Research Finds Possible Root Cause Of Pessimism

Research done by a team of scientists at MIT has identified a possible source for pessimistic reactions and negative moods in the brain, a region called the caudate nucleus. Animal tests have revealed that the caudate nucleus seems to play a substantial role in viewing things in a pessimistic light, and the research offers the possibility of creating better treatments for mood disorders like depression and anxiety.


Role Of The Caudate Nucleus

Anxiety, depression and similar negative mood disorders are frequently defined by pessimistic reactions to stimuli, and researchers hope that if they could influence that reaction better treatments for disorders could be created. The research team investigated the role of the caudate nucleus in making decisions and found that when the caudate nucleus was stimulated in macaques, the monkeys were much noticeably more likely to consider potential drawbacks of a decision versus the benefits of the decision and to make negative decisions in general.

An interesting finding was that the length of the pessimistic episode went well beyond when the stimulation ended, with the pessimistic mood persisting for a day or so after the stimulation ceased. The researchers say this implies that the caudate nucleus is playing a role in, or is acting as a proxy for, depression or anxiety.

Previous research has established that the caudate nucleus plays a role in emotional decision-making. In this specific experiment, the researchers stimulated the caudate nucleus with a mild electrical current, while the monkeys were presented with the option to receive a reward (some tasty juice) at the same time as an aversive stimulus (a blast of air to the face).

The researchers aimed to replicate a mental state often witnessed in patients of anxiety, obsessive-compulsive disorder, or depression. Those who have these disorders often place much more weight on the possible negative outcomes of a situation over the possible positive outcomes.


Overestimating Cost

The experimenters conducted various trials and in each trial, they varied the amount of juice and the strength of the puff of air. This allowed there to be various reward/consequence ratios, and for the animals to be able to weight the cost/benefits of taking the juice or not taking the juice. The stimulation of the caudate nucleus seemed to have a pronounced effect on the macaques, skewing their decisions in the negative direction so that the monkeys started rejecting ratios of juice/air that they would have accepted prior to the stimulation.

Photo: RyanMcGuire via Pixabay, CC0

Ann Graybiel, the lead researcher on the study and member of MIT’s McGovern Institute for Brain Research, says that the stimulation of the caudate nucleus seems to make animals devalue the rewards they were previously interested in and focus more on the potential drawbacks or costs of the events they are engaging with.

“This state we’ve mimicked has an overestimation of cost relative to benefit,” Graybiel says.

When examining the brainwave activity within the caudate nucleus the researchers found that the basal ganglia, a structure that contains the caudate nucleus, changed along with the changes in decision making patterns. Doctors could potentially use this to determine how responsive a person would be to treatments which target the caudate nucleus.


The exact nature of the relationship between the caudate nucleus and the basal ganglia, when it comes to things like anxiety and depression, isn’t fully understood. It was thought that the frontal cortex was mainly responsible for the control of these emotions, but the new study challenges this idea and gives some insight into how the basal ganglia is involved in pessimistic decision-making. Professor of neuroscience at the University of California, Santa Barbara, Scott Grafton (not involved in the research) explains the importance of the work.

“We know that the frontal cortex and the basal ganglia are involved, but the relative contributions of the basal ganglia have not been well understood. This is a nice paper because it puts some of the decision-making process in the basal ganglia as well,” Grafton says.

Future Research

Photo: Free-Photos via Pixabay, CC0

The brain’s complexity makes teasing apart mechanisms and effects from one part of the brain on another difficult, but the research team thinks that the results of the initial study are promising and that the caudate nucleus is quite likely to play a role in the disruption of dopamine in the brain. Dopamine helps regulate our mood and our sense of pleasure and reward. The caudate nucleus has regions of it that are linked with the brain’s limbic system, known to regulate mood, and it sends signals to dopamine-producing regions, as well as ask motor areas.

The abnormal spike of activity tracked within the caudate nucleus, Graybiel explains, could quite be inhibiting activity. Graybiel explains that many circuits are involved with pessimistic outlooks and the suppression of dopamine and that the study suggests we are “so delicately balanced” that a few changes to the system can have profound effects on behavior.

While the initial tests focused on how macaques responded to stimulation of the caudate nucleus, the next phase of research will be investigating if humans respond similarly to stimulation of the nucleus. Previous research has indeed noted a link between the caudate nucleus and abnormal brain activity, but much more work will have to be done to understand the exact mechanisms responsible for the link and to determine what therapeutic interventions might be possible.

Graybiel explains that the next steps of the project involve collaborating with psychiatrists at the Mclean Hospital to research those who have anxiety or depression disorders. The goal is to see if the brains display abnormal activity within the caudate nucleus and neocortex when they engage in pessimistic decision making. MRI studies have found abnormal activity in two areas that connect the caudate nucleus and medial prefrontal cortex.

As always, more research will have to be done to see if the findings of this study hold true under other conditions. For now, those who work on treatments for anxiety and depression may want to pay close attention to future research involving the basal ganglia and caudate nucleus.



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