At birth, infants show the ability to respond to human voices and speech. In one experiment, infants only a few days old were videotaped as an adult spoke to them. By carefully studying the infants' small movements and changes in facial expressions, researchers found that infants moved in patterns that corresponded with the human voices they heard. When the infants heard other sounds like tapping or disconnected vowel sounds, they did not respond in the same way as when they heard speech.