Read Online Welcome to Your Child's Brain: How the Mind Grows From Conception to College by Sam Wang, Sandra Aamodt - Free Book Online Page B
Authors: Sam Wang, Sandra Aamodt
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receptors, transformed into electrical impulses, and transmitted to the brain. (As we will show you in chapters 10–12, all our knowledge of the world comes in the form of these electrical impulses.) Interactions with parents and other caregivers are only part of the rich tapestry of available stimuli. Physical changes in your infant’s brain can also occur when she watches the mobile that hangs over her crib, when she sticks her toes in her mouth, and when sirens pass on the street outside. Later, her universe expands to include social interactions with other children, finding her way around the neighborhood, learning to play sports, going to school, and much more. All these experiences leave their traces in her brain, some very long-lasting and others transient.
Because neural circuits mature at different times, there are a variety of sensitive periods in development, each corresponding to a particular brain function. Sensitive periods are most common in infants and toddlers because the brain is undergoing such dramatic growth at this stage, but they can occur at other times as well. Some sensitive periods occur before birth, such as the maturation of the sense of touch based on the baby’s experience within his mother’s womb (see chapter 11 ). Many occur soon after birth, as when early interactions with caregivers shape the circuits of the brain that respond to stress (see chapter 26 ). Other sensitive periods, such as the one for the grammatical aspects of language learning, continue well into childhood and adolescence.
As we described in chapter 2 , preprogrammed chemical cues direct axons to their target regions and orchestrate the formation of a large number of synapses. Once those basic elements are in place, experience can influence the further development of the circuit by controlling the activity of those axons and synapses. Synapses that are more effective at activating their target neuron are more likely to be retained and strengthened, through biochemical plasticity pathways in the target cell, while those that are ineffective become weak or disappear. Synaptic activity can also trigger the growth or retraction of axonal or dendritic branches. Cells that fire together, wire together (see chapter 21 ).
Once these plastic changes have occurred, the brain architecture often becomes more difficult to modify in the future, either because the extra axons and synapses are no longer available or because the biochemical pathways that modify synaptic strength change with age. In this way, the brain uses sensory experience to shape the connections within a neural circuit, pruning away the ones that areunnecessary while maintaining those that are strongest and most active to produce the perceptions and behaviors that are appropriate for that child’s individual environment.
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Experience isn’t something that happens passively, even to babies. Your child’s brain has definite preferences about what it should learn at various stages of development.
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Unnecessary synaptic connections are pruned throughout childhood. In the primary visual cortex, the total number of synapses increases rapidly from birth to its peak at eight months old, and then declines slowly to age five, as visual ability is maturing (see chapter 10 ). The biggest reduction in synapse number in this region happens sometime between ages five and eleven. (We don’t know exactly when this change occurs because children ages six to ten have not been studied.) In the frontal cortex, synapse density remains high at least through age seven, falls somewhat by age twelve, and reaches adult levels in the middle teenage years (see chapter 9 ). It is not clear what happens between ages seven and twelve.
Synapse elimination has been studied in much more detail in other primates, and the results are roughly consistent with the sparse human data. In rhesus monkeys, an explosive increase in synapse density in the first few months after birth is followed
Because neural circuits mature at different times, there are a variety of sensitive periods in development, each corresponding to a particular brain function. Sensitive periods are most common in infants and toddlers because the brain is undergoing such dramatic growth at this stage, but they can occur at other times as well. Some sensitive periods occur before birth, such as the maturation of the sense of touch based on the baby’s experience within his mother’s womb (see chapter 11 ). Many occur soon after birth, as when early interactions with caregivers shape the circuits of the brain that respond to stress (see chapter 26 ). Other sensitive periods, such as the one for the grammatical aspects of language learning, continue well into childhood and adolescence.
As we described in chapter 2 , preprogrammed chemical cues direct axons to their target regions and orchestrate the formation of a large number of synapses. Once those basic elements are in place, experience can influence the further development of the circuit by controlling the activity of those axons and synapses. Synapses that are more effective at activating their target neuron are more likely to be retained and strengthened, through biochemical plasticity pathways in the target cell, while those that are ineffective become weak or disappear. Synaptic activity can also trigger the growth or retraction of axonal or dendritic branches. Cells that fire together, wire together (see chapter 21 ).
Once these plastic changes have occurred, the brain architecture often becomes more difficult to modify in the future, either because the extra axons and synapses are no longer available or because the biochemical pathways that modify synaptic strength change with age. In this way, the brain uses sensory experience to shape the connections within a neural circuit, pruning away the ones that areunnecessary while maintaining those that are strongest and most active to produce the perceptions and behaviors that are appropriate for that child’s individual environment.
----
Experience isn’t something that happens passively, even to babies. Your child’s brain has definite preferences about what it should learn at various stages of development.
----
Unnecessary synaptic connections are pruned throughout childhood. In the primary visual cortex, the total number of synapses increases rapidly from birth to its peak at eight months old, and then declines slowly to age five, as visual ability is maturing (see chapter 10 ). The biggest reduction in synapse number in this region happens sometime between ages five and eleven. (We don’t know exactly when this change occurs because children ages six to ten have not been studied.) In the frontal cortex, synapse density remains high at least through age seven, falls somewhat by age twelve, and reaches adult levels in the middle teenage years (see chapter 9 ). It is not clear what happens between ages seven and twelve.
Synapse elimination has been studied in much more detail in other primates, and the results are roughly consistent with the sparse human data. In rhesus monkeys, an explosive increase in synapse density in the first few months after birth is followed
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