Brain Mechanisms

Cerebral play of forces in offensive-defensive mechanisms. Dr Adams's excellent review of the specific neuronal circuitry of intraspecific aggressive behavior in mammals is most useful. I would, however, question his main working hypothesis about the anatomical identification of three motivational systems for offense, defense, and submission, and propose instead the involvement of a constellation of brain structures, with a continuous dynamic equilibrium, in the control of the various manifestations of agonistic behavior.

To locate a defense motivational system in the midbrain central gray is controversial, because offensive-defensive responses depend on the play of forces in neuronal pools located not in one but in several structures, including the central gray, the tectum, some thalamic nuclei, and the amygdala, plus inhibitory influences from the septum, the caudate nucleus, parts of the amygdala, and other cerebral structures.

Electrical stimulation, chemical injections, and localized destructions of the brain may act upon the equilibrium of the whole system, which is also influenced by its past history, by present sensory inputs, and by individual interpretation of received information, We should not place functional labels on anatomical areas without taking into consideration the environmental context. Research should be oriented toward evaluating the role of specific structures and factors involved in each type of offensive-defensive reactivity . Adams's reference to " ... points in the forebrain where there are inhibitory interactions between the defense and submission systems " appears to be in agreement with the idea of constellations and not centers.

Another controversial issue is Adams's concept of the "consociate modulator which switches the behavior of the animal when its opponent is familiar, from defense to submission," In our experience the essential factors determining an animal's response are hierarchical position and social context, not familiarity. This has been demonstrated in monkey colonies where the occurrence, direction, and expression of aggressive responses elicited by aversive radio stimulation of the brain or by radio-controlled skin shock depended on the social rank of the stimulated monkey, which would attack submissive monkeys but showed no aggression in the presence of a dominant partner. Exactly the same brain stimulation elicited aggression or submission, depending on the presence of a low- or high-ranking partner (Plotnik, Mir, and Delgado 1970).

In other experiments with gibbons, electrical stimulation of the same point in the central gray in the same animal produced aggressive behavior if the animal was in a laboratory colony but not when tested in a tree ecological situation on the island of Hall. In this last case the stimulated animal would run out of sight into the bushes without showing hostility against other gibbons. These studies also demonstrated the establishment of dynamic equilibrium between excitatory central gray and inhibitory caudate nucleus stimulations, with relative predominance dependent on relative strength of stimulation (Delgado, Sanguinetti, and Mora 1973).

In our opinion, e1ectrical stimulation of the brain acts as a trigger of preestablished cerebral functions, influencing the processing of sensory inputs and the release of previously acquired motor patterns and experiences. In some experiments the motor effects elicited by brain stimulation may be considered as fragments, often involved in offensive-defensive behavior, but lacking negative reinforcing properties (Delgado 1967).

As a working hypothesis we have proposed that emotional expression, including offense and defense, is composed of groups of fragments that comprise autonomic responses, vocalization, facial expression, and both tonic and phasic motor activity. Depending on the cerebral area stimulated, these fragments may be evoked as isolated or as organized sequences. Emotional states depend on the activation of constellations of brain structures determined by the decoding of specific sensory inputs. Brain stimulation may produce similar activation, allowing the artificial manipulation of emotions as well as of emotional reactivity (Delgado and Mir 1969).

Some of these ideas may coincide with Adams's distinction between motivating, releasing, and directing stimuli, with the difference that the distinction is not in the stimuli but in the cerebral frame of reference and in the social context, which may be decisive for the interpretation of sensory inputs. Hopefully Adams's thought-provoking review will provide the basis for further studies to increase our understanding of the brain mechanisms involved in offensive, defensive, and submissive behavior.