How hard should I work? Ask dopamine
Why do I bother getting up in the morning? What is it that prods my brain into gear, convinces it — me — to shift out of the warm, duvet-y cocoon, and into the shower. Then get the kids up. Then feed the cats. Turns out the answer might be: dopamine. Dopamine — I hate you.
If you know one thing about dopamine then you know it is how the brain signals reward. Actually it doesn’t quite do that. Rather than signal how rewarding something is, it signals how unexpected the reward is. But alongside reward, studies in animals have repeatedly pointed to another mysterious role of dopamine, this time in motivation. Healthy rats will happily press a lever many times for a single chocolate wheat hoop. Rats with less dopamine won’t bother. If it’s not easy, then they’ll go do something more interesting. Like licking themselves.
This raises an obvious, but tricky, question. How can dopamine, a chemical released by brain cells, signal both reward and how bothered you are to get it? A recent study by Josh Berke and colleagues has made this a lot less mysterious.
They trained rats to get reward only after a torturous series of steps. “Alright Ratty,” they said, “if you’re hungry, pay attention: every time you see the start light go on put your nose in this hole here and hold it there until two lights go on; then take your nose out of the hole, and put it in one of the holes under a light. For one of the holes there will be a high likelihood of a “clunk” — that’ll be the chocolate arriving in the dispenser on the other side of the cage. Only then can you go and get the tiny sliver of chocolate. For the other hole-under-a-light, the likelihood of a clunk will be smaller. Sometimes much smaller.”
“Got that? If you mess up any stage, you have to start again.”
“Oh, and one more thing,” they added, “once you’ve worked out which hole has the highest likelihood of getting food, we’ll change them.” Off Ratty trotted, wondering if there was an easier way to get food. Meanwhile, the grad student down the corridor was trying to remember if you first put the money into the vending machine, then keyed in the number for the Hershey’s bar, or keyed in the number, then put in the money.
Rats are smart, so they got this task down. But clearly its complexity meant they needed to be motivated to do it. So Berke and co recorded the amount of dopamine in their striatum, a brain region crucial for deciding which actions to do next. Here dopamine influences the way neurons in the striatum respond to their inputs, and changes how strongly they are linked to those inputs. The striatum is then the best candidate for understanding how dopamine can be both an error signal and a motivation signal.
They found that rapid changes in the amount of dopamine happened at key points in the task: when the rat put its nose in the first hole; if it heard the clunk; and when collecting its reward. These rapid changes fitted with the expected error signal at these times. But, and this is a crucial but, over the whole task the amount of dopamine fitted best with the value of each key point in the task.
That is, the amount of dopamine recorded how valuable each point was to the rat. On trials where the rat figured out that a large reward was due at the end, the amount of dopamine increased continuously from the start to the end of the task, when it claimed its reward. If it was clear reward was not coming, because there was no clunk, then the amount of dopamine fell. This value signal is all we need to work out whether it is worth doing a task or not.
As a clincher, Berke and co manipulated the amount of dopamine in the striatum, to show that changing value could change the rat’s motivation. And it could: when they artificially increased the amount of dopamine at the start light, the rats were faster at putting their nose in the starting hole.
Berke and co’s work — not to mention the rats’ hard work — has shown how dopamine can be both error and motivation. It suggests we work hard for things that we find valuable, and dopamine tells us what we find valuable. More dopamine sloshing around translates to harder, more engaged, work. I’m off to find me some dopamine for the winter mornings….
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Reference: Hamid, A. A., Pettibone, J. R., Mabrouk, O. S., Hetrick, V. L., Schmidt, R., Vander Weele, C. M., Kennedy, R. T., Aragona, B. J. & Berke, J. D. (2016) Mesolimbic dopamine signals the value of work. Nature Neuroscience, 19, 117–126. dx.doi.org/10.1038/nn.4173
