The Guess Worker

No one knows much about pain. Nothing is known about how pain arises in the consciousness. Almost nothing is known about the physiology and biochemistry of how pain is created in the brain. When it comes to pain, science is ignorant.

Of course we can expect that, in time, discoveries will be made which will make everything clearer. Until then the only thing we can do is speculate. Given the little knowledge we have, it might be thought that speculation wouldn't get us very far. But actually, just on the basis of a few experiments and by considering what pain does, we can make some good guesses. With this approach I believe it's possible to understand even some of the biochemistry of pain.

What motivates us?

Let's start with a biochemical called dopamine. Dopamine is a neurotransmitter found in the brains of many animals, including humans. This substance has been shown to be necessary for motivation. For example, rats deficient in dopamine are so unmotivated that they barely feed themselves and starve to death if they are not force-fed. (See last post "A conversation with Richard Palmiter").

Now, we can say that motivation is caused by wants. Our wants motivate us to do things. The dopamine-deficient rats don't want food and so they aren't motivated to eat food. What this means is that if dopamine is necessary for motivation, we can also say that dopamine is necessary for wanting.

In "Pain and pleasure" I discussed the idea that wants are a kind of pain. If this is true, then there must be a connection between dopamine and pain. What this connection is, however, is not immediately obvious. We might think that because dopamine-deficient rats don't have wants, they also don't have pain. Therefore dopamine causes pain - and the more dopamine there is, the more pain there is. But this reasoning is wide off the mark. Judging by dopamine-deficient rats' reactions to unpleasant stimuli, they can feel pain. And they apparently experience pleasure much less than normal rats.

Dopamine used to be the pleasure biochemical

In fact, until about ten years ago, it was thought that dopamine worked in the opposite way - dopamine was considered to be the biochemical which caused pleasure. High levels of dopamine in the brain were thought to result in happiness and the enjoyment of life, and low levels were thought to cause misery and depression. One reason for believing this was the reactions of human beings to certain drugs. Drugs which stimulated dopamine release in the brain caused feelings of euphoria or pleasure. Another reason came from experiments in rats: rats with higher dopamine levels appeared to enjoy rewards more than those with dopamine deficiency.

But this idea has fallen out of favour since. In subsequent experiments, rats with very high levels of dopamine don't show increased liking (that is, pleasure) to sweets. But they do seem to show an increase in wanting. In contrast, rats with very low levels of dopamine still show responses of liking and disliking to stimuli but show very little wanting.

These results led some scientists (notably Professor Kent Berridge of Michigan University) to conclude that dopamine doesn't have anything to do with pleasure at all. Dopamine, they argued, is only involved in wanting and motivation. Wanting, then, must be an experience quite separate from pain and pleasure. Kent Berridge went on to propose a concept of wanting which he called "incentive salience". According to this idea, wanting is caused by attention being focussed on a particular object. So for example, the aroma from a bakery causes someone to want to eat because he or she focuses on the smell.

Big question marks

This concept of wanting is, of course, quite different to mine. I can see several problems with it. First, it is hard to see why focussing on an object by itself would be motivating. Second, pleasure does appear to have some role in motivation and so it's not easy to understand what role that would be if pleasure and wanting are entirely separate. Then, because pain doubtlessly also motivates, there would have to be at least two motivational systems - one involving pain and another involving wanting. (Conceivably there even might be a third system involving pleasure.) Evolution is unlikely to have developed two or three systems, if one system can do the job.

There are other objections. If I am in pain, I want to get rid of the pain. Therefore wanting and pain sometimes can be linked. Why would wanting at times be separate from pain and at others times involve pain? Furthermore how could we explain with the incentive salience model that wants can become painful if we neglect them? If we neglect the want to eat, this want becomes hunger - a painful want.

Big question marks, then, hang over the incentive salience model. But how else could the apparent separation of wanting and liking be explained? How could dopamine have any relationship to pain and pleasure given the results of the experiments with rats?

To answer these questions, I'm going to return to the idea that dopamine causes pleasure. But instead of a static system - in which a high level of dopamine means pleasure and a low level means pain - I'm going to suggest that the process is dynamic. I'll describe how this could work in the next post.