Neuroscientists at MIT believe that they have discovered why patients with neuro-psychiatric disorders, such as anxiety or depression, might experience moods that lead them to focus on the negative side of things.
The researchers claim to have pinpointed a region in the brain that can generate this type of pessimistic mood and, in tests in animals, showed that stimulating this region - known as the caudate nucleus - induced animals to make more negative decisions.
According to the tests, the animals appeared to give far more weight to the anticipated drawback of a situation than its benefit, compared to when the region was not stimulated, with the pessimistic decision-making sometimes continuing throughout the day after the original stimulation.
We feel we were seeing a proxy for anxiety, or depression, or some mix of the two
The researchers say the findings could help scientists better understand how some of the crippling effects of depression and anxiety arise, and guide them in developing new treatments.
"We feel we were seeing a proxy for anxiety, or depression, or some mix of the two," said MIT Institute Professor Ann Graybiel, who is also a member of MIT's McGovern Institute for Brain Research, and the senior author of the study.
"These psychiatric problems are still so very difficult to treat for many individuals suffering from them."
The paper's lead authors are McGovern Institute research affiliates Ken-ichi Amemori and Satoko Amemori, who have been studying emotion and how it is controlled by the brain. McGovern Institute researcher Daniel Gibson, an expert in data analysis, is also an author of the paper.
These patients tend to engage in ritualistic behaviours designed to combat negative thoughts
In the study, the researchers wanted to see if they could reproduce an effect that is often seen in people with depression, anxiety, or obsessive-compulsive disorder.
"These patients tend to engage in ritualistic behaviours designed to combat negative thoughts, and to place more weight on the potential negative outcome of a given situation," the report says.
It's this kind of negative thinking that the researchers suspected could influence approach-avoidance decision-making.
To test this hypothesis, they 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," the report states.
Graybiel is now working with psychiatrists at McLean Hospital to study patients who suffer from depression and anxiety
This type of decision-making required cost-benefit analysis. So, if the reward is high enough to balance out the puff of air, the animals would choose to accept it, but when that ratio is too low, they would 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.
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 added.
Graybiel is now working with psychiatrists at McLean Hospital to study patients who suffer from depression and anxiety, to see if their brains show the same abnormal activity in the neocortex and caudate nucleus during approach-avoidance decision-making.
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