"The neural circuits involved with regulating emotions may be damaged in people with this condition," said Tom Johnstone, a neuroscientist at the University of Wisconsin-Madison's School of Medicine and Public Health and lead author of the study published in the journal Neuroscience.
One of the hallmarks of depression is that people with the condition seem to be unable to pull themselves out of a funk or black mood.
In order to figure out what goes awry in these situations, Johnstone and his colleagues conducted an experiment on a group of 21 depressed people and 18 healthy controls.
Researchers tried to manipulate them into a negative state of mind and then watched how well they could bounce back.
Specifically, they presented their guinea pigs with a series of images of things such as car accidents or threatening-looking animals and then asked them to consciously modulate their responses, by envisioning more positive outcomes than the one implied.
They also asked them to try imagining the situation was acted out rather than real.
"We ask them to reframe the content of what they're seeing," said Johnstone. "We hope to engage cognitive areas in re-interpreting the emotional content of a stimulus."
As expected, all of the individuals had increased brain activity in the prefrontal cortical areas that are known to regulate the emotional parts of the brain.
In the healthy individuals, high levels of prefrontal cortical activity correlated with low levels of activity in the amygdala, the almond-shaped structure that appears to regulate fear and anxiety.
In other words, they were able to quell their emotional response to the images.
In the depressed individuals, high levels of activity in the amygdala persisted in spite of the intense activity in the regulatory regions, and even increased in response to it, suggesting that their conscious effort to recalibrate their emotions was thwarted by dysfunctional brain circuits.
The researchers speculated that signals from the prefrontal cortical area of the brain are not getting through to the amygdala in the depressed individuals for reasons still unknown.
The findings suggest that cognitive behavioral therapies that hold that an individual can change the way they feel about a situation by changing the way they think about it may be counterproductive for some people.
"Our results suggest that there is a subgroup of patients with depression for whom traditional cognitive therapy may be contraindicated," said senior author Richard Davidson.
He noted that in some cases, the depressed individuals' mental effort to recalibrate their response only resulted in greater emotional activity.