One Part Of Your Brain Can Lead To Depression When Pessimistic And Tracking Negative Events
Our pessimism and how we respond to negative events is a significant determiner of our mental health and how we experience joy and sorrow. A study published in Molecular Psychiatry, found that a pea-sized region of the brain that responds to bad experiences has the opposite reaction to expectations of aversive events in people with depression compared to healthy adults.
People who expect bad things to happen may not actually be able to control a pessimistic outlook. Recently, British researchers discovered an area of the brain called the "habenula" warns us that something bad will happen. Although this tiny (smaller than a pea) sized region is useful for helping us learn from bad past experiences, if it is overactive, it can promote depressive thinking and pessimistic attitudes.
"A prominent theory has suggested that a hyperactive habenula drives symptoms in people with depression: we set out to test that hypothesis" says senior author Professor Jonathan Roiser (UCL Institute of Cognitive Neuroscience). "Surprisingly, we saw the exact opposite of what we predicted. In people with depression, habenula activity actually decreased when they thought they would get a shock. This shows that in depressed people the habenula reacts in a fundamentally different way. Although we still don't know how or why this happens, it's clear that the theory needs a rethink."
The habenula is involved in many functions including: pain processing, reproductive behaviors, nutrition, sleep-wake cycles, stress, and learning. Lateral habenula function tends to have links to reward processing. It helps encode negative feedback or negative rewards. It is thought that the habenula may interact with the basal ganglia, and other neurotransmitter systems. The lateral habenula in particular is able to signal information-prediction errors and reward-prediction errors.
Activity: It becomes overactive when we have worse expectations for a particular scenario. The activity is thought to decrease in those with more optimistic outlooks.
Signaling: It tends to signal how much we expect negative outcomes. There is greater signaling when we expect significantly worse scenarios.
Tracking experiences: It is able to track our experiences and help us learn from past negative events.
The researchers scanned the brains of 25 people with depression and 25 never-depressed individuals using high-resolution functional magnetic resonance imaging (fMRI). The participants were shown a sequence of abstract pictures while they lay inside the scanner. Over time they learned that different pictures were associated with a chance of different outcomes - either good or bad. Images predicting electric shocks were found to cause increased habenula activation in healthy volunteers, but decreased activation in depressed people.
There were no differences in average habenula size between people with depression and healthy volunteers. However, people with smaller habenulae, in both groups, were found to have more symptoms of anhedonia, pessimism and a loss of interest or pleasure in life.
"The habenula's role in depression is clearly much more complex than previously thought," explains lead author Dr. Rebecca Lawson (UCL Wellcome Trust Centre for Neuroimaging). "From this experimental fMRI study we can draw conclusions about the effects of anticipated shocks on habenula activation in depressed individuals compared with healthy volunteers. We can only speculate as to how this deactivation is linked to symptoms, but it could be that this ancient part of the brain actually plays a protective role against depression. Animal experiments have shown that stimulating the habenula leads to avoidance, and it is possible that this occurs for mental as well as physical negative events. So one possible explanation is that the habenula may help us to avoid dwelling on unpleasant thoughts or memories, and when this is disrupted you get the excessive negative focus that is common in depression."