Hemispheric Asymmetry
• Anatomy and pathways
– animal comparisons
• Tests of hemispheric specialization
• Variations in organization
• Theories of hemispheric specialization
• Development
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Hemispheric Asymmetry
• Pathology
– unusual patterns of asymmetry
– dysfunction of one hemisphere
• Cognitive Style
– education
– occupation
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Anatomy and Pathways
• The brain appears symmetric although divided down
the center from front to back
• There are four main lobes on each side
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frontal - thought and planning
parietal - motor and touch
temporal - hearing
occipital - vision
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Anatomy and Pathways
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Anatomy and Pathways
• In general the brain receives from and sends
information to the opposite side of the body
– contralateral
• The organization in vision is more complex
– optic chiasm
– right visual field is projected to left hemisphere and left visual field to right
hemisphere
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Anatomy and Pathways
Nasal
hemiretina
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Anatomy and Pathways
• Not completely symmetric
– temporal lobe is slightly larger in LH
– men often have slightly larger RHs
• First notice functional difference in aphasia
– more likely with LH damage
– women more likely to suffer less and recover more than men
• Animal comparisons
– some “handedness” evidence (paws and tail curling)
– song bird that loses and re-grows left hemisphere temporal lobe each mating
season
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Tests of Hemispheric Specialization
• Brain damage
– look at location of loss and cognitive ability lost
– aphasias
– problem - not controlled, patients often recover functions
• Split-brain patients
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cut corpus callosum (band of fibers connecting the two hemispheres of the cortex)
present stimuli to each hemisphere
initially large differences
over time only subtle differences
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Tests of Hemispheric Specialization
• Brain activity assessment
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EEG - correlate electrically activity with cognitiive activity
PET scan - (CAT scan) usually look for damage
Evoked potentials - neuro-electrical response to stimuli
fMRI - watch ongoing change in activity with cognitive task
• The Wada test
– sodium amytal injection - put one hemisphere to “sleep”
• Dichotic listening
– Kimura Effect - right ear dominance for speech
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Tests of Hemispheric Specialization
• Tachistoscopic presentation
– different information to each visual field
– presented too fast for an eye-movement
• Kinsbourne tests
– lateral eye movements
• triggering activation in one hemisphere causes eye movements in the
opposite direction
– multiple tasks
• vocalize and use limbs at the same time
• hard to do right handed balancing or tapping when speaking
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Variations in Organization
• Left-handedness
– inverted left-hand writers often have language in right hemisphere
– left-handers who do not do inverted writing may be less lateralized
• Gender
– men are more lateralized, women are less lateralized
– women suffer less language loss from left hemisphere damage because
language is more evenly distributed
– it has been argued that men’s superior spatial skills are due to their right
hemisphere being totally dedicated to spatial tasks
– the basis for the gender difference is unclear
• genetic
• hormonal
• environment (social)
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Variations in Organization
• Maturation rate
– people who reach puberty earlier are less lateralized
– interaction with gender difference since women tend to mature faster
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Theories of Hemispheric Specialization
• Language theory
– LH specialized for language
– RH specialized for spatial tasks
• Coding theory
– LH uses analytic code
– RH uses analog code
• Processing theory
– LH - analytic, parts, logic
– RH - gestalt (wholistic), global, emotional
• Processing theory 2
– LH - high resolution , later analysis (details)
– RH - low resolution, early analysis (gist)
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Development
• Some differences at birth
– anatomy
– evoked potentials
• Plasticity
– children recover from very early brain damage
– mono-hemispheric children
– brain anomolies show early rewiring
• Critical Periods
– early exposure to stimuli critical for normal development
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Pathology
• Unusual patterns of asymmetry (controversial)
– stuttering
• competition between the hemispheres for the control of speech
• left hand inverted writers are more likely to stutter
• left handers forced to use their non-preferred hand
– dyslexia
• left-ear advantage in dichotic listening
• language in right hemisphere
• some have argued that a lack of lateralization may influence dyslexia,
but men are more likely to be dyslexic than women (and women are
generally less lateralized)
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Pathology
• Dysfunction of one hemisphere
– schitzophrenia
• more likely after damage to LH
• EEGs show more abnormal activity in LH
• may be lack of normal communication between hemipsheres
– depression
• more likely after damage to RH
• EEGs show more abnormal activity in RH
• shock treatment need only be done in RH
– LH damage
• usually lose language
– RH damage
• emotional disruption
• unilateral neglect syndrome
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Cognitive Style
• Left Hemisphere
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verbal
sequential
time oriented
digital
logical
analytical
rational
western
• Right Hemisphere
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non-verbal
simultaneous
spatial
analog
Gestalt
synthetic
intuitive
emotional
eastern
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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Cognitive Style
• Cultural Differences
– what style of thinking is valued
• Education
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college majors
teaching and learning styles
gender differences
types of creativity
does American education short change the RH
RH as pattern finder and LH for critical analysis
• Occupations
– artists, architects, counselors
– lawyers, scientists
Cognitive - hemispheric_asymmetry.ppt © 2001 Laura Snodgrass, Ph.D.
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