Everything humans do involves using one or more senses. It is through the senses that infants discover the world. Without one's senses, the brain would be an eternal prisoner within the confinement of one's skull. Humans experience these sensations through interactions with the environment; interpreting the meaning of these sensations for actions is called sensory processing. When a child uses her senses to discover a new object, she creates a neuronal pathway in the brain. The more often she stimulates her senses from her environment, the more likely she is to create new neuronal pathways and strengthen old neuronal pathways in the brain.
Sensory development begins during gestation and continues throughout childhood. There are seven sensory processes: taste, smell, touch, hearing, seeing, body position sense (called proprioception), and movement sensations (called vestibular input). Below is a brief discussion of each sense, its purpose, and the stages of its development; how infants stimulate their senses; and why sensory stimulation is important for infants.
Several touch receptors make up the somatosensory system. The infant experiences the sense of touch by any direct contact to the skin. The sensory receptors for touch send messages to the brain, through neurons, concerning temperature, pain, and the texture and pressure of objects applied to the skin.
The somatosensory system begins to develop during gestation. The nervous system, which is the message carrier to the brain for the senses, begins to develop at the third week of gestation. At the ninth week of gestation the sensory nerves have developed and are touching the skin. By the twenty-second week of gestation, the fetus is sensitive to touch and temperature. At birth, the sense of touch can be observed through the infant's reflexes when it comes in contact with different stimuli. One example is the rooting response. This is when an infant will reflexively turn its head in the direction of a touch to its cheek.
It is important for adults to understand what types of touch stimulation a specific infant needs. For example, infants who fall asleep only when rocked and like to be cuddled may prefer firm pressure against the body. One way to apply this pressure is by swaddling infants. This firm pressure relaxes excited neurons that are sending messages back and forth from the surface of the skin to the brain. Some infants are content to lie or sit and play in one spot; this does not mean that they are not as curious as other infants, but that they can absorb only so much stimulation at one time. By contrast, other infants who are constantly exploring by reaching out to touch various objects and textures are more likely seeking stimulation.
Taste and smell are chemical senses; they process information by processing chemical changes in the air and in objects on the tongue. These are primitive sensory systems that are intimately involved with early developmental activities such as feeding, eating, and recognizing family members compared to strangers. In this way, these are protective senses; they enable the organism to survive, both through recognizing familiarity for safety purposes and by enabling the infant to identify food for nourishment.
The taste buds become apparent during the eighth week of gestation, and by the fourteenth week the taste sensation is formed. At birth, infants express positive and aversive facial responses to tastes. The sense of smell is apparent at birth as an infant begins to recognize and prefers its mother's scent. As infants begin to develop, it is important to observe their reaction to the different sensations of sweet, sour, bitter, and salty, as well as to textures, to know what they like or dislike.
The movement sensations, or vestibular system, is a sensory area that is not often discussed in literature but is important to development. The vestibular system involves one's balance and works in conjunction with other senses. The vestibular system is designed to answer questions that relate to the human body, such as "Which way is up?" and "Where am I going?" This is accomplished by measuring the position of the head through the combined efforts of the five sensory organs in the inner ear, a process that enables one to maintain one's balance.
During gestation, the vestibular system is immature but operating by the ninth week and continues to mature throughout gestation and after birth. The vestibular system is important for an infant to be able to hold its head steady when being held upright, sitting up, standing, and walking. It is easy to recognize when the vestibular system is sending different messages to the brain than what is actually taking place. Examples include infants falling over when sitting and falling down when walking. In these instances, the vestibular system is sending a different message to the infant's brain in relation to what is happening with its body.
The auditory system begins to develop next. Around the fifth week of gestation the ear begins to form, and by the twenty-fourth week of gestation all hearing structures are in place. By the end of gestation, the auditory system is reasonably mature and continues to develop throughout the first year after birth. Infants demonstrate this sense by turning their head or eyes toward a sound. Newborns are more likely to respond to higher frequencies than lower frequencies. Also, repetition and longer duration increase the likelihood of infants hearing and responding to a sound. Adults can encourage infant stimulation through musical toys that use repetitive sounds and higher pitched tones.
The visual system begins to develop around the ninth and tenth week of gestation and continues developing until three years after birth. At birth, infants are able to detect motion, can focus on an object about eight inches away, are sensitive to brightness, and have red and green color vision. By the end of the second month, infants are able to track smooth pattern movements and begin to discriminate between colors. During the third month, infants are better able to focus on objects farther away and are beginning to develop depth perception, both of which continue to develop until age two or three. Many toy companies gear toys that have geometric shapes and are black and white for newborns, and toys that are brightly colored and have patterns for infants about three months and older. These toys encourage development as the infant's neuronal pathways are being established.
Body Position Sense
The seventh sense, body position sense, or proprioception, works in conjunction with other senses. Proprioception is the movement and position of the limbs and body in relation to space. Proprioceptors are located in muscles and joints and are triggered by bodily movements. Proprioceptors, combined with vision, the sense of touch, and input from the vestibular system, help infants reach such milestones as rolling over, crawling, and walking.
Sensory Systems in Concert
The sensory systems work in concert with each other to enable an infant to engage with the environment and gain control over the body and its capabilities. Consider a sensory explanation of what one might observe when watching a six-month-old infant playing ball using all seven senses. As he touches and hits the ball, the infant receives cues as to whether the ball is hard or soft, smooth or rough. He discovers the texture and taste using his tongue to lick the ball when it is near or in his mouth, as well as any odors by smelling the ball. The infant receives visual stimulation by the color of the ball and by watching it roll or bounce. The auditory system is triggered when a noise, such as a thud, ring, or squeak is made as the infant hits the surface of the ball, throws it, or shakes it. The vestibular system is activated as the infant is sitting up, maintaining his balance. Finally, the infant uses proprioception as he is moving his arms and legs in space while throwing, pushing, or hitting the ball and watching it with his eyes and maintaining his balance.
Adults can encourage sensory development in their infants by providing a safe and stimulating environment for discovery. Sensory exploration is important to infant development, specifically for establishing new neuronal pathways in the brain and strengthening already developed neuronal pathways.
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Kandel, Eric, James Schwartz, and Thomas Jessell. Principles of Neural Science, 4th edition. New York: McGraw-Hill, 2000.