The book Babyhood, by Penelope Leach, states: “Many studies have been made of the sights which interest an infant most, the sounds which attract and hold his attention, the sensations he most clearly seeks to repeat.
Buku Babyhood, karya Penelope Leach, menyatakan, ” Ada banyak penelitian tentang apa saja yang paling menarik bagi mata si bayi, suara apa saja yang memikat dan menyita perhatiannya, dan sensasi apa saja yang paling ingin ia rasakan lagi.