Brain Mechanisms

My remarks will address three types of fundamental issues that are raised by the commentaries: (1) the general conceptual framework and terminology of motivational systems as I have defined them; (2) other levels of analysis that are alternatives to the neuroethological analysis employed here; and (3) specific questions concerning the offense, defense, and submission systems.

On the most general level, I am gratified that some of the commentators, especially the Blanchards, Miczek, and Panksepp, seem to agree that a classification of behaviors ought to be based upon the neural circuitry involved. This has not been a common view in the past.

A "neural" classification of behaviors, such as I propose, does not necessarily correspond to traditional logical distinctions, as Miczek points out. Nor does it necessarily provide an exhaustive classification of behaviors that have been grouped together traditionally. For example, Brain, in the useful schema he provides in his commentary, includes categories of predatory aggression and reproduction termination that are not included in my classification. And, as Brain notes, submission is not a category of aggression, although it is related to aggressive behaviors. However, I am pleased to see that there is considerable correspondence between his "self-defensive" category and my "defense," his "social aggression" and my "offense," and his "maternal aggression" with what in my classification consists of both offense and defense. I am also pleased to see considerable correspondence between my categories and those of Ursin. Ursin distinguishes fear, defense, attack, and prey-killing. These appear to correspond to submission, defense, offense, and predatory aggression in my terminology.

Conceptual framework and terminology of motivational systems. I have provided a new and, I hope, clearly defined terminology for a conceptual framework for the brain mechanisms of social behavior. These terms include "motivational system," "motivational mechanism," "motivating stimuli," releasing and directing stimuli," "motor patterning mechanism," and "motor pattern." The commentators are free, of course, to disagree with this choice of terminology, but it is also important that my own use of it should be represented accurately.

There is considerable confusion in the commentaries in the use of the terms "motivational mechanism" and "motivational system." I have arbitrarily defined a motivational mechanism as a hypothetical set of homogeneous neurons responsible for the motivational state of the animal (Lehman and Adams 1977) and I have in the present target article supported this definition with data on a hypothetical defense motivational mechanism. In the terms of the Baenninger commentary, it is a "neural center" with one function and one anatomical locus. In terms of Isaacson's question, such a mechanism is hypothesized to be homogeneous in all of his categories: anatomical structure, input, output relations, biochemical nature, and responsiveness to circulating neuromodulators. I have arbitrarily defined a motivational system, on the other hand, as a more complex assemblage of neural mechanisms involving many anatomical structures, of which a motivational mechanism is only one component, albeit the defining component. In Figures 1, 2, and 4, the motivational mechanisms are shown as one box per figure, while each motivational system consists of the entire figure. Many commentators do not accept this distinction; instead, they often use the term "motivational mechanism" to represent all or part of what I have called the "motivational system." Thus, Decsi & Nagy say that a defense motivational mechanism is "not only there," meaning in the midbrain central gray. Delgado and Isaacson do not follow my definitions when they maintain that a defense motivational system is not confined to the central gray, and similarly, Karli cannot accept the idea that a motivational mechanism could be limited to the central gray. Koolhaas suggests that a motivational mechanism must be represented by many structures in the brain. Leyhausen asserts that motivational systems are far from being unitary or homogeneous; although he uses the word "system," he appears to be criticizing my concept of "motivational mechanism." Wiepkema also cannot accept the idea that forebrain structures fail to form an essential part of a defense motivational mechanism.

There are two sources for the confusion surrounding the use of the terms "motivational mechanism" and "motivational system," one factual and one semantic. The factual question can only be answered by future research. Are there homogeneous sets of neurons in a single anatomical locus responsible for the motivational state of the animal? I have hypothesized and given data to support the existence of such neurons. 'The semantic question is also important, however. If the existence of such neurons is confirmed; should they be called a motivational mechanism? Many of my commentators obviously wish to use this term for more complex neural assemblages. What term would they like to use for a homogeneous set of neurons? I am not dogmatically committed to "mechanism," but I cannot, at the moment, think of a better term.

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