In a study of four-day-old infants, researchers measured the average length of breaks between non-nutritive sucking periods. Then, they made the infant hear the mother’s voice or a foreign voice, depending on whether the break between periods was lengthened or shortened. In this study, the child learned that it is somehow possible to hear the mother’s voice by changing the length of the breaks (De Casper & Fifer, 1980). The study shows not only that the newborn recognizes the mother’s voice, but also that it is possible to control the environment by playing with duration.