Transcript Early Experience and Developmental Learning Overview • Increasing differentiation of areas of cortex • Infant is born during height of brain development • Tertiary sulci develop from 1 month.

Early Experience and
Developmental Learning
Overview
• Increasing
differentiation of
areas of cortex
• Infant is born during
height of brain
development
• Tertiary sulci develop
from 1 month before
to 12 months after
birth
Four (very brief) Levels of Brain
Development
Creation of a tube
Neural migration
• Many elements of initial neural migration specified
genetically
• By 20 weeks gestation, 100 billion neurons!
50,000 – 500,000 neurons per minute
• Neurons follow path of glial cells outward from
ventricles
– To form 6 layers of cortex
Neural development: Synaptogensis
• Once in place, synapses are overproduced somewhat
haphazardly
– 1 year old has 150% more synapses than adult
• These are pruned (diminish) during development
• Repetition of sensory-motor patterns create more
specific set of experience dependent synaptic
linkages
Increase in complexity of neural
connections
Like a
growing
forest
How do the correct synapses form?
• 15,000 synapses for every cortical neuron
– 1.8 million per second in first 2 years!
• Cerebral cortex triples in thickness in 1st year
• Sensory and motor neurons must extend to correct
brain are and form correct synapses
• This quantity of information cannot be genetically
micro-managed
Three models
What does individual development look
like?
Individuals
Group
Two Types of Experience in Brain
Development
• Experienceexpectant
• Experience dependent
Experience-expectant
• How common early experiences provide essential
catalysts for normal brain development
– Early visual stimulation, hearing, exposure to language,
coordinating vision and movement,
• The developing brain “expects” and requires these
typical human experiences, and relies on them as a
component of its growth.
Experience-dependent
• How individual experience fosters new brain growth and
refines existing brain structures
• Can be unique to an individual
– Reading
– Singing, music
Neural Darwinism (Edelman)
• Use it or lose it
– What is not used, is pruned
– What is used, develops stronger connections
• Organism & environment are system that shapes
brain
– Brain development is guided by environment
• Brain enables behavior which shapes brain
– Synaptic development is not teleological
The fetus as constructing its own
development
• Fetal behavior impacts physical development
– In chicks prevented from moving, cartilage turns
to bone
• Fetal sensory experience impacts sensory
development
– Mice whose tongues were anesthetized had
malformed cleft palates
Prenatal sensory experience impacts
sensory development
• Hearing typically develops before sight
• Rats, ducklings, and quail chicks exposed to visual
stimulation prenatally
– before they normally would
• lose hearing ability at birth
Normal sensory development contingent on extrafetal environment
• Differences in the timing of augmented prenatal stimulation led to
different patterns of subsequent auditory and visual responsiveness
following hatching.
• No effect on normal visual responsiveness to species-typical maternal cues
was found when exposure to tactile and vestibular stimulation coincided
with the emergence of visual function (Days 14-19)
• When exposure took place after the onset of visual functioning (Days 1722), chicks displayed enhanced responsiveness to the same maternal
visual cues.
• When augmented tactile and vestibular stimulation coincided with the
onset of auditory function (Days 9-14), embryos subsequently failed to
learn a species-typical maternal call prior to hatching.
•
Honeycutt, H. & R. Lickliter (2003). Developmental Psychobiology 43: 71-81. The influence of prenatal tactile and vestibular stimulation and visual responsiveness in
bobwhite quail: A matter of timing
Prenatal behavioral development
•
•
•
•
•
•
9 weeks - movement
16 weeks - frowning, grimacing
25 weeks - moves to drumbeat
26 weeks - remembers sounds
32 weeks - all brain areas functioning
34 weeks - can habituate
1st Trimester
• Behavioural Repertoire:
–
–
–
–
8 weeks: Startle (arms and legs shoot outward)
9 weeks: “graceful” general movements of the head, trunk, limbs
10 weeks: Stretch (head moves back, trunk arches, arms lifted)
11 weeks: Yawning
• Cause and Function of Prenatal Movement
– Unable to inhibit movement; inhibition comes with the connection to
higher brain centres
– Fetal movement is necessary for the physical systems to develop
normally (stimulate development of muscles, tendons, ligaments);
– Breathing movement important for lung development
– Changes in position may promote better circulation & help prevent skins
from sticking together
– Motor behaviour moves amniotic fluid
• structural growth of fetus
– Some behaviours (e.g., sucking) may be preparatory
»
http://web.uvic.ca/psyc/coursematerial/psyc435a.f01/435A/Week%202%20Lecture%20Notes.pdf
Role of experience
Overview of brain growth
• Subcortical areas responsible for reflexes develop
first
– E.g. spinal cord
• Followed by cortical areas in a specific progression
– What is most human develops last
• Most but not all neurons present at birth
– Synapses develop
– Myelin develops
At the same time - Myelination
• Fatty sheaths develop and insulate neurons
• Dramatically speeding up neural conduction
• Allowing neural control of body
– General increase in first 3 years is likely related to speedier
motor and cognitive functioning
• allowing activities like standing and walking
• Endangered by prenatal lead exposure
“Promoting early brain development”?
