I conjecture that the answer lies in a fashionable conception of the relation between mind and body. There are several competing philosophical theories about that relation. Let us accept, for the sake of argument, the most extreme "materialist" theory: the psychophysical identity theory. Accordingly, every mental event corresponds to a physical event, because it is a physical event. The relation between mind and the relevant parts of the body is, therefore, like the relation between heat and molecular motion: They are precisely the same thing, observed in two different ways. As it happens, I find this view of the relation between mind and body very congenial.
However, I think it is often accompanied by a serious misunderstanding: the notion that when we find a parallel between physiological processes and mental or personality processes, the physiological process is what is really going on and the mental process is just a passive result of the physical process. What this overlooks is the reality of downward causation, the phenomenon in which an emergent property of a system can govern the position of elements within the system (Campbell, 1974; Sperry, 1969). Thus, the complex, symmetrical, six-pointed design of a snow crystal largely governs the position of each molecule of ice in that crystal.
Hence, there is no theoretical obstacle to acknowledging the fact that thoughts, desires, values and other mental phenomena can dominate bodily functions. Suppose that a man's mother dies, and he undergoes the agonizing trauma we call unbearable grief. There is no doubt that if we examine this man's bodily processes we will find many physical changes, among them changes in his blood and stomach chemistry. It would be clearly wrong to say that these bodily changes cause him to be grief-stricken. It would be less misleading to say that his being grief-stricken causes the bodily changes, but this is also not entirely accurate. His knowledge of his mother's death (interacting with his prior beliefs and values) causes his grief, and his grief has blood-sugar and gastric concomitants, among many others.
There is no dispute that various substances cause physiological changes in the bodies of people who ingest them. There is also no dispute, in principle, that these physiological changes may themselves change with repeated doses, nor that these changes may be correlated with subjective mental states like reward or enjoyment.
I say "in principle" because I suspect that people sometimes tend to run away with these supposed correlations. For example, changes in dopamine(Drug information on dopamine) levels have often been hypothesized as an integral part of the reward/reinforcement process. Yet research shows that dopamine in the nucleus accumbens does not mediate primary or unconditioned food reward in animals (Aberman and Salamone, 1999; Nowend et al., 2001; Salamone et al., 2001; Salamone et al., 1997). According to Salamone, the theory that drugs of abuse turn on a natural reward system is simplistic and inaccurate: "Dopamine in the nucleus accumbens plays a role in the self-administration of some drugs (i.e., stimulants), but certainly not all" (personal communication, Nov. 26, 2001).
Garris et al. (1999) reached similar conclusions: "Dopamine may therefore be a neural substrate for novelty or reward expectation rather than reward itself." They concluded:
- [T]here is no correlation between continual bar pressing during [intracranial self-stimulation] and increased dopaminergic neurotransmission in the nucleus accumbensåour results are consistent with evidence that the dopaminergic component is not associated with the hedonistic or 'pleasure' aspects of rewardåLikewise, the rewarding effects of cocaine do not require dopamine; mice lacking the gene for the dopamine transporter, a major target of cocaine, will self-administer cocaine. However, increased dopamine neurotransmission in the nucleus accumbens shell is seen when rats are transiently exposed to a new environment. The increase in extracellular dopamine quickly returns to normal levels and remains there during continued exploration of the new environmentådopamine release in the nucleus accumbens is related to novelty, predictability or some other aspects of the reward process, rather than to hedonism itself.
Perhaps, then, some people have been too ready to jump to conclusions about specific mechanisms. Be that as it may, chemical rewards have no power to compel--although this notion of compulsion may be a cherished part of clinicians' folklore. I am rewarded every time I eat chocolate cake, but I often eschew this reward because I feel I ought to watch my weight.