Critical periods in development - “nature” vs. “nurture ...raghav/pdfs/neurobiology1/...2nd...
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2nd September 2013 Bio 334 - Neurobiology I - Critical periods in development 2 1
Critical periods in development - “nature” vs. “nurture” - Part 2
Raghav RajanBio 334 – Neurobiology I
September 2nd 2013
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2nd September 2013 Bio 334 - Neurobiology I - Critical periods in development 2 2
Critical periods
● How do we know it is a critical period?
● What starts the critical period?
● What closes it?
● What determines that it is closed?
– what are the changes that can be made during the critical period?
– what changes cannot be made outside of the critical period?
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Closing one eye changes representations in the visual cortex only when done early
● Kittens
● 2 days of monocular deprivation
● Shows that there does exist a critical period
Mark F Bear, Barry W Connors, Michael A Paradiso. Neuroscience: Exploring the brain (2007) – Chapter 23
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Theoretical models predicted that the balance of excitation and inhibition could shape column width
● First shown in cats through injections into visual cortex
● Agents that reduced or increased inhibition
Takao Hensch, Nature Reviews Neuroscience 2005http://henschlab.files.wordpress.com/2012/06/hensch-nat-rev-neuro-2005.pdf
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Ocular dominance plasiticity in mouse visual cortex (without ocular dominance columns)
● Individual cells in the binocular zone can be tested for their response to contralateral and ipsilateral visual stimuli
● Mice provide an excellent genetic system to work out the molecular mechanisms
Levelt and Hubener. Critical period plasticity in the visual cortex. Annual Reviews in Neuroscience 2012
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Mice knockout for GAD-65 (reduced inhibition) show no ocular dominance plasticity
● GAD-65 – one of two isoforms of an enzyme that synthesizes GABA – an inhibitory neurotransmitter
● No shift in ocular dominance after monocular deprivation
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2851625/
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Increasing inhibition restores ocular dominance plasticity
● Injecting diazepam (increasing inhibition) restores ocular dominance plasticity
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2851625/
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Change in the amount of inhibition controls ocular dominance plasticity
● Red circles mark the start, peak and end of normal plasticity
● Modifying inhibition can advance or delay the critical period
Takao Hensch, Nature Reviews Neuroscience 2005http://henschlab.files.wordpress.com/2012/06/hensch-nat-rev-neuro-2005.pdf
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Synaptic changes occur after monocular deprivation
● Monocular deprivation triggers increase in spine motility
● Spines can then be eliminated
● Axons retract
● Spines can then recover
● Other axon outgrowth
Takao Hensch, Nature Reviews Neuroscience 2005http://henschlab.files.wordpress.com/2012/06/hensch-nat-rev-neuro-2005.pdf
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Current understanding of ocular dominance plasticity – molecular mechanisms
Takao Hensch, Nature Reviews Neuroscience 2005http://henschlab.files.wordpress.com/2012/06/hensch-nat-rev-neuro-2005.pdf
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Can ocular dominance plasticity be re-induced in adulthood
● Onset of this plasticity determined by maturation of inhibitory interneurons
● Structural modifications are associated with critical period
● Few methods to induce ocular dominance plasticity again
– One is to dissolve the extracellular matrix that inhibits axonal sprouting and growth
– Another is to transplant inhibitory interneuron precursors from donors
http://www.jneurosci.org/content/30/45/14964/F1.expansion.html
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Facets of critical periods
● Functional competition between inputs – tunes circuits to individual and its environment
● Structural modifications become impossible
● Regulation of onset and duration by experience, not age
● Timing is variable for different systems – one critical period may open only when another one is done
● Inhibition plays a key role
● Attention, motivation are also very important
● Potential for reactivation in adulthood – lifelong learning!!
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Critical periods for higher functions like language – learning a second language as an adult is difficult
● Evidence for a critical period for language acquistion
– Socially isolated children lose ability to acquire normal language later
– Language recovery is better after cerebral damage only if damage occurs early in life
– Second language learning is difficult in adulthood
Takao Hensch. Critical Period Regulation. Annual Review of Neuroscience 2004http://henschlab.files.wordpress.com/2012/06/hensch-ann-rev-neurosci-2004.pdf
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Broadly language learning involves perception and production
● Perception
● Production
●
● Both these aspects are affected later in life
– If you can perceive sounds properly, you can produce them better – and vice versa
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Only some animals are vocal learners and among them songbirds are the most experimentally tractable
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Early work done on the chaffinch, a common European songbird
http://www.birdsongs.it/songs/fringilla_coelebs/fringilla_coelebs.html# Spectrogram 4http://en.wikipedia.org/wiki/Common_Chaffinch
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Song learning requires sensory experience
● Birds raised in auditory isolation have abnormal songs
● Birds deafened early also have abnormal songs
Fernando Nottebohm. Ontogeny of bird song. Science 1970http://www.psy.fsu.edu/~mnl/CNL/private/Neuroeth/Nottebohm%201970.pdf
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Song learning requires intact auditory feedback
● Deafening early in life (107 or 88d after hatching) results in poor songs as adults
●
● So, song is an example of vocal learning
Fernando Nottebohm. Ontogeny of bird song. Science 1970http://www.psy.fsu.edu/~mnl/CNL/private/Neuroeth/Nottebohm%201970.pdf
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Existence of sensitive period for song learning
● Hand-raised white crowned sparrows learned from tape tutors only during 10-50 days post-hatch
●
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Some birds have only one sensitive period, while others have multiple sensitive periods
Brainard and Doupe What songbirds teach us about learning. Nature 2002http://www.summer10.isc.uqam.ca/Page/docs/readings/WHITE_Stephanie/Brainard%20and%20Doupe%20review.pdf
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Sensitive periods are not age-dependent but instead rely on sensory experience
● Isolated birds will learn well after their sensitive period
● Birds reared in white noise conditions also have an extended sensitive period
● Castrated birds with lower testosterone also have an extended sensitive period
●
●
● In all cases, sensitive period is not extended indefinitely
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Hormones appear to be an important determinant of sensitive periods
● Testosterone early in life for a zebra finch can prematurely crystallize song
● In open-ended learners, testosterone can trigger changes in brain nuclei size, etc. in the breeding season
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Testosterone levels, brain nuclei size and song change with breeding season
http://people.eku.edu/ritchisong/birdcommunication.htmlhttp://classes.uleth.ca/201001/biol4420a/BirdPaper.Iwaniuk.Tramontin2000.pdf
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Neural substrates for song learning
Brainard and Doupe What songbirds teach us about learning. Nature 2002http://www.summer10.isc.uqam.ca/Page/docs/readings/WHITE_Stephanie/Brainard%20and%20Doupe%20review.pdf
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Structural modifications post first tutor song exposure can be imaged
● Image spines on GFP labelled neurons
● Isolated juvenile birds are tutored and spine stability is imaged that night
●
● Blue spines remain
● Yellow spines are lost
● Green spines are addedhttp://www.ncbi.nlm.nih.gov/pmc/articles/PMC2918377/
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Better song learning is correlated with increased spine turnover
● Lower spine turnover in adult birds that have finished learning
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2918377/
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Spines get stabilised within 24hrs of tutor song exposure
● Most changes in spine stability occur within 48 hours of tutor song exposure
● Low spine turnover birds learn less
● High spine turnover birds learn more
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2918377/
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Spines also get bigger after tutor song exposure
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2918377/
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Thus, overall, for sensory, motor and higher functions, critical periods exist
● Critical periods represent periods when circuits tune themselves to the environment and the individual
● A period of “nurture” to adapt to “nature”!! (both genes and environment)
● An understanding might help facilitate life-long learning