The Aggression Systems
Brain Mechanisms of Aggression - Introduction Page 16

Table of Contents


Preface Pages 1 - 2


Human aggression - introduction Pages 3 - 4 - 5 - 6 - 7 - 8


Evolution of aggression - introduction Pages  9 - 10 - 11 - 12 - 13 - 14


Brain mechanisms of aggression - introduction Pages 15 - 16 - 17 - 18 - 19 - 20


Dynamics of aggression - introduction Pages 21 - 22 - 23


In summary, it is now possible to distinguish offense, defense, and predation on the basis of lesions that differentially abolish these behaviors which indicates that each is based on a fundamentally different brain organization. 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 seen if a specific lesion can be found that will abolish predation without abolishing offense or defense.

It is proposed that at the heart of the brain organization of offense and defense are two sets of neurons that I have called the "offense motivational mechanism" and the "defense motivational mechanism." Each of these mechanisms are presumably located in a particular place in the brain and the neurons within each are interconnected by facilitative synapses so that the mechanism becomes active as a unit. The degree of their activity determine the strength of that particular motivation at a particular moment. The inputs to the set of neurons are assumed to come from the various analyzers and synthesizers of motivating stimuli, while their outputs go to the motor patterning mechanisms appropriate to that motivational system.

Our most recent data, described in the preceding section on evolution, distinguish competitive and territorial fighting as two types of offense. This suggests that the offense motivational mechanism is subdivided into two similar, but independent sub-units.

Brain research presented here supports the hypothesis that there is a defense motivational mechanism located in the midbrain central gray, as discussed in the Author's Response in the paper published from The Behavioral and Brain Sciences: 1) Defense may be elicited by both electrical and chemical stimulation of the midbrain central gray (for electrical stimulation data, see the following paper on single cell activity in the cat). 2) Single neurons are active during defense in the midbrain central gray. 3) The behaviors of defense are all abolished by total lesions of this brain area, as shown in the paper reprinted here. 4) An animal will no longer learn a response to escape from stimulation of this area. And 5) pharmacological manipulations such as those involving morphine have their effects upon defense in this brain area.

As for the offense motivational mechanism, it may be located in the ventromedial tegmentum, since lesions of that region abolish offense that occurs in isolated male rats and mice according to the paper reprinted here. Offense can be produced by chemical stimulation of the anterior hypothalamus (see paper reprinted here). However, according to further unpublished work from our laboratory, offense was not abolished following lesions of this area. Further studies will be needed to establish the existence and location of the offense motivational mechanism.

According to the motivational systems analysis it should be possible to distinguish the brain organization of two types of stimulus analyzers and synthesizers: (1)those of motivating stimuli should influence the motivational mechanism directly and should influence all the motor patterning mechanisms of the system indirectly in an equal way; while (2) those of releasing and directing stimuli should influence individual motor patterning mechanisms directly and discretely - without having any influence upon other motor patterning mechanisms of the system.

(Continued on next page)

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