New research has solved the mystery behind why abstract things like a payday make us so happy.
The neuroscientists suggest that animals 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.
According to neuroscientist Geoffrey Schoenbaum of the University of Maryland in Baltimore, it's hard to explain why people work for things that are not intrinsically gratifying.
"People are not normally working for primary rewards, such as food or sex, but for proxies, such as money," Nature quoted him, as saying.
Schoenbaum added that individuals are able to plan their behaviour with distant goals in mind.
"You work harder when you want a certain thing, like a new car," he said.
In the study, the researchers first 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.
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 study is published in Nature 1.