Effective defense against natural threats in the environment is essential for the survival of individual animals. Thus, instinctive behavioral responses accompanied by fear have evolved to protect individuals from predators and from opponents of the same species (dominant conspecifics). While it has been suggested that all perceived environmental threats trigger the same set of innately determined defensive responses, we tested the alternate hypothesis that different stimuli may evoke differentiable behaviors supported by distinct neural circuitry. The results of behavioral, neuronal immediate early gene activation, lesion, and neuroanatomical experiments indicate that the hypothalamus is necessary for full expression of defensive behavioral responses in a subordinate conspecific, that lesions of the dorsal premammillary nucleus drastically reduce behavioral measures of fear in these animals, and that essentially separate hypothalamic circuitry supports defensive responses to a predator or a dominant conspecific. It is now clear that differentiable neural circuitry underlies defensive responses to fear conditioning associated with painful stimuli, predators, and dominant conspecifics and that the hypothalamus is an essential component of the circuitry for the latter two stimuli.defensive behavior ͉ dorsal premammillary nucleus ͉ periaqueductal gray A nimals have evolved a set of basic, genetically preprogrammed physiological responses and behaviors to ensure survival of individuals and of the species as a whole. Effective defense against threats in the environment is one obvious function essential for survival of the individual and is coordinated by the brain. A simple explanation is provided by the species-specific defensive reaction (SSDR) theory, which postulates that the same innately determined defensive behaviors (like freezing and flight) are triggered by all perceived environmental threats-from natural predators to electric foot shocks in a laboratory protocol (1). This seemingly limited response repertoire suggested the plausible hypothesis that an animal's defensive behavior system processes cues about predators and an artificial threat in the same way and has led to a unitary view of the neural network mediating fear responses, with the central amygdalar nucleus playing a critical role in linking fear processing and defensive responses (2). However, it is also possible that differentiable mechanisms are engaged. Animals are naturally selected to protect themselves from dangers associated with the presence of a predator or a dominant conspecific, which evokes the sensation of fear and associated behavioral responses (3), whereas it is reasonable to postulate that, in contrast, physically harmful stimuli alone may evoke pain with or without fear.Fear responses to predators or dominant conspecifics are comparable to other forms of goal-oriented behavior like feeding, drinking, and mating in the sense that they appear to be accompanied by strong motivation or drive followed by behaviors critical for maint...

Aggressiveness has a high prevalence in psychiatric patients and is a major health problem. Two brain areas involved in the neural network of aggressive behavior are the amygdala and the hypothalamus. While pharmacological treatments are effective in most patients, some do not properly respond to conventional therapies and are considered medically refractory. In this population, surgical procedures (ie, stereotactic lesions and deep brain stimulation) have been performed in an attempt to improve symptomatology and quality of life. Clinical results obtained after surgery are difficult to interpret, and the mechanisms responsible for postoperative reductions in aggressive behavior are unknown. We review the rationale and neurobiological characteristics that may help to explain why functional neurosurgery has been proposed to control aggressive behavior.