• Re-discovery of importance of early experience
– “How brain connections grow and change as a
result of stimuli from the environment.
– How early stress can be harmful to the
developing brain.
– Principle of "use it or lose it"
– Seven ways to support brain development:
• http://www.pitc.org/
“Considerable misunderstanding of
early brain development occurs
when neurons and synapses are
considered independently of the
development of thinking, feeling,
and relating to others.”
–Thompson, 2001, p. 29
Is it all over after 3?
• Is the course of development set in infancy?
• Early experience is important
• But, with some exceptions, human beings remain
open to the positive effects of additional experience
– The same is true for the impact of experience on brain
development
• How important is it to ‘stimulate your child’s brain’?
What kind of stimulation is best?
• Running rats …
• Adult neurogenesis …
Implications for practice
• It is important to provide a safe, warm, supportive,
stimulating environment for infants
• But its never too late to improve developmental
outcome for an individual
• At any point, current conditions are as important as
past conditions
• No flashcards
Brain Overgrowth in the First Year of
Life in Autism
• The clinical onset of autism appears to be
preceded by 2 phases of brain growth
abnormality: a reduced head size at birth
and a sudden and excessive increase in
head size between 1 to 2 months and 6 to
14 months. Abnormally accelerated rate of
growth may serve as an early warning
signal of risk for autism
• Courchesne, Carper, Akshoomoff, (2003)
• Why overgrowth?
• Later developing processes more susceptible
to the effects of experience
• Motor development more plastic than
language development
• Sensitive periods
• Genetics and experience: Indissoluble
Synapse Rearrangement
•Active synapses likely
take up neurotrophic
factor that maintains the
synapse
•Inactive synapses get too
little trophic factor to
remain stable
Synapse Rearrangement
Time-lapse imaging of synapse elimination
Two neuromuscular
junctions (NM1 and
NMJ2) were viewed in
vivo on postnatal days 7,
8, and 9.
Myelination
MYELIN AND SALTATORY CONDUCTION
Myelin is an electrical insulator sheath wrapped around axons
Oligodendrocytes produce myelin on CNS axons
Schwann cells produce myelin on PNS axons
Short gaps in myelin along axons called nodes of Ranvier
Myelin’s function is to speed action potential propagation down long axons
MYELIN SHEATH COMPOSED OF MANY LOOPS OF A GLIAL PROCESS
Each oligodendrocyte has several
processes, each of which produces
a myelin sheath on a different axon
Schwann cells each form only a
single myelin sheath
MYELIN SHEATH GENERATED BY CONTINUED MIGRATION
OF PROCESS LEADING EDGE AROUND AXON
While the leading glial process continues to encircle the axon,
the earlier-formed loops undergo compaction
to form the contact myelin sheath
MYELINATED FIBERS VIEWED IN CROSS-SECTION
Low magnification
Light microscopy
Electron microscopy at
very high magnification
reveals alternating
major dense lines and
intraperiod lines
High magnification
electron microsopy
ORGANIZATION OF THE MYELIN REPEAT PERIOD
PLP is the most abundant protein in CNS myelin
P0 is the most abundant protein in PNS myelin
THE PARANODE IS SITE OF TIGHT AXON-GLIAL ADHESIONS
ROLE OF MYELIN IN FAST ELECTRICAL TRANSMISSION
Unmyelinated
Axon
(SLOW CONDUCTION)
Myelinated
Axon
(FAST CONDUCTION)
SODIUM CHANNELS ONLY AT NODES
AT VERY HIGH DENSITY
Action potential at one point along unmyelinated axon produces current that only
propagates short distance along axon, since current is diverted through channels
in axon membrane. So action potential can only next occur short distance away
Myelin reduces effective conductance and capacitance of
internodal axon membrane.
Action potential at node of Ranvier produces current that propagates
0.5-5 mm to next node of Ranvier, generating next action potential
THIN AXO-GLIAL SPACE AT PARANODE LOOPS CREATES HIGH
NODE-INTERNODE PERIAXONAL RESISTANCE WHICH
ELECTRICALLY ISOLATES INTERNODAL MEMBRANE
Only 20 Angstrom gap between
mature paranodal loop
and axonal membrane
SINCE
Rparanode >>>> Raxial & Rleak
Rparanode
CHARGING OF
INTERNODAL
MEMBRANE
VERY SLOW
AND CHANGE
IN INTERNODE
Raxial
VM IS
INSIGNIFICANT
NODE PARANODE
Tight junctions between
mature loops
Rparanode
Raxial
INTERNODE
PARANODE
NODE
POTASSIUM CHANNEL SHUNT NOT REQUIRED IN
MOST MATURE MYELINATED AXONS
Myelinated axons conduct action potentials at ~ 50 mm/msec
Total refractory period of nodal Na+ channels after inactivation
is ~ 5 msec.
