Newborns also have a remarkably efficient mechanism for defending themselves against over-stimulation. Something that causes a startle reaction, such as a bright flash of light in the eyes or a sudden bang, speeds up the baby's breathing and heart rate. If the shocks are repeated he continues to startle, putting a strain on his physiological systems. But very shortly he protects himself; he closes his eyes, curls his body into a snug bundle, and "pretends" to be asleep. What he has actually done is to control his reaction to the too-intense stimulus by shutting himself off from it. Psychologically, this is termed "habituation." It may remind you of the protective defense system of certain insects that "play dead" when gently pocked. More poking, however, won't reactivate the insect; it remains comatose until you are long out of range. It's a giant leap from insects to human babies, but the contrast, rather than comparison, is interesting.

The baby can be reactivated by introducing a new pleasing stimulus, such as crooning voice or a rattle. He'll "come awake" and respond calmly to the new interest. This capacity to control his responses is a valuable and necessary ability that the newborn uses to organize his world. He needs to shut out disturbing stimuli in order to make choices and focus his attention on important infant matters.