Free Welcome to Your Child's Brain: How the Mind Grows From Conception to College by Sam Wang, Sandra Aamodt Page A

characteristics like head size and ear shape, which change as the animal grows, it can’t be specified in advance, so this mapping is learned during development.
    The owl’s brain learns this map by using visual experience to calibrate the auditory map. To study this process, researchers equip baby owls with prism glasses, which make objects appear to be shifted to one side. At first the animals make a lot of mistakes as they try to move around with the glasses on, but gradually the brain adapts by changing its visual map to reflect the new reality. The auditory space map also shifts in response to prism glasses, even though the auditory information is unchanged.
    The shift happens because the neurons that bring in timing and loudness information extend their axon branches to connect with new neurons in a different part of the map. The former connections remain in place, though their synapses are weakened, allowing the owls to return to the old mapping once the prism glasses are removed. This plasticity occurs in a sensitive period, until about seven months of age. In adults, whose sensitive period has ended, it is more difficult to rearrange connections because their axon arbors are limited to a smaller area of the midbrain and thus the wiring is not already in place to carry signals outside the range established in youth.
    One of the basic principles of brain development is that the simplest building blocks are finished first. Later, more complex processes build upon earlier ones. For example, the areas of visual cortex that detect edges and shading must become functional before other visual areas can start to interpret these patterns as objects. For this reason, there is not a single sensitive period for vision, but a series of sensitive periods, each requiring experience for the maturation of a different region of the visual brain. If the experience required to complete an early developmental process is not available, the sensitive period is normally extended for a while, resulting in delayed maturation of that brain circuit and all the others that depend on it. Eventually, though, the window of opportunity closes, and any resulting damage may become permanent.
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    DID YOU KNOW? THE LIMITS OF BRAIN PLASTICITY

    Optimistic popular writers have extolled the wonders of neural plasticity. The idea that experience can produce large changes in the brain is encouraging, as it supports the hope that people can learn and grow throughout life, overcoming obstacles along the way. Stories of untapped potential have a nearly unlimited appeal to the American character. But it’s time to step back and take a careful look at the evidence.
    Even infants are not blank slates whose brains and behaviors are infinitely modifiable. Before sensory experience can act on a child’s brain, the neurons need to be able to talk with each other via synaptic connections. Developmental programs specify particular patterns of connectivity, which are standard for all individuals. Unless there is a genetic error or a developmental accident, the output cells of the eyes will send their axons to the visual areas of the thalamus, which will pass the information along to the primary visual cortex. Axons that carry signals from the touch-sensitive receptors in the fingertips will occupy more space in the somatosensory cortex than axons carrying signals from the less sensitive elbow, and so on.
    Under most circumstances, these connection patterns are adaptive, but in unusual cases, this may not be true. In people who cannot see, parts of the visual cortex can be taken over by adjacent regions and used for other functions. Similar types of plasticity allow people to recover from impairments due to strokes by using another part of the brain to compensate for the damaged region. But if the damage is extensive, recovery is likely to be incomplete.
    Plasticity outside a sensitive period, if it is possible at all, usually requires more than simple exposure to