1. Critical periods in development - “nature” vs. “nurture”
Raghav Rajan
Bio 334 – Neurobiology I
August 29th 2013
29th August 2013
Bio Neurobiology I - Critical periods in development

2. Bio 334 - Neurobiology I - Critical periods in development
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 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
29th August 2013
Bio Neurobiology I - Critical periods in development

4. 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 Neurobiology I - Critical periods in development

5. Bio 334 - Neurobiology I - Critical periods in development
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
29th August 2013
Bio Neurobiology I - Critical periods in development
Mark F Bear, Barry W Connors, Michael A Paradiso. Neuroscience: Exploring the brain (2007) – Chapter 23

6. 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
29th August 2013
Bio Neurobiology I - Critical periods in development

7. 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 Neurobiology I - Critical periods in development

8. Bio 334 - Neurobiology I - Critical periods in development
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
29th August 2013
Bio Neurobiology I - Critical periods in development
Kandel, Schwartz and Jessell, Principles of Neural Science – Chapter 56

9. Bio 334 - Neurobiology I - Critical periods in development
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
29th August 2013
Bio Neurobiology I - Critical periods in development
Kandel, Schwartz and Jessell, Principles of Neural Science – Chapter 56

10. Bio 334 - Neurobiology I - Critical periods in development
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
29th August 2013
Bio Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9

11. Bio 334 - Neurobiology I - Critical periods in development
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
29th August 2013
Bio Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9

12. 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?
29th August 2013
Bio Neurobiology I - Critical periods in development

13. Bio 334 - Neurobiology I - Critical periods in development
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
29th August 2013
Bio Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9

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
29th August 2013
Bio Neurobiology I - Critical periods in development

15. 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
Fig. 5
29th August 2013
Bio Neurobiology I - Critical periods in development

16. Bio 334 - Neurobiology I - Critical periods in development
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
29th August 2013
Bio Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9

17. Segregation of eye-specific inputs in layer 4 develops after birth
29th August 2013
Bio Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9

18. Even horizontal connections are influenced by experience
Normally horizontal connections connect columns with different ocular preference
Not so, even after two days of strabismus
29th August 2013
Bio Neurobiology I - Critical periods in development
Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 – Chapter 9