Critical periods in development - nature vs. nurture

Transcription

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

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? 2

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 3

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 4

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 postsynaptic neuron fire repeatedly, those connections are strengthened Conversely, connections that are not effective are lost 5 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 6

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 7

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 8

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 9

10 Most cat V1 cells (outside of layer 4) are binocular respond to both eyes All expts. in the binocular zone only Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 Chapter 9 10

11 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 11

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? 12

13 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 13

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 14

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 15

16 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 16

17 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 17

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 Dan H Sanes, Thomas A Reh, William A Harris. Development of the Nervous System 2005 Chapter 9 18