Plasticity in the visual cortex

Plasticity in the visual cortex

1. Segregation of retinal axons in LGN

   • 2 Magnocellular Laminae
   • 4 Parvocellular Laminae

2. Occular dominance columns

   • How left/right eye LGN input segregated in Layer IV of Striate Cortex
   • Development
     • LGN axons in layer 4 retract to form smaller arborisations
       • Restricted separate cortex domains become established

3. Occular dominance plasticity

   • Critical period after birth; "sensitive period"
     • If signal to both eyes blocked, OD columns desegregate
     • If one eye closed (monocular depravation)
       • Size of columns corresponding to eye shrinks
       • Potentiation of input from open eye
       • Depression of input from deprived eye
         • Eye = physiologically fine, but blind
     • Factors responsible for period
       • Space and molecular obstacles
         • Perineuronal nets
           • Responsible for synapse stabilisation
         • NOGO (Reticulon-4)
           • Inhibits neurite outgrowth
       • Ratio of excitation to inhibition
   • Strabismus (cross-eye)
     • Very few cells driven by both eyes
       • Almost complete lack of binocular representation
   • Plasticity dependent on molecular mechanisms
     • NMDA Receptors
     • CAMKII
     • Kinase A
     • ERK
     • tPA
     • CREB
     • Dentritic spines dynamics
     • Protein synthesis

4. Effects of mononuclear deprivation (in cortex)

   • Young animals
     • Depression from closed eye, dependent on presynaptic activity
     • Delayed potentiation from open eye, independent of depression
   • Adult animals
     • No depression from closed eye
     • Delayed potentiation from open eye

5. Hofer et al. experiment

   • 1st monocular deprivation doubled rate of spine formation
     • Increased spine density
     • Increased response to non-deprived eye
   • Spines return to baseline level when binocular vision restored
   • When eye deprived second time, spine density remains same
     • Prior experience stored in cortical circuits

6. Sleep and Plasticity

   • Sleep enhances plasticity

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