Researchers have made up their minds on which part of the brain is most responsible for decision making.

One of the smallest parts of the brain is getting a second look after new research suggests it may be in charge of our internal cost-benefit see-saw.

A study published in Nature Neuroscience says the lateral habenula, a region of the brain linked to depression and aversive behaviours, may be calling quite a lot of the shots.

The lateral habenula is one of the oldest regions of the brain, evolutionarily speaking.

“These findings clarify the brain processes involved in the important decisions that we make on a daily basis, from choosing between job offers to deciding which house or car to buy,” says Professor Stan Floresco from the University of British Department of Psychology and Brain Research Centre.

“It also suggests that the scientific community has misunderstood the true functioning of this mysterious, but important, region of the brain.”

Scientists trained lab rats for the study, teaching them to choose between a consistent small reward (one food pellet) and a potentially larger reward (four food pellets) which only appeared sporadically.

Just like humans, the rats chose larger rewards when costs - the amount of time they had to wait before receiving food - were low. The mice preferred smaller rewards when such risks were higher.

Previous studies had suggested that turning off the lateral habenula causes rats to choose larger, riskier rewards more often, but that was not the case.

In turns out the rats were selecting either option at random, no longer showing the ability to choose the best option for them.

The findings have important implications for treating depression.

“Deep brain stimulation - which is thought to inactivate the lateral habenula - has been reported to improve depressive symptoms in humans,” Floresco says.

“But our findings suggest these improvements may not be because patients feel happier. They may simply no longer care as much about what is making them feel depressed.”

More information is available in the full report, now published in Nature Neuroscience.