Learning and Memory across the Lifespan

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Chapter 12

• Learning and Memory across the Lifespan

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12.1 Behavioral Processes
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12.1 Behavioral Processes

• The Developing Memory Infancy through
  Adolescence
• Learning and Memory in Everyday Life Can
  Exposure to Classical Music Make Babies Smarter?
• Sensitive Periods for Learning
• The Aging Memory Adulthood through Old Age

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Developing Memory in Infancy Some Learning Can
Occur Before Birth!

• Gestational age (GA)time since conception.
• By 25 weeks GA, enough development in fetuss
  brain and sense organs to perceive.
• In studies (play sounds to human fetus)
• Fetus (3436 weeks GA) moved in response to a
  sound habituated (reduced response) by trial 13.
• Moved to 2nd stimulus habituated by trial 11.
• Back to 1st stimulus habituated by trial 8.

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Habituation to Sound (in 10 Human Fetuses)
Adapted from Hepper Shahidullah, 1992.
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Time (sec.)
Polygraph recording of sucking on nipple
Figure adapted from Figure 1 of DeCasper
Spence, 1986.
Short pause
Long pause
Long pause
Unfamiliar story plays
Familiar story plays
Familiar story plays
Details of DeCasper Spence (1986)
paradigm Before birth, infants were played
familiar story in mothers voice memory for
story was tested after birth by playing
recordings of this story or unfamiliar story
while infants sucked on artificial nipple.
Infants tend to suck in bursts punctuated by
pauses (interburst intervals, IBIs). First,
researchers took baseline measurements of average
IBI. Then, conditioned some infants that long
IBIs would be reinforced with familiar story
while sucking short IBIs would be punished with
unfamiliar story while sucking. (Other infants
in counterbalanced conditioning, short intervals
were punished.)
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Conditioning and Skill Learning in Young Children

• Explosion of learning in first few years of life!
• Most learning present in adults, present in
  infants.
• But, perceptual and motor systems immature.
• Until input/output systems mature, infants
  cannot fully learn or express.

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Rovee-Collier Studies

• (1993) Rovee-Collier studied instrumental
  conditioning in infants
• 2-month-old infants learned to kick to move a
  colorful mobile (hung over the crib).
• Illustrates instrumental conditioning.
• With no reminders, Infants remembered foot-kick
  technique for 13 days.
• With reminders, up to 21 weeks.

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Rovee-Collier Studies

• If crib liner with new pattern was used, babies
  didnt kick.
• Illustrates context-dependent learning.

Courtesy of Carolyn Rovee-Collier
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Infants and Classical Conditioning

• Other studies show infants have basic components
  of classical conditioning.
• Human and rat infants learned delay eyeblink
  conditioning.
• But, use more trials than adults of their
  species.
• Trace conditioning improved from infancy to
  early adulthood.
• Shows that, with more mature development,
  organism can learn more efficiently (under
  increasingly difficult conditions).

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Development of Episodic and Semantic Memory

• Elicited imitationinfants ability to imitate an
  action at a later time.
• From single observational learning training
  session.
• In study, 10-month-olds are shown how to operate
  a toy puppet.
• 4 months later, showed more interest in the
  puppet than control group (same age, no prior
  showing).
• At 5 years, showed more interest and dexterity
  with the puppet than control group, though most
  could not recall previous exposure.

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Development of Episodic and Semantic Memory

• In study, 4-, 6-, and 8-year-olds taught 5 facts
  from an experimenter and 5 from a puppet.
• One week later, 6- and 8-year-olds recalled and
  recognized more facts than 4-year-olds.
• 6- and 8-year-olds had better recall of source,
  with more intra-experimental errors (i.e., knew
  it was learned in experiment, confused source).
• 4-year-olds made more extra-experimental errors
  (i.e., thought learning was outside experiment,
  for example at school).

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Episodic Memory in Children
Data from Drummey Newcombe, 2002.
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Development of Working Memory

• Working memory lifespan progression
• English-speaking children 56 years can hold
  average digit span of 34 digits in working
  memory.
• By 910 years, can hold 56 digits.
• By 1415 years, can hold 7 digits (adult
  average).
• Similar working memory progression seen with
  words and visual patterns.
• Why fewer for children?
• Lack of exposure.
• Childrens performance improves with familiarity.

