BRAIN
LATERALIZATION
LANGUAGE AND
COGNITION
CEREBRAL LATERALIZATION
Significant Events in History
• Marc Dax (1836)
Dax was the first to report left hemisphere
involvement in speech disorders caused by
brain damage.
• Paul Broca (1860’s)
Unaware of Dax’s work, Broca made key
discoveries regarding left hemisphere
involvement in aphasia.
• Broca’s area, Broca’s aphasia
• Hugo-Karl Liepmann (1900’s)
Apraxia is almost always caused by left
hemisphere damage.
CEREBRAL LATERALIZATION
Assessments of Lateralization
• Sodium Amytal Test
• Dichotic Listening Test
• Functional Brain Imaging
CEREBRAL LATERALIZATION
Speech lateralization and handedness
• The left hemisphere is dominant for speech in
majority, both right- and left-handed, although
greater variability among left-handed individuals.
Neurological studies of aphasics (Russell & Esper,
1961).
• Right handed aphasics: 60% left, 2% right hemisphere
damage
• Left handed aphasics: 30% left, 24% right hemisphere
damage
Results of Sodium amytal tests (Milner, 1974).
Left-hemisphere dominance for speech in:
• 92% of right-handed individuals
• 69% of left-handed or ambidextrous individuals
• 30% of left-handed or ambidextrous individuals with early
left hemisphere damage
CEREBRAL LATERALIZATION
Sex Differences and Lateralization
• Some evidence suggests that the male
brain is more lateralized than female brain.
• e.g., McGlone’s (1977, 1980) studies of
unilateral stroke victims and WAIS
subscore tests
• Some fMRI studies show that females tend
to use both hemispheres in languagerelated tasks more so than males.
SPLIT-BRAIN STUDIES
Meyers’
and Sperry’s work in cats
Split-Brain Patients
• Commisurotomy to reduce seizure
spreading.
Tests involving verbal identification of
stimuli presented to the left or right
hemisphere.
Tests involving spatial stimuli presented to L
or R hemisphere.
Myers and Sperry (1953)
Testing Split-Brain Patients
HEMISPHERIC SPECIALIZATIONS
Examples of Lateralization
• Left Hemisphere Specializations
Language
Controlling ipsilateral movement
• Right Hemisphere Specializations
Spatial ability
Emotion
Musical ability
• See table 16.1 in Pinel for more examples
NEUROANATOMICAL ASSYMETRY
Planum Temporale
• Larger in left hemisphere in most individuals
Geschwind and Levitsky (1968) study
Witelson (1983) study
• Asymmetry is present at infancy
• Asymmetry of planum temporale in chimps and other apes
Left planum temporale and perfect pitch
APHASIA
Definition:
Acquired disorders of
language secondary to brain damage
Common Subtypes
• Broca’s aphasia
• Wernicke’s aphasia
• Global Aphasia
• Conduction Aphasia
LANGUAGE AREAS
BROCA’S APHASIA
Characteristic
symptoms
• labored and poorly articulated speech
• agrammatism (telegraphic speech)
• anomia
• agraphia (writing impairment)
Region
of brain damage
• left inferior frontal cortex, 3rd frontal gyrus,
anterior to face region of motor cortex
(Broca’s area)
WERNICKE’S APHASIA
Characteristic
symptoms
• poor comprehension of spoken and written
language
• fluent and spontaneous speech, but
incoherent
• paraphasia (sound and word substitutions)
• alexia (reading impairment)
Region
of brain damage
• left superior temporal gyrus (Wernicke’s
area)
GLOBAL APHASIA
Characteristics
• Total loss of comprehension and
expressive abilities, involving both
spoken and written language.
• Some automatic speech, such as
emotional exclamations retained
Damage
is extensive
• involves both B. and W. areas, large
portions of frontal, temporal and
parietal cortex.
CONDUCTION APHASIA
Characteristics
• fluent speech, comprehension only slightly
impaired
• repetition primarily impaired (esp. novel or
nonwords, or sentences)
Brain
regions damaged
• arcuate fasciculus
(connection between B and W
area)
• or primary auditory cortex
APHASIAS: SUMMARY
APHASIAS: SUMMARY
WERNICKE-GESCHWIND MODEL
Connectionist Model
for the anatomical
analysis of aphasias
DYSLEXIA
Developmental
Dyslexias
• Some controversy in categorizing this
disorder
• Sensory-processing problem?
• Memory disorder?
Acquired
Dyslexias
• surface dyslexia (whole word reading
impaired)
• deep dyslexia (phonological dyslexia)
NEURAL ABNORMALITIES IN DYSLEXIA
Anomalies
in cortical cell arrangement
• Ectopias: unusual groupings of cells in
outer layers
• Micropolygyria: excessive cortical folding
• Disoriented cells
These
abnormalities probably occur
during neural migration during fetal
development
BRAIN IMAGING DYSLEXIA
fMRI
studies show different patterns
of brain activation in dyslexics and
nondyslexics.
Dyslexic subjects:
• showed less activation in posterior
regions (e.g. Wernicke’s area) and
overactivity in anterior regions
compared to nondyslexics.
• showed less activation of visual cortex
in response to written words.
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