What generates pessimistic mood?

Scientists have identified a brain region that could generate pessimistic moods in disorders such as anxiety or depression that leads people to focus more on the possible downside than the potential benefit in a stressful situation.

In a study tested on animals, researchers at the Massachusetts Institute of Technology (MIT) have found that stimulating caudate nucleus -- a brain region linked to emotional decision-making --induced animals to make negative decisions. 

The caudate nucleus, has within it regions that are connected with the limbic system, which regulates mood, and sends input to motor areas of the brain as well as dopamine-producing regions. 

The study showed that the animals gave far more weight to the anticipated drawback of a situation than its benefit, compared to when the region was not stimulated. 

This pessimistic decision-making could continue through the day after the original stimulation.

"We feel we were seeing a proxy for anxiety, or depression, or some mix of the two," said Ann Graybiel, a professor at the MIT. 

In the study, which appeared in the journal Neuron, the team wanted to see if they could reproduce an effect that is often seen in people with depression, anxiety, or obsessive-compulsive disorder. 

The team stimulated the caudate nucleus with a small electrical current as animals were offered a reward (juice) paired with an unpleasant stimulus (a puff of air to the face). 

The results showed that the cost-benefit calculation became skewed, and the animals began to avoid combinations that they previously would have accepted.

This continued even after the stimulation ended, and could also be seen the following day, after which point it gradually disappeared.

This result suggests that the animals began to devalue the reward that they previously wanted, and focused more on the cost of the aversive stimulus. 

"This state we've mimicked has an overestimation of cost relative to benefit," Graybiel noted.

The researchers also found that brainwave activity in the caudate nucleus was altered when decision-making patterns changed.

"There must be many circuits involved," she said. 

"But apparently we are so delicately balanced that just throwing the system off a little bit can rapidly change behaviour."