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Introduction to the Laboratory
The major goal of our lab is to understand relationships
between development and learning at the level of both
brain and behavior.
Development in animals is frequently characterized
by periods of heightened capacity for both neural
and behavioral change. Sensitive periods of development
are those in which brain and behavior are most susceptible
to modification by experiential factors in the external
environment and/or changes in internal milieu (such
as levels of hormones and growth factors). Certain
types of learning occur only during sensitive periods
of development, and coincide with heightened phases
of neural plasticity. In humans, for example, children
are much more adept at learning languages than are
adults, and the time at which the capacity for language
acquisition decreases seems to correlate with the
end of the period of maturation of the cerebral hemispheres.
One
of the few groups of organisms other than humans that
learn vocal sounds used for communication during a
sensitive period of development are songbirds. Vocal
learning in songbirds, as in humans, is highly developmentally
regulated - a specific vocal pattern emerges gradually
during development as juvenile birds listen to and
learn to copy a "tutor" song. Adult birds are unable
to modify their previously established song patterns
or learn new ones. Song learning and production are
controlled by circuits of highly localized, interconnected
brain nuclei, including regions of cortex and basal
ganglia. This brain-behavior system provides an ideal
model in which to address basic questions pertaining
to relationships between neural development and complex
processes of learning and behavior.
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An adult male zebra finch (note the sexy
orange cheek patches) |
This is a zebra finch brain, in a nutshell;
note the huge cortex |
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Overview: Song is a Learned Behavior
Our studies take advantage of a model system: vocal
learning in zebra finches. Like many songbirds, zebra
finches learn a specific vocal pattern during a sensitive
period of development. Examples of song produced by
adult male zebra finches are shown below; these spectrograms
plot the frequency of song sounds as a function of time.
Young birds must hear songs produced by members of their
species; this auditory experience engenders specific
activity-dependent processes in the brain to guide the
process of vocal learning. Normally juvenile birds listen
to the song of their fathers and gradually learn to
produce a copy of that song. However, they can learn
to copy songs from other “tutors”, even
from birds of other species if they are cross-fostered
to them. Thus, juvenile zebra finches learn a specific
vocal pattern by imitating the song of an adult with
whom they interact socially during a sensitive period
of development.
These
figures show the stereotyped song production of a normal
adult zebra finch. Individual syllables of the song
pattern are produced in a highly stereotyped pattern.
Juvenile male zebra finches produce their first song-related
vocalizations starting around 30-35 days of age. These
sounds bear little resemblance to the tutor songs they
are learning to copy, but become progressively more
similar until they achieve their final, stable form
between 80-90 days of age.
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Neurons in Area X of the basal ganglia project
to the thalamus |
"Barrels" in mouse cortex represent individual
whiskers and form during a sensitive period of
development |
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Overview: The Neural Circuits that Control Song Learning
The neural substrate that underlies vocal learning and
behavior is highly anatomically localized. The schematic
diagrams of the neural song control system shown below
depict the major brain regions that control the ability
to learn and produce learned vocalizations. The view
on the top shows the brain regions in their approximate
location when viewing an outline of the brain from the
side. The view on the bottom shows a “flow-chart”
view of connections between these same brain regions.
The
brain regions shown in blue are important for controlling
already-learned songs in adult birds: HVC and RA appear
to encode the learned programs for specific songs in
adults. The brain regions shown in orange actively contribute
to refinement of song behavior during the sensitive
period. Lesions within this circuit disrupt song behavior
during early stages of vocal learning, but have no effect
in normal adult birds following the period of song learning.
Projection neurons in lMAN show complex auditory properties,
and become selectively tuned to the bird’s own
song around the time that lMAN lesions lose the ability
to disrupt song behavior
Interestingly, both the morphology and the function
of these brain regions change dramatically during the
sensitive period for song learning. For example, lMAN
grows during early stages of vocal development, but
then regresses; this growth and regression is accompanied
by dramatic synaptic re-modeling. Lesions of lMAN disrupt
song learning during the period of growth, but damage
to lMAN has no effect on song behavior at later stages
of development when regression is occurring.
We study factors that regulate basic processes of neural
development, such as neurodegeneration and changes in
synaptic specificity and synaptic strength. Ultimately,
we hope to unravel the cellular and molecular mechanisms
by which learned behaviors emerge during senstitive
periods of development.
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