Differentiation means that an infant's abilities becomes increasingly distinct and specific. He will gain more and more mastery of movement. As he grows his movements will become more specific. He will learn to respond to specific noises and voices. This will form his infant language, which is very complex.
Growth progresses from the center of the body toward the periphery.
Movement will seem to be traced from a baby's arm as he becomes more and more accurate in reaching out on a finger or an object. He will at one point seem to cry less as he becomes more interested in sounds and in making his own sounds. He will pay more attention to what is happening around him. He will also start to have more visual involvement in the world around him. He will begin to show signs of boredom or habituation when you might show the same rattler over and over to him.