Plasticity in the Visual system

Question 1

what is the chemoaffinity hypothesis (Sperry)

Answer 1

cells and fibres in the brain and SC carry individual identification tags (cytochemical in nature)

Question 2

issue with the chemoaffinity hypothesis

Answer 2

genome does not have enough information to encode tags

instead protein gradients are used

Question 3

classical eye rotation experiment

Answer 3

ventral RGC projects to medial tectum
dorsal RGC projects to lateral lectum

after optic nerve is cut and eye is rotated:
ventral retina (now up) projects to dorsal tectum
dorsal retina (down) projects to ventral tectum

Question 4

half tectum vs half retina

Answer 4

half tectum - nasal axons have no target (compressed topographic maps)
half retina - posterior target available (expansion of topographic maps)

Question 5

sensitive period

Answer 5

periods of heightened susceptibility to modification occurs at particular times in development

Question 6

critical period

Answer 6

time when environmental input is required for proper development of a particular brain circuit
if no input - circuits are permanently damaged

Question 7

how do microglia promote synaptic plasticity

Answer 7

remodel the EM using IL-33
critical periods are upregulated by perineuronal nets PNN (block plasticity)
microglia removes PNN (via IL-33 secretion) and remodel existing connectivity
develops binocular dominance

Question 8

binocular projections

Answer 8

projections of binocular field view onto the retina
cross at the chiasm
mixing of pathways occur at the striate cortex V1 (L2/4)

Question 9

V1 selectivity

Answer 9

highest in L4

Question 10

where does segregation of eye input occur

Answer 10

LGN

Question 11

monocular deprivation

Answer 11

normal - contralateral/ipsilateral/bilateral cells activated
monocular deprivation (kitten) closed from birth - only ipsilateral cells activated
monocular deprivation (adults 12-38 months) - binocular vision

few days of deprivation sufficient to induce shift in cortical wiring

Question 12

kitten critical period

Answer 12

3 months

Question 13

how to observe ocular dominance in V1

Answer 13

radioactive aa injected in eye (LGN and V1 label inputs)
trans-synaptic transport through LGN to L4 V1
terminations visible

Question 14

axons in normal vision vs monocular dominance

Answer 14

normal vision - firing and connectivity
monocular deprivation - axons have no branching

critical periods allow thalamic/cortical neurons to remodel their connections in response to environment

Question 15

what is the hebbian synapse

Answer 15

cells that fire together, wire together

cortical cells preferentially receive input from the ipsilateral eye (less from the contralateral eye)

Question 16

what is the hebbian molecule

Answer 16

NMDA R
coincidence detector

Question 17

3 eyed frog

Answer 17

Tranplanted eye
periodic pattern of eye specific stripes over the double innervated tectum
mediated via NMDAR dependent mechanisms

Question 18

orientation/direction selectivity

Answer 18

certain stimulus orientation/direction produces spiking

Question 19

vertical vs oblique electrode placement

Answer 19

vertical placement in V1 - no change in RF position/orientation tuning curve
oblique placement in V1 - changes in RF/tuning curve

Question 20

primate V1 vs mouse V1

Answer 20

primate V1 - vertical column of cells
mouse V1 - no columnar organisation

different species process visual information differently

Question 21

4 response properties of V1

Answer 21

orientation preference
direction preference
ocular dominance (L&R)
visuotopic location