1998 Childhood is the time for learning. Many cognitive and motor skills are gained quickly during childhood and are not mastered as easily if the learning process begins later. However, environmental factors such as drugs, alcohol, and viral illnesses are able to disrupt normal brain development during critical periods of CNS development.
Environmental factors become critically important during later stages of brain maturation.
In humans and other mammals, the number of synapses increases dramatically after birth. The specificity of neuronal connections is then refined during early postnatal life. Experimental data have shown conclusively that neuronal activity is critical for the elaboration of synaptic territories, as well as for making proper synaptic connections. Thus, once the initial circuitry of the CNS is guided by intrinsic factors into roughly correct patterns, after birth environmentally derived activity takes over to refine connections between neurons. In the 1960s and 1970s, Torsten Wiesel and David Hubel conducted an influencial series of experiments on this topic, for which they received the Nobel Prize in 1981.