Variations in Cerebral Asymmetry or Individual Differences

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Variations in Cerebral Asymmetry or Individual
Differences
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Is hemispheric specialization present at birth or
occur with development?

• Evidence for lateralization or specialization
  from birth
• (1) neuroanatomical asymmetries are evident
  between 10th and 31st weeks after conception
  (i.e., prenatally)
• (2) asymmetries do not become proportionately
  bigger with development of skills (e.g, language,
  reading)

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• (3) evidence of lateralization of differential
  activation in infants that is task specific
• (4) young children with hemispherectomies exhibit
  specific deficits related to hemisphere that was
  removed

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Individual differences and variations in cerebral
asymmetry

• Handedness
• Gender
• Environmental factors/ culture

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Handedness

• Why are most people right-handed and left
  hemisphere dominant for speech?
• May need a single motor center
• Benefit for both hemispheres to receive
  information but may be detrimental if each
  hemisphere chooses different responses
• Left hemisphere may be especially important for
  controlling sequential actions
• For generating representations based on a small
  number of primitive units (Corballis)

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• The left hemispheric dominance would allow
  language to have a generative quality the the
  left hemisphere allowed language to take on its
  generative quality and allowed motor functions to
  be more precise and faster (Corballis)
• Alternatively, it is possible that the mechanisms
  producing language and motor performance are
  unrelated
• e.g. not all right-handers are left hemisphere
  language dominant and not all left-handers are
  right hemisphere language dominant

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What is the relation between handedness and
cerebral asymmetry?

• Some relation between handedness and anatomy, but
  variations exist.
• Compared to right-handers, left-handers more
  likely to show no asymmetry or a reversal of
  asymmetry

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Handedness and Asymmetry
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What is the relationship between function and
cerebral asymmetry?
Milner, 1977
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Bozeman (N1234)
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• Left-handers Incidence of aphasia about 70 that
  of right-handers and deficits are less severe
• Functional organization more heterogeneous in
  left-handers, although most still have same
  organization as right-handers

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Summary of handedness data

• Weak relation between handedness and structural
  asymmetry
• Weak relation between handedness and functional
  asymmetry

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What causes left-handedness?

• May be a genetic bias favoring right-handedness
• Theories that left handedness caused by a defect
  in cerebral development (e.g., early trauma or
  immune dysfunction)
• If true, should find a high incidence of
  cognitive deficits in left-handers but this is
  not the case

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• Hormonal variation causing handedness?
• Testosterone is inhibitory for left hemispheric
  development (Geschwind and Galaburda)
• High does in utero slows development,
  particularly for the left hemisphere, thereby
  allowing the right to grow more rapidly
• This alters cerebral organization and in some
  people, produces left handedness
• Testosterone affects immune system, with boys
  more likely to suffer from neurological and
  psychiatric disorders

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Summary of Cause of Handedness

• Cerebral asymmetry and immune function can be
  altered early in life thereby influencing
  handedness

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Gender

• Gender differences in cognitive skills
• e.g., girls acquire language abilities about 1
  month prior to boys by 11 years girls
  consistently perform better on tests of verbal
  abilities (including memory) boys do better on
  visuospatial tasks
• Gender difference not large considerable overlap
  between groups but is consistent

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• Gender differences in behavior (aggressiveness)
• Gender differences in vulnerability to adversity
• Beginning with gestation, males more prone to
  neurological and psychiatric disorders
• These differences may be due to physiological
  than cultural reasons

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Gender and Lateralization

• Hiscock et al. (1994) conducted a meta-analysis
  of 352 dichotic listening studies
• Most had no information regarding gender
  differences thereby suggesting that gender
  differences were absent or accounted for little
  variability
• 28 (8) had data on gender differences that were
  significant
• 21/28 (75) showed males to be more lateralized
  than females

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• Hiscock also conducted a meta-analysis of 516 VF
  studies
• 33 (6) reported gender differences
• 27/33 (82) showed males more lateralized
• Findings are consistent with notion of weak
  gender differences at level of population

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Lesion Studies
Meta-analysis of 16 studies (Inglis and Lawson)
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• Incidence of aphasia following left hemisphere
  lesions
• 77 of women aphasic
• 89 of men aphasic

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Basis for gender differences?

• Hormones may affect functioning of brain
• Estrogen facilitates verbal recall evidence
  from menstrual cycle
• Just after ovulation (estrogen high), females
  perform better on fine motor tasks and poorer on
  spatial tasks opposite pattern tends to occur
  when estrogen low

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Does culture affect asymmetry?

• Bilingualism
• If both languages rely on same neural system,
  both languages should be impaired following
  lesion
• If different neural systems then no correlations
  between language losses
• Findings contradictory

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• At present, no strong evidence that bilinguals
  less lateralized for language than unilinguals
• Carotid amytal studies of bilinguals (Chinese/
  English), left hemisphere language in majority

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• Hearing Impaired (Signing)
• Kimura 11 cases of signing disorders following
  brain damage
• Right-handers 9/9 impaired following left
  hemisphere damage
• Left-handers
• 1/2 -impaired after right damage
• 1/2 impaired after left damage

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Implications

• Development of language based on signs has the
  same anatomical basis as vocal language
• Language has a preferential base for the left
  hemisphere

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Extreme Cultural Deprivation

• Young girl who had many years of extreme
  deprivation
• Showed a strong left ear advantage for both
  verbal and non-verbal sounds
• Did the left hemisphere degenerate or was it
  actively inhibited?