Variations in Cerebral Asymmetry

1
Bryan Kolb Ian Q. Whishaws

• Fundamentals of
• Human Neuropsychology,
• Sixth Edition
• Chapter 12
• Lecture PPT

Prepared by Gina Mollet, Adams State College
2
Variations in Cerebral Asymmetry
3
Portrait Individual Responses to Injury

• A.B. vs. L.P.
• A.B.
• Damage to the posterior part of the left temporal
  lobe
• Difficulties in reading, speaking and remembering
  words
• L.P.
• Similar injury
• Trouble recognizing faces, and drawing pictures
• What could account for the differences?

4
Handedness and Functional Asymmetry

• About 10 of the population is left-handed
• Marion Annett

5
(No Transcript)
6
Anatomical Studies

• Hand preference correlated with differential
  asymmetry patterns
• Left-handers show more variation in asymmetry
• More fibers descend to the right hand
• MRI
• Little evidence of differences in anatomy in
  right and left-handers
• Greater hemispheric interaction in left-handers

7
(No Transcript)
8
Functional Cerebral Organization in Left-Handers

• Majority of left-handers have left lateralized
  speech
• Larger incidence of left-handedness among
  mentally defective children and children with
  neurological disorders
• Little known about cerebral organization of those
  with right hemisphere speech

9
Theories of Hand Preference Environmental
Theories

• Behavioral utility
• Proposes left hand is better for holding a shield
  or holding a baby, thus allowing the free right
  hand to perform skilled tasks
• Environmental Reinforcement
• Bias in the environment
• Environmental Accident
• Genetic bias towards right-handedness
• Left-handedness develops through a cerebral
  deficit

10
Snapshot Genetic Influences on Brain Structure

• Thompson and colleagues
• Examined gray matter distribution in unrelated
  people and monozygotic and dizygotic twins
• High similarity in monozygotic twins, but not in
  unrelated people

11
(No Transcript)
12
Theories of Hand Preference Anatomical Theories

• Right-handedness
• Enhanced maturation and greater development of
  the left hemisphere
• Most species have a left-sided developmental
  advantage

13
Theories of Hand Preference Hormonal Theories

• Geschwind and Galaburda
• Different levels of testosterone influence
  cerebral asymmetry
• Higher testosterone increased likelihood of
  left-handedness

14
Theories of Hand Preference Genetic Theories

• Annett
• Dominant gene rs for speech in the left
  hemisphere and right handedness
• Recessive gene rs- that results in no systematic
  bias for speech or handedness
• Model predicts about 12.5 left-handers

15
Sex Differences in Cerebral Organization

• Sex Differences in Behavior
• Motor Skills
• Men superior at throwing and intercepting objects
• Women superior at fine motor tasks
• Both present in young children

16
Sex Differences in Cerebral Organization

• Sex Differences in Behavior
• Spatial Analysis
• Men superior at mental rotation and in spatial
  navigation
• Women superior at spatial memory

17
(No Transcript)
18
(No Transcript)
19
Sex Differences in Cerebral Organization

• Sex Differences in Behavior
• Mathematical Aptitude
• Men superior at mathematical reasoning
• Women superior at computation
• Perception
• Women more sensitive to all sensory stimuli
  except vision
• Men superior at drawing mechanical objects

20
(No Transcript)
21
Sex Differences in Cerebral Organization

• Sex Differences in Behavior
• Verbal Ability
• Women superior verbal fluency and verbal memory
• Aggression
• Physical aggression more prevalent in men than
  women
• Genes or Experience?
• Effects are found in young children and adults
• Even after training women perform poorly on a
  water-level task

22
(No Transcript)
23
(No Transcript)
24
Sex Differences in Brain Structure

• Male brain larger than the female brain
• Females larger volumes in areas associated with
  language, in medial paralimbic regions and some
  frontal lobe regions
• Men larger
• Medial frontal and cingulate region
• Amygdala and hypothalamus
• Ventricles and overall white matter

25
Sex Differences in Brain Structure

• Differences in volume and organization in gray
  matter
• Influence of sex hormones
• Sex differences in the brain appear due to the
  distribution of estrogen and androgen receptors
  during development

26
(No Transcript)
27
Sex Differences in Brain Structure

• Established Asymmetries
• Larger left planum temporale found more often in
  men
• Men have larger asymmetry in the Sylvain fissure
• Planum parietale is about twice as large in men
• Women have more interhemispheric connections
• Women more likely to show an atypical fingerprint
  asymmetry

