NEURAL ORGANIZATION OF THE DEFENSIVE BEHAVIOR SYSTEM RESPONSIBLE FOR FEAR

This paper applies the behavior systems approach to fear and defensive behavior, examining the neural circuitry controlling fear and defensi ve behavior from this vantage point. The defensive behavior system is viewed as having three modes that are activated by different levels of fear. Low levels of fear promote pre-encounter defenses, such as meal -pattern reorganization. Moderate levels of fear activate post-encount er defenses. For the rat, freezing is the dominant post-encounter defe nsive response. Since this mode of defense is activated by learned fea r, forebrain structures such as the amygdala play a critical role in i ts organization. Projections from the amygdala to the ventral periaque ductal gray activate freezing. Extremely high levels of fear, such as those provoked by physical contact, elicit the vigorous active defense s that compose the circa-strike mode. Midbrain structures such as the dorsolateral periaqueductal gray and the superior colliculus play a cr ucial role in organizing this mode of defense. Inhibitory interactions between the structures mediating circa-strike and post-encounter defe nse allow for the rapid switching between defensive modes as the threa tening situation varies.