The predation and defense, which were undisturbed after the lesions, served as behavioral controls, indicating that there was no simple motor incapacity that interfered with the performance of offense. Predation includes approach, chasing, and biting attack similar in many respects to the bite-and-kick attack of offense. Defense included a defensive upright posture that is quite similar to offensive upright posture.
For offense and predation, each animal's pre-operative results served as behavioral controls, the offense being abolished and the predation remaining unchanged. With regard to defense, a separate series of tests were conducted on unlesioned rats of the same strain and housing conditions and tested in the same shock-testing apparatus. The range of thresholds was similar, being 0.18 to 0.75 mA for the lesioned animals and 0.35 to 0.75 mA for the unlesioned controls.
By fulfilling the prediction that brain stem lesions should be found that would abolish offense rather than other types of aggressive behavior, these results support the motivational systems analysis upon which the prediction was based [2]. The motivational systems analysis suggests that offense, defense, and predation represent three distinct systems organized around discrete sets of neurons, called motivational mechanisms, whose activation is responsible for the motivational state of the organism. The model for this analysis comes from defense, where the motivational mechanism was said to be located in the midbrain central gray, a location where: (1) lesions permanently abolish defense; (2) electrical and chemical stimulation produce defense; and (3) single neurons are differentially active during defense [2]. More recent data have shown that offense is not abolished by midbrain central gray lesions [16]. Although predation is interrupted by central gray lesions, the interruption affects only the final killing bite and not the other motivated components of the behavior such as orienting, tracking, and lunging towards the prey [20].
Further research is needed to determine if the ventromedial tegmentum contains the neurons of a motivational mechanism for offense, in the same sense that the central gray appears to have such a mechanism for defense. The fact that brain lesions of the ventromedial tegmentum abolish offense and not other types of aggression is one of the necessary criteria, but others remain untested as yet. Is the lesion effect permanent? Can offense be elicited by electrical and chemical stimulation of the ventromedial tegmentum, and are single neurons differentially active in that region during offense?
In conclusion, it is now possible to distinguish offense, defense, and predation on the basis of brain stem lesions that differentially abolish these behaviors. Both offense and defense can be abolished by discrete lesions in the ventromedial tegmentum and midbrain central gray, respectively, that do not abolish the other types of aggression. It remains to be determined if predation can be abolished by a brain stem lesion that does not disturb offense or defense. Temporary or partial abolition of predation has been reported as a result of various brainstem lesions, but none have been shown to abolish predation totally and permanently [5, 6, 18, 20].
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