Transcript ppt - IISER Pune

Critical periods in development - “nature” vs.
“nurture”
Raghav Rajan
Bio 334 – Neurobiology I
August 29th 2013
29th August 2013
Bio 334 - Neurobiology I - Critical periods in development
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The story so far .....
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Neural tissue is induced during gastrulation
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Neurons differentiate from precursor cells
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Gradients of signaling molecules pattern the neural tissue
into different parts
Axons of neurons find their way to specific partners
Neurons form connections with each other and with
muscles
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On a larger scale, maps form within the brain
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All this so far, is largely driven by genes - “nature”
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What about “nurture”? How important is sensory
experience?
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Bio 334 - Neurobiology I - Critical periods in development
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There exist “critical” or “sensitive” periods during
development
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Critical period – A strict time window
during which experience provides
information that is essential for normal
development and permanently alters
performance.
Sensitive period – A limited time during
development, during which the effect of
experience on brain function is
particularly strong
http://www.cerebromente.org.br/n14/experimento/lorenz/index-lorenz.html
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Bio 334 - Neurobiology I - Critical periods in development
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During this period, experience can refine synaptic
connections
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Sensory experience in early life is extremely important
and can shape the nervous system
While the nervous system continues to be plastic later on,
experience does not have as great as influence
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Visual system
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Auditory system
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Song learning in songbirds
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Bio 334 - Neurobiology I - Critical periods in development
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Donald Hebb proposed Hebbian plasticity rules - “Fire
together, wire together”
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Such synapses are called
Hebbian synapses
If a pre-synaptic neuron
can make the postsynaptic neuron fire
repeatedly, those
connections are
strengthened
Conversely, connections
that are not effective
are lost
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Mark F Bear, Barry W Connors, Michael A Paradiso. Neuroscience: Exploring the brain (2007) – Chapter 23
Visual perception is shaped by early experience
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People with congenital cataracts (opaque covering of
lens) – impaired vision from birth
Cataracts typically removed between 10 and 20 years of
age
Have difficulty perceiving shape and form
One form of plasticity that has been extensively studied
is OCULAR DOMINANCE PLASTICITY
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Bio 334 - Neurobiology I - Critical periods in development
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Monocular and binocular zones in the visual field
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Depending on
the placement of
the eyes, the
size of individual
zones can vary
Kandel, Schwartz and Jessell, Principles of Neural Science
Chapter 27
Bio 334 - Neurobiology I - Critical periods in development
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In primary visual cortex, eye specific inputs are
segregated in layer 4
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Normal primary visual cortex has ocular dominance
columns – i.e. each column is dominated by input from
one of the two eyes
Cells outside of layer 4 receive input from both eyes
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Bio 334 - Neurobiology I - Critical periods in development
Kandel, Schwartz and Jessell, Principles of Neural Science – Chapter 56
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Most cells in monkey primary visual cortex (outside of
layer 4) are responsive to stimuli in either eye
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Only cells in the binocular zone have been considered
29th August 2013
Bio 334 - Neurobiology I - Critical periods in development
Kandel, Schwartz and Jessell, Principles of Neural Science – Chapter 56
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Most cat V1 cells (outside of layer 4) are binocular –
respond to both eyes
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All expts. in
the binocular
zone only
Bio 334 - Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9
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Monocular deprivation results in more responses to the
non-deprived eye
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One eye kept
shut for
about 2
months after
birth
Recordings
immediately
after eyelid
is opened
LGN responds
to deprived
eye,
although
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System
2005 – Chapter 9
area
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Bio 334 - Neurobiology I - Critical periods in development
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Does disuse lead to loss of connections?
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Monocular deprivation results in loss of connections from
the deprived eye – could be due to disuse
What happens during binocular deprivation? Do all cells
become unresponsive?
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Bio 334 - Neurobiology I - Critical periods in development
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Surprisingly binocular deprivation does not alter the
ocular dominance histogram
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A good
fraction of
cells are
unresponsive
to light
Of the
responsive
cells, a lot of
them show
abnormal
responses
Bio 334 - Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9
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Competition hypothesis emerges
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Maybe connections from both eyes compete with each
other in cortex
Retinal synapses in LGN are not affected by deprivation
because they're monocular
In the cortex, monocular deprivation result in active
afferents from one eye and lower activity from other eye
– latter at a disadvantage
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Strabismus to test the hypotheis
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Misalignment of eyes
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Bio 334 - Neurobiology I - Critical periods in development
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Organization of binocular inputs
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Corresponding points in the
two retinas receiving inputs
from the same location
project to nearby locations
in layer 4
Strabismus affects this
Normally input from both
eyes will be active at the
same time
Not so in strabismus
http://www.answers.com/topic/visual-system-organization. Fig. 5
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Bio 334 - Neurobiology I - Critical periods in development
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Kittens raised with aritificial strabismus have very few
binocular neurons
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Misalignment of eyes induced by surgery
Visual stimulus does not fall on corresponding portions of
the retina
Cortical neurons are rarely activated by both eyes at the
same time
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Bio 334 - Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9
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Segregation of eye-specific inputs in layer 4 develops
after birth
29th August 2013
Bio 334 - Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9
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Even horizontal connections are influenced by
experience
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Normally horizontal
connections connect
columns with different
ocular preference
Not so, even after two
days of strabismus
Bio 334 - Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9
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