The function of the lateral habenula, one of the smallest brain regions and evolution-wise considered one of the oldest, has been somewhat of a mystery to science, although there is evidence that it is involved in depression and avoidance behavior. Now, a new Canadian study of rats suggests it plays an important role in making cost-benefit decisions.

Researchers from the University of British Columbia (UBC) write about their findings in a recent online issue of Nature Neuroscience.

Co-author Stan Floresco, a professor in UBC's Department of Psychology and Brain Research Centre, says:

"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."

He says they also suggest scientists have misunderstood the true role of this mysterious but important brain region.

Researchers studied rats with active and inactive lateral habenula

Together with Colin Stopper, a PhD candidate in his lab, Prof. Floresco trained laboratory rats to choose between a consistent small reward (a single food pellet) or a potentially larger reward (four food pellets) that appeared sporadically.

Like humans, the rats tended to opt for the larger reward when the cost of doing so - in this case, the amount of time they had to wait for the food to appear - was low, and they preferred the smaller rewards when the cost was high.

Then they examined what happened to the rats' choosing behavior when they inactivated the lateral habenula.

Previous studies have suggested this would make the rats choose the larger, riskier reward more often. But this is not what they found.

Instead, when the lateral habenula was inactivated, the rats appeared unable to decide the best option for themselves when the choices had different cost-benefit amounts.

The researchers write:

"... [lateral habenula] inactivation abolished choice biases, making rats indifferent when choosing between rewards associated with different subjective costs and magnitudes, but not larger or smaller rewards of equal cost."

Implications for deep-brain stimulation as treatment for depression

Given that the lateral habenula is known to be involved in depression, the researchers suggest their findings have important implications for the use of deep-brain stimulation in the treatment of the disease.

Prof. Floresco says:

"Deep brain stimulation - which is thought to inactivate the lateral habenula - has been reported to improve depressive symptoms in humans. 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."

The researchers say more studies are now needed to fully understand what is going on in the brain during cost-benefit decisions and related behavior.

There could be further implications for the understanding and treatment of conditions including schizophrenia, stimulant abuse and depression, they note, because these are also linked to impairments in cost-benefit decision processes.

Another study published recently in Psychological Science suggests that higher emotional intelligence leads to better decisions. One of the researchers said people with high emotional intelligence are able to remove emotions that have nothing to do with the decision.