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Critical Period for the Development of the Child: The first trimester of pregnancy and the first three years of life

I have discussed the "various stages of development," "the diversity of development" and "the natural direction of development" in previous articles. In this article, I would like to take this issue a step further and discuss the possibility of a critical period in developmental theory.

The word I use here to express the critical state or period in Japanese (rinkaiki) is often used in the world of physics to describe the period when a substance moves from one state to another. In biological developmental theory, it refers to the period when there is a definite factor in influencing a change in development. During this critical period, if there is an obstacle blocking the structure or function of the factor influencing development, a disability arises and this process is considered the "vulnerable period." If the factor that is supposed to kick a developmental pattern into place, for some reason is not activated, it also causes a disability but the process is considered the "sensitive period."

The most representative example of the "vulnerable period" in development is the thalidomide event that broke out nearly 50 years ago. Children who were born to mothers, who took this sedative during the early months of pregnancy, had birth defects in their hands and feet. This drug was developed and manufactured in Germany, so the widespread use of it caused an outbreak of these defects in the children born to the mothers who took this drug. In the United States, however, it is important to note that there was no such outbreak because of the FDA control. The Japanese government, on the other hand, had approved this drug and unfortunately, there were many children who were born with severe defects. Needless to say, thalidomide is no longer used.

While I was studying medicine, the first research studies were presented by an Australian group reporting that when the pregnant mother caught rubella in the first months of pregnancy, there were frequent occurrences of cataract and heart deformity in the newborn born to that mother. Therefore, the rubella vaccine was developed and today, women are strongly encouraged to be vaccinated in order to avoid such tragedy.

For physical development, the first trimester of pregnancy is considered the "vulnerable period." This is the period when the fertilized egg is actively developing and forming the internal organs and if the rubella virus or the thalidomide bind with the cell, it causes a disturbance in the developmental process, and thus, a physical defect is likely to occur. Generally, if the virus is contracted after this period, there is hardly any problem.

Gender is determined by the combination of the X and Y chromosomes. Therefore, the combination of XX is a female, and XY is a male. In this process, the determining chromosome is the Y chromosome, which produces a differentiating factor in the testis. Thus, any abnormal combination which may occur, will produce a body that resembles that of a female if the Y chromosome is not present. Whether this factor is produced or not, the primitive sexual gland develops into an ovary or the testes, which eventually leads to the development of the external sexual organs. However, even if the Y chromosome is present, if there is an abnormality in the differentiating factor and it is not secreted, or if there is no acceptor for this factor, then, a hermaphrodite is born. This is a representative example of the "sensitive" period.

However, when referring to this "sensitive" period, it is more often pointing to the emotional aspect of development, rather than the physical. For example, it is said that up to the first year, the child builds an emotional bond with the primary caregiver (most of the time, the mother) and those who do not have this opportunity often have problems in their emotional, as well as social development. There is a saying in Japanese which says that "the spirit of the three-year-old is the foundation for that person until the age of 100." Specifically, it means that whatever influences the child may have until age three is the same "soul" that carries that person throughout his/her life. This is the representative model of the "sensitive" period.

Furthermore, there is the example of an animal experiment by the Nobel Prize winner Heubner who researched the above relationship with animals (mice and cats). His experiment was to sew the eyes of a newborn kitten shut for a period of time and to block out light. The result was that the structure of the brain that is supposed to develop for sight was abnormal or underdeveloped. Cutting the whiskers of a newborn mouse had a similar effect for the tactile sense, as it affected the cluster of nerve cells that catches the tactile information into the brain. Whiskers are even more sensitive than the human fingertip, and can sense what something is by just brushing across an object. Thus, it seems that stimulation for the tactile sense to develop is critical, after, for the completion of the system.

At least in the case with animals, when visual and tactile stimulation are denied of the newborn, the development of these senses becomes abnormal. The emotional bond between mother and child is strengthened by the mother-child interaction through skinship and an affectionate environment. We cannot research what happens to development when these elements are deprived, as easily as we can with animals and the above experiments. Hence, we will never know for sure if these effects on brain development and emotional development are permanent or how it changes their structure. However, it is reported that parents who abuse their child (ren) often were abused or were neglected themselves. Therefore, perhaps we can speculate that affection which is critical in emotional development is also something that "does not develop without experience."

There is also a possible argument that points to the plasticity of the brain. I believe, the brain, until the age of three is as soft as clay, and once it is "pushed" upon, it does not change for life. Perhaps the "sensitive" period and the "critical period" are in sync with the theory of the plasticity of the brain.

Thus, the saying, "the soul of the three year old is for life."
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