Let’s talk about the opponent-process theory, which is 40 years old and essentially discarded, but has got that magical “it” factor, by which I mean I’ve always thought it was cool.
First, step back in time to a 19th-century theory of color perception. The crux of that old theory is Ewald Hering’s observation that while we can perceive some color combinations, like red-blue or yellow-green, others, like red-green or yellow-blue are inconceivable. Hering’s theory—the original opponent-process theory—explained this phenomenon by describing how information is process by cones (color receptors) in the eyes. Hering suggested that red + green (and blue + yellow) are opponent processes, meaning that looking at a red object causes red-sensitive cones to respond and simultaneously depresses responses from green-sensitive cones. A green object does the converse.
In other words, perception of red and green are not independent. They are antagonistic “opponent” responses that act like a seesaw: seeing red inhibits green, seeing green inhibits red (this is an oversimplificaton, see here for more). Because of that interdependence, “reddish green” is inconceivable—you can’t have both red and green receptors active simultaneously. “Blue-yellow” is the same. And this theory also explains why looking at red things produces a green afterimage: red receptors “tire out” and lead green to be overrepresented. For similar reasons, red-green colorblindness occurs, but not red-blue colorblindness.
In the 1970s, Richard Solomon applied similar concepts to his opponent-process theory of acquired motivation, which emphasizes the role of opponent emotional processes in drug tolerance and addiction. In Solomon’s theory, taking a drug produces two opposing internal processes, generically called the A-process and the B-process. The A-process is the direct physical effects of the drug: euphoria, sedation, analgesia, behaving like the town drunk in some early Irish novel, face-melting hallucinations, etc. In jargon of the theory, the A-process is said to have a positive hedonic value (that is, it feels good). When the drug kicks in, we feel good—maybe our digestion has slowed and heart rate decreased—but the body wants and tries to maintain homeostasis, and its going to counteract those effects with the B-process. To get back to “baseline,” the B-process has to have negative hedonic value. Or, in other words, it’s unpleasant.
The interesting part of the theory is not the existence of the A and B processes, but how they change over time. The A-process is the direct physical effect of the drug, so it’s going to onset quickly and dissipate relatively quickly as the drug is metabolized. The B-process is the body’s response to the A-process, so it must lag behind. On the left side of the picture, we see that after taking the drug, the A-process kicks in quickly and offsets quickly. The B-process, though, takes longer to kick in and taper off:
The net effect on hedonic state can be determined by adding the A and B together. On the right side above, the net effect is a quick positive peak before the B-process kicks in, then a leveling off, and finally a slow negative return to baseline when the B-process occurs in isolation.
By Solomon’s theory, drug tolerance and addiction occur because the B-process changes. At a given strength or dosage, a drug will always have the same direct effect; the A-process never changes, no matter how many “pots you’ve smoken.” But if one keeps taking the drug, the B-process gets stronger, onsets quicker, and fades away more slowly (left side of picture):
This changes the net effect. First, opponent processes interact to produce drug tolerance: the “high” from the A-process is smaller and therefore less pleasant than previously (left circle). It takes a bigger does to achieve the same effect. They also produce withdrawal symptoms: the negative state induced by the B-process takes longer to wear off (right circle). That unopposed B-process on the right-hand side is drug cravings and “dope sickness” and the serotonin rebound that ruins the Mondays of countless ravers. By this view, we can understand addiction almost entirely as a response to withdrawal symptoms: self-medicating to get back to an emotional baseline and short-circuit that unpleasant, isolated B-process. Tolerance occurs because the B-process kicks in faster and cuts into the positive hedonic value offered by the drug; withdrawal occurs when the B-process takes longer to dissipate.
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The really interesting part of the theory is the way it can be used to explain other behaviors. In drug addiction, the A-process is pleasant and the B-process is unpleasant. But what happens when the A-process is unpleasant? If they oppose, then when A is unpleasant, B must be pleasant. Follow this logic through and you arrive at the conclusion that, when the initial action is unpleasant, tolerance and withdrawal can be pleasant. Framed differently: the pleasure of some activities might derive not from the action itself, but the post-coital “rebound-effect” bliss once it’s over.
Here’s an example: if you “take a sauna,” the A-process is unpleasant heat, and the B-process the sense of cool relaxation once you leave. If we follow opponent-process theory, then an inveterate sauna taker develops a “tolerance” for searing heat, thus reducing its negative impact. Simultaneously, the experience an extended “withdrawal” that in this case is relaxing post-sauna bliss. Skydivers experience terror during the jump (A-process) and euphoria afterwards (B-process). Over time, the terror lessens and the euphoria extends. How about runner’s high? Giving blood? Watching horror movies? Bungee jumping, base jumping, and cow tipping? Perhaps all opponent-processes. (I suspect it’s possible that some serious thrill seekers manage to “double dip” in enjoying the “terror” of a bungee jump, whereas I’d douse my drawers). One 1970s book even suggests that romantic love is an addiction governed by opponent processes, which is either really cool or just depressing, I’m not sure which. In any case, it is a view in which we may do things not because we enjoy the action itself, but because the aftereffect is pleasant.
Solomon’s opponent-process theory is about 40 years old now, and it’s largely been left behind. This is not so much because it is wrong, but because it is so oversimplified that it turns out not to have a lot of meat on its bones. Despite that, it remains one of my favorite theories/findings in psychology.