Early Experience and Sensitive Periods

1
Early Experience and Sensitive Periods
genes are good for setting up general patterns of
connections specific connections/modifications
based on environmental conditions these
adjustments take place during particular times in
development songbirds have a genetic
predisposition to sing learning their specific
song comes from experience sparrows and zebra
finches learn their songs during a sensitive
period if they don't learn it, they cannot
ever learn it in the future
2
Birdsong
birds from different areas have different songs
these songs have patterns birds that are raised
isolated have simplified song patterns birds
that can't hear their own song can't sing
properly-- becomes disjointed must hear their
own species' song-- other species songs will
cause isolated song patterns learning is limited
to a particular repetoire- it is not an
unlimited range of responses
3
Birdsong
birdsong learning takes place in 2 steps song
memorization hearing the actual songs must
occur early happens during 2-10 weeks after
birth happens very slowly after 3-4
months vocal learning actual production of the
sounds-- requires adequate growth only
happens 26-50 weeks of after hatching sensitive
periods can be extended by partial but
incomplete experience learning pathway must be
primed to continue
4
Birdsong
a sexually dimorphic birdsong learning pathway
exists in zebra finches separate pathways for
song production and song learning
exist posterior HVc-RA-nXIIts (motor neurons)
are for song production anterior areaX-DLM-LMAN
pathway needed for song development believed
to compare the bird's singing to a stored song
representation
5
Birdsong
androgens close the sensitive period for song
learning- as the sexual dimorphism develops,
learning is less capable artificial hormone
injections close the critical period
early castration prevents proper song
development size and survival function
6
Birdsong
birdsong learning parallels human language
acquisition along with differential
processing fully learning a language ends around
age 7 ability decreases through
adolescence grammar, syntax, and phonetics
(vocal\ production) are age dependent
meaning and vocabulary are not age
dependent people who learn a language later use
a different part of their brain based on fMRI
7
Sound Localization in Owls
owls form a map of space (and frequency) based on
their hearing integrates sight and hearing by
using time/intensity difference between the
two ears must change as the organism
grows young animals adapt to changes such as
earplugs old animals have more limited
adaptability prisms in front of the owl's eyes
can cause the sound/vision map overlap to
shift out of alignment
8
Sound Localization in Owls
young owls shift more than old owls-- young can
shift by 70 ms off, old owls shift my only up
to 10 ms sensitive period ends near sexual
maturity (ie. full growth)
9
Sound Localization in Owls
neurons in the ICC (auditory thalamus) make a
topographic projection to the ICX (inferior
colliculus) and the ICX projects
topographically to the tectum before vision is
altered visual alteration causes new axon
growth in the ICX from ICC neurons to cover
more territory removing prisms later allow a
shift back to the natural pattern can
switch back and forth
10
Sound Localization in Owls
NMDA receptors are required to generate the
switched pathway blocking receptors by AP-5
reduces new synapse intensities greatly while
only partly affecting older, natural
connections AMPA receptors are also important,
but increase in number after the initial new
connections form
11
A Sensitive Period for the Temprament of Rats
the way a rat mother grooms her pups determines
the rat's temprament determined by the
glucocorticoid receptor and hippocampus set
during the first week lasts for the life of the
animal extensive grooming and easy access to
milk make the rats less fearful and more
adventurous when placed in a new environment
12
A Sensitive Period for the Temprament of Rats
foster mothers determine the temprament of their
offspring-- genetics have little
influence genetics constrains the range of
options- environment determines which trait(s)
are expressed glucocorticoid levels determine
the stress in the blood high levels
stress low levels relaxed reversed in the
cerebellum low levels low stress high
stressed stressed mice do not show the same
amount of LTP than unstressed ones
13
A Sensitive Period for the Temprament of Rats
levels of GR (glucocorticoid receptor) in the
hippocampus is the result of grooming
behavior levels rise when pups are temporarily
removed, mothers quickly groom only works
during the first 2 weeks GR level is controlled
by promoter methylation mothering causes
demethylation and higher gluocorticoids in the
cerebellum methylation is only plastic during
early postnatal days
14
Principles of Developmental Learning
environmental conditions during upbringing
permanently shape a lot of neural circuits to
obtain pathways appropriate for their
experience early damage/neglect may be
irrepairable because the circuits are able to
change only early in development sensitive
periods allow a short time window to dramatically
change the properties of the neural circuit
within a gentically defined range
15
Principles of Developmental Learning
sensitive periods require a certain level of
maturation 1) supplied information must be
reliable and precise 2) information must be
processed postsynaptically 3) plasticity must
be active (or reactivated) anatomical and
chemical changes may be same as during
development (ie. BDNF during visual cortical
remodeling) critical periods end when molecular
mechanisms of plasticity are lost multiple
mechanisms (ie. axon growth, LTP/LTD,
chromosomes) might be included in the 'lost'
capability functional tuning or synaptic
sharpening/elimination are related to the
close of sensitive periods-- new periods have
fewer options to work on deprivation often
prevents tuning and extends critical periods