Decades of reinforcing neural connections can make the adult brain stubbornly resistant to rapid changes. Should our brain's structure trap us in cycles of dark moods and thoughts, disorders like chronic depression can be extremely hard to shake.

According to new research, some patients with major depressive disorder (MDD) can have their brains 'rewired' within weeks, given the right treatment.

Anti-depressants, behavioral therapy, and electroconvulsive therapy don't work for everybody with MDD, but scientists in Germany claim these treatments have the power to change brain structures. How long those changes last remains to be determined.

People who experience MDD often have trouble regulating negative emotions and their physiological responses to stress. Under such heavy conditions, enjoying even the most pleasant activities in life can feel burdensome.

In the past, brain imaging studies have found serious depression is associated with changes in the volume of gray matter (made up of neuron bodies) and white matter (made up of nerve fibers). It's also associated with increases in amygdala activity, which influences emotional experiences; shrinkage in the hippocampus, which plays a major role in long-term learning and memory; and shrinkage in the basal ganglia, which helps process emotions.

Treatment-resistant MDD, on the other, is marked by changes in the basal ganglia and the lobe that processes sensory information.

If there really is a strong link between the structure of a human brain and the function of depression, it could seriously help improve diagnosis and treatment.

Today, however, researchers disagree on whether this link is consistent or robust enough to rely on.

Researchers in Germany think it is.

Their new study was presented at the 35th annual European College of Neuropsychopharmacology in Vienna. The findings suggest that some of the structural brain features found in MDD patients were alleviated when antidepressant treatment was successful.

Before receiving depression treatment, 109 patients with MDD had their brains scanned using a magnetic resonance imaging ( MRI) machine. Then, patients were treated with either electroconvulsive therapy, psychological therapy or anti-depressant medication, or a combination of all the therapies.

Six weeks after the first MRI scan, patients had their brains scanned again. The 'before and after' results were then compared with the brains of 55 healthy participants.

Ultimately, the authors found patients with the greatest symptom improvements also showed the most structural brain changes. After six weeks, the connectivity between neurons in certain parts of their brain had increased, and these effects were independent of treatment choice.

"We were surprised at the speed of response," says psychiatrist Jonathan Repple from the University of Frankfurt.

"We don't have an explanation as to how these changes take place, or why they should happen with such different forms of treatment".

Randomized controlled trials have shown that electroconvulsive therapy, cognitive behavior therapy and antidepressants can all significantly improve depressive symptoms, but it's much harder to connect this improvement to structural changes in the brain.

ECT appears to be the most effective and rapid treatment of the bunch, though it does come with more side effects and scientists are still trying to figure out the most successful regime.

ECT works by passing an electrical current through a patient's brain while they are under general anesthetic, and in mouse studies, the therapy seems to improve communication between neurons in certain parts of the brain. In fact, researchers from Johns Hopkins University recently found new brain cells developing in the hippocampus of ECT-treated mice.

Studies among humans have found similar results. In 2015, for instance, ECT was found to 'mould' the brain structures of some MDD patients, re-shaping neural connections in their amygdala and hippocampus.

Antidepressant drugs, meanwhile, are associated with neural plasticity in the hippocampus and prefrontal cortex, and CBT is associated with altered brain activation in the prefrontal cortex and precuneus, which is linked to mental imagery and memory.

Despite being evidence-based, these depression treatments are not always equally effective, nor do they result in consistent levels of structural brain changes.

The brain is an incredibly complex structure, and human emotions are pretty darn intricate themselves. Piecing the two together is incredibly challenging work, but that hasn't stopped scientists like Repple from trying.

The psychiatrist Eric Ruhe, who was not involved in the current study, commends Repple and his colleagues for the difficulty of their most recent paper.

He says the study still needs to be replicated in independent samples, but that the results "align very much with our current belief that the brain has much more flexibility in adaptation over (even short) time than was previously thought."

"This means that the brain structure of patients with serious clinical depression is not as fixed as we thought, and we can improve brain structure within a short time frame, around 6 weeks," explains Ruhe.

"This gives hope to patients who believe nothing can change and they have to live with a disease forever, because it is 'set in stone' in their brain"."

The work was presented at the 35th annual European College of Neuropsychopharmacology in Vienna.