Forty years ago, Stellar (1954) proposed his ‘two-centre’ theory of motivation, according to which actions such as eating, drinking, and copulating are under the control of mutually antagonistic hypothalamic centres. Like all good review papers Stellar’s article highlighted what was not known as much as what was known at the time. Although he was writing about the hypothalamus Stellar was acutely aware that, whatever role it had in motivation, it acquired that function as a result of being located in a much larger system. That system had to include a ‘final common path for behaviour’, inputs from exteroceptive systems, inputs from the internal environment, and inputs from the neural mechanisms underlying cognitive function; but Stellar was unable to specify these in detail. Thanks to the diligence of all those studying motivation and to advances in technique our understanding of organization of this wider neural system has since flourished. We now know, for example, something of how visual information about food is processed in the brain, and the sites at which this information could be integrated with olfactory and visceral information. We also know more about the brainstem mechanisms controlling the motor side of eating.
In addition to the things known by Stellar, we have also acquired an impressive body of information about systems not included in his model. Of these the most significant is the existence of a neural system within which chemical or electrical stimulation can mimic many of the reinforcing effects of natural rewards such as food, water, or sexual contact. The phenomenon of intracranial self-stimulation has revolutionized the way psychologists and physiologists think about motivation since it was first described by Olds and Milner (1954).
There have also been major advances in the way in which psychologists think about motivation. Stellar’s model was significant for incorporating cybernetic concepts. The two hypothalamic centres he described were part of a negative feedback loop that was completed by the impact of the final common path for behaviour on the internal and external environment. The assumption behind this approach is that the link between the hypothalamic excitatory centre and the final common path for behaviour is direct, and that motivated behaviour is somehow switched on and off by the waxing and waning of the motivational state. So long as psychologists and physiologists contented themselves with studying the eating behaviour of rats presented with a single food (p.viii) source in a monotonous environment with no competing demands on their time or energy, this is a reasonable simplification but it does not do justice to the complexities of human motivation. Psychological research over the past 40 years has confirmed what is enshrined in our day-to-day language, that motivation involves a disposition to act rather than the activation of specific behaviour. Moreover, the disposition to act is translated into behaviour through a process of choice. Except in pathological circumstances we choose whether to eat, drink, or have sex. Having made that basic choice, we then choose how we will indulge ourselves. The indirectness of the link between what Morgan (1960) called the ‘central motive state’ and the motivated choices we make is encapsulated in the term ‘appetite’.
This book was conceived as a celebration of what we have learned about motivation since the publication of Stellar’s paper. It combines chapters that review our current knowledge of the psychological bases of appetitive behaviour with those that review recent advances in our understanding of the neural systems involved. Wherever possible, the authors were asked to focus on work on appetitive processes in humans or, where that was not available, to consider the implications of work on other species for appetitive processes in humans. We have intentionally defined ‘appetite’ in the loosest possible way in order to encourage debate about the nature of appetite, particularly about whether there are common mechanisms underpinning the diverse appetites we experience or whether it is more appropriate to talk about appetites.
Morgan, C.T. (1960). Physiological theory of drive. In Psychology: A study of a science (ed. S. Koch). McGraw-Hill, New York.
Olds, J. and Milner, P. (1954) Positive reinforcement produced by electrical stimulation of septal area and other areas of rat brain. Journal of Comparative and Physiological Psychology, 47, 419–27.
Stellar, E. (1954). The physiology of motivation. Psychological Review, 61, 5–22.