Infancy

At birth, infants' sensory systems are available for processing perceptual information from the world and from their own bodies, but each system operates, for the most part, within a more limited range than later in infancy. Newborns' senses of taste and smell are particularly well established; in the first two weeks of life, infants can discriminate among sweet, sour, and bitter tastes, and can recognize the smell of their mother's milk. Infants' hearing is also relatively mature at birth, although the loudness threshold for detecting sound is ten to twenty decibels higher in newborns than in adults. Young infants are highly attuned to human voices, especially their mother's. In addition, infants localize sounds, and by the end of their first month, if not sooner, they differentiate speech sounds in a manner comparable to adults. In fact, one-month-olds across cultures differentiate speech sounds not evident in their particular native languages. Thus, young infants demonstrate a wider range of speech sound sensitivity than adults; by ten to twelve months, however, infants' sensitivity to speech sounds narrows and conforms to their native language.

From birth, infants demonstrate distinct preferences for the human face and are especially attuned to moving rather than static stimuli. By two months, infants' color vision is well established. At the same time, infants can process depth information by some cues, a capacity that continues to develop into the middle of the first year. Infants' overall visual acuity, however, is much more limited than that of adults. In the first two to three months, infants cannot discriminate In a visual acuity test, an infant sits on his mother's lap and looks at gray and striped circles. Of all the sensory systems, infants' visual capabilities have been the most thoroughly investigated. (Laura Dwight/Corbis) fine visual detail and tend to focus on areas of a stimulus where the contrast between light and dark is greatest, such as the hairline or the eyes of a face. By six to eight months, however, visual acuity nears adultlike maturity.

Infants' perceptual development is inextricably linked to their motor, or action, development. Like adults, infants' action in the world guides their perception of the world, just as their perception of the world guides their action in it. For example, the ability to perceive through touch the size, texture, and hardness of objects develops over the first six to nine months of life in parallel with changes in how infants manually explore objects. At birth, infant action is limited, rather inflexible, and reflexive, largely because they lack control of their heads and trunks. When provided with postural support, however, newborns demonstrate rudimentary reaching abilities, directing their arms in the general direction of objects. Within the first weeks of life, infants are able to support their own heads, facilitating gaze and the scanning of the environment. By three to four months, infants begin using vision to guide their reaching efforts, resulting in successful contact with objects. With experience in visually guided reaching, infants increasingly coordinate their grasping of an object with the movement of their arm toward the object. Between five and seven months, infants develop sufficient trunk control to support their independent sitting, which in turn provides a solid position for head and arm activity.

Infants around seven to nine months begin to move themselves in the environment by means of crawling, which opens up a whole new world of exploration for them. Between eight and twelve months, infants begin to stand, first by using furniture or other objects to support themselves and then by establishing postural control to support independent standing. Walking soon follows as infants move into their second year. With each new motor transition, infants gain new means of perceptually apprehending the world. At the same time, infants rely on their perceptual development to establish more efficient means of acting on the world. For example, infants must rely on what they see and feel when crawling or walking over surfaces in order to continuously update their action and make their action fit the ever-changing demands of the environment.

Enormous individual differences mark the timing of developmental changes in both perceptual and motor development. Increasingly, developmental psychologists have shifted their focus away from simply documenting infants' perceptual and motor milestones toward understanding how these changes occur and how the domains of perception and action constantly and seamlessly interact to produce unique, individual pathways of development.