Brain Mechanisms

The distinction between motivating and releasing/directing stimuli is central to the motivational systems analysis presented here. Yet, despite the fact that I drew the distinction between motivating and releasing/directing stimuli from Tinbergen (1951, pp. 122-23), Leyhausen, who quotes Tinbergen in his commentary, says that he "cannot agree with the distinction between the motivating and releasing function of stimuli." Waldbillig also questions my proposal that many motor patterns do not occur until their patterning mechanisms receive simultaneous motivating and releasing/directing inputs. Waldbillig suggests that there is an alternative model consisting of what he calls "response chains," but he does not describe this model in detail in his commentary or in the papers to which he refers. The example he gives, the abolition of biting and the retention of approach following midbrain lesions in the rat, is similar to the example of selective abolition of hissing in cats with midbrain lesions that I have cited as an instance of destruction of a pathway from a motivational mechanism to one particular motor patterning mechanism. (See the fourth paragraph in the section on motor patterning mechanisms for defense in the target article.)

My proposal that motivational mechanisms are usually activated by motivating stimuli does not rule out the possibility of activation by "internal" stimuli as well. Laborit, Leyhausen, and, by implication, Ursin consider my model to depend too much upon the former and to ignore the latter. It is true that I have emphasized the role of motivating stimuli in offense, defense, and submission, but I think that this reflects the fact that intraspecific aggression is usually confined to specific and transitory environmental situations. There are at least two exceptions, however. One is aggressive play, which is not considered in this review. The other is the offense component of maternal aggression, which I have argued is activated by prolactin in the absence of motivating stimuli (but cf. Gandelman commentary for a contrary view). Other motivational systems may be more dependent upon internal stimuli. I have argued elsewhere (Adams, submitted for publication) that the activation of the motivational mechanism of exploration/marking can occur from hormonal stimulation alone, that is, in the absence of motivating stimuli. Therefore, in Leyhausen's terms, I do not consider myself to be "one of the last believers" in a rigid stimulus-response model of behavior.

Motor patterns, according to the analysis I present here, may be "ambivalent," that is, activated by more than one motivational mechanism. This can lead to some logical confusion. If an animal flees or adopts an upright posture, the observer cannot determine from that alone whether its defense or submission motivational mechanism is activated. This explanation is in reply to the question of Miczek about the significance of flight and of Rodgers concerning the upright posture.

The proposal that there may be a "master switch," located perhaps in the paleocerebellum, that chooses among contradictory motor patterns, has been elaborated by Berntson. Although I would emphasize motor patterns rather than motivational mechanisms, I appreciate not only the discussion of his own original data, but also the clear analysis of the existing literature. It was in a personal discussion with him that I became sufficiently emboldened with the idea to commit it to print. Berntson's commentary is the best reply I can give to the disbelief expressed by Baenninger that there might be one locus in the brain with such a powerful function.

(End of section)