29th august 2013bio 334 - neurobiology i - critical periods in development1 critical periods in...
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29th August 2013 Bio 334 - Neurobiology I - Critical periods in development 1
Critical periods in development - “nature” vs. “nurture”
Raghav RajanBio 334 – Neurobiology I
August 29th 2013
29th August 2013 Bio 334 - Neurobiology I - Critical periods in development 2
The story so far .....
● Neural tissue is induced during gastrulation
● Neurons differentiate from precursor cells
● 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
● On a larger scale, maps form within the brain
● All this so far, is largely driven by genes - “nature”
● What about “nurture”? How important is sensory experience?
29th August 2013 Bio 334 - Neurobiology I - Critical periods in development 3
There exist “critical” or “sensitive” periods during development
● 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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During this period, experience can refine synaptic connections
● 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
● Visual system● Auditory system● Song learning in songbirds
29th August 2013 Bio 334 - Neurobiology I - Critical periods in development 5
Donald Hebb proposed Hebbian plasticity rules - “Fire together, wire together”
● Such synapses are called Hebbian synapses
● If a pre-synaptic neuron can make the post-synaptic neuron fire repeatedly, those connections are strengthened
● Conversely, connections that are not effective are lost
Mark F Bear, Barry W Connors, Michael A Paradiso. Neuroscience: Exploring the brain (2007) – Chapter 23
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Visual perception is shaped by early experience
● 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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Monocular and binocular zones in the visual field
● Depending on the placement of the eyes, the size of individual zones can vary
Kandel, Schwartz and Jessell, Principles of Neural Science Chapter 27
29th August 2013 Bio 334 - Neurobiology I - Critical periods in development 8
In primary visual cortex, eye specific inputs are segregated in layer 4
● 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
Kandel, Schwartz and Jessell, Principles of Neural Science – Chapter 56
29th August 2013 Bio 334 - Neurobiology I - Critical periods in development 9
Most cells in monkey primary visual cortex (outside of layer 4) are responsive to stimuli in
either eye
● Only cells in the binocular zone have been considered
Kandel, Schwartz and Jessell, Principles of Neural Science – Chapter 56
29th August 2013 Bio 334 - Neurobiology I - Critical periods in development 10
Most cat V1 cells (outside of layer 4) are binocular – respond to both eyes
● All expts. in the binocular zone only
● All expts. in the binocular zone only
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
● One eye kept shut for about 2 months after birth
● Recordings immediately after eyelid is opened
● LGN responds to deprived eye, although area occupied is smaller
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9
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Does disuse lead to loss of connections?
● 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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Surprisingly binocular deprivation does not alter the ocular dominance histogram
● A good fraction of cells are unresponsive to light
● Of the responsive cells, a lot of them show abnormal responses
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9
29th August 2013 Bio 334 - Neurobiology I - Critical periods in development 14
Competition hypothesis emerges
● 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
● Strabismus to test the hypotheis● Misalignment of eyes
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Organization of binocular inputs
● 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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Kittens raised with aritificial strabismus have very few binocular neurons
● 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● Very few binocular neurons – results later in poor depth perception
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
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
● Normally horizontal connections connect columns with different ocular preference
● Not so, even after two days of strabismus
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9
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