After your birth, your neurons continued to reproduce at a rapid pace, finally slowing down around 12 months of age (Huttenlocher, 1994). For many years it was assumed that neurons do not reproduce after early infancy. Recent research, however, has de- finitively documented the growth of new neurons throughout the lifespan (Eriksson et al., 1998). These adult neural stem cells (NSCs, Gage, 2000) are generated through- out adulthood in two principal brain areas, the subventricular zone (SVZ) located near the ventricles and in part of the hippocampus called the subgranular zone. SVZ neurons migrate to the olfactory bulb where they appear to maintain its functioning by generating interneurons. New hippocampal neurons appear to integrate into exist- ing networks that involve learning and memory. The location, migration patterns, and the ways adult neural stem cells integrate with existing neural networks are subjects of intense current investigation given the potential contribution this knowledge can make to disease prevention and remediation (Curtis, Kam, & Faull, 2011).
Brain growth after birth is also due to the formation of synapses, new con- nections among neurons. The growth spurt in synapses reflects the vast amount of learning that typically occurred for you and for most babies in the early months of postnatal life. Some areas of your developing brain experienced periods of rapid synaptic growth after birth, such as in the visual and auditory cortices, which in- creased dramatically between 3 and 4 months. In contrast, the synapses in your pre- frontal cortex developed more slowly and reached their maximum density around 12 months. As you will see in the following sections, infants make rapid strides in cognitive development at the end of the first year, at about the time when prefrontal synapses have reached their peak density.
The growth of these connections was the product of both internal and external factors. Certain chemical substances within your brain, such as nerve growth fac- tor, were absorbed by the neurons and aided in the production of synapses. Your own prenatal actions, such as turning, sucking your thumb, and kicking, as well as the other sensory stimulation you experienced, such as sound, light, and pressure, all contributed to synaptic development. However, as we noted earlier, the major work of synaptogenesis, the generation of synapses, took place after birth, when much more sensory stimulation became available.
You arrived in the world with many more neurons than you would ever need. Over the next 12 years or so, through a process known as neural pruning, many neurons would die off and many synaptic connections would be selectively discarded. Some of these neurons migrated incorrectly and failed to make the proper connections, rendering them useless. Some of the synaptic connections were never established or were rarely used, so they ultimately disappeared as well. What counts most after birth is not the sheer number of neurons, but the number and strength of the interconnec- tions. Those branching points of contact that remained to constitute your brain would be a unique reflection of your genetics and epigenetics, the conditions of your prenatal period, the nutrition you received, and your postnatal experience and environment. This rich network of connections makes your thinking, feeling brain, and its structure depends heavily upon what happens to you both before and after your birth.
You may be wondering how to account for the simultaneous processes of synaptogenesis and pruning, which seem to be acting at cross-purposes. What is the point of making new synaptic connections if many neurons and connections will just be culled eventually? Greenough and Black (1992) offer an explanation for the apparent contradiction. They argue that synaptic overproduction occurs when it is highly likely that nature will provide the appropriate experience to structure the development of a particular system. For example, many animal species, as well as humans, go through a predictable sequence of activities designed to provide information for the brain to use in the development of vision. These include open- ing eyes, reaching for and grasping objects, and so on. This type of development depends upon environmental input that is experience-expectant because it is ex- perience that is part of the evolutionary history of the organism and that occurs
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Publishing Servicesreliably in most situations. Hence, it is “expected.” Lack of such experience results in developmental abnormalities, as we saw in the kitten experiments performed by Wiesel and Hubel. The timing of this particular kind of experience for nervous system growth is typically very important; that is, there is a critical period for such experience-expectant development. Nature may provide for an overabundance of synapses because it then can select only those that work best, pruning out the rest.
In contrast to overproducing synapses in anticipation of later experience, some synaptic growth occurs as a direct result of exposure to more individualized kinds of environmental events. This type of neural growth is called experience-dependent. The quality of the synaptic growth “depends” upon variations in environmental op- portunities. Stimulating and complex environments promote such growth in rat pups and other mammals (e.g., Kolb, Gibb, & Robinson, 2003). It seems likely that the same is true for infants and children. Imagine what might be the differences in synaptic de- velopment between children raised by parents who speak two different languages in their home and children raised by those who speak only one. Or imagine the synaptic growth of an infant raised in impoverished conditions with very little verbal interaction from adults, and very little chance to explore the environment, as compared to that of an infant reared in an environment rich in adult attention and opportunities for en- gagement with interesting objects. Experience-dependent processes do not seem to be limited to sensitive periods but can occur throughout the life span. Connections that re- main active become stabilized, whereas those that are not used die out. This type of ex- perientially responsive synaptic growth and the concomitant changes in brain structure it induces have been linked to learning and the formation of some kinds of memory. This process fine-tunes the quality of brain structure and function and individualizes the brain to produce the best person-environment fit (Paus, Keshavan, & Giedd, 2004).
Clearly, your early experiences played a vital role in the functional and struc- tural development of your brain. Your experiences helped stimulate the duplication of neurons in some parts of your brain, and it prompted synaptogenesis and prun- ing. These processes contribute to the plasticity of brain development, which can be quite remarkable. Suppose, for example, that you had suffered an injury to your left cerebral cortex during infancy. You can see in Box 2.1 that ordinarily, the left cerebral cortex serves language functions. Yet when the left hemisphere is damaged in infancy, the right hemisphere is very likely to take over language functions. Your language acquisition might have been delayed by an early left hemisphere injury, but by 5 or 6 years old, you would most likely have caught up with other children’s language development. Adult brains also exhibit some plasticity after brain injury, but nothing as dramatic as we see in children (e.g., Kolb & Wishaw, 2006; Stiles, 2001).
Understanding of postnatal brain development has improved dramatically over the last few decades, spurred by modern technologies. In Table 2.4, we describe sev- eral of the newer approaches that are recurrently mentioned in this and later chapters.
TABLE 2.4 Studying the Developing Brain
TECHniquES pROCEDuRES AnD pRODuCTS
electroencephalography electrodes attached to the scalp can allow recording of the electrical (eeG) activity of the cerebral cortex. Record- ings from the electrodes graph the frequency and amplitude of brain waves. event related potentials (eRPs), which are specific brain wave changes in response to sights, sounds, or other experiences, can be recorded as well, helping to identify areas of cortex that process the type of input used.
Functional magnetic resonance imaging (fMRI)
A computerized system for recording activity anywhere in the brain by imaging responses to input to input. Stimulation causes the affected areas of the brain to increase blood flow and metabolism. A magnetic scanner detects these changes and sends the information to a computer, which produces an image that shows the differential activity of brain areas. fMRI is one of several MRI technologies, generally referred to, along with Pet techniques, as “brain imaging.” Positron emission
tomography (Pet)
Like the fMRI, a type of brain scan that detects changes in blood flow and metabolism in any part of the brain, and produces an image that shows the differential activity of brain areas. Requires either injection or inhalation of radioactive material.
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