Therefore, by the time Na+ channels return to rest after an action
potential, the spike has propagated 25 cm away
(which is terminated in most cases)
K+ channel inhibition in mature myelinated fibers
does not alter conduction or promote misfiring.
FORMATION OF NODAL, PARANODAL, AND JUXTANODAL
PROTEIN CLUSTERS DURING MYELINATION
Kv1
Kv1
• Na+ channels cluster early at wide immature nodes. As nodes narrow and
mature, Na+ channel density increases.
• K+ channels cluster later and shift their position. They first appear at nodes,
But move to paranode and then juxtaparanode as structure matures.
• K+ CHANNELS ARE OF CONTINUED IMPORTANCE DURING MATURATION OF MYELIN,
SINCE ONLY FULLY MATURE FIBERS CONDUCT FAST ENOUGH TO MAKE THEM UNNEEDED.
PERSISTENCE OF K+ CHANNELS IN MATURE JUXTAPARANODES MAY FUNCTIONALLY
PROTECT FIBERS IN CASE OF PARTIAL DE-MYELINATION
MUTATIONS CAN CAUSE MINOR OR MAJOR MYELIN LOSS
“SHIVERER” mutant mouse has almost
complete absence of myelination,
due to a failure of precursor cells
to differentiate into oligodendrocytes
Other mutations which impair
myelination are mutations in the
major protein components of
the myelin sheath
MUTATIONS IN PLP GENE CAUSING HYPOMYELINATION IN CNS
Similarly, structural mutations in PNS myelin protein genes
cause defective myelination of the PNS
Myelination Lasts for up to 30 Years
Brain Weight During Development and Aging
Critical Periods
Sensitive Period
Anatomy and physiology are especially sensitive to
modulation by experience.
Critical Period
An extreme form of Sensitive Period.
Appropriate expression is essential for the normal
development of a pathway or set of connections
(and after this period, it cannot be repaired).
e.g., There was a critical period for the formation of
ocular dominance columns, based on neuronal
activity/input from both spontaneous firing and
visual inputs from the eyes.
If appropriate information is not received during
the critical period (from experience), this
pathway never attains the ability to process
information in a normal fashion, and as a
result, perception or behavior can be
permanently impaired.
E.g., development of appropriate social and
emotional responses to others.
E.g., development of language skills in humans.
Models of Developmental Learning and the
Importance of Early Experiences
1. Sound localization in the owl.
- a “map” of auditory space is developed in the
midbrain of the barn owl.
- This map integrates auditory and visual info
so that movements of the eyes and head can be
oriented towards auditory stimuli (and catch
mice and rats!).
To create a map of auditory space, the midbrain
nucleus has to learn (of inferior colliculus)
spatial cues based on auditory signals it receives
from the 2 ears.
Auditory space map and its inputs
What are the cues?
Differences in timing and in the level (and
pitch) between the 2 ears.
Why are these things not just genetically
encoded?
Individual differences in size, shape of head,
sensations and speed of head movement, etc.
In young animals, this plasticity allows the
pathway to respond and adjust to change or
disruptions (e.g., growth, damage to ear, etc.).
Tuning of inferior colliculus neurons is adjusted
in response to visual cues.
The location of plasticity is the ICX (external
nucleus of the inferior colliculus).
If a stable shift in visual field occurs during the
critical period (i.e., owls raised with prisms
over their eyes), the auditory receptor field of
the icx will realign with the shifted visual field.
Now, the owl will have (correctly coordinated
visual/auditory stimuli with the prisms on).
The sensitive period for the owl, during which
large shifts can occur, is throughout juvenile
life (until reaching sexual maturity).
The experience induces the growth and
elaboration of axons into the icx to sites where
they can support appropriate responses.
This response depends upon activation of
NMDA receptors (involved in plasticity).
Note: in our example, the power of genetically
programmed pattern is still great (though an
adult owl cannot adjust to a large shift in
visual field, one that has been shifted can
return to normal in the adult (over a period of
weeks) when the prisms are removed.
NMDA receptors
Plasticity of axons in the ICX
2. Development of Birdsong.
A special form of communication developed by
birds to i.d. their own, defend territories, or
attract mates.
Birdsong is complex and has a periodic structure
(like music).
