Theme 2: changing brain
Type Notes
Due @March 31, 2025
Status Done
Experience-Dependent Plasticity in the Developing
Brain (Purves, Chapter 25)
After basic neural structure is built, a final phase of development begins in late prenatal to early
postnatal life
This phase is driven by neural activity, often triggered by environmental stimuli—i.e., the
newborn’s experience
These time-limited periods of postnatal change are known as critical periods
During critical periods:
Neural circuits are especially plastic and adaptable
Afterward, the capacity to change connectivity declines
Critical periods help optimize individual brain circuitry to suit personal experiences
The cellular mechanisms during critical periods resemble those in learning and memory, involving:
Neurotrophins and neurotransmitters
Second messengers influencing gene expression, synapse growth, and pruning
Most knowledge comes from visual system studies, particularly regarding how input from each eye
shapes the visual cortex
Postnatal changes in cortical size reflect experience-driven growth in some brain areas more than
others
Developmental disorders (e.g., autism, schizophrenia) may involve disrupted experience-
dependent mechanisms
Neural activity and Brain Development
Hebb’s postulate ⇒ coordinated activity between presynaptic and postsynaptic neuron
strengthens the synaptic connection
Synapses with correlated activity are retained or expanded
Synapses with uncorrelated activity are weakened or eliminated (pruning)
Theme 2: changing brain 1
, Synaptic strengthening = when presynaptic inputs repeatedly activate a postsynaptic neuron, the
connections between them are retained, and new branches may form
This principle explains:
Emergence of new behaviors in early life
Enhanced skill acquisition in childhood
Brain growth continuing after birth, tracking with cognitive development
Postnatal brain growth is characterized more by synapse formation and the growth of dendrites
and axons
Not by the addition of neurons (neurogenesis)
Sensory experience validates initial wiring:
Preserves or adjusts early connections
Lack of sensory input can hinder normal connectivity and behavior
These activity-driven mechanisms decline with age, limiting the brain’s ability to reorganize
Educational and psychiatric implications: Early experience is vital for optimal development
Critical Periods
Critical periods = temporal windows during which specific environmental stimuli must be
experienced for normal development
Examples:
Parental imprinting in birds
Language acquisition in humans
Song learning in songbirds
Features:
Specific behaviors require certain environmental inputs during these times
Once the period ends, further experience has little effect on those behaviors
Missing the window leads to permanent deficits
Especially dependent on the cerebral cortex
Theme 2: changing brain 2
, The Role of Oscillations in Establishing Critical Periods
Before sensory experience, the brain generates subthreshold oscillations to help guide early brain
development
Oscillations = spontaneous waves of electrical activity that help organize circuits
Example: Retinal waves in the developing visual system
Spontaneous activity (e.g., calcium waves) in the retina before eye opening
Not correlated between the two eyes
Helps segregate left vs. right eye inputs in the lateral geniculate nucleus (LGN)
Disruption of these waves (genetically or pharmacologically) prevents proper eye-specific
segregation
These spontaneous oscillations:
are relayed to visual cortex
prepare circuits for experience-driven refinement
influence synapse and circuit maturation
Critical Periods in Visual System Development
The visual system is ideal for studying experience-dependent plasticity
Easy to manipulate input (e.g., close one eye)
Visual pathways are anatomically distinct
Ocular dominance columns = stripes of neurons in the visual cortex that respond preferentially to
input from one eye or the other
Alternating eye-specific input stripes in layer 4 of visual cortex
In normal animals there is equal width, equal activation from each eye
Theme 2: changing brain 3
Type Notes
Due @March 31, 2025
Status Done
Experience-Dependent Plasticity in the Developing
Brain (Purves, Chapter 25)
After basic neural structure is built, a final phase of development begins in late prenatal to early
postnatal life
This phase is driven by neural activity, often triggered by environmental stimuli—i.e., the
newborn’s experience
These time-limited periods of postnatal change are known as critical periods
During critical periods:
Neural circuits are especially plastic and adaptable
Afterward, the capacity to change connectivity declines
Critical periods help optimize individual brain circuitry to suit personal experiences
The cellular mechanisms during critical periods resemble those in learning and memory, involving:
Neurotrophins and neurotransmitters
Second messengers influencing gene expression, synapse growth, and pruning
Most knowledge comes from visual system studies, particularly regarding how input from each eye
shapes the visual cortex
Postnatal changes in cortical size reflect experience-driven growth in some brain areas more than
others
Developmental disorders (e.g., autism, schizophrenia) may involve disrupted experience-
dependent mechanisms
Neural activity and Brain Development
Hebb’s postulate ⇒ coordinated activity between presynaptic and postsynaptic neuron
strengthens the synaptic connection
Synapses with correlated activity are retained or expanded
Synapses with uncorrelated activity are weakened or eliminated (pruning)
Theme 2: changing brain 1
, Synaptic strengthening = when presynaptic inputs repeatedly activate a postsynaptic neuron, the
connections between them are retained, and new branches may form
This principle explains:
Emergence of new behaviors in early life
Enhanced skill acquisition in childhood
Brain growth continuing after birth, tracking with cognitive development
Postnatal brain growth is characterized more by synapse formation and the growth of dendrites
and axons
Not by the addition of neurons (neurogenesis)
Sensory experience validates initial wiring:
Preserves or adjusts early connections
Lack of sensory input can hinder normal connectivity and behavior
These activity-driven mechanisms decline with age, limiting the brain’s ability to reorganize
Educational and psychiatric implications: Early experience is vital for optimal development
Critical Periods
Critical periods = temporal windows during which specific environmental stimuli must be
experienced for normal development
Examples:
Parental imprinting in birds
Language acquisition in humans
Song learning in songbirds
Features:
Specific behaviors require certain environmental inputs during these times
Once the period ends, further experience has little effect on those behaviors
Missing the window leads to permanent deficits
Especially dependent on the cerebral cortex
Theme 2: changing brain 2
, The Role of Oscillations in Establishing Critical Periods
Before sensory experience, the brain generates subthreshold oscillations to help guide early brain
development
Oscillations = spontaneous waves of electrical activity that help organize circuits
Example: Retinal waves in the developing visual system
Spontaneous activity (e.g., calcium waves) in the retina before eye opening
Not correlated between the two eyes
Helps segregate left vs. right eye inputs in the lateral geniculate nucleus (LGN)
Disruption of these waves (genetically or pharmacologically) prevents proper eye-specific
segregation
These spontaneous oscillations:
are relayed to visual cortex
prepare circuits for experience-driven refinement
influence synapse and circuit maturation
Critical Periods in Visual System Development
The visual system is ideal for studying experience-dependent plasticity
Easy to manipulate input (e.g., close one eye)
Visual pathways are anatomically distinct
Ocular dominance columns = stripes of neurons in the visual cortex that respond preferentially to
input from one eye or the other
Alternating eye-specific input stripes in layer 4 of visual cortex
In normal animals there is equal width, equal activation from each eye
Theme 2: changing brain 3
