Sensitivities of the Infant
The best criterion that can be used to determine the presence or absence of sensory capacity is the motor response to sensory stimuli that would normally arise when these sense organs are stimulated. However, it is often difficult to tell whether a motor response is made to a stimulus or whether the reaction is a part of general mass activity. Nor does absence or response necessarily mean absence of sensitivity. It may mean only that the stimulus used was too weak to elicit a response.
An even more serious problem in studying the sensitivities of the infant is finding a suitable method of doing so. To date, for example, no reliable test for color vision in infants has been devised. Consequently, the knowledge of whether infants can see color or whether they are capable of only black-white vision has been determined indirectly from the study of the development of the cones in the eyes of infants who die at birth or shortly afterward. Because the cones are very undeveloped, it is assumed that newborn infants are color blind.
There is ample evidence that the state of the infant influences reactivity. When infants are dazed by the birth experience, when they are drowsy or asleep, or when they are preoccupied with feeding, they will react differently to sensory stimuli than when their physical condition is better. The intensity of the stimulus likewise has a profound influence on infant’s reactivity to different sensory stimuli. This explains, in part least, the variations in the sensory abilities of different infants, just as it explains the variations in the sensory abilities of the same infant from time to time during the infancy period.
While present knowledge of sensory reactions of infants is somewhat limited, it is now believed that infants have a greater capacity for sensory discrimination than formerly was thought. With refinements in experimental techniques and with scientist’s increased interest in studying the sensory experiences of newborn infants, more and more knowledge about this area of development has been accumulating. What is known about the sensory capacities of infants up to the present time is summarized in below:
Sensory Capacities of Infants
Newborn infants are far from blind but their visual field is only about one-half that of adults because the rods are undeveloped except around the fovea. Color vision is either totally absent or minimal, due to the undeveloped state of the cones. Because of muscle weakness, the infant cannot focus both eyes on the same object simultaneously and, as a result, everything is seen as a blur. There is some evidence that infants respond to differences in brightness but this evidence is inconclusive. The ability to follow a moving object and then move the eyes backward – optic nystagmus – appears during the first week of life for horizontal movements and later for vertical movements.
It is believed that hearing is the least developed of all the senses at birth partly because the stoppage of the middle ear with amniotic fluid for several days after birth makes it impossible for sound waves to penetrate to the inner ear where the cells for hearing are located and partly because these cells are only partially developed. Low-frequency tones can be heard sooner than those of high frequency and infants respond sooner to the human voice than to other sounds. Hearing normally improves within the first three or four days after birth, as the amniotic fluid drains from the middle ear. The infant can then determine the direction from which a sound comes and can discriminate pitch and intensity.
The cells for smell, located in the upper part of the nose, are well developed at birth. That the infant can distinguish odors is shown by crying, head turning, and attempts to withdraw from unpleasant stimuli and by sucking and relaxing the body in response to pleasant stimuli.
Because taste is markedly influenced by smell and because the cells for taste, located on the surface of the tongue and in the cheek areas, are well developed, the infant’s sense of taste is keen. The infant has generally positive reactions – relaxing the body and sucking – in response to sweet stimuli and generally negative responses – crying and squirming – in response to salty, sour, and bitter stimuli.
Sensitivity to hunger is fully developed at birth and hunger contractions occur within the first day of life. Thirst also appears then.
The sense organs for touch, pressure, and temperature are well developed at birth and lie close to the surface of the skin. The skin of the new-born’s lips is especially sensitive to touch, while the skin on the trunk, thighs, and forearms is less sensitive. Sensitivity to cold is more highly developed than sensitivity to heat. Sensitivity to pain is weak for the first day or two or postnatal life and then improves rapidly. Pain responses develop earlier in the anterior than in the posterior end of the body.