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.