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9
8
7
6
Girls
5
Mean Digits
Boys
4
3
2
1
0
5
6
7
8
9
10
11
12
13
14
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Age (years)
Memory for digit span increases with age (reach
adult levels by about age 12 or 13) no
significant gender difference. Figure plotted
from data in Gardner, R. (1981). Digits forward
and digits backward as two separate tests
Normative data on 1567 school children. Journal
of Clinical Child Psychology, Summer 1981,
131135.
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Learning and Memory in Everyday Life Can
Exposure to Classical Music Make Babies Smarter?

• Limited intellectual benefits from exposure to
  classical music (no true Mozart effect).
• Research shows little evidence that supports
  benefits zero evidence that the effect lasts
  longer than 1015 minutes.
• So, why did scores increase?
• Music may prime or prepare brain regions for
  abstract spatial reasoning or mental imagery.
• Music may improve mood and subsequent
  performance.

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Sensitive Periods for Learning

• Sensitive periodstime ranges during which
  learning is enhanced or possible.
• Examples
• In male sparrows, 30100 days is a sensitive
  period for song learning.
• In cats, 3 weeks to 60 days is a sensitive
  period for visual development.
• But, for monkeys, all of the first 6 months are
  important for visual development.

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Sensitive Periods for Learning

• In study of 28 human infants who had cataract
  surgery at age 1 week to 9 months
• ACUITY improved significantly over 1 month,
  with some improvement apparent after as little as
  1 hour of visual input.
• Unlike older children, improvement was the same
  for eyes treated for monocular and binocular
  deprivation.
• Visual input necessary for postnatal
  improvement its onset initiates rapid functional
  development.

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Imprinting

• Imprintingphenomenon in which some species
  (e.g., newborn goslings, turkeys, sheep, deer,
  buffalo) form a social bond with the first
  object they see.
• Imprinting involves critical period for
  permanent change to occur.
• http//www.youtube.com/watch?vLGBqQyZid04

Thomas D. McAvoy/ Time Magazine
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Social Attachment Learning

• Primates do not appear to imprint, but there is
  evidence of sensitive period for social
  attachment.
• In study, Harry Harlow rears rhesus monkeys
  isolated from mothers in adolescence moved to
  group cages, show social retardation.
• For rhesus monkeys, first months sensitive
  period for learning social interactions.
• http//www.youtube.com/watch?vAn02zCsVEpY

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Social Attachment Learning

• Sensitive period for social attachment in humans?
  Consider children under Ceausescu regime (1970s)
• Romanian children (RC) reared from infancy (up
  to 42 months) in depriving institutions, then
  placed in UK adoptive homes.
• Compared with nondeprived UK-born children
  adopted before 6 months.
• past
• http//www.youtube.com/watch?vrXivHuugp3c
• present
• http//www.youtube.com/watch?vFWKQNMZa--Yfeature
  related

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Social Attachment Learning

• Findings
• RC tested at time of entry to UK showed
  developmental impairment in cognitive function.
• Tested again at 4 years all RC show
  improvement.
• RC adopted before aged 6 months showed normal
  cognitive and social functioning.
• But, RC adopted at 6 months or later still showed
  some cognitive deficits, mild social problems.
• Suggests biological programming or neural
  damage from institutional deprivation varied
  outcomes related to early environmental
  stimulation.

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A Sensitive Period in Humans
Data from Rutter et al., 1998.
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Aging Memory Adulthood to Old Age

• Working memory capacity is particularly
  vulnerable in old age.
• Average STM capacity for digits drops from 7 (in
  early to middle adulthood) to 66.5 in elderly
  adults.

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Conditioning and Skill Learning DeclineBut
Well-Learned Skills Survive

• Learning decline begins around age 4050 in
  humans.
• Also seen in elderly rabbits, rats, and cats.
• However, well-established, highly practiced
  skills tend to be maintained or improved (chess
  and bridge experts).

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Conditioning and Skill Learning DeclineBut
Well-Learned Skills Survive

• In studies
• Eyeblink conditioning may take twice as many
  trials in elderly.
• Skill learning for rotary pursuit task and
  computer use take more time.

