The important cells in the cortex are neurons (see Figure 2). All 100 billion neurons that an individual ever will possess are present in the brain at birth. Each first resembles a spindly young tree before it develops its elaborate system of branches and roots. Each is fairly isolated and does not communicate with other neurons through its branches (dendrites) or roots (axons). As infants begin to receive appropriate stimulation -- stimulation that is sensory, novel, and challenging, such as the sight and sound of a new rattle -- the neurons begin to branch out. When babies begin to realize that two objects are similar ("I can suck a breast, and I can suck a bottle") or that two events are related ("When mommy comes in my room, I get picked up"), neurons begin to communicate with one another. The more communication that occurs, the more branching that occurs, and the denser the forest of neurons becomes. Even though no new neurons are created, the cortex becomes thicker because of the extensive network of branches and roots that develop among the existing neurons when children receive appropriate stimulation.

The development of neurons, and the attendant change in brain interconnectedness, does not happen with equal ease throughout one's life. It is during the first three years of life that brain growth occurs most quickly and easily: Multitudes of new connections are made every day. This is not surprising if we consider the external changes that occur from birth to three years.

During this first three years, normally developing children learn to speak, think, and perform sophisticated movements and build interpersonal relationships. There is no other three-year period in life during which we come close to matching the rate of these accomplishments. PET scans comparing the brains of healthy and neglected three year olds clearly show that this growth occurs as a function of the environment rather than heredity

After age three, it becomes somewhat more difficult for neural connections to be made, but until about age nine, when the hormones associated with puberty come into play, the brain still has good potential to grow and change. It is, in fact, during this time -- from about three to nine -- that the brain uses the most energy in its work (see Figure 4). The brain of a child in this age range daily uses twice as much glucose energy as it will at any other time in his/her life. Almost 50 percent of the calories that young children consume are used to support this intense brain activity, much of which has to do with consolidating the growth of neural pathways.

In the first three years, pathways proliferate wildly as each new experience and stimulus lead to the growth of new connections. The more connections that are made, the more possibilities that exist. Beginning about age three, the connections are pruned and refined -- consolidated -- with the result that only those that are well used and meaningfully connected to the child's life remain.

Go on to The Early Years.