In 1899 Darwin pioneered the effort to describe some of the complex interactions between genetic, reflexive, physiological and experiential factors that control individual differences in emotions and their behavioral manifestations. In recent years some of these interactions have been clarified by means of selective breeding experiments with rats and mice, and, at the molecular level, by means of knockout and other physiological and genetic techniques. However, even with our most sophisticated genetic techniques, explication of the genetic and other factors that influence emotion depends on how we measure the expression of emotion in the behavior of our nonverbal animal models.

In this paper we present experiments, which use the open-field and conditioned-suppression (CER) paradigms, to study innate and conditioned fear, respectively. We compare the selectively bred Syracuse High- and Low-Avoidance (SHA/Bru and SLA/Bru) strains of rats, which differ genetically in their expression of fear and anxiety. The experiments demonstrate how our measurement techniques determine whether or not genetic differences are detected. Specifically, the traditional open-field procedure was used to establish the unlearned genetic difference in emotional defecation between animals of the SHA/Bru and SLA/Bru strains, without the artifact of differences in level of locomotor or other activity. However, the parameters of the CER paradigm determined whether or not those genetic differences are expressed in conditioned fear. We also show that differences in the morphology and physiology of the hypothalamic pituitary-adrenocortical (HPA) system are closely linked to the phenotypic difference in avoidance learning and the associated differences in innate and conditioned fear. These morphological and physiological differences may mediate those differences in the expression of fear. Corticotropic-releasing hormone (CRH) is known to activate the HPA axis and to have independent anxiogenic behavioral effects, e.g., enhancement of conditioned suppression. We also show that whether or not manipulation of CRH results in the predicted behavioral differences in emotional expression in the Syracuse animals is a function of the parameters of the CER procedure.

These experiments indicate that the genetic difference in expression of conditioned fear in response to exogenous CRH depends on the presence of behaviorally measurable conditioned fear, which, in turn, depends on the parameters of the conditioning paradigm.

The implications of these findings for further research on emotion are explored, and ways of improving the reliability, validity and sensitivity of some of the established techniques for measuring emotion are suggested.

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