Critical Period

Historical Overview

The embryological critical/sensitive period concept originated in two classic research programs in the 1920 and 1930s:

• 1.
  Working with fish embryos, Stockard (1921) found that physical and chemical agents created “monsters” if presented during rapid cell proliferation and differentiation, but had essentially no effect if presented earlier or later. Permanent damage occurred when an agent interrupted normal development during “sensitive periods” or “critical moments.”
• 2.
  Studying differentiation in amphibian embryos, Spemann (1938) transplanted cells at various times from their original donor site to a different host site. Cells transplanted early developed appropriate to the host site, whereas those transplanted later developed appropriate to the donor site. Equipotential development was followed by determined development. Spemann viewed differentiation as a process of “cellular induction” occurring during a “critical period” in normal development.

The critical period concept quickly became applied to behavior. Lorenz apparently first used it in 1937 in his account of imprinting, the process through which newly hatched precocial birds presumably develop filial attachments and species identification (but see Gottlieb for an alternative approach). He suggested that imprinting's “two chief characteristics,” restriction to a narrow time period and irreversibility, were “in common with Spemann's inductive determination….” In 1962, Scott stated, “Critical periods determine the direction of social, intellectual, and emotional development.” For Scott, experience during such periods influenced development in at least two ways: (1) acting at a “turning point” leading toward normal or abnormal development, and (2) producing an irreversible effect that subsequent experience cannot modify. Essentially, experience at a critical time directed the organism down a particular one-way developmental path. Scott's general principle of organization suggested that as any system becomes organized, from differentiating embryonic cells to developing behavior patterns of young animals, reorganization of the system becomes progressively more difficult. That is, “organization inhibits reorganization.”[Page 325] Indeed, he claimed that major modification can occur only during organization.

Other mid–20th century factors brought critical periods to wide attention. Consider:

• 1.
  They fit with Freudian psychodynamic theory's emphasis on the role of early experience in determining personality, then dominant in many areas of psychology and psychiatry.
• 2.
  In his influential 1951 review of maternal deprivation, Bowlby stated that depriving young children of maternal care for a prolonged period may have lifelong adverse consequences: “[I]t is a proposition exactly similar in form to those regarding the evil after-effects of rubella in foetal life….”
• 3.
  Hebb's theory proposed that first learning was necessary for optimal brain development and functioning.
• 4.
  Lenneberg (1967) proposed a critical period for language acquisition. Its beginning owed to lack of maturation, and its end apparently “related to a loss of adaptability and inability for reorganization in the brain….”
• 5.
  A variety of experimental nonhuman research supported the apparent existence of critical periods. For example, infant rats reared in enriched environments had larger brains and better problem-solving abilities than those raised in restricted environments, female goats needed contact with their newborn kids to develop maternal attachment, Harlow's infant monkeys deprived of maternal care showed aberrant social behavior, young songbirds needed particular early experience to develop normal song, and early hormone manipulation permanently modified sexual behavior in rodents. The latter is of particular interest because it clearly demonstrated the importance of timing: Testosterone injections in newborn female rats or castration of newborn male rats produced opposite-sex behavior (with appropriate hormone replacement) in adulthood, whereas similar manipulations even in later infancy had no such effect. Some of this research is reprinted in books of readings edited by Denenberg and Scott.

Current Embryological Concept

Critical/sensitive periods are associated with the development of virtually all organ systems. In this section, critical period and sensitive period refer to times when teratogens may cause major morphological damage and minor morphological/functional damage, respectively, as suggested by Moore and Persaud, who have an excellent figure of human critical/sensitive periods. The embryonic period in humans is often described as “the critical period” since most organogenesis and severe effects of teratogens occur then. Such a description is oversimplified, however, because critical periods for various organ systems have different onsets and durations. The longest, for the central nervous system (CNS), extends to the 16th week of pregnancy, well into the fetal period. Sensitive periods for four systems—CNS, eyes, teeth, and external genitalia—extend for years after birth, resulting in potential adverse effects of abnormal environmental influences or positive effects of supportive environmental influences well into childhood or adolescence. Examples of adverse effects include effects of tetracycline in infancy/early childhood on mottling of permanent teeth and of heavy metals and infections on CNS development. Examples of positive effects include early surgical removal of congenital cataracts on visual acuity and depth/pattern perception and a low phenylalanine diet in infancy on CNS development of those with phenylketonuria.

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