Critical periods, stimulus input, and emotional reactivity: A theory of infantile stimulation.

Abstract

Experiments, using rats and mice, do not support the critical period hypothesis that there are certain limited time periods in infancy during which a particular class of stimuli will have profound effects upon subsequent behavior. Where findings are consistent with the hypothesis, further research has shown that the "critical period" is a complex function of amount of infantile stimulation. The central hypothesis of this paper is that amount of stimulus input in infancy acts to reduce emotional reactivity in a monotonic fashion. From this it follows that an inverted U function should be obtained between amount of infantile stimulation and adult performance for tasks involving some form of noxious element and which are of "moderate" difficulty. For tasks which are "easy" or "difficult," the relationships between performance in adulthood and infantile stimulation should be monotonic, though opposite in slope. Data supporting this theory are discussed. This paper has two purposes. The this hypothesized change in emotional first is to question certain aspects of reactivity offers a useful mechanism the critical period hypothesis (Scott, by which one can explain a number 1958, 1962). The second, and more of disparate and seemingly unrelated important, purpose is to develop a changes in adult performance, somewhat different hypothesis: thatThe data to be considered, and the stimulation in infancy reduces the generalizations therefrom, come speorganism's emotional reactivity; this cifically from research with rats and reduction is a monotonic function of mice which have received various forms amount of stimulus input. In turn, of stimulation between birth and weaning.

The Effects of Early Rearing Environment on the Development of GABAA and Central Benzodiazepine Receptor Levels and Novelty-Induced Fearfulness in the Rat

“…The classical studies of Denenberg, Levine, and colleagues showed that postnatal handling decreased the expression of fearrelated behaviors under stressful conditions (see Denenberg 1964;Levine 1975). More recent studies have shown that these effects are particularly notable under conditions of novelty (e.g., Bodnoff et al 1987;Nunez et al 1996;Escorihuela et al 1992).…”

“…The development of behavioral and endocrine responses to acute stress is greatly influenced by the early postnatal rearing environment (for reviews see Levine 1975;Denenberg 1964;Meaney et al 1996). These environmental effects persist throughout the life of the animal, resulting in stable individual differences in stress reactivity.…”

“…Postnatal handling has been shown to decrease fearfulness to novelty (Levine 1962(Levine , 1957Denenberg 1964;Bodnoff et al 1987). As adults, handled (H) rats show reduced novelty-induced suppression of appetitive behavior and increased exploration in novel environments.…”

“…As adults, handled animals showed reduced startle responsivity, increased exploration in a novel open field, and decreased novelty-induced suppression of feeding relative to the handled (H) and/or maternal separation (MS) groups. As compared with handled animals, both nonhandled (NH) and MS animals displayed: (1) reduced GABA A receptor levels in the locus coeruleus (LC) and the n. tractus solitarius (NTS); (2) reduced CBZ receptor sites in the central and lateral n. of the amygdala, the frontal cortex, and in the LC and NTS; and (3) The development of behavioral and endocrine responses to acute stress is greatly influenced by the early postnatal rearing environment (for reviews see Levine 1975;Denenberg 1964;Meaney et al 1996). These environmental effects persist throughout the life of the animal, resulting in stable individual differences in stress reactivity.…”

“…3 contribute to vulnerability to disease in later life (Seckl and Meaney 1994). The critical question, then, concerns the mechanisms for these early environmental effects on the development of behavioral and endocrine responses to stress.Postnatal handling has been shown to decrease fearfulness to novelty (Levine 1962(Levine , 1957Denenberg 1964;Bodnoff et al 1987). As adults, handled (H) rats show reduced novelty-induced suppression of appetitive behavior and increased exploration in novel environments.…”