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The least known aspect of the neural mechanism for aggression consists of the locus and functioning of the motor patterning mechanisms. The locus of these mechanisms could be determined by tracing the projections from the various motivational mechanisms using Nauta degeneration or autoradiographic histological procedures. Discrete lesions in areas to which these fibers project would be expected to eliminate specific motor patterns of the various motivational systems while leaving others intact. The time is coming when it should be possible to link our knowledge of sensory processes and motivational systems on the afferent side of the brain with the growing knowledge of motor systems such as those of locomotion (Shik & Orlovsky 1976; Orlovsky & Shik 1976).
Releasing and directing stimuli for the motor patterning mechanisms have also received too little attention. The method of research by which we have investigated the releasing and directing stimuli for the upright posture (Kanki & Adams 1978) could be used to investigate similar stimuli for other motor patterns as well.
A major question remains that of the functional significance of the forebrain pathway for defense. Does it reflect motivating stimuli of defense pheromones, neophobia, unfamiliar conspecific opponents, or still other motivating stimuli? This question could be answered by lesion studies on suitable behavioral preparations in which defense could be elicited reliably in response to the various motivating stimuli listed above. Since these stimuli are not usually effective in laboratory rodents raised under standard conditions, either wild animals or laboratory rodents raised under special conditions should be used. Raising animals with nest boxes (Clark & Galef 1977) might provide suitable experimental subjects.
Following successful lesion experiments on defense motivating stimuli, it should be feasible to employ single-neuron recording to investigate the neural basis of aggression. The principle challenge is to develop behavioral preparations that respond with defense, submission, and offense to various motivating stimuli in rapid succession while fitted with chronic recording microelectrodes (see Adams 1968; Pond et al 1977). In this way it should be possible to determine the precise functional neural circuitry of the septum, amygdala, hypothalamus, and midbrain central gray.
Ideally, by means of experiments that combine behavioral techniques, chronic microelectrodes and chronic stimulating electrodes, it should be possible to classify neurons on the basis of both behavioral and physiological characteristics; let us suppose, for example, that neurons in the ventromedial hypothalamus that fired maximally during submission were also found to be those that respond to central gray stimulation. Providing that the physiological characteristics were not abolished by certain anesthetics (and this could be tested by giving anesthetics to a functioning chronic preparation), it would then be possible to conduct acute neurophysiological experiments on neurons with known behavioral functions, and thereby to extend our knowledge far beyond its present confines.
The study of the brain mechanisms of aggression could gain a great deal from new developments in the understanding of basic mechanisms of olfaction. Although it appears from behavioral research that the olfactory qualities of familiar versus unfamiliar stimuli, and qualities of pheromones based on the presence or absence of gonadal hormones in the opponent, are critical in motivating stimuli for social behavior, the neural mechanisms for these processes are not known at all.
An evolutionary perspective would be strengthened by data on sequences of motor patterns related to aggression in opossums and primates obtained in experimental paradigms such as we have used on the rat (Lehman & Adams 1977). Such data might help determine whether these species have motivational systems of offense, defense, and submission homologous to those of cats and rats. At the present time several of us are conducting such experiments in stumptail macaques, and, hopefully, other species will be studied in other laboratories as well.
Finally, there is a possibility that there is a brain mechanism, not yet studied to any extent, by which the organism chooses among possible motor patterns that might be performed at any given instant. To some extent the activation of a motor patterning mechanism is presumably due to activation of motivational mechanisms and the presence of the requisite releasing and directing stimuli. It may be possible for more than one motor patterning mechanism to be active simultaneously. Thus, for example, vocalization, locomotion, and piloerection may accompany a number of postures in the rat and cat. But, beyond this there appear to be situations in which there is a sharp discontinuity among the activations of related motor patterning mechanisms such as those of freezing, fleeing, and lunge-and-bite attack during defense in muroid rodents. It is possible that there may be a "master switch" in the brain for such motor patterns that ensure that only one is activated at a time. Where would one look for such a "master switch?" I would suggest looking in old parts of the cerebellum that appear to receive projections from and send projections to most or all of the motor initiation centers of the brain (Nieuwenhuys 1967).
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