Motivational Systems in Muroid Rodents

The terminology of the components of a motivational system follows that used in the previous neural analysis [Adams, 1979a, see especially "author's response"]. A motivational mechanism consists of a set of homogeneous neurons in a particular neuroanatomicallocation which simultaneously facilitates a number of motor patterning mechanisms. There are only a small number of motivational mechanisms, four of which are discussed in the present review, and their activity is responsible for what is commonly called the motivational state of the organism. A motor patterning mechanism is a somewhat more complicated assemblage of neurons which produces a coordinated movement, posture, autonomic effect or hormonal secretion which constitutes a basic unit of behavior, eg, the acts and postures cataloged for laboratory rodents by Grant and Mackintosh [ 1963]. The stimuli which affect motivational mechanisms and motor patterning mechanisms are divided into three categories, depending upon which of these mechanisms they directly affect. Motivating stimuli directly affect motivational mechanisms. Releasing stimuli and directing stimuli directly affect motor patterning mechanisms. Releasing stimuli trigger the all-or-none release of a preprogrammed movement; while directing stimuli provide continuous feedback and guidance of the orientation of movement, a distinction which is derived from classical ethology [Tinbergen, 1951, p 83]. The particular neurons and brain pathways involved in the neural processing of motivating, releasing, and directing stimuli are presumably different, despite the fact that the same physical stimulus which is perceived by the organism may function in several or all capacities. The time course of motivating stimuli often endures for minutes or even hours, while that of releasing and directing stimuli may be very brief, often in milliseconds. Usually, a convergence of types of stimuli upon the animal is necessary before a response may be seen: Motivating stimuli are usually necessary to activate a motivational mechanism which provides a necessary motivational input to motor patterning mechanisms; at the same time, releasing stimuli are necessary to provide a releasing input to that particular motor patterning mechanism which will produce the response. Finally, I have chosen the term motivational system to denote the entire assemblage related to a particular motivational mechanism, ie, the sensory analyzers and synthesizers of its motivating stimuli, the motor patterning mechanisms that it activates, and the sensory analyzers and synthesizers of the releasing and directing stimuli of those motor patterning mechanisms.

There is one change from the terminology employed in the previous neural analysis [Adams, 1979a]: The term "sensory filters" has been replaced by "sensory analyzers and synthesizers." The term "analyzer" was suggested by Gandelman in his commentary in the above reference and I think he was correct. The term "filter" implies that all of the sensory information is furnished by the immediate stimulus, that the animal plays a passive role, and that its behavior is totally determined by the immediate stimulus situation. Instead, the stimuli are analyzed and synthesized with memory of the results of previous activity, eg, the determination of familiar versus unfamiliar conspecific stimuli, neophobic stimuli, and perception of an escape route that depends upon an "internal map" of the environment. If anything, the present analysis underestimates the significance of developmental factors and experience and seems overly deterministic, a bias that unavoidably reflects the undeveloped state of research on flexibility and individual differences in agonistic behavior [cf Wiepkema commentary in Adams, 1979a].

A model of the organization of the four motivational systems, reflecting the form but not the anatomical detail of the hypothetical neural substrate, is presented in Figure 1. Details will be found in the appropriate sections of the text.

Despite the unity of each motivational system there may be considerable overlap and interactions between components of the systems. This will be considered in an additional section of the paper following those sections devoted to the four motivational systems.