chewed if it were hungry; and hungry animals are clearly guided by environmental cues as they search for food. In fact, any behavior must be caused by some combination of both internal and external factors.
Many years ago, Lorenz proposed a motivational model of behavior that illustrates the interdependence of internal and external factors. This model is shown in Figure 3.2. According to Lorenz, each behavior pattern (motor mechanism) is associated with a reservoir that can hold a certain amount of energy. Whenever the behavior pattern occurs, energy is used up; but when the behavior pattern does not occur, energy can build up in the reservoir. The higher the level of energy, the more pressure it exerts on the valve. When the value opens, as a result of external stimulation, energy is released and the behavior occurs. Thus, in this model, a particular behavior pattern cannot occur without at least some internal causal factors as well as some external ones. Further, the model makes it clear that internal and external factors can substitute for each other in determining the intensity of a behavior pattern: a strong stimulus can compensate for weak internal factors and vice versa.
Figure 3.2 Lorenz’ model of motivation. The tap (T) supplies a constant flow of endogenous energy to the reservoir (R). The valve (V) represents the releasing mechanism and the spring (S) the inhibitory functions of the higher coordinating mechanisms. The scale pan (Sp) represents the perceptual part of the releasing mechanism, and the weight applied corresponds to the impinging stimulation. When the valve is open, energy flows out into the trough (Tr), which coordinates the pattern of muscle contractions. The intensity of the response can be read on the gauge (G). (From Lorenz 1950).
The fact that both internal and external factors are essential for any behavior to occur does not imply, of course, that one cannot study the effects of internal and external factors separately. The effects of varying various internal factors can be determined if the external situation is kept relatively constant, as can the effects of external factors if the internal state of the animal is held constant. A classic example of such a study is the one by Baerends et al. (1955) on the courtship behavior of the male guppy (Lebistes reticulatus). Courtship by the male comprises a number of behavior patterns, including posturing in front of the female, a special sigmoid posture, and copulation attempts. These authors were able to derive a scale of internal motivation using the relation of marking patterns on the body of the male to the number of copulation attempts; external stimulation was considered to be proportional to the size of a female. Figure 3.3 shows the results of an experiment in which females of different sizes were presented to males at different levels of internal motivation. The points plotted on the graph represent the relationship between the measures of internal and external stimulation at which particular patterns of behavior were observed. If it is assumed that the total motivation necessary for a specific behavior pattern to occur is always the same, the patterns “posturing,” “sigmoid intention,” and “sigmoid” can be seen to represent increasing values of courtship strength. The lines connecting these points of equal motivation have been called motivational isoclines by McFarland and Houston (1981).
Figure 3.3 Results of an experiment on guppy courtship. (a) Relationship between the intensity of the external stimulation, the intensity of the internal stimulation, and the kind and degree of development of the resulting activity. (b) “Calibration curve” for determining the place of the different marking patterns on the abscissa of (a). CA, copulation attempt; S, sigmoid posture; Si, sigmoid intention; Pf, posturing in front of the female. (From Baerends et al. 1955).
Specific versus general effects of causal factors
Ever since the psychologist Hull (1943) postulated a general drive, a recurring question in the study of motivation has been whether causal factors have general or specific effects. Does a hungry dog merely eat its food more quickly and accept less preferred foods more readily, or does it also attack a stranger more fiercely and copulate more vigorously? There is evidence to support either point of view, but common sense suggests that some causal factors are likely to have broad effects whereas others will have only limited effects. A man who is worried about difficulties at work may show exaggerated or even inappropriate responses in feeding, aggressive, and sexual situations. On the other hand, the same man will probably only drink an extra glass of water if he has lost more body fluid than usual on a warm dry day.
In general, any particular causal factor will most likely have both specific and general effects; which effects are more important will depend on the question of interest. For example, specific effects of causal factors are implied in Lorenz’s model of motivation. The model posits that the fluid in the reservoir is specific to the particular behavior pattern with which it is associated: Lorenz spoke about action-specific energy . On the other hand, the circadian clock will be seen to have an important influence on many behavior systems. I will examine specific and general effects of causal factors in some detail in the section on displacement activities.
Central versus peripheral locus of action
A fourth pervasive issue in motivation concerns the locus of action of causal factors. Do causal factors operate within the central nervous system (CNS) or at a more peripheral level? Once again, common sense suggests that they must act in both places; nonetheless, this has also been a controversial issue. Historically, the controversy arose as a reaction by the early behaviorist school in psychology to the views of the introspectionists, who thought one could understand behavior by reflecting on one’s own experiences (see Chapter 1). The behaviorists were skeptical of internal causes that could not be investigated directly, and they attempted to explain as much behavior as possible in terms of stimuli and responses that could be measured physically. However, as it has become more and more possible to measure and manipulate events that occur within the CNS, one major objection to the postulation of central factors has been removed. Nonetheless, some researchers continue to emphasize central or peripheral factors.
Causal Factors
Motivation is concerned with the factors that control the activity of the behavior mechanisms of the individual. These factors are generally considered to be stimuli, hormones and other substances in the blood, and the intrinsic activity of the nervous system. Each of these factors will be briefly discussed.
Stimuli
Stimuli can control behavior in many ways: they can release, direct, inhibit, and prime behavior. Chapter 2 discussed many examples of stimuli that release and direct various behavior patterns. Some stimuli can have exactly the opposite effect: rather than facilitate behavior, they inhibit it. A good example is provided by the nest-building behavior of many species of birds. Birds typically build their nests using specific behavior patterns. The stimuli that release and direct their behavior have been studied in several cases and conform to the general principles already discussed. However, at a certain point the birds stop building and no longer react to the twigs, lichens, or feathers with which they construct their nest. There are several possible reasons why they stop, but one reason is that the stimuli provided by the completed nest inhibit further nest building. This can be seen when a bird takes over a complete nest from the previous season and shows very little nest-building behavior. Other birds, in the same internal state, that have not found an old nest show a great deal of nest-building behavior (Thorpe 1956).
Another example of the inhibitory effects of stimuli is seen in the courtship