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(B) Rotary pursuit skill learning
(A) Eyeblink classical conditioning
60
50
Mean distance covered
40
Trials to Criterion
30
20
10
0
18-29
30-39
40-49
50-59
60-69
70-70
Age (in years)
Age (in years)
Classical conditioning and skill learning decline
with aging.
(A) Plotted from data in Solomon et al., 1989,
Table 1. (B) From Kausler, D. (1994). Learning
and Memory in Normal Aging. New York Academic
Press, p. 38 fig 2.4 (top), which cites adapted
from Ruch, 1934.
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Episodic and Semantic Memory Old Memories Fare
Better than New Learning

• Healthy elderly adults tend to retain semantic
  knowledge, and recall many episodic memories.
• In paired associates test
• Elderly may be able to recognize words or
  images previously studied.
• May have difficulty with recall.
• May recall more information if given more time
  or allowed to self-pace rate of presentation.

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Paired associate learning is impaired in elderly
adults relative to young adults when items are
presented at a rate of one every 1.5 seconds
impairment decreases if presentation rate is
slowed. Recall best when learning is self-paced,
though elderly subjects never quite reach same
performance as young subjects. From D. Kausler
(1994) Learning and Memory in Normal Aging, NY
Academic Press, p. 88, which cites adapted from
Canestrari, 1963, Table 2.
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12.1 Interim Summary

• Just about every kind of learning and memory
  observed in adults can also be observed in very
  young children.
• Some simple kinds of learning (e.g.,
  habituation, recognition) can be observed before
  birth.
• Other kinds of memory (particularly episodic
  and working memory) may be present at a very
  young age, but do not fully mature until late
  childhood or adolescence.

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12.1 Interim Summary

• Development of learning and memory abilities at
  least partially reflects brain development.
• Sensitive periods time windows early in life
  when certain kinds of learning advance most
  rapidly.
• Includes imprinting, social attachment learning.

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12.1 Interim Summary

• Many kinds of learning and memory show some
  decline in healthy aging.
• Working memory is especially vulnerable.
• In other memory domains (e.g., skills,
  conditioning, episodic and semantic memory) old,
  well-formed memories tend to survive well may be
  harder to acquire new memories.

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12.2 Brain Substrates
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12.2 Brain Substrates

• The Genetic Basis of Learning and Memory
• Neurons and Synapses in the Developing Brain
• Gender Differences in Brain and Behavior
• The Brain from Adulthood to Old Age

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The Genetic Basis of Learning and Memory

• DNAmaterial in cell nucleus instructions for
  replication.
• Looks like twisted ladder sides sugar and
  phosphate molecules, rungs base pair.
• Four kinds of DNA
• Adenine
• Thymine
• Cytosine
• Guanine

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The Genetic Basis of Learning and Memory

• DNA organized into chromosomes.
• Humans have 23 chromosome pairs (one set from
  each parent).
• 23rd pair determines gender.
• XX female
• XY male
• Chromosomes subdivided into genes segment of DNA
  with information for building proteins from amino
  acids.
• Probably 20,000 to 25,000 genes in humans.

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Genes and DNA
CNRI/Photo Researchers, Inc.
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Genetic Variation among Individuals Affects
Innate Learning Abilities

• Mutationaccidental changes in DNA sequence.
• Possibly from outside causes (e.g., radiation,
  viral infection) or copying error.
• Mutations can
• Be harmless.
• Lead to cell malfunction, disease, death.
• Be beneficial to the species.
• New characteristics for reproduction or survival.

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Genetic Variation among Individuals Affects
Innate Learning Abilities

• Because of mutation over time, most genes have
  allelesnaturally occurring variations.
• e.g., eye color

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Genetic Variation among Individuals Affects
Innate Learning Abilities

• Brain function also influenced by variations can
  affect learning and memory.
• Examples
• BDNF protein (the Val allele) may facilitate
  long-term plasticity.
• Tyr allele (variant of His allele on 5-HT2AR
  gene) results in less-efficient serotonin
  receptors.
• Perform slightly worse on delayed word recall
  task.

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Genetic Influences on Learning and Memory in
Humans
(a) Data from Egan et al., 2003 (b) adapted from
de Quervain et al., 2003.
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Selective Breeding and Twin Studies

• Tryon (1940) Can animals be bred for learning
  ability?
• Bred discrete groups of maze-bright and
  maze-dull rats in 7 generations.
• By 7th generation, maze-bright offspring
  routinely out-perform rats bred from maze-dull
  line.

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Data shown are hypothetical, based on Tryon, 1940.
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Selective Breeding and Twin Studies

• Multiple genes control characteristics of
  learning ability.
• No single gene.
• Human twin studies suggest that over half of the
  variation in memory scores may be genetic.
• Identical twins have more similarity than
  fraternal.

