Information is available about the time at which the infant makes the first interpretable responses to each of the different modes of stimulation. The most nearly perfect senses in the newborn are those of touch and pain. A series of pinpricks cause distress, whereas an abrasion of the skin seems not to do so. The sense of touch clearly plays a role in feeding behavior. Newborn infants react vigorously to irritating odors such as ammonia and acetic acid, but discrimination between olfactory stimuli is not evident until much later. Sugar solutions initiate and maintain sucking from birth on, whereas quinine (bitter) solutions seldom do, and the latter stimulus elicits avoidance behavior. Hearing in the newborn is manifest within the first few postnatal days. Sharp, quick sounds elicit responsive blinking and sometimes startle. In some infants, the human voice appears to cause similar reactions by the second week. Strong light and objects held before the face evoke reactions in the neonate; later, visual searching is an integrating factor in most projected motor activities.
Sensation in the newborn infant must be judged largely by its motor reactions, so that sensory and motor developments seem to run in parallel but this may be partly artifactual. There are nonetheless discernible maturational stages that constitute sensory milestones, so to speak. This is most apparent in the visual system, which is more easily studied than the other senses. Sustained ocular fixation on an object is observable at term and even in some preterm infants; at these ages it is essentially a reflexive phototropic reaction. It has been observed that the neonate will consistently gaze at some stimuli more often than others, suggesting, according to Fantz, that there must already be some elements of perception and differentiation at this early stage. Voluntary fixation (i.e., following a moving object) is a later development. Horizontal following occurs at about 50 days; vertical following, at 55 days; and following an object that is moving in a circle, at 75 days. Preference for a colored stimulus over a gray one was recorded by Staples by the end of the third month. By 6 months the infant discriminates between colors, and saturated colors can be matched at 30 months. Perception of form, at least as judged by the length of time spent in looking at different visual presentations, is evident at 2 or 3 months of age (Fantz). At this time infants are attracted more to certain patterns than to colors. At 3 months, most infants have discovered their hands and spend considerable time watching their movements. The ages at which infants begin to observe color, size, shape, and numbers can be determined by means of the Terman-Merrill and Stutzman intelligence tests (see Gibson and Olum). Perception of size becomes increasingly accurate in the preschool years. An 18-month-old child discriminates among pictures of familiar animals and recognizes them equally well if they are upside down.
Visual discrimination is reflected in manual reactions, just as auditory discrimination is reflected in vocal responses. Much of early visual development (first year) involves peering at objects, judging their position, reaching for them, and seizing and manipulating them. The inseparability of sensory and motor functions is never more obvious. Sensory deprivation impedes not only the natural sequences of perceptual awareness of the child’s surroundings but also the development of all motor activities. Auditory discrimination—reflected in vocalizations such as babbling and, later, in word formation—is discussed further on in connection with language development.
The Development of Intelligence (See Also Chap. 20)
The subject of intellectual endowment and the development and testing of intelligence were touched upon in Chap. 20. There it was pointed out that although intelligence is modifiable by training, practice, and schooling, it is much more a matter of native endowment and not simply a question of environment and providing the stimulus to learn, although these are clearly factors. It is evident early in life that some individuals have greater intelligence; they also clearly maintain this differentiation all through life, and the opposite pertains in others.