Depression changes the circuits in the brain, study finds

Depression alters the brain’s response to positive and negative stimuli, particularly in the amygdala, a new study has found.

The study, carried out by the Institut Pasteur and France’s National Centre for Scientific Research (CNRS), found that depression reduces neural activity linked to positive perceptions while increasing it for negative perceptions.

The researchers investigated the amygdala, a part of the brain involved in processing emotions, with a view to finding out how it functions during depressive episodes.

The study involved looking at the behaviour of mice exhibiting behaviour characterised by anxiety and stress (for example, they stopped self-grooming, stayed close to walls and preferred to be in the dark). These mice also responded differently to smells – appearing less attracted by a smell that would normally be attractive while being strongly repulsed by smells from predators.

The scientists measured the activity of neurons involved in response to smells. Their findings, published in the journal Translational Psychiatry, suggest that a depressive state alters certain neural circuits. These lead to a decrease in the activity of neurons indicating a pleasurable response to a positive stimulus, but an increase in the activity of neurons related to the distress felt in response to a negative stimulus.

The researchers hope that their findings might pave the way for the development of new drugs that could work for the 30% of people with depression who don’t respond to anti-depressant drugs.

“We now know that the amygdala is not only involved in our emotional response to environmental stimuli, fostering attraction or repulsion, but that it also plays a role in depression,” said Mariana Alonso, co-author of the study and head of the emotional circuits group at the Institut Pasteur.

She added: “Recent research has demonstrated the role of certain specific neural circuits in the amygdala in the positive or negative perception of environmental stimuli, but the alteration of these circuits during a depressive episode had not previously been observed.”

People with depression much more likely to experience mind-wandering

A separate study, published in the Journal of Affective Disorders, has found that individuals with major depressive disorder report mind-wandering twice as often as healthy adults.

Mind wandering is the spontaneous shift of attention away from a current task or external environment to internal thoughts or daydreams, and typically occurs when people are engaged in routine or low-demand activities.

In people with depression, mind wandering tends to focus on negative thoughts, regrets, or worries, contributing to a persistently low mood. Studies suggest that people with depression experience more frequent and uncontrollable mind wandering, increasing cognitive load and interfering with concentration and productivity.

Study author Matthew Welhaf and his colleagues at Washington University aimed to better understand the frequency of mind wandering in individuals with major depressive disorder (MDD) in everyday life compared to healthy individuals. They also aimed to explore the content of mind wandering. The study required the 106 participants to report their experiences several times per day. Fifty-three were healthy controls with no history of mental health disorders, and the other 53 had been diagnosed with major depressive disorder.

Eight prompts a day in the form of beeps required the participants to report whether they were mind wandering at the time of the prompt, what they were thinking about and how they felt. Results showed that participants with major depressive disorder reported mind wandering in 37% of prompts, compared to only 17% for healthy controls.

Participants with major depressive disorder also reported that their mind wandering had a negative emotional tone in 42% of cases, compared to only 10% among healthy participants.