Critical Periods in Physical Development:
The concept of critical periods originated in the study of embryological development. As the embryo grows, the various organ systems develop in a fixed time sequence, and each system has a critical period during which it is maximally sensitive to growth stimulation and maximally vulnerable to disruptive factors. The organ system that does not develop normally during its critical period does not get a second chance, because the focus of growth shifts to other systems.
Thus, if the mother contacts German measles during the first three months of pregnancy, the effect on the embryo depends on the exact time of infection. The infant may be born blind, deaf, or brain-damaged, depending on which organ system in its critical phase of development at the time of the infection.
Critical Periods in Emotional Development:
Critical periods have been identified in the postnatal emotional development of animals. There appears to be a critical period for taming dogs so that they will respond to human beings. Puppies were raised in a large field with tame mothers but without human contact.
They were removed at various ages to spend 10 minutes a day with a handler who was passive and waited for them to approach him. If this experience was delayed as long as 14 weeks the puppies had become wild dogs, fearful of human beings.
The best time for taming was between five to seven weeks of age. When tamed at these ages, the dogs remained tame, as was seen when they were tested again 14 weeks later.
It is possible, but much harder to demonstrate, that the psychological development of the child has critical periods. It has been suggested that a child must form a satisfactory emotional attachment to an adult during the first three years of life in order to be capable of normal affection relationships in later life (Bowlby, 1969). If this hypothesis is true, then these years would constitute a critical period for the development of social relations.
Critical Periods in Sexual Identification:
A study of children born with sexual organs inappropriate to their true (genotypic) sex suggests that there is a critical period for sexual identification. These children (called pseudo hermaphrodites) can often be helped by surgery.
If the sex-change operation takes place before the child is two years old there appear to be no personality difficulties; that is, a boy who has been initially raised as a girl can assume the male sex role and vice versa. In a child older than two, however, switching identity to the opposite sex usually causes serious problems in adapting to the new sex role.
The notion of a critical period in development implies that at a certain time during a person's development he is optimally ready to acquire certain behavior. If the appropriate experiences do not occur during this period, then the behavior can be learned later only with great difficulty, if at all. As we examine different areas of development we will look for evidence of critical periods.
(b) Motor Development:
The human foetus develops within the mother's body according to a relatively fixed time schedule and fetal behavior (such as turning and kicking) also follows an orderly sequence, depending on the stage of growth. Premature infants who are kept alive in an incubator develop at much the same rate as infants who remain in the uterus full term. The regularity of development before birth provides a clear picture of what is meant by maturation.
Motor development after birth-using the hands and fingers, standing, walking-also follows a regular sequence. For example, activities, such as rolling over, crawling, and pulling up to a standing position, that prepare the child for walking occur in the same order in most children.
Unless we believe that all parents subject their offspring to the same training regime (an unlikely possibility), we must assume that growth processes determine the order of behavior.
As you can see from Figure not all children go through the sequence at the same rate; some infants are more than four or five months ahead of others in standing alone or walking. But the order in which they go from one stage to the next is generally the same from one infant to the next.
Because the child's mastery of the movements necessary for sitting, standing, walking, and using Ms and fingers follows such as orderly sequence, and because children in all cultures accomplish these skills at roughly the same age, motor development appears to be primarily a maturational process little Influenced by the culture in which the child is reared.
Psychologists have done much research on the roles maturation and experience play in development. The most reasonable conclusion from the available data is that human development involves a continuous interaction between the organism and its environment. No behavior develops solely because of maturation or solely because of learning.
Certain brain and body structures must mature before a particular behavior can occur, but experience is required to develop the brain's capacity to organize and process incoming information and to signal the appropriate response.
(d) Early Deprivation:
Infant monkeys were reared in total darkness from birth to three months, except for a brief period each day when they were exposed to light while wearing special goggles that permitted only diffuse, unpatented light to reach the eyes. When the monkeys were first exposed to light without, goggles, they showed serious deficiencies in visual-motor behavior.
They could not track moving objects with their eyes, did not blink when threatened by a blow to the face, nor put out their arms when moved rapidly toward a wall.
However, these skills improved with continued visual exposure, and by the end of several weeks the dark-reared monkeys were performing as well as normal monkeys. In most instances they acquired the responses in less time than had the normal monkeys at an earlier age.
This study provides for the importance of both maturation and experience. Fully adequate use of vision depends on neuromuscular growth continuing after birth as well as on practice in the use of vision. The dark- reared monkeys required experience in light before they could develop the proper response; but the fact that they required much less experience than newborn monkeys once the goggles were removed is evidence of the role of maturation (Riessen, 1965).