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The Influence of Environment

• Rats raised in enriched environment (good
  sensory stimulation) have more dendrites and
  synapses.
• Males had the most growth in visual cortex.
• Female rats had the most growth in frontal
  cortex.

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Neurons and Synapses in the Developing Brain

• Neurons are overproduced, then weeded out.
• Neurogenesis (neuron birth) most active during
  prenatal development continues to a limited
  degree throughout life.
• Not uniform throughout brain some neurons form
  earlier than others.

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Neurons and Synapses in the Developing Brain

• In early development, glia guide cell migration
  produce molecules that modify growth of axons and
  dendrites.
• Some glia (oligodendrocytes) produce myelin
  sheath, from birth to 18 years.
• Neurotrophic factors (e.g., BDNF protein) help
  cells properly locate and specialize.
• Without these chemical compounds, about 1/3 of
  neurons die (apoptosis), a natural phenomenon.

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Neurons and Synapses in the Developing Brain

• Synapses are also formed, then pruned.
• Synaptogenesis (formation of new synapses)begins
  during gestation, but most active after birth to
  about age 6.
• Tiny dendrite spines come and go if stimulated
  by neurotransmitters, synapses may form.
• Unused synapses die (pruning).
• New synapses may strengthen during non-REM
  sleep and unused may die during REM sleep.

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Most synapses Occur on Dendritic Spines
(a) Adapted from Hof Morrison, 2004 (b)
adapted from Trachtenberg et al., 2002.
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Sensitive Periods for Learning Reflect Sensitive
Periods for Neuronal Wiring

• Neural pathways (and specific receptors) may
  develop rapidly during sensitive periods.
• Apoptosis may then clean up neurons not used in
  in this sophisticated development.

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The Promise of Stem Cells for Brain Repair

• Young brains highly plastic older brains less
  able to adjust.
• Can stem cells be integrated into adult brains?
• Stem (especially from fetal tissue) cells have
  ability to develop into many cell types.
• e.g, skin, liver, brain cells
• Fetal stem cell transplant research still
  preliminary.
• Tried in Parkinsons disease patients.
• New neurons do not cure the underlying disease.

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Embryonic Stem Cell Transplants in Brains of
Parkinsons Patients
Adapted from Freed et al., 2001.
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Gender Differences in Brain and Behavior

• In studies
• Women often perform better than same-aged men
  on
• List recall.
• Story recall.
• Memory for object location.
• Men can outperform women in maze learning.
• Men and women studied a fictitious town map
• Men tended to learn a route more easily.
• Women remembered more landmarks.

54
Gender Differences in Brain and Behavior

• Male and female rats also show gender
  differences.
• Sex hormones may contribute to gender-based
  learning differences.

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Effects of Sex Hormones on Brain Organization

• Pubertybodys physical change to sexual maturity
  in adolescence.
• Surge in release of sex hormones.
• Primarily estrogens in woman, androgens in men
  (especially testosterone).
• In mammals and birds, testosterone surges in
  female fetuses and even more in male fetuses just
  before birth.

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Effects of Sex Hormones on Brain Organization

• During infancy, testosterone influences sex
  differences in brain development.
• Larger in women
• Lateral frontal cortex
• Language areas (supramarginal gyrus)
• Hippocampus
• Larger in men
• Visual and spatial processing areas

57
Effects of Sex Hormones on Adult Behavior

• Gender differences in memory performance appear
  after puberty from circulating estrogen and
  testosterone.
• Estrogen stimulates adult rats neuronal growth
  and synaptic plasticity (LTP), especially in the
  hippocampus.

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Effects of Sex Hormones on Adult Behavior

• Estrogen may increase verbal learning
  testosterone may increase spatial learning.
• But, relationship between sex hormones
  (especially testosterone) and learning is
  complex.
• Studies show male-to-female transsexual persons
  taking estrogen scored higher on paired-associate
  task.
• Compared to similar group who had not yet
  started estrogen treatment.

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Adulthood to Old Age Parts of the Aging Brain
Lose Neurons and Synapses

• Slow human brain shrinkage, including the
  cerebellum, begins in young adulthood.
• By age 80, average adult loses about 5 percent
  of brain weight.
• Studies show
• Cerebellum-dependent classical eyeblink
  conditioning slows with age.
• However, there is little loss of hippocampal
  neurons in the healthy elderly.
• Reductions in neurons disease warning signs.