“Dialects” or varieties of song can specify a
geographic area, where the basic song structure
is common to a species.
Song is developed by a combination of genetic
instructions and learning (early experiences).
The latter often takes place during a critical
period.
1) Characteristics of song learning.
*How can the extent of the critical period be
determined in a species?
[raise birds in acoustic isolation and expose them to
song for brief periods at different points of
development]
“Isolate song”: a flat, species-specific pattern with
complexity that a bird can develop if raised in
isolation during the critical period.
“Developmental song”: abnormal pattern
developed in a bird that is unable to hear itself
and get auditory feedback during critical period.
2 major pathways for birdsong learning
Birdsong characteristics
2) Shows importance of a critical period during
song memorization (learning songs of
conspecifics: 2-8 weeks).
3) Illustrates importance of critical period during
vocal learning (bird hears and evaluates his
own song so that it matches the memorized
song pattern).
[auditory feedback is essential for shaping the
pattern of connectivity in the song-production
pathway]
• Importance of genetic background for the types of
patterns that can develop:
- note isolate song (earlier)
- what does a baby bird develop when raised with an
alien (other species) birdsong during the critical period
for memorization?
[genetically detrimental “filters” within pathway are
responsible for song memorization].
If too distinct from normal, isolate song develops.
Regarding song memorization, the critical period is closed
after the appropriate stimuli have been received.
After 8 weeks plasticity decreases; So, stimulus must be
prolonged and rich (i.e., live bird and not a recording)
for any memorization to occur.
Closure of critical period and decreased plasticity are also
related to sexual maturity.
*What happens if a bird receives testosterone early?
(song is fixed in “immature” state).
*What happens if bird is castrated prior to learning to
sing?
[song production inconsistencies and unstable for rest of
life – critical period never closes?]
Neural Pathway for Song Learning
Still an active area of research
We know a little regarding song production in species
when only males sing (*how is this studied?)
[Sexual dimorphism]
Song System – 2 groups of nuclei:
Motor – posterior forebrain: song production
Feedback – anterior forebrain: oral learning
Posterior pathway: - necessary for products of
learned sounds. Higher visual center (HVC)  RA
arhistriatum  hypoglossal nucleus:
motor neurons centrally
vocal muscles.
Anterior pathway: neurons respond maximally to
bird’s own song.
HVC  “area X”  DCM (a thalamic nucleus) 
LMAN (lateral magnocellular nuc of ant striatum)
Sexual dimorphism-song nuclei
Sexual Dimorphism
In Mammals:
A lesion in LMAN during critical learning period
“freezes” song much like early testosterone.
However, lesions in adult birds after learning is
complete have zero effect.
LMAN then sends input to RA  hypoglossal n.
 muscles mediated by NMDA receptors.
These synapses will compete selectively and
several will be eliminated just prior to closure
of critical period (as sex hormones rise).
3. Imprinting
Learning of cues through i.d. a parent –
important for survival involves learning
multiple sensory cues that i.d. the parent
(visual, auditory, olfactory, etc.) during brief
critical periods.
Recall the classic experiment by Konrad Lorenz
(“mother goose”).
Critical periods can be brief and can begin
shortly before birth in some species.
What happens if baby is in isolation during critical
period? [never responds appropriately to social
signals from members of its own species].
What happens when raised by another species?
What is the genetic filter here?
During the (critical) imprinting period, a duck will
choose a duck over another species, or the
closest equivalent (i.e., goose > human).
One Neural Path
Auditory stem  nucleus of anti forebrain
activated as in previous examples of selective
elimination of inputs occurs during late critical
period in response to experience.
Binocular Vision
Ability to “fuse” the image from 2 eyes to create
a 3-D image with depth.
The consequence of visual inputs onto neurons
of the visual cortex is guided by early
experience.
A critical period exists, during which monocular
deprivation can prevent the development of
stereoscopic vision.
Review the development of the ocular
dominance columns for the projection of LGN
to Layer IV of visual ctx.
Critical period for binocular vision development
Equal input from both eyes  equally wide
columns with competition based on neural
activity:
If 1 eye is occluded, open eyes inputs are pruned
and activity of majority of visual ctx is driven by
LGN afferents from normal eye.
*Competition is driven by:
amount of neural activity
degree of synchrony
Therefore, the stimulation of both optic nerves with
equal but asynchronous stimuli  one
dominating , and impaired binocular vision.
Amount and synchrony of synaptic activation
shapes the synaptic function and architecture
by adjustments in synaptic strength through a
process depending on activation of NMDA
receptors: LTP.
*Rats: This dual process is not as rapid and
plastic as the other examples we have
reviewed. Disruption are not completely
universal during critical period and patterns
are not as ‘pre-set’.