Critical Periods and Plasticity


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Critical Periods and Plasticity

  1. 1. Critical Periods and Plasticity
  2. 2. Critical Period for Flial Imprinting Heinroth, 1911; ducklings vs. goslings Konrad Lorenz, 1970; critical period for imprinting Critical period for parental imprinting in ducklings
  3. 3. Harry Harlow, 1950s Surrogate mother experiments Critical Period for Attachment
  4. 4. Stoeckel et al., 2005, J Neurophysiology Thalidomide toxicity during 1st trimester. Critical Period for Prenatal Toxicity on Limb Development
  5. 5. Critical Period for.. Language acquisition: 6 months (onset) Phoneme selectivity: 6 months (end) Critical Period for Language Acquisition
  6. 6. Kuhl, 2004 Do American infants hear Swedish vowels?
  7. 7. Language Acquisition Development of language is independent of the mode of expression.
  8. 8. Language Acquisition Critical Period for Acquisition of Second Language w/o an accent: 7-8 years
  9. 9. Critical Period for Song Acquisition Critical period ends at puberty Selectivity for songs of own species. In the lack of own species’ song ….adaptation.
  10. 10. …imprinting in the chick …brain sexual differentiation …extraocular muscle development …visual plasticity …monocular deprivation …addiction vulnerability …wing pattern induction in the polyphenic tropical butterfly …GABAergic receptor blockade for induction of a cAMP- mediated long-term depression at CA3-CA1 synapses …methamphetamine-induced spatial deficits …second-language acquisition …experience-dependent Plasticity in Visual Connections in Xenopus …lung cancer susceptibility …cross-modal plasticity in blind humans …nicotine exposure effects …disruption of primary auditory cortex by synchronous auditory inputs …functional vestibular development in zebrafish …right hemisphere recruitment in American Sign Language processing …barrel cortex critical period plasticity …feminization in tilapia …developmental climbing fibre plasticity …sensory map plasticity …sensitivity to juvenile hormone …language acquisition …LTP at thalamocortical synapses …caste determination in Bombus terrestris and its juvenile hormone correlates …deafferentation-induced apoptosis …nicotine-induced disruption of synaptic development in rat auditory cortex …activity-dependent synapse elimination in developing cerebellum …conversion of ectodermal cells to a neural crest fate …psychosis …verbal language development …reduced brain vulnerability to injury. …chorda tympani nerve terminal field development …the sensitivity of basal forebrain cholinergic neurones to NGF deprivation …light-induced phase advances of the circadian locomotor activity rhythm in golden hamsters …the influence of peripheral targets on the central projections of developing sensory neurons …the specification of motor pools in the chick lumbosacral spinal cord …axon regrowth through a lesion in the developing mammalian retina …long-term potentiation in primary sensory cortex …song learning in the zebra finch …restoration of normal stereoacuity in acute-onset comitant esotropia …transcription for induction of a late phase of LTP. …regeneration capability of adult rat retinal ganglion cells after axotomy ..synaptogenesis ..experience-dependent synaptic plasticity in rat barrel cortex ..peripheral specification of dorsal root ganglion neurons ..androgenic block of neuromuscular synapse elimination speech lateralization in deaf populations Critical period for ……
  11. 11. What is a critical period? “During a critical period, a pathway awaits specific instructional information in order to continue develop normally. … If appropriate experience is not gained during the critical period, the pathway never attains the ability to process information in a normal fashion and, as a result, perception or behavior is impaired permanently.” Alternatively…… “During a critical period, a pathway maintains a state of vulnerability to unusual stimuli. …. If an inappropriate experience is maintained during the critical period, the pathway sustains adaptive changes permanently.”
  12. 12. Critical period for stereopsis… in humans Unbalanced binocular stimulation during a critical period disrupts normal vision. Neonatal unilateral cataract Corrections.. before 4 months: minimal visual loss …between 6-30 months: finger counting acuity Cataract forming after 3 years: responsive to correction Strabismus: Correction after 1 years: loss of stereopsis Correction after 4 years: strabismic amblyopia
  13. 13. Visualization of ocular dominance columns Critical period for stereopsis … in monkey
  14. 14. Effect of Visual Deprivation on Ocular Dominance Columns Visual cortex of normal adult monkey 18 months old. MD at 2 weeks. Label injected into nondeprived eye. Autoradiographs from visual cortex, 10 days after tracer injected into one eye 18 months old. MD at 2 weeks. Label injected into deprived eye. Hubel, Weisel and LeVay, 1977
  15. 15. Horton and Hocking, 1999 Lid sutured when 1 week old. Lid sutured when 7 weeks old. Critical period (window of opportunity for successful surgery): postnatal weeks 1- 5.