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Neuron Loss in Prefrontal Cortex of Aging Monkeys
Adapted from Smith et al., 2004.
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Synaptic Connections May Be Less Stable in Old Age

• Barnes (et. al.) suggest total number of neurons,
  synapses does NOT decrease rather, decrease in
  ability to maintain changes in synapse strength.
• Rat and monkey studies suggest that synapses
  may be less stable in old age.
• In studies, young rat and an old rat learned a
  figure 8-shaped maze. In second session,
  hippocampal LTP in the old rat was unstable.
• Instability could contribute to spatial and
  episodic memory declines.

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Hippocampal Neurons Encoding Location in Old and
Young Rats
(be) adapted from Barnes et al., 1997.
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New Neurons for Old Brains? Adult Neurogenesis

• Adult brain may be able to grow new neurons.
• Adult neurogenesis has been studied (and
  reliably observed) in birds, fish, amphibians,
  reptiles.
• Neurogenesis in mammals?
• Studies show limited neurogenesis in brains of
  adults macaque monkeys and human cancer patients.
• Most new neurons die within a few weeks.

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12.2 Interim Summary

• Development of learning and memory abilities at
  least partially reflects brain development.
• Temporal and frontal cortex are among the last
  brain areas to fully mature.
• May help explain why memory processes dependent
  on these areas are among last to reach full adult
  potency.

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12.2 Interim Summary

• Genes play a large role in determining learning
  and memory abilities.
• Enriched environment studies show that
  experiences can also impact brain organization
  and an individuals abilities.
• Before birth, the brain overproduces neurons and
  synapses.
• Unnecessary neurons and synapses are gradually
  eliminated.

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12.2 Interim Summary

• Sensitive periods may reflect times when external
  inputs can easily and profoundly alter brain
  connectivity.
• After sensitive period, large-scale
  organization of brain area in question may be
  fixed, and further learning (of the kind in
  question) may be limited to fine-tuning.

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12.2 Interim Summary

• Sex hormones, like estrogen and testosterone, can
  influence development and performance.
• Leads to gender differences among adults in
  various kinds of learning and memory.
• Influence on developing brain leads to gender
  differences even in very young individuals.

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12.2 Interim Summary

• Working memory declines in healthy aging.
• Vulnerability may reflect normal frontal cortex
  shrinkage in healthy aging.
• Pattern of memory loss in healthy aging may
  reflect loss of neurons and synapses.
• Also, may reflect decrease in ability to
  maintain changes in synapse strength.
• Thus, newly encoded information may be lost.

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12.2 Interim Summary

• New neurons produced throughout the lifespan.
• But, particularly in humans, there is as yet
  little evidence that adult neurogenesis could
  provide large-scale replacement for damaged or
  aging neurons.

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12.3 Clinical Perspectives
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12.3 Clinical Perspectives

• Down Syndrome
• Alzheimers Disease
• A Connection between Down Syndrome and
  Alzheimers Disease?
• Unsolved MysteriesTreating (and Preventing)
  Alzheimers Disease

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Down Syndrome

• Down syndromecongenital form of mental
  retardation which occurs equally in girls and
  boys.
• Retarded speech and language development low
  IQ scores.
• Usually caused by trisomy 21 (extra copy of a
  chromosome 21).
• During embryo formation, parents (usually
  mothers) chromosome fails to split properly.

Laura Dwight
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Brain Abnormalities and Memory Impairments

• In Down syndrome, brain size may be average at
  birth, but growth in some areas (e.g.,
  hippocampus, frontal cortex, cerebellum) may be
  stunted.
• Individuals tend to have profound deficits in
  hippocampal-dependent memory abilities.
• Young adults with Down syndrome performed at
  the 5-year-old level on mental abilities tasks.
• Also, performed much worse on
  hippocampal-dependent memory tasks.

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Hippocampal-DependentLearning and Down Syndrome
Data from Vicari, Bellucci, Carlesimo, 2000).
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Brain Abnormalities and Specific Memory
Impairments in Down Syndrome
Figure summarizes performance on battery of tests
that require hippocampal function (like list
learning and spatial learning) compared with a
battery of tests that require prefrontal function
(like working memory). Adapted from Pennington
et al., 2003, Figure 2.
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Animal Models of Down Syndrome

• Mice bred for segmental trisomy (Ts65Dn mice)
  showed deficits in hippocampal-dependent tasks
  (e.g., location of maze goal).
• Enriched environment improved spatial memory in
  female Ts65Dn mice.
• Exacerbates impairment in Ts65Dn males.

