Neuroskeptic readers will know that I'm a big fan of theories. Rather than just poking around (or scanning) the brain under different conditions and seeing what happens, it's always better to have a testable hypothesis.
I just found a 2007 paper by Israeli computational neuroscientists Niv et al that puts forward a very interesting theory about dopamine. Dopamine is a neurotransmitter, and dopamine cells are known to fire in phasic bursts - short volleys of spikes over millisecond timescales - in response to something which is either pleasurable in itself, or something that you've learned is associated with pleasure. Dopamine is therefore thought to be involved in learning what to do in order to get pleasurable rewards.
But baseline, tonic dopamine levels vary over longer periods as well. The function of this tonic dopamine firing, and its relationship, if any, to phasic dopamine signalling, is less clear. Niv et al's idea is that the tonic dopamine level represents the brain's estimate of the average availability of rewards in the environment, and that it therefore controls how "vigorously" we should do stuff.
A high reward availability means that, in general, there's lots of stuff going on, lots of potential gains to be made. So if you're not out there getting some reward, you're missing out. In economic terms, the opportunity cost of not acting, or acting slowly, is high - so you need to hurry up. On the other hand, if there's only minor rewards available, you might as well take things nice and slow, to conserve your energy. Niv et al present a simple mathematical model in which a hypothetical rat must decide how often to press a lever in order to get food, and show that it accounts for the data from animal learning experiments.
The distinction between phasic dopamine (a specific reward) vs. tonic dopamine (overall reward availability) is a bit like the distinction between fear vs. anxiety. Fear is what you feel when something scary, i.e. harmful, is right there in front of you. Anxiety is the sense that something harmful could be round the next corner.
This theory accounts for the fact that if you give someone a drug that increases dopamine levels, such as amphetamine, they become hyperactive - they do more stuff, faster, or at least try to. That's why they call it speed. This happens to animals too. Yet this hyperactivity starts almost immediately, which means that it can't be a product of learning.
It also rings true in human terms. The feeling that everything's incredibly important, and that everyday tasks are really exciting, is one of the main effects of amphetamine. Every speed addict will have a story about the time they stayed up all night cleaning every inch of their house or organizing their wardrobe. This can easily develop into the compulsive, pointless repetition of the same task over and over. People with bipolar disorder often report the same kind of thing during (hypo)mania.
What controls tonic dopamine levels? A really brilliantly elegant answer would be: phasic dopamine. Maybe every time phasic dopamine levels spike in response to a reward (or something which you've learned to associate with a reward), some of the dopamine gets left over. If there's lots of phasic dopamine firing, which suggests that the availability of rewards is high, the tonic dopamine levels rise.
Unfortunately, it's probably not that simple, as signals from different parts of the brain seem to alter tonic and phasic dopamine firing largely independently, and this would mean that tonic dopamine would only increase after a good few rewards, not pre-emptively, which seems unlikely. The truth is, we don't know what sets the dopamine tone, and we don't really know what it does; but Niv et al's account is the most convincing I've come across...
Niv Y, Daw ND, Joel D, & Dayan P (2007). Tonic dopamine: opportunity costs and the control of response vigor. Psychopharmacology, 191 (3), 507-20 PMID: 17031711