Why does payday feel good? You can’t eat money, and it can’t have your babies — so how did that 'ker-ching!' feeling become so sweet?
Working with rats, neuroscientists have gained an insight into how the brain comes to take pleasure in abstract rewards. Animals, they suggest, have a reward system that focuses on specific outcomes — what an action would achieve — which in turn plugs into a more general system that lets us know what feels good.
Understanding how these two systems interact could help us understand what happens when they go wrong, such as in drug addiction or in general failures of willpower. The results are reported in Nature 1.
It’s hard to explain why people work for things that are not intrinsically gratifying, says neuroscientist Geoffrey Schoenbaum of the University of Maryland in Baltimore. “People are not normally working for primary rewards, such as food or sex, but for proxies, such as money.”
And, he says, they are able to plan their behaviour with distant goals in mind. “You work harder when you want a certain thing, like a new car.”
Sound and light
Separating the cognitive (goal-oriented) and general (emotional) systems is difficult, because achieving your goal makes you feel happy. Schoenbaum and his colleagues achieved it by using an ingenious variation on classical pavlovian conditioning.
First, the researchers taught rats to associate one light with a grape-flavoured sucrose pellet, and a different light with a banana-flavoured pellet. Such conditioning makes the lights gratifying on their own — animals will work to experience the cue, even if they don’t get a pellet.
Then, the team played sounds along with the lights. The ‘grape’ light with a sound still delivered a grape pellet. In this situation, animals tend to ignore the extra information and do not learn to associate the sound with food.
But the ‘banana’ light plus a sound led to a different reward – a grape-flavoured pellet. So in this case, the sound adds information. The light means something nice is coming and the sound tells you what flavour it will be.
Rats like the two flavours equally, so the sound says nothing about the treat’s value, only its details.
The team next tested the rats on sounds and lights alone. The animals, they found, will press a bar to obtain either the light or the sound on its own, even if no food pellet follows on. The generalized reward of the treat and the abstract property of its flavour were equally strong motivations.
But rats with damage to an area of their brain called the orbitofrontal cortex, which is thought to be involved in decision-making, would work to see the treat-associated light, but not to hear the grape-associated sound. That is, they will work for a cue associated with positive emotions, but not one linked only to a specific outcome.
It’s a bit like separating Homer Simpson’s “Mmm… donuts”, into a generalized expression of pleasure (“Mmm”), and the specific object of his desire (the "donuts"), and working out the brain regions responsible for each thought.
“It’s quite a brilliant experiment,” says Trevor Robbins, a neuroscientist at the University of Cambridge, UK. “It’s a very clever dissection of the learning mechanisms by which the stimuli that are associated with rewards also become rewarding.”
The brain police
Schoenbaum suggests that the orbitofrontal cortex, which lies at the front of the brain, just above the eyes, is the home of the brain’s cognitive reward system. It acts as a forecaster, predicting the value of different behaviours, learns which ones are ultimately rewarding, and triggers a corresponding emotional response.
“The orbitofrontal cortex is the apex of the brain’s reward system,” says Robbins.
Normally the two systems will give the same 'answer'. But the orbitofrontal cortex could also act as a kind of policeman, says Schoenbaum, diverting the pursuit of immediate gratification in favour of longer-term goals.
Sometimes, however, cues override goals, as when the paraphernalia of drug-taking can trigger cravings in addicts. “Addicts can become euphoric just from putting a needle in their arm, before the drug hits the brain,” says Robbins. In a milder form, our eyes might see the golden arches of McDonald's and our feet march us up to the counter, despite our resolution to cut down on milkshakes.
Next, Schoenbaum hopes to go after the brain regions that provide the emotional reward. Ultimately, he says, we may be able to “manipulate and correct” the two systems when they get dangerously out of balance.