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Dopamine and Motivation

The research on the association between dopamine and reward has led to a general conception that dopamine is a pleasure chemical in the brain.

Dopamine and pleasure

This association now permeates the popular media: major news outlets present stories with titles like music releases the brain’s pleasure chemical and addiction in the brain’s pleasure pathway. But does the activity of dopamine neurons really correspond to pleasure?

One of the great things about observing animals in experiments that you get to know their tendencies pretty well. Some researchers led by the psychologist Kent Berridge knew that when rats receive a squirt of juice they tended to take on a standard sort of facial expression: they leaned forward made around mouth and stuck their tongue out.

You’ve seen the sort of expression before: it’s the same expression that a young child makes when they taste something good. Rats, monkeys, humans and many other mammals all show the same sort of facial expression when they like something.

Berridge and colleagues also noticed that when rats received something unpleasant like juice tinted with a bitter additive they made a different facial expression: they leaned back they gape their mouth and they use their tongue to make a barrier across its open.

You can simulate this yourself just imagine that you lifted what he thought was a glass of cold freshwater to your lips took a sip and then realize that it was actually bad. If you imagine tasting something unpleasant, not something disgusting or revolting but unpleasant, then you’ll likely generate a facial expression of dislike.

Dopamine and wanting

Just as for expressions of pleasure facial expressions associate with their version are similar in rats monkeys humans and many other mammals. These researchers realize that they can rely on facial expressions to assess whether a rat found some juice pleasant or unpleasant so they ran the critical test to evaluate whether dopamine neurons were associated with pleasure.

They used a very selective neurotoxin that lesioned or selectively damaged neurons in the part of the dopamine system of the rats. Without those neurons in good working function the rats became aphagic which means they wouldn’t eat. More specifically they wouldn’t exert any effort to obtain food or even juice.

They wouldn’t press levers and experiments, they wouldn’t run through a maze, they wouldn’t even walk across a room even if they’re hungry or thirsty. Without an intact dopamine system these rats no longer wanted rewards like food or juice, but they would still eat or drink if the food or juice was placed directly into her mouth.

So what happened when these aphagic rats? Those with a damage dopamine system received something pleasurable like sugar water. They leaned forward made around mouth and licked their face and pause, exactly like neurologically normal rats do. And, they liked the sugar water!

And, when the aphagic rats received a better solution, lithium water, they gave their mouth protruded their tongue and shook their head again exactly like the neurologically normal routes. They dislike the bitter solution! So, if part of the rats dopamine system is disabled it will not walk across his cage to consume food, but still shows normal hedonic responses the pleasant and unpleasant stimuli.

These results provided compelling evidence that dopamine neurons are not associated with experiences of pleasure or liking but they are associated with motivating animals to seek out rewards what researchers call wanting.

A different kind of addiction

So what causes liking of is not dopamine? Current research suggests that feelings of liking are associated with the activity of a different sort of neuron, one that uses opioids as neurotransmitters. But there’s still much more to be learned. The distinction between wanting and liking can be seen in several sorts of pathological decision-making.

Consider drug addiction. We now know that essentially all addictive drugs act on the brain’s dopamine system in some way. Some addictive drugs cause dopamine neurons to fire more frequently, other drugs because those neurons to release more dopamine when they fire and still others cause dopamine to have greater effects on other neurons.

Take cocaine for example. Normally, after a dopamine molecule is released by a neuron it floats around and influences adjacent neurons in until is pumped back into the original dopamine neurons, but when cocaine is ingested it prevents those re-uptake pumps from doing their job. So, the rewarding effects of dopamine become more intense and longer lasting.

The feeling of wanting, especially for cocaine itself, becomes greatly exaggerated
when addiction becomes chronic, addicts may no longer experience pleasure from consuming their drug of choice but their overactive dopamine system still pushes them to want more and more. Drugs become more valuable to addicts even as they become less pleasurable.

What dopamine does

So I described two features of these dopamine:

  1. First, there firing isn’t associated with rewards themselves, but with changes in reward from expectations: this is called reward prediction error, and
  2. Second, dopamine neurons aren’t necessary for the pleasure associated with reward or liking, but are necessary for the motivation to consume rewards, or for wanting.

So, to understand many real world decisions we need to first understand why people value some things more than others. One possibility is that value comes from experienced or anticipated pleasure as introduced by Bentham, but that isn’t consistent with our brain’s reward system is actually set up.

The key dopamine neurons aren’t necessary for pleasure. Instead these dopamine neurons respond in a way that encourages us to seek out new and better rewards. They increase their firing rate when a reward is better than our expectations but they decrease their firing rate when a reward is worse than our expectations and they don’t change their firing when our expectations are exactly met.

Our brain response to new information about rewards not necessarily to rewards themselves.