28
(No Transcript)
29
(No Transcript)
30
The Homosexual Brain

• Homosexual men outperform all groups on verbal
  fluency
• Homosexual women throw more accurately than
  heterosexual men

31
Sex Differences Revealed in Imaging
32
(No Transcript)
33
Research with Neurological Patients

• Degree of asymmetry in the lesion effects
• Differences in verbal IQ and performance IQ in
  men and women after left and right hemisphere
  damage
• Intrahemispheric Organization
• Differences in the appearance of aphasia and
  apraxia in men and women after left hemisphere
  damage

34
(No Transcript)
35
(No Transcript)
36
Explanations of Sex Differences Hormonal Effects

• Organizing Effect
• Effects of hormones on brain organization
• Leads to sexual differentiation
• Assumed to take place during development

37
Explanations of Sex Differences Hormonal Effects

• Functional effects of hormones seen in adulthood
• High estrogen associated with depressed spatial
  ability
• Estrogen affects levels of catecholamines and
  neuron structure
• Low testosterone associated with high spatial
  ability in men
• Hormone replacement in women increases verbal
  fluency and verbal and spatial memory

38
(No Transcript)
39
Explanations of Sex Differences

• Genetic Sex Linkage
• Recessive gene on the X chromosome accounts for
  spatial ability in girls
• Maturation Rate
• Brain matures faster in girls
• The slower the brain develops, the more asymmetry
  appears

40
Explanations of Sex Differences

• Environment
• Girls and boys are encouraged to engage in
  different activities leading to different
  abilities
• Not supported by a bulk of the evidence
• Preferred Cognitive Mode
• Women prefer to solve problems using a verbal
  strategy

41
Explanations of Sex Differences

• Conclusions
• Six significant behavioral differences appear
  between men and women
• Verbal ability
• Visuospatial analysis
• Mathematical ability
• Perception
• Motor skills
• Aggression

42
Environmental Effects on Asymmetry

• Culture and Language
• Asian languages might promote more
  right-hemisphere participation
• Reading pictorial Chinese produces right
  hemisphere activation not seen in reading English
• Exposure to multiple languages may influence
  cerebral organization of language

43
(No Transcript)
44
Sensory or Environmental Deficits

• Brain organization in nonhearing people
• Congenitally deaf subjects do not show a right
  visual field superiority for language tasks
• Deaf signers may acquire language in the left
  hemisphere, which may lead to development of
  visuospatial abilities in the left hemisphere

45
Sensory or Environmental Deficits

• Environmental Deprivation
• The case of Genie
• Rescued from an experiential deprivation and
  malnutrition at the age of 13
• Developed cognitive abilities, but language
  lagged
• Found to be processing verbal and nonverbal
  stimuli in the right hemisphere
• Romanian orphanages
• Children had little environmental stimulation
• Long lasting damage to their brain development

46
Sensory or Environmental Deficits

• Epigenetics
• Changes in gene regulation that take place
  without changes in the DNA sequence
• Alterations in gene expression can influence
  cerebral function

47
Effects of Hemispherectomy

• Both hemispheres specialized at birth
• Removal of the right hemisphere
• Deficits on complex visuospatial deficits
• Removal of the left hemisphere
• Deficits in understanding spoken language
• Equipotentiality
• Equal potential for the hemispheres to perform
  any task
• Hemispherectomy provides evidence against
  equipotentiality

48
(No Transcript)
49
Ontogeny of Asymmetry

• Cerebral asymmetries are present before birth and
  become more pronounced in adulthood
• Left hemisphere shows a greater response to
  language at 1 week of age
• Parallel development theory
• At birth functional overlap between hemispheres
• Hemispheres becoming increasingly specialized for
  more complex tasks

50
(No Transcript)
51
Asymmetry in Nonhumans

• Asymmetry in Birds
• Left hemisphere dominance for song
• Lateralization for visually guided functions
• Asymmetry for memory formation
• One hemisphere sleeps at a time
• Asymmetry in Rodents
• Larger right hemisphere
• Lateralized modulation of the immune system
• Left hemisphere dominance for species-specific
  calls

52
Asymmetry in Nonhuman Primates

• Chimps show asymmetries in Brocas area and in
  the planum temporale
• Chimps use the right hand for manual gestures
• Hemispheric specialization for processing visual
  input
• Japanese macaques use the left hemisphere for
  communication

53
(No Transcript)