The neurobiology of conditioned and unconditioned fear: a neurobehavioral system analysis of the amygdala.

A neurobehavioral system approach to conditioned and unconditioned fear is presented. By employing reproducible fear behaviors in Pavlovian conditioning and unconditioned fear paradigms, it has been possible to delineate some differences in neural circuitry and cellular biology for conditioned and unconditioned fear. It is suggested that the basolateral complex of the amygdala and the central nucleus of the amygdala are part of the neural circuitry for fear conditioning but not for unconditioned fear to a predator odor. Furthermore, changes in expression of the transcription factor early growth response gene 1 in the lateral nucleus of the amygdala are shown to be important for contextual fear conditioning but not for unconditioned fear to a predator odor. In addition, data suggest that although conditioning to a synthetic predator odor, trimethylthiazoline, has been difficult to demonstrate, conditioning can occur by modifying by the environment. Finally, the relevance of the animal studies to human anxiety disorders is discussed. Copyright 2004 Sage Publications