The Scicurious Brain

The Scicurious Brain

The Good, Bad, and Weird in Physiology and Neuroscience

Dopamine goggles make the glass half full


When it comes down to it, most humans are pretty optimistic. Yeah, we know the Titanic sank, but our boat is better. We know that driving a car is really pretty dangerous, but we're more careful, it won't happen to us. This is not just a cultural thing, we generally tend to place more importance on positive information about something than on negative. We're more optimistic than we should be on everything, from the future of our current relationship to the stock market.

But what is it that makes us so optimistic? And what happens when it goes wrong? Because not everyone is optimistic. People with major depressive disorder, for example, are more pessimistic (sometimes they are just pessimistic enough to be realistic, but they can also be unrealistically pessimistic). What is it that determines how optimistic we are?

It's time to take another look at dopamine.

Sharot et al. "How Dopamine Enhances an Optimism Bias in Humans" Current Biology, 2012.

I often talk about the neurotransmitter dopamine in the context of addiction. But dopamine is a much more subtle signal than just the "reward" or "pleasure" you see thrown around in the media. Dopamine is extremely important in detecting prediction errors: when you've made the wrong choice. Basically, dopamine can spike in the presence of reward, and can spike when a reward is expected. Conversely, you often see decreases in dopamine when something unexpected happens and you fail to get your reward, like with a Rickroll. That's a prediction error, you predicted a reward and it didn't happen.

Dopamine's role in predicting successes and reacting to errors is incredibly important for our behavior. It is one of the many processes that allow us to learn from our experiences and revise our future actions accordingly. But even dopamine tends to lean positive! When give the dopamine precursor L-DOPA (you can't give dopamine, it does not cross the blood brain barrier. L-DOPA does, so you can give it to people and it will be broken down in to dopamine when it reaches the brain) to patients with Parkinson's, they show increased learning. But it's asymmetric, with the patients in general showing preference for positive outcomes over negative ones.

So does dopamine specifically increase our preference for positive outcomes? To test this, the authors of this study took 21 healthy patients and gave them either a placebo or L-DOPA (each participant did the task twice over a period of two weeks, and received both treatments in randomized order), to increase the dopamine in their brains. They then had them do a series of belief update tasks. These tasks basically ask you about the likelihood of something horrid happening to you, say, the likelihood of getting hit by a low flying helicopter as you sunbathe on your roof. You guess, and then it tells you how much that likelihood really is in percentages. Then you have to update your beliefs. How likely do you think it is now that you'll get hit by a low flying helicopter?

You can see here that people make two kinds of estimation errors. Either they estimate positively (thinking that they have a higher chance of being hit by a helicopter), or they estimate negatively (thinking they have a lower chance). In general, people will estimate they have a lower chance. Then the authors asked the participants to revise their estimates. Most people who are optimistic will still think they have a lower chance of being hit by a helicopter.

But if you give these patientis L-DOPA, increasing their dopamine, they get even MORE optimistic.

What you can see here are the revised predictions the subjects made after they had seen the real statistics. The L-DOPA bars are the paler ones (which is somewhat confusing). You can see that when the real statistics were desirable (0% chance of getting hit by a low flying helicopter!), people changed their predictions to match no matter what. After all, the reality sounded great. But when the statistics presented were undesirable (what you MEAN 80% of roof sunbathers suffer helicopter distress?!), the placebo group updated their opinions to match the facts (the right red bar). The L-DOPA group, on the other hand, updated their beliefs a lot less than the placebo group. They saw the numbers, but it didn't change their gut feeling on being hit by a helicopter. Their belief updates were impaired.

But is it dopamine itself? The authors tried the same thing with an acute dose of the antidepressant citalopram, which selectively increases serotonin. There they saw no differences, people were just as optimistic or pessimistic as before.

This means that when dopamine is increased, we have a lowered ability to take in negative outcomes. We see them, and we remember them...but we just don't think they can happen to us. And this may be important when you consider people with depression, who are often far more pessimistic than average. It's possible that increased dopamine might help them nurture a more positive view, which might in turn help mood, though it's definitely too early to say until it is tested. But this study does show that optimism may be influenced by dopamine levels even in healthy people. And that's a pretty glass half-full kind of study.

Sharot, T., Guitart-Masip, M., Korn, C., Chowdhury, R., & Dolan, R. (2012). How Dopamine Enhances an Optimism Bias in Humans Current Biology, 22 (16), 1477-1481 DOI: 10.1016/j.cub.2012.05.053

The views expressed are those of the author and are not necessarily those of Scientific American.

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