A little over 15 years ago, biopsychologist and neuroscientist Dr. Kent Berridge, of the University of Michigan, along with his colleague Dr. Terry Robinson, set out to show that dopamine is not the pleasure transmitter neuroscientists once thought it was. Now that dopamine has been proven not to be the source of all pleasure, Berridge is busy pinpointing exactly where and how pleasure does occur in the brain.
Brain World: So what exactly is happiness?
Kent Berridge: Happiness is complicated, because there are different ingredients. Pleasure is only one ingredient; a sense of meaningfulness and engagement in life are also important. Focusing on the pleasure ingredient, people have many types of pleasures — there’s sexual, intellectual, and sensory, to name a few. But the brain mechanisms of all these pleasures seem to overlap.
So the brain mechanisms that generate sensory pleasures may also help generate the pleasures of happiness, even though happiness isn’t tied to any sensation or event; it’s free-floating, it can come and go, or be frequent or infrequent. You can’t guarantee a happy response to a certain experience. The important thing about whether an experience will have a happy response lies in the brain of the person.
Different experiences will make different people feel happy. For some people, that means looking at art, thinking about moral pleasures, even meditating — all these experiences can produce the same physical “happy” response in the brain, but via very different triggers. But because sensory pleasures are more easily assessed in a scientific setting, we study them to find the brain mechanisms, and then can extrapolate to other kinds of pleasures.
BW: Does that mean that sensory pleasure is the only real pleasure?
KB: No, even “normal” sensory responses can change with experience. What makes something is how the hot spots in the brain respond. So if you get nausea after eating a sweet food for the first time, that same sweet food might taste bad to you in the future — giving you displeasure. Similarly, “unpleasant” flavors such as the bitter taste of coffee or beer, for example, can become pleasurable sensations because you learn to like them. You learn to like them because of the way the brain responds to them, and we’re still discovering how and why this is happening.
BW: What elements go together to make happiness from an experience?
KB: What we’ve found is that there is liking, wanting, and learning in reward, all factors in happiness that are interconnected within the brain. If we tweak the brain, these systems come apart and show us that the component mechanisms are different. Wanting is not the same as liking, for example. This is very clear when it comes to addiction. Dopamine, for example, is important in the reward mechanism in the brain, and for a long time it was thought to be what made us like things.
But in some addictive individuals, the dopamine system becomes hyperreactive, so that more drugs, say, turns on more dopamine. If dopamine provided us with happiness, it would make these addicts happier than nonaddicts or let them get more pleasure from a smaller dose of drugs. That’s clearly not the case. When we looked deeper into this, we found that dopamine provided the wanting mechanism in the brain, rather than the pleasure mechanism. That’s why addicts want more and more, whether it be in the form of binge-eating, gambling, or drugs. And when there’s an imbalance between wanting and pleasure, such as for addicts, it actually leads to unhappiness.
BW: So if dopamine isn’t the source of pleasure, what is?
KB: That’s what we’re trying to find out. We’re focused on both liking and wanting as brain mechanisms. We’ve pinpointed these hedonistic hot spots in the brain, areas that are specifically designed to amplify pleasures, producing natural opioids and cannabinoids. These are like a circuit — one hot spot can’t turn on without the other; unanimity is needed in these pleasure-generating mechanisms.
BW: So does everyone experience happiness in the same ways?
KB: When psychologists ask people if they’re happy, most will say they are. On a scale of 1 to 10, most will put themselves at 5 or 6, but some people put themselves as above 8 — those are the lucky ones. Perhaps surprisingly, people who score themselves as being 9 or 10 might not be as socially engaged, or as affluent or successful in life. So maybe there’s such a thing as being too happy, at least from that point of view. When you probe further into how people rate their own happiness, you find that pleasure isn’t the only factor, it’s also based on satisfaction.
This is why a person who, for example, eats chocolate all day long or masturbates nonstop, say, may not describe themselves as happy, even when they are getting a lot of pleasure. Pleasure without satisfaction is meaningless. This goes back to Socrates — it’s not a new insight. But still, pleasure is needed to be happy. There’s very little happiness if there’s not any pleasure. So my job is to find out how pleasure comes about in the brain, and we hypothesized that there are hot spots in the brain relating to pleasure for both nice sensations and happiness.
BW: How do you go about pinpointing these hedonistic hot spots?
KB: We work on rats, painlessly manipulating their brains. First we put a cannula [a thin tube] in the brain, using micro-injection under anesthesia. That way we can add a tiny droplet of chemical into the brain structure and the rat doesn’t feel it, but it will activate the systems we want to pinpoint.
We also use optogenetics, a technique devised by Karl Deisseroth of Stanford University. We inject a special virus into the brain, which carries a gene for a photoreceptor molecule, and it only affects particular neurons. Then, as it is now sensitive to light, we can use a laser light to trigger responses. This may be even better than adding drugs, because we can turn off the effect from one moment to the next, whereas when we add chemicals to the brain the effect can last for half an hour or more, making it more difficult to pinpoint exactly how the rat has responded.
We’ve found that the hedonistic hot spots form a looping circuit, running from the prefrontal cortex down to the nucleus accumbens, downwards further to the ventral pallidum and lower structures, and then back up to the top through the mesolimbic dopamine system, that adds the wanting component to a pleasure that’s liked. Now we’re looking at each of these different stages of the circuit, figuring out exactly how they work together to produce pleasure and reward. We’ll assess a rat’s response to cocaine, morphine, sugar — tweaking its brain each time so we can see how differently it responds.
In time, we hope to know exactly how the pleasure generators work together to create pleasure and, importantly, what puts a person into a state where pleasure is no longer present. Understanding the wanting side would also help us understand addiction better, and understanding the pleasure-liking side may also give a bit more insight into happiness.
This article was first published in Brain World Magazine’s Fall 2012 issue.