Sensitive Periods in the Development of the Brain and BehaviorAbstractHypothesis: experience during sensitive period modifies the architecture of circuit in fundamental ways, causing certain patterns of connectivity to become highly stable and energetically preferredPlasticity that occurs beyond sensitive period alters connectivity patterns within architectural constraints established during sensitive periodIntroduction“Sensitive period” – effects of experience on brain strong during limited period in developmentAdverse effects of atypical experience throughout critical period cannot be remediated by restoring typical experience later in lifeSensitive periods are property of neural circuitsExamples of Sensitive Periods4 systems studied to illustrate properties of sensitive periods:1) ocular representation in cortex of mammalsInformation from eyes conveyed to cortical layer by axons from thalamus which are shaped during first months after birth2) auditory space processing in midbrain of barn owlsNot a critical periodNeural connections shaped by juvenile experienceImprinting after juvenile period ends shows why it is not a critical period3) filial imprinting in forebrain of ducks and chickensNeurons in nucleus in forebrain undergo changes to become functionally selective for imprinted stimulusAfter imprinting period ends, preference for imprinted stimulus does not change4) song learning in forebrain of songbirdsArchitectural and functional changes in forebrain nucleusOpening of Sensitive PeriodsInitial ConditionsSome circuits established by innate mechanisms with no experienceInnate influences establish initial pattern of connectivity but then is shaped by experience during sensitive periodWhen circuit can select from large range of potential patterns of connectivity, experience has big impact on connectivityWhen patterns constrained by genetic influences, effect of experience is smallerPrerequisitesSensitive period cannot open until:Info provided to circuit must be reliable and precise to allow circuit to carry out functionCircuit must contain adequate connectivityMust have activated mechanisms that enable plasticityExperience before these 3 things will have no effectTiming of InitiationConditions required for sensitive period to open may result from development or enabled by experienceDepriving animals of experience can delay opening of sensitive periodsComplex behaviors may have multiple sensitive periodsHigher levels of sensitive periods depend on info from lower levels to accomplish functionsDuring a Sensitive PeriodProperties of Sensitive Period PlasticityExperience provides precise info about individual or about environment that often cannot be predicted and genetically encodedPredisposition of circuit instructed by typical experience reflects selectivity of circuit’s various inputs and innate connectivity of circuitMechanisms of Sensitive Period PlasticityAxon and synapse formation and elimination can alter circuits during sensitive periodsDistribution of stabilized synapses shapes growth patterns of axons and dendritesSynaptic connections that fail to predict activity of postsynaptic neurons are weakened and eliminated and axon branches are prunedEliminating axons based on experience only during sensitive periodSynapses that are eliminated are presumed to not contribute to representation of learned stimulusAs found in song learning in birds and filial imprinting in birdsSynapse consolidation may influence sensitive period plasticity; occurs from experience during these periodsExternal nucleus of barn owl, multiple representations of auditory cues can be acquired through experience during sensitive periodOwls that acquire alternative representations as juveniles are able to re-express as adultsSuggests that these synapses are invulnerable to elimination due to the kind of Cell Adhesion Molecules that insert into synapses and drive these neurons during sensitive periodThe Unique Advantage of Initial ExperienceExperience in sensitive period has advantage in shaping connectivity of circuitIntense and repeated activation of circuit can change it (synapses that drive postsynaptic neurons become strong and less likely to change)Synapses not driving postsynaptic neurons are depressed and sometimes eliminatedIf sensitive period remains open, subsequent experience has ability to cause further changesBehavioral and emotional states can have big impact on changes that result from experience during sensitive periodWith heightened levels of arousal from experience, plasticity may still occur in later stagesSongbirds can still learn songs when conversing with other adultsStability Landscape as a Metaphor for Sensitive Period PlasticityStability landscape – represents range of possible connectivity patterns that a circuit might acquire and degree to which any particular pattern is preferredBall at low point at center of landscape – represents innate mechanisms establish initial pattern of connectivity that is appropriate to process the neural activity that results from typical experienceDownward arrows refer to patterns resulting from experience that cause structural and functional changes that modify, refine, reinforce initial patternChanges can alter patterns that circuit can acquire and create high and low points in stability landscapePattern becomes more sharply defined and highly preferred even though pattern may be atypicalRepeated experience instructs new pattern, creating new low point in landscapeSome circuits only able to maintain single highly preferred patternImprinting and song memorizationEnding of Sensitive PeriodsSensitive period ends when mechanisms responsible for heightened state of plasticity no longer operate or operate at low efficiencyPoint in development where landscape becomes resistant to changeEnd as result of progress of developmentAcquisition of language proficiency in humansSome sensitive periods end as result of experienceIf fundamental changes…Depend upon axonal or dendritic growth – loss of any mechanisms that enable neurite outgrowth would end sensitive periodRequire synapse formation or elimination – loss of key mechanisms support processes end sensitive periodRely on structural stabilization of selected synapses by particular CAM – loss of capacity to produce molecule end sensitive periodHost of mechanisms can contribute to ending sensitive periodIncrease in effectiveness of inhibitory circuitryMyelination of axonsAppearance of