Brain Mechanisms

This paper represents a preliminary attempt to analyze the intraspecific behavior of mammals in terms of specific neural circuitry. Despite the large number of studies and reviews on the brain mechanisms of intraspecific aggression in cats and rats, there is still no agreement on the general outline of the neural circuitry involved. Hopefully, this paper will help to frame a working model that can stimulate and help in the design of future experiments.

My own involvement in this problem began with a study in which I recorded from individual neurons during affective defense behavior in the cat (Adams 1968). Some of the results of that study were encouraging; in the midbrain central gray neurons were found that fired if and only if the animal showed affective defense. Those results were expected, since other investigators such as Hunsperger (1956) and Skultety (1963) had already established that the central gray plays a critical role in affective defense. Other results were discouraging; despite the fact that electrical stimulation of the hypothalamic ventromedial nucleus produced affective defense, the neurons of that nucleus were not active during affective defense elicited in a semi-natural situation. Puzzled, I came to the conclusion that our behavioral control was not adequate to the demands of our neurophysiological techniques.

For the past ten years I have followed a strategy of developing refined behavioral understanding that might be applied to neurophysiological experiments. I have concentrated on the behavior of muroid rodents, which are the best-studied of all mammals in terms of behavior, and which are amenable to neurophysiological techniques. I felt that a more thorough behavioral analysis was needed in order to provide the kind of functional information needed to evaluate the neurophysiological data.

In particular, I am greatly indebted to the techniques and terminology of ethology as it has been developed by Lorenz (1970), Tinbergen (1951), Leyhausen (1956), and Eibl-Eibesfeldt (1970). [See also Eibl-Eibesfeldt: "Human Ethology" BBS 2(1) 1979.] From them I have taken a critical distinction between two aspects of stimuli: motivating stimuli, which bias the organism towards the performance of a number of functionally related motor patterns such as those of sexual behavior, defense, and so forth; and releasing and directing stimuli, which trigger or orient the performance of one particular motor pattern without regard to its functional or temporal relationship to other motor patterns. Whereas the time course of the action of a motivating stimulus is usually on the order of minutes or more, the time course of a releasing stimulus may be on the order of milliseconds. In muroid rodents, and to some extent in cats, the motivating stimuli for social behavior are often olfactory, while releasing and directing stimuli are most often visual or tactile. It should be noted that this distinction is not a classification of stimuli, but a distinction between two ways in which stimulus information is processed by the nervous system. One and the same physical stimulus may function simultaneously as a releasing, directing, and motivating stimulus. For example, the vibrissal perception by one rat of the presence and vibration of the vibrissae of a second rat may release and direct a motor pattern of upright posture and, at the same time serve as a motivating stimulus to the "consociate modulator" that ensures that the rat will not lunge and bite the nose of the opponent (Kanki & Adams 1978). By "consociate," I mean a familiar individual of the same or a different species. The "modulator" (discussed further in the section on Submission) is a hypothetical mechanism for processing social cues of this sort.

Because the present analysis refers to specific, albeit hypothetical sets of potentially identifiable neurons, the terminology for these neurons does not correspond exactly to earlier ethological terminology, which was not intended to be so specific. I have continued to use the term "motivation," however. Although this terminology is not always welcome among neurophysiologists (for an exception. see Bindra 1969), an ethological analysis such as the one used here would be incomplete without referring to the concept represented by motivation, and the attendant scientific tradition seems to warrant using the word itself. In the present terminology, a critical role is played by motivational mechanisms, which are hypothetical sets of homogeneous neurons whose activity is held to be responsible for the motivational state of the animal. These are the neurons that are activated in the presence of motivating stimuli, and which are in turn responsible for the activation of a number of related motor patterning mechanisms. Motor patterning mechanisms coordinate the production of the motor patterns of the animal that are its observed or measurable behavior, including not only postures and acts, but also vocalizations, autonomic effects, and hormonal and pheromonal secretion. Motor patterning mechanisms are activated by simultaneous inputs from a motivational mechanism and from sensory filters responsive to releasing and directing stimuli specific to that motor pattern. I have coined the term motivational system to refer to the entire complex of motivating stimuli, releasing and directing stimuli, the neural mechanisms that alter all these stimuli, motor patterning mechanisms, and the particular motivational mechanisms through which they are all related.

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