Brain Mechanisms

Neural circuitry for motivational systems. Specifying the sets of neurons and interconnections that integrate motivated behaviors furnishes a useful framework for further considering the neuropsychological basis of aggression. This commentary will discuss the defense, submission, and offense circuits of the septum and amygdala and their connections, and particularly the effects of localized lesions within these systems.

Defense. The exclusion of the septal area from the diagram for the neural circuitry for defense is surprising, in view of the well known hyperdefensiveness of septal rats toward their human experimenters. Perhaps the exaggerated form of the reaction or the lack of data on the motivating stimuli for the septal syndrome precludes inclusion of these behaviors as part of the animal's "natural" behavioral repertoire. The following experiment gives food for thought concerning septal hyperreactivity and the stimulus properties that evoke it.

Max, Cohen, and Lieblich (1974) noted the reactions of septal rats when approached from above or below by the gloved hands of the experimenter. In rats approached from below, resistance to capture, vocalization, and biting occurred much less frequently than in those approached from above. These authors interpreted approach from above as representing the sort of stimulation that might come from a dangerous predator, or, in Adams's terminology, dorsal tactile stimulation capable of activating the defensive motivational system in muroid rodents. According to this interpretation, the septum might be viewed as a sensory link between peripheral sensory stimulation and the midbrain defense motivational mechanism.

A somewhat more complicated role for the septum may be inferred from its relation with the amygdala. King and Meyer (1958) discovered a reciprocity between the two areas, in that septal hyperreactivity was totally abolished by a subsequent lesion of the amygdala. Perhaps this experiment should now be repeated, but in a more elegant variant made possible by the proposed neural circuitry for defense. The replication and extension would involve making lesions in the ventromedial amygdala (defense zone), or in the perifornical hypothalamus, in hyperreactive septal. rats, to determine whether interruption of the pathways facilitating defense would block !he septal syndrome. Several cautions are indicated in an experiment of this sort. First the amygdalo-hypothalamic defense pathway is apparently inferred from data in cats, not rats. Second, while the amgydala defense/zone may be identified using electrophysioiogical criteria, later research (Ursin 1965) involving lesions of the defense zone in cats did not result in reduction of defensive behavior. Finally, lesions of the perifornical hypothamus that abolished defense reactions from the amygdala stimulation in one experiment (Hilton and Zbrozyna 1963) appeared to encroach upon the lateral hypothalamic-medial forebrain bundle area. The latter area, when damaged in rats, has been found to severely limit the expression of all forms of behavior (LHA syndrome).

Defense and submission. The nature of the influence upon consociate modulation of the ventromedial hypothalamus may also be considered in connection with septal and amygdaloid functions. Jonason and Enloe (1971) found that pairs of septal rats, following an initial period of fighting, spend significantly more time on socially cohesive behavior in an open field than did normal control pairs. Amygdaloid-amygdaloid pairs averaged about one-half the contact time of normals. Thus it appears that septal rats display defense reactions to conspecifics in this situation but then settle down to a more docile contact-seeking form of behavior, whereas rats with amygdaloid lesions show no defense and much less contact-seeking behavior.

Does contact-seeking between rat pairs in an open field represent submissive behavior? To this commentator's knowledge, a detailed analysis of posturings between pairs of septal or amygdaloid rats has not been published. One might assume that obvious submissive encounters are not likely to be seen as consequences of attacks, since very few attacks indeed occur in this situation. Meyer, Ruth, arid Lavond (1978) have presented an interesting analysis of contact-seeking behavior that may be relevant to the present discussion. Recall from Adams's model that familiarity with the individual is a prerequisite tor consociate submissive behavior. Meyer, Ruth, and Lavond maintain that "because septal rats are likely to fight when they are first put into the open field, [they] require substantial periods of time to establish their social contacts" (p. 1028). Furthermore, based on the data with septal rat pairs and with septal rats, choices among other rats, furry rabbits, or cats, or nothing, Meyer, Ruth, and Lavond have concluded that social cohesiveness in the septal rat may represent the release of a contact-comfort motive akin to that postulated by the Harlows for infant monkeys [see Rajecki et al "Toward a General Theory of Infantile Attachment" BBS 1(3) 1978]. To return once again to the combined-lesion paradigm, consociate behaviors of septal rats should be abolished by a subsequent lesion to the ventromedial hypothalamus (consociate modulator). The specific effect should either be a failure to give up defensive fighting or flight (septal-lesion effect) or a tendency to engage in less contact-seeking behavior (amygdaloid-lesion effect).

Offense. Paradoxically, septal rats appear to be neither offensively aggressive nor contact-seeking in reaction to a strange intruder in a colony situation (Blanchard et al 1977b). Whether hippocampal lesions would produce a similar effect, due to the loss of spatial recognition, as Adams suggests, is a question awaiting investigation. Surgical interruption of hippocampal-septal connections by fornix lesions prevents the occurrence of hyperdefensiveness to handling when septal lesions are subsequently made in the same animal (Olton and Gage 1974). Whether fornix lesions would also block hypo-offensiveness in septal rats would seem a paradox on a paradox and is not reasonable to expect if spatial appreciation of territory is an essential element of the offense motivational system.