  16. 16. Normal Deprivation leads to shrinkage of deprived axons, followed by expansion of non-deprived axons. What explains the shrinkage of columns?
  17. 17. Do the anatomical changes affect the physiology? More cells in cortex become monocular
  18. 18. How about a short term deprivation (which shrinks the deprived axons but does not expand the nondeprived axons)?
  19. 19. Can this be just an outcome of reduced activity from one eye? No. Monocular deprivation in the adult does not change the physiology or the anatomy.
  20. 20. Does visual activity have anything to do with it? Yes. When the visual activity is totally prevented by keeping the animals in complete darkness, critical period for plasticity is delayed. Visual activity is necessary to open the critical period.
  21. 21. Determining the critical period 1) Do you have an assay for plasticity or stability? e.g. MD changes ODC wiring. 2) Close one eye for a given duration e.g. 2-7 days of MD is sufficient to induce OD changes 3) At suture removal day, asses OD. Onset of critical period: The youngest age at which initiation of MD leads to at least 50% ocular dominance shift. End of critical period: Oldest age at which initiation of MD leads OD shift.
  22. 22. Determining the critical period Issa et al., 1999, JNeurosci.
  23. 23. Finding #1: Ocular dominance columns segregate by 5-6 weeks in the cat. Finding #2: Critical period of plasticity in cats end by 6 weeks. Then: critical period ends when the columns segregate. So, what makes geniculo-cortical axons vulnerable to deprivation? Model 1: Deprivation stops a normal developmental process. LeVay, Stryker, Shatz, 1978 Katz and Shatz
  24. 24. Hebbian Mechanisms for development Beautiful model. But,…... the data do not validate the predictions.
  25. 25. Model #2: ocular dominance column formation and critical period plasticity are independent events. Evidence #1: ODC develop earlier then the onset of critical period Monkey at birth Ferret at P18 CP:P32-P60 Cat at P14 CP:P21-P40
  26. 26. Evidence #2: Dark-rearing delays the critical period, but not ODC segregation. monocular enucleation
  27. 27. Fine. Then, what does enable the plasticity of the brain during a critical period? Why do deprived axons retract? Active synapses hold onto their targets (remember NMJ?). Why do nondeprived axons bother to invade the territory? And, how do they know about what is happening to the deprived axons?
  28. 28. What prevents or facilitates plasticity of the brain during a critical period? Factors that may be involved in Critical Period Plasticity: * NMDAR * metabotropic glutamate receptors PKC activation * CaMKinase * BDNF prevents MD effects prevents DR effects * GABAergic activity * ACh/NE * Hormones
  29. 29. Properties of critical periods: 1) Functional competition between inputs 2) Neuronal activity 3) Structural consolidation of pathways 4) Onset and duration is defined by activity 5) Sequence of critical periods across systems 6) Diversity of molecular mechanisms 7) Particular roles of excitation and inhibition 8) Attention and motivation 9) Potential for reactivation
  30. 30. Properties of critical periods: 1) Functional competition between inputs Binocular lid suture ==> no OD shift Activity blockade in cortex during MD ==> nondeprived axons shrink! Hata, Tsumato, Stryker, 1999 Nondeprived Deprived
  31. 31. 2) Neuronal Activity Dark-rearing ==> Critical period delayed 3) Structural consolidation of pathways Plasticity may persist in some pathways throughout development. But, not in others… 4) Onset and duration is defined by activity Dark rearing, enriched environments, BDNF..
  32. 32. 5) Sequence of critical periods across systems barrel ctx plasticity…. Visual cortex plasticity Layer 4 CP … layer 2/3 critical period 6) Diversity of molecular mechanisms TC to layer 4 plasticity Layer 4 to layer 2/3 plasticity ocular dominance vs orientation column plasticity
  33. 33. 7) Inhibition is important. Result: When GABA is reduced, CP doesn’t start. When GABA is replaced, CP starts. Conclusion: GABA is necessary for initiation of CP.
  34. 34. Enhancement of GABAergic activity initiate CP.
  35. 35. MD shifts OD responses Restoration of MD, restores OD Short MD
  36. 36. OD is restored even after MD into post CP ages Long MD But not if MD is initiated before eye opening. Early-Long MD
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