WASHINGTON, Aug. 9 (Xinhua) -- American neuroscientists identified a brain region that can generate pessimistic mood of many patients with anxiety or depression.

A study published on Thursday in the journal Neuron revealed that, in animal tests, stimulating a region known as the caudate nucleus could cause more negative decisions.

Animals would give far more weight to the anticipated drawback of a situation than its benefit, compared to when the region was not stimulated, and the pessimistic decision-making could continue through the day after the stimulation, according to the study.

"We feel we were seeing a proxy for anxiety, or depression, or some mix of the two," said the paper's senior author Ann Graybiel, a member of McGovern Institute for Brain Research at Massachusetts Institute of Technology (MIT).

The researchers stimulated the caudate nucleus, a brain region linked to emotional decision-making, with a small electrical current as animals were offered a reward (juice) paired with an unpleasant stimulus (a puff of air to the face).

In each trial, the ratio of reward to aversive stimuli was different, and the animals could choose whether to accept or not. If the reward is high enough to balance out the puff of air, the animals will choose to accept it, but when that ratio is too low, they reject it.

When the researchers stimulated the caudate nucleus, 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, according to the study.

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.

Also, they found that brainwave activity in the caudate nucleus was altered when decision-making patterns changed.

This change is in the beta frequency and might serve as a biomarker to monitor whether animals or patients respond to drug treatment, according to Graybiel.

Graybiel is now working with psychiatrists at McLean Hospital to study patients who suffer from depression and anxiety, to see if their brains show abnormal activity in the neocortex and caudate nucleus during approach-avoidance decision-making.

Magnetic resonance imaging (MRI) studies have shown abnormal activity in two regions of the medial prefrontal cortex that connect with the caudate nucleus.

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

The researchers suggested that the abnormal activity seen in the caudate nucleus in this study could be somehow disrupting dopamine activity.

"Apparently we are so delicately balanced that just throwing the system off a little bit can rapidly change behavior," said Graybiel.