In another experiment, dogs reared in confined quarters were perfectly healthy but in some respects appeared "stupid" (Scot. 1968). For example, they seemed insensitive to pain. They did not respond to a pin prick or to having their tail stepped on. Time after time, they would investigate a lighted match by putting their nose into the flame.
Whatever the felt experience may have been, certainly the pain stimulus did not evoke the avoidance responses found in normal dogs. Subsequent studies with other species have led to the conclusion that restriction or deprivation of stimulation generally produces anima's that in later life do not learn new tasks as quickly as their normal counterparts.
What will be the psychological effects if we provide an organism with an unusual amount of simulation instead of a restricted or deprived environment? Young gerbils (small mouse-like rodents) housed together in a large cage equipped with various kinds of toys differ significantly after thirty days from gerbils kept singly in small, bare cages. They perform better on learning tasks; their brains weigh more and show a higher concentration of some of the chemicals associated with learning.
Human infants may also benefit from an enriched environment, even in the first weeks after birth. One study showed the effect of early stimulation upon visually directed reaching-a visual-motor response that develops in clearly specified maturational steps (White, 1971).
A month-old baby lying on his back will stare at an attractive object held above him but will make no attempt to reach for it. By two months he will swipe at it accurately but with a closed fist. By four months he will alternate glances between his raised open hand and the object, gradually narrowing the gap. By five months he will accurately reach and successfully grasp the object.
Although the universality of this response sequence indicates a large degree of maturational dependence, the rate of development can be accelerated, the environment of a group of month-old Infants In a state hospital was enriched by
4. Hanging an elaborate ornament over the cribs featuring contrasting colors and forms to look at and explore with the hands.
Infants receiving this kind of treatment succeeded in visually directed reaching at an average age of three and a half months as contrasted with five months for a control group reared in the relatively unstimulating conditions of normal hospital routine.
Interestingly enough, the enriched-environment infants were delayed in one aspect of their development; they did not begin visually studying their hands until around two months, as contrasted with a month and a half for the control infants.
With virtually nothing else a look at, the control group discovered their hands earlier than the experimental group.
Note, however, that increased stimulation will not result in accelerated development unless the infant is maturation ally ready. In fact, too much stimulation too soon may be upsetting.
During the first five weeks of the above experiment, infants in the enriched group spent less time looking at their surroundings (seeming to ignore the ornament and patterned bumpers) and engaged in much more crying than did the control infants. It may be that a month-old infant is actually distressed by being surrounded by more stimulation than he is able to respond to.
A subsequent study found that providing infants with only a simple but colorful object mounted on the crib rails for the first two months of life and then introducing more complex ornaments during the third month seemed to produce optimal development.
These infants showed no signs of unusual distress, were consistently attentive to their surroundings, and achieved visually directed reaching at less than three months. Thus we see the importance of providing stimulation appropriate to the level of maturation (White, 1971).
Early experiences seem to be important in providing the background necessary to cope with the environment at a later age. The parents who are proud of the "good baby" lying quietly in the crib may not be giving that baby what is best for him. The importance of a stimulating environment in the early years can be further illustrated by a classic study by Skeels and Dye (1939).
A group of orphaned children (whose development at the age of nineteen months was so retarded that adoption was out of the question) was transferred to an institution for the mentally retarded.
In this institution, in contrast to the overcrowded orphanage, each child was placed in the care of an older, mildly retarded girl who served as a surrogate mother, spending great amounts of time playing with the child, talking to him, and informally training him.
In addition, the living quarters were spacious and well equipped with toys. As soon as the children could walk, they began to attend a nursery school where conditional play materials and stimulation were provided.
After a period of four years this experimental group showed an average gain in intelligence of 32.1 Q. Points; a control group that remained in the orphanage showed a loss of 21 points. A follow-up study over 20 years later found the experimental group to be still superior to the control group (Skeels, 1966). Most of the experimental group had completed high school (one-third had gone to college), were self-supporting, and had married and produced children of normal intelligence.
Most of the control group, on the other hand, had not progressed beyond third grade and either remained institutionalized or did not earn enough to be self- supporting
Although the number of subjects in this study was small and the possibility of some innate intellectual differences between the experimental and control group cannot be completely ruled out, the results are sufficiently impressive to indicate the importance of a stimulating early environment for later intellectual development.