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Alzheimers Disease

• Alzheimers Disease (AD)a form of progressive
  cognitive decline from accumulating brain
  deterioration.
• AD affects about 4.5 million people in U.S.
• As many as 50 percent of people over age 85 are
  afflicted.
• http//www.youtube.com/watch?v7-P9lbTJ9Hw

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Progressive Memory Loss and Cognitive
Deterioration

• AD progression
• Earliest symptoms of AD occur in episodic
  memory, such as forgetting recent visitors.
• Later, there are declines in semantic memory
  (e.g., forgetting familiar names, locations).
• Next, conditioning and skill memory
  deteriorate.
• In late-stage AD, there is often a lack of
  awareness and daily living skills.
• http//www.youtube.com/watch?voTEbq4h-kvQ

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Patients with AD show marked impairment in many
forms of memory, including list learning. Over
three trials with a 10-word list, AD patients
recall fewer items than same-aged healthy
controls after a 10 minute delay, the patients
recall almost none of the studied words. Adapted
from Figure 1 of Moulin et al. (2004).
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Plaques and Tangles in the Brain

• Amyloid plaques deposits of beta-amyloid
  (abnormal byproduct of amyloid precursor protein,
  or APP kills adjacent neurons).
• Plaques are fairly evenly distributed across
  cerebral cortex.
• Neurofibrillary tangles collapsed protein
  scaffolding within neurons.
• Early in AD, accumulate in hippocampus and MTL,
  relating to semantic and episodic memory
  deficits.
• Hippocampal shrinkage early AD warning sign.

81
Plaques and TanglesHallmarks of Alzheimers
Disease
a) Amyloid plaque (dark center spot) surrounded
by residue of degenerating cells.
b) Neurofibrillary tangles (seen as darkened
areas).
(a) Cecil Fox/Science Source/ Photo Researchers.
(b) Adapted from Figure 3 of Hardy Gwinn-Hardy,
1998.
82
Plaques and Tangles in the Brain

• Verification of presence of plaques and tangles
  (to confirm AD diagnosis) can only happen at
  autopsy.
• 10 to 20 percent of probable AD diagnoses
  (based on MRI, PET, lumbar puncture, etc.) are
  incorrect.
• Many other conditions (some treatable) mimic
  AD, so better diagnostic test needed.
• e.g., vitamin B deficiency, hypothyroidism,
  depression

83
Genetic Basis of Alzheimers Disease

• Several genes implicated in AD.
• Most progress understanding genetic cause of
  early-onset AD (begins at 3550 years).
• Less than 1 percent of AD cases early-onset.
• Caused by genetic mutations, which are
  autosomal dominant (meaning, just one mutated
  gene from either parent will trigger early-onset
  AD).

84
Connection Between Down Syndrome and Alzheimers
Disease?

• Chromosome 21 (implicated in Down syndrome)
  contains APP (implicated in AD).
• By age 3540, adults with Down syndrome develop
  neural plaques and tangles.
• Half of Down syndrome patients show memory
  decline and other symptoms of AD other half do
  NOT show cognitive decline.
• Why? Unclear. Explanation will help in
  understanding both pathologies.

85
Unsolved MysteriesTreating (and Preventing)
Alzheimers Disease

• Cholinesterase inhibitors treat forgetfulness and
  anxiety.
• Inhibiting breakdown of neurotransmitter
  acetylcholine (depleted in patients with AD).
• Memantine blocks glutamate receptors.
• May help protect neurons from
  glutamate-mediated damage, slow cognitive decline.

86
Unsolved MysteriesTreating (and Preventing)
Alzheimers Disease

• Risk factors for AD include
• Type-II diabetes
• High LDL (bad cholesterol)
• Previous head injury
• Stroke
• High blood pressure
• High levels of cognitive activity may slow AD
  symptoms.

87
12.3 Interim Summary

• Down syndrome condition in babies born with
  extra copy of chromosome 21.
• Children with Down syndrome have cognitive
  impairments.
• Includes memory impairments.
• Some brain areas tend to be abnormally small.
• Includes hippocampus, frontal cortex,
  cerebellum.

88
12.3 Interim Summary

• In Alzheimers disease, plaques and tangles
  accumulate in the brain.
• Memory symptoms are prominent early in the
  disease.
• Consistent with finding that hippocampus and
  nearby MTL areas suffer pathology early in
  disease.
• Several genes may contribute to an individuals
  risk for the common, late-onset form of the
  disease.