Critical periods
A time period when environmental factors
have especially strong influence in a
particular behavior.
– Language fluency
– Birds- Are you my mother?
– eye-specific layers in lgn and visual cortex
There is a Critical Period for Learning Language
http://www.aeiou.at/aeiou.film.f/f050a
Manual "Babbling" in Deaf Infants With Deaf, Signing
Parents Compared to Manual "Babble" in Hearing Infants
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Konrad Lorenz and his geese (Photo by Tom
McAvoy; courtesy of LIFE Magazine, ©1955, Time, Inc.)
Birdsong
Development as an active process
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Frog brain
QuickTime™ and a
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Frog
Development as an active process
Eye
Eye
Optic tectum
Development as an active process
Eye
Eye
?
Optic tectum
Development as an active process
Critical periods in visual
development
• Visual system is a model system to study
mechanisms of plasticity.
• Retina to LGN to visual cortex. LGN and
layer 4 of cortex have monocular inputs.
• Cells in other layers of cortex can get inputs
from both eyes.
Columnar Organization of Ocular Dominance (review)
Effect of Early Closure of One Eye on Distribution of
Cortical Neurons Driven By Stimulation of Both Eyes
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Consequences of brief Monocular Deprivation at the
Critical Learning Period in the Cat
Ocular Dominance: Single-cell Recordings
in Normal and Strabismic Cats
Cat cortex,
mask-raised.
Each row is a
different cell.
Other experiments
• Closing both eyes > relatively normal distribution
of inputs. Implies that it is not atrophy of unused
circuits that causes the deprived eye to lose out,
but competition that allows one eye to win.
Other experiments
• Closing both eyes > relatively normal distribution
of inputs. Implies that it is not atrophy of unused
circuits that causes the deprived eye to lose out,
but competition that allows one eye to win.
• Tracing of single neurons in LGN shows that
visual activity influences arborization of axons,
lack of activity leads to less branching.
Terminal Arborizations of LGN Axons Can Change Rapidly
in Monocular Deprivation
reduced number of branches in deprived eye
Transneuronal Labeling With
Radioactive Amino Acids
Ocular Dominance Columns in Layer IV
of the Primary Visual Cortex of an Adult
Macaque Monkey
How do neurons know that they are around others of
the same eye?
Hebb’s postulate
• When an axon of cell A is near enough to excite a cell B and
repeatedly or persistently takes part in firing it, some growth
process or metabolic change takes place in one or both cells
such that A's efficiency, as one of the cells firing B, is
increased.
How do neurons know that they are around others of
the same eye?
Hebb’s postulate
• When an axon of cell A is near enough to excite a cell B and
repeatedly or persistently takes part in firing it, some growth
process or metabolic change takes place in one or both cells
such that A's efficiency, as one of the cells firing B, is
increased.
• Correlated activity between presynaptic and postsynaptic
cells strengthens synaptic connections between them.
• Cells that fire together, wire together.
Also used in learning and memory, called long term potentiation
(LTP).
Hebb's Postulate During Visual System Development
Long Term Potentiation [LTP]
• Brief high frequency train of inputs leads to a
long-lasting increase in EPSP amplitude.
• LTP does not occur at all synapses, only those that
are stimulated. > Learning and memory. Would
not want to strengthen synapses that didn’t
correlate with a stimulus.
• Time scale: days, weeks ????
What does Calcium do?
• Ca stimulates multiple Ca++ dependent protein
kinases
• Mice mutant in these kinases have defects in
LTP, learning, and memory.
– Kinases
• Get more AMPA receptors into the spine.
More receptors = more depolarization in
response to glutamate.
• Grow more spines = increases number of
synapses with a give neuron
Long-Term Depression
Long-term depression ( LTD) is the opposite of long-term
potentiation (LTP). Cells become less sensitive to input.
•Occurs when communication across the synapse is silenced or
weakened. (stimulation at a low rate over long period)
•LTD is important in the cerebellum, in procedural memory,
where the neurons involved in erroneous movements are
inhibited by the silencing of their synaptic connections.
•LTD allows us to correct our motor procedures when learning
how to perform a task. (e.g. – forgetting the way that was a
mistake so you don’t keep doing it)
•Like LTP – can occur in several brain regions – especially
hippocampus and cerebellum
LTD in cerebellum:
•Depolarization of post-synaptic
cell
•Glutamate activates both AMPA
and Mglu
•Indirectly triggers clatharindependent internalization of AMPA
receptors
•This weakens the synapse
•In hippocampus – similar
mechanism.
Long-term depression
• Weakens inputs (forgetting?).
• Low frequency tetanus leads to long-term
depression of synapses. Has all the same
properties as LTP. Requires NMDA receptors, Ca.
• Small rises in Ca++ lead to depression whereas
large increases in Ca++ trigger potentiation.
• Cam Kinase required for LTP, Ca++ dependent
phosphatase required for LTD.
Conclusions
• Experience can change behavior.
• Experience can change patterns of synaptic
connections.
• Neural activity is required to make these
changes.
• Changes are easier early in development,
during critical periods.