The development of a smoothly controlled motor system is a major childhood priority. Suckling is almost the first mobile act of an infant, followed by the brain-outward maturation of the arm and leg systems—eating before grasping before walking. Since mobility is a central human characteristic, these innate systems must develop early at the survival level without formal instruction. This motor development includes specific currently ill understood periods during which various key specialized brain systems generally develop (such as walking at about one, talking at about two).

How infants begin their mastery of complex motor behaviors is a fascinating developmental phenomenon. Consider a behavior that most parents observe. If you stick out your tongue to an observant infant shortly after birth, the probability is high that she will reciprocate the behavior.

Sticking out our tongue is an uncommon act for humans, and it requires the activation of a complex motor neuron sequence. Our tongue is a very important muscle that is used to facilitate eating and speech, so we normally keep it inside our mouth. I suppose it would be possible for an infant to randomly fire the appropriate motor neurons for tongue projection, but that's not what occurs when an infant sticks out her tongue in immediate mimicry of a parent's action. How can an infant possibly master such a complex motor act immediately after observing it?