Most brain cells form before birth. With maturation and experience, their interconnections multiply rapidly and become more complex after birth. A pruning process strengthens heavily used links and weakens unused ones, and we seem to have a critical period for some skills, such as language.Complex motor skills—sitting, standing, walking—develop in a predictable sequence. Timing may vary with individual maturation and with culture.We have few conscious memories of events occurring before age 4, a blank space in our conscious memory psychologists call infantile amnesia.