left and Right Hemispheres

  • Language is the most lateralized of all abilities; the left-hemisphere is better than the right at most language-related tasks
  • however, the right hemisphere proved to be able to understand single written and spoken words; also right-hemisphere detects prosody and discourse

Differences Between The Left and Right Hemispheres

  • The right hemisphere proved better than the left at a variety of tasks involving spatial ability, emotional stimuli and musical tasks

Differences Between The Left and Right Hemispheres

  • The two hemispheres seem to engage different types of memory processing; LH attempts to place its experience in a larger context (relation of parts that make up the whole), while the RH attends strictly to the Gestalt perceptual characteristics of the stimulus (parts or whole but not relation between)
  • This is usually termed analytical (LH) versus holistic (RH)

Differences Between The Left and Right Hemispheres

Differences Between The Left and Right Hemispheres

  • There are also anatomical asymmetries in the human brain; for example the planum temporale and frontal operculum (language related areas) are larger in LH
  • However, Heschl’s gyrus (also language related) in larger in RH

Differences Between The Left and Right Hemispheres

  • (not in book) left-handers seem to have symmetrical planum temporales, suffer less severely from LH aphasia, and suffer more severely from RH aphasia
  • This suggests left-handers may have a more diffuse representation of language and is evident in differential grammatical strategies in sentence processing

Three Theories of Cerebral Asymmetry

  • Analytic-synthetic theory
  • Motor theory
  • Linguistic theory

Analytic-Synthetic Theory

  • Suggests that there are two fundamentally different modes of thinking, an analytic mode (LH) and synthetic mode (RH), and that the neural circuitry for each is fundamentally different
  • LH (pieces of the whole) operates in logical, sequential, analytic fashion
  • RH (the whole) makes immediate, overall synthetic judgments

Motor Theory

  • Posits that LH is specialized for fine motor movement of which speech is but one example
  • Two lines of evidence:
    • Lesions of the LH disrupt facial movements more than do RH lesions, even when they are not related to speech
    • Degree of disruption of nonverbal facial movements is positively correlated with the degree of aphasia

Linguistic Theory

  • Based on the view that the primary function of the LH is language; this is based on studies of deaf people who communicate using ASL; this ability is lost if these people suffer damage to the LH, even when they are able to make the movements required
  • (or is this just showing ASL is a language, and that language is highly analytical?)

Broca’s Area

  • Inferior left prefrontal lobe in left hemisphere
  • Damage leads to deficits primarily speech production (problems with expression) and also grammatical comprehension

Wernicke’s Area

  • Left temporal lobe, just posterior to the primary auditory cortex
  • Damage leads to deficits to semantic language comprehension (problems with reception) and speech is imcomprehensible, despite having correct grammar, rhythm an intonation (word salad)

Conduction Aphasia

  • Damage to pathway connecting Broca’s and Wernicke’s areas called the arcuate fasciculus
  • Comprehension and spontaneous speech are intact but patient not able to repeat words they have just heard

Alexia

  • Damage to the left angular gyrus (area of left temporal and parietal cortex jost posterior to Wernicke’s)
  • Inability to read despite intact language comprehension and production

Agraphia

  • Also due to damage to the left angular gyrus
  • Inability to write despite intact language comprehension and production
  • Involvement of LAG in alexia and agraphia show its responsible for language related visual input

Broca’s Aphasic Video (shown in class)

Wernicke-Geshwind Model

  • Seven components in Left hemisphere: primary visual cortex, angular gyrus, primary auditory cortex, Wernicke’s area, arucate fasciculus, Broca’s area, and primary motor cortex

Responding to a heard question

  • Primary auditory cortex to Wernicke’s area where comprehended
  • To respond, concept generated in Wernicke’s area, goes via arcuate fasciculus to Broca’s area, then to primary motor cortex and articulatory areas (face, lip, and tongue muscles, voice box, and muscles assoicated with lungs)

Reading aloud

  • Primary visual cortex to left angular gyrus, which transmits visual code to auditory code
  • Then to Wernicke’s area to arcuate fasciculus to Broca’s to primary motor cortex to articulatory areas

Evidence against W-G Model

  • Damage to these boundaries has little lasting effect on language
  • Damage to other brain areas can produce aphasia
  • Broca’s and Wernicke’s aphasia are rarely “pure” - aphasia is both receptive and expressive
  • Major individual differences for cortical localization for language

Cognitive Neuroscience Approach to Language

  • Cannot perform lesion studies because humans are only known species with language
  • Use Cognitive Neuroscience (brain imaging) to study relation of brain and language

Cognitive Neuroscience Approach to Language

  • Each of of the components in W-G model can be broken down further into constituent cognitive processes
    • Phonological analysis (sounds)
    • Grammatical analysis (structure)
    • Semantic analysis (meaning)

Cognitive Neuroscience Approach to Language

  • (2) Areas of brain involved in language are not solely dedicated to language; many of the constituent cognitive processes also play roles in other behavior
  • Example - some areas involved in short-term memory and visual pattern recognition are involved in reading, too

Cognitive Neuroscience Approach to Language

  • (3) W-G model assumes that brain areas involved in language are large, circumscribed, and homogenous but Cog neuro assumes they are small, widely distributed, and specialized

Dyslexia and Cognitive Neuroscience

  • Dyslexia is pathological difficulty in reading, does not result from general visual, motor, or intellectual deficits
  • Developmental dyslexia- apparent in childhood
  • Acquired dyslexia - damage in individuals who were already capable of reading

Developmental Dyslexia

  • (1) Differences between brains of dyslexic and non-dyslexic readers have been reported, however none seems to play a critical role
  • Example - dyslexics do not display the asymmetry of the planum temporale

Developmental Dyslexia

  • (2) Several types of dylexias and thus likely to have different causes and brain areas susceptible

Developmental Dyslexia

  • (3) Difficult to determine cause-and-effect of brain abnormalities
  • Are these abnormalities the cause of dylexia or the result of lack of reading experience (brain develops differently as a result of different experience)?

Acquired Dyslexia

  • Two strategies for reading aloud:
    • Lexical procedure - based on specific stored information that has been acquired about written words - looks at it, recognizes it and says it
    • Phonetic procedure - looks at words, recognizes letters, sounds them out and says word

Acquired Dyslexia

  • Surface Dyslexia - patients lose ability to to pronounce words based on the specific memories of the words (they lose their lexical procedure)
  • Can pronounce non-words - wug
  • Example: can pronoun words consistent with rules ( fish, river, or glass) but can’t pronounce unusual words (have, lose, and steak are like cave, hose, and beak)

Acquired Dyslexia

  • Deep Dyslexia - patients lose ability to apply common rules of pronunciation ( they lose their phonetic procedure)
  • Can’t pronounce non-words
  • Example: can say phonetically unusual words like aisle and yacht but not pronunciation rule-consistent words like fish, river, or glass

Acquired Dyslexia

  • Where are lexical and phonetic processes in the brain?
  • Deep dyslexia (lose phonetic procedure) due to damage in LH
  • Surface dyslexia (lose lexical) due to partial LH damage or RH damage


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