Most of us, at some point in our lives, have experienced or displayed a pessimistic attitude. But what exactly is pessimism? Is there an area of the brain responsible for pessimism? Pessimism is a mental attitude in which an undesirable outcome is anticipated from a situation. Pessimists tend to focus on the negative aspects of that situation or even life in general.
Many patients with psychological disorders, such as anxiety or depression, experience negative mood states that lead them to focus on the potential drawbacks of a given situation rather than on the potential benefits.
A team of neuroscientists has identified a region of the brain that can generate this type of pessimistic mood. Research suggests that both anxiety and depression are caused by an overstimulation of the caudate nucleus .
A new study, led by Ann Graybiel, a professor at MIT (Massachusetts Institute of Technology) in Cambridge and published in the journal Neuron, examines the neurological underpinnings of pessimism in mice and also finds clues to anxiety and depression in humans.
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.
The area of the brain responsible for pessimism
Researchers have shown that stimulating the caudate nucleus can generate negative mood states that lead to irrational decision-making. According to the results of the study, stimulation of the caudate nucleus causes animals to give much more weight to the anticipated disadvantage of a situation than its potential benefit.
For the study, Graybiel and his colleagues focused on a type of decision-making process known as approach-avoidance conflict. Approach-avoidance conflict describes situations in which people (or mammals) have to decide between two options, weighing the positive and negative aspects of each alternative.
The caudate nucleus is considered the area of the brain responsible for pessimism due to overstimulation.
Previous research by this same team had already identified a neural circuit that underlies a specific type of decision-making known as approach-avoidance conflict. These types of decisions, which require weighing options with both positive and negative elements, tend to provoke high levels of anxiety.
They had also shown that chronic stress dramatically affects this type of decision making : more stress generally leads animals to choose high-risk, high-reward options.
In the new study, the researchers wanted to see if they could replicate an effect often seen in people with depression, anxiety or obsessive-compulsive disorder. These patients tend to engage in ritualistic behaviors designed to combat negative thoughts and to give more weight to the possible negative outcome of a given situation. The researchers suspected that this type of negative thinking might influence approach-avoidance decision making.
To recreate the scenario in which rodents have to choose by weighing the positive and negative aspects, the scientists offered the mice a little juice as a reward, but combined it with an aversive stimulus : a puff of air in the face.
Emotional decisions
To test this hypothesis, the researchers stimulated the caudate nucleus, a region of the brain linked to emotional decision-making . Over the course of several trials, the researchers varied the ratio of reward to unpleasant stimuli and gave the rodents the ability to choose whether to accept the reward with the aversive stimulus or not.
As the researchers explain, this model requires rodents to perform a cost-benefit analysis. If the reward is high enough to balance the air puff, the animals will choose to accept it, but when the ratio is too low, they reject it.
When the researchers stimulated the caudate nucleus, the cost-benefit calculation was thrown off course and the animals began to avoid combinations they would have previously accepted . This continued even after the stimulation had ended, and could also be seen the next day, after which it gradually disappeared.
This result suggests that the animals began to devalue the reward, and focused more on the cost of the aversive stimulus. Graybiel explains that this state they had imitated involves an overestimation of the cost in relation to the benefit.
Anxiety and depression, a delicate balance
The researchers also found that brain wave activity in the caudate nucleus was altered when decision-making patterns changed . This change is in the beta frequency and could serve as a biomarker to monitor whether animals or patients respond to drug treatment, the researchers explain.
Researchers are working on studying patients suffering from depression and anxiety to see if their brains show abnormal activity in the neocortex and caudate nucleus during approach avoidance decision making. MRI studies have shown abnormal activity in two regions of the medial prefrontal cortex that connect to the caudate nucleus.
The caudate nucleus has regions within it that are connected to the limbic system that regulates mood and sends information to motor areas of the brain as well as dopamine-producing regions. Researchers believe that the abnormal activity seen in the caudate nucleus in this study could somehow be altering dopamine activity.
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- Amemori, K., Amemori, S., Gibson, D., and Graybiel, A. (2018). Striatal Microstimulation Induces Persistent and Repetitive Negative Decision-Making Predicted by Striatal Beta-Band Oscillation. Neuron , 99 (4), 829-841.e6. doi:10.1016/j.neuron.2018.07.022