The first area of development to consider is physical development, which occurs in several important ways. Obviously, children grow in size and weight. As time goes on, they also become better able to move themselves around and to manipulate objects. Their senses become more refined. Each of these important types of physical development is covered in the following discussion.
Physical Development: Sensory Development
Piaget made infancy his "Sensorimotor" stage because he recognized that infants learn about their world by interacting with it through their senses. They don't understand their environment very well at first, but are born exquisitely prepared to explore and learn. They learn how to make purposeful movements, how to make sense of things, how to speak, and how to perform other skills. All of these developments require babies to use all their senses: touch, taste, smell, hearing, and sight.
Babies can feel and respond to pain and touch from birth, and this is an important first connection between infants and caregivers. They can feel hot and cold, hunger and satisfaction, soft and rough textures, pain and comfort, and cuddling and abandonment. This is why babies can often be soothed at birth by their caregivers' warm hugs or a warm bottle.
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As infants grow, they begin to touch objects in their environment with their hands, feet, and mouths to learn about them. When babies put toys and other things in their mouths, they are not trying to taste them as much as they are trying to feel the texture and structure. It is important for caregivers to keep babies' environments clear of dangerous objects such as small objects or poisonous substances.
While babies learn about their environment through feeling things with their mouths, they also learn by tasting. The senses of taste and smell senses are intertwined. When infants are born, they have the ability to distinguish sweet, sour, and bitter tastes, but they will prefer sweet tastes and aromas, such as breast milk. In fact, a baby's ability to taste is so specific that he or she can tell the difference between her own mothers' breast milk and that of another woman. As babies start to get older, between ages 1 and 6 months, they begin to have a taste for saltier solutions. This will prepare them to eat solid foods later on. When babies begin to eat solid foods, somewhere around 6 months, they will prefer sweet foods to bitter foods, and fruits to vegetables. As more and more foods are added over the coming weeks, they will begin to develop their own individual taste preferences.
Babies can hear at birth, and doctors can test infants for hearing problems right after birth. As infants grow, their mental ability to process and use information they hear improves. At birth, babies will turn their heads toward sounds in their environment. Research has also shown that babies prefer more complex sounds, such as speech and music, to simple tonal sounds. Furthermore, babies can even begin to distinguish different speech sounds soon after leaving the womb. As babies begin to mature, between ages 1 and 6 months, they are able to locate where sounds come from in their environment and to compile sounds into more complex chunks, such as musical phrases. By age 6 months, babies begin sorting out speech sounds from their own language and ignoring speech sounds that they recognize as not from their own language.
While some senses are fully developed at birth, others require time to mature before they become refined. Unlike their abilities to smell or hear, babies are not able to see as well as adults do. They develop their acuity, color perception, and ability to focus as they mature in the first months. At birth, visual acuity is only 20/600, which means that most objects farther away only look like dark shadowy objects. Newborns can best see objects and faces that are held 8 to 14 inches from their face, which is about how far away a caregiver's face is when holding a baby. Babies' eyes develop quickly, and by age 2 or 3 months they have the ability to see a full spectrum, or range, of colors and can focus on objects just like adults. At this point, they can also recognize their caregiver's face and can tell the difference between other people's faces. By about 6 to 8 months, they develop the visual acuity of that of adults, about 20/20, and can track or follow objects in their line of sight with increasing accuracy. By about 9 months, they also develop depth perception, or the ability to see and understand that different objects are different distances away. They will be able to understand that they are sitting on a couch and will have to climb down to reach the floor.
Babies are not simply passive consumers of sensory information. They actively make sense of the information they take in through their senses. This process has an actual effect on the quality of their brain development. Babies that are properly stimulated, cared for, and loved actually develop better (faster, more robustly, etc.) than babies who are neglected. Babies' senses can be stimulated in many ways: listening to caregivers speaking, looking at different objects and colors, and playing with toys that have different textures. Babies literally need touch and affection from caregivers in order to grow and to thrive properly. Babies who do not receive appropriate touch and affection may ultimately have developmental problems.