What are primitive reflexes?
Primitive reflexes range from very simple reactions such as the blinking and startle reflexes to more complex patterns of behavior such as the stepping and crawling reflexes. Primitive reflexes appear to be important for orienting the infant to its environment and for protecting it against potential threats to its safety. The reflexes are automatically elicited when an appropriate stimulus is present, such as liquid in the mouth or pressure on the bottom of the feet, and they take the same form on every occasion.
Several primitive reflexes exist for the head and face. Blinking is elicited when an object approaches the eye. This reflex functions to protect the eye from harm. The head-turning reflex is seen when an infant is placed face down on a soft surface such as a mattress. The infant will turn its head to one side to allow breathing to continue. Similarly, the defensive reaction reflex is seen when a cloth is placed over an infant’s face. The infant will turn its head and make swiping movements with its arms in an attempt to clear away the obstruction. Rooting and sucking are two important reflexes of the face that assist in feeding. The rooting reflex is elicited when an object gently touches an infant’s cheek; the infant will turn its mouth in the direction of the touch. Sucking is elicited whenever a nipple-sized object is placed in an infant’s mouth.
The reflexes of the arms include the Moro reflex and the tonic neck reflex. The Moro reflex is seen when an infant’s head is dropped slightly. The infant’s legs and its arms, with hands extended, will spread wide, then come together in an embracing movement as the hands clench. This sequence of behaviors appears to be a defense against falling, as the reflexive response could allow the infant to grasp on to something. The Moro reflex is seen in other primates as well. The tonic neck reflex is seen when an infant, lying on its back, has its head turned to one side. The infant’s arm on the same side will extend, and the other arm will move up to the back of its head, to assume a “fencing position.” This reflex is also called the fencer’s reflex.
The primitive reflex of the hand is the palmar or grasping reflex. If an object touches an infant’s palm, then the hand will clench into a fist. If the object is small enough for the infant to get a grip on it, then the infant’s grip will be strong enough to support its own weight. A newborn infant grasping an adult’s fingers with both hands can be pulled to a sitting or standing position.
The swimmer’s reflex is seen when an infant is lying on its stomach. If the infant is gently tapped along its side above the waist, then it will turn its torso in the direction of the touch. Another primitive reflex of the torso is the crawling reflex. If an infant is lying on its stomach and pressure is applied against the soles of its feet, then it will move its arms and legs in a crawling movement. This reflex may be a precursor for later voluntary crawling behavior.
Two reflexes of the legs also appear to prefigure later-developing motor behavior. If an infant is held upright under the arms and its feet are allowed to touch a surface, then the infant will demonstrate the standing reflex by straightening its legs and supporting some of its body weight. If the infant is then tilted forward and moved forward slightly, then it will make high stepping movements characteristic of the stepping reflex.
The plantar reflex is similar to the palmar reflex. If an infant’s foot is stimulated with an object touching the sole just below the toes, then the toes will curl around the object as if to grasp it. The Babinski reflex is another reflex of the foot; if the outside edge of an infant’s foot is gently stroked, then the toes will fan out. The Babinski reflex, unlike many of the other primitive reflexes, does not have an obvious function; the lack of a Babinski response, however, may indicate neurological dysfunction.
Some of the primitive reflexes, such as sucking and grasping, have been detected in utero. Most of the reflexes disappear within a few months after birth, although some primitive reflexes, such as blinking, persist throughout life. Still other reflexes, such as the crawling and walking reflexes and the defensive reaction, are later replaced by behaviors that are voluntarily controlled.
Because the absence of primitive reflexes can indicate neurological damage, most infants are tested for some of the reflexes soon after birth. Reflexes of the face, hands, and feet are components of neurological examinations such as the Brazelton Neonatal Behavioral Assessment Scale.
Despite the fact that the primitive reflexes are involuntary, infants show variability in their reflexive responses. There are individual differences among infants, such that some infants may show stronger or weaker primitive reflexes than others. Within individual infants, there is variation in the strength with which the reflexes may be elicited on different occasions. The age of the infant, its state of arousal, and the interval since its last feeding are all factors that can affect an infant’s reflexive responses.
Traditionally, it was believed that primitive reflexes disappeared with brain development, as the higher centers of the brain took over some motor functions from the subcortical centers of the brain. Recent investigations of the stepping reflex, however, have shown that other factors can affect the development of the reflexes.
Esther Thelen and Linda B. Smith, in their book A Dynamic Systems Approach to the Development of Cognition and Action (5th ed., 2002), report that infants who no longer show the stepping reflex under typical circumstances do demonstrate stepping if their legs are submerged in water. Similarly, the stepping reflex can be suppressed in very young infants by adding weights to their legs. Another study, by Philip Zelazo, as described in an article found in Developmental Psychobiology: The Significance of Infancy (1976), provided infants with daily stepping practice and showed that under those conditions, the stepping reflex did not disappear. These findings suggest that experience, growth, and weight gain, as well as brain development, influence the developmental course of the primitive